Tag Archives: quantum physics

Ultracold atoms spun on a string form quantum tornadoes

Like weather patterns on Earth, the spinning of a fluid of quantum particles led to the formation of swirling ‘quantum crystals’. Credit: MIT.

The familiar universe around us behaves in largely predictable ways, physically speaking. It’s why we’re confident to board a flight or we don’t act surprised when a computer performs a task exactly as instructed. But when zooming into the world of the very small, at the atomic level, our assumptions about how the universe works — a model known as classical physics — start to break down. Case in point, MIT physicists have coaxed a bunch of ultra-chilled atoms to exhibit a never before seen phenomenon: a crystal made of ‘quantum tornadoes’.

Welcome to the bizarre world of quantum physics. Get in, no time to explain!

Before delving into the specifics of this latest research, it’s worth taking a trip down memory lane, back to the 1980s. This was a fruitful decade in physics, with a lot of particle physics activity that eventually led to the discovery of a new family of matter known as quantum Hall fluids and consisting of clouds of electrons suspended in magnetic fields.

Classical physics dictates that the electrons in the Hall ‘fluid’ should repel each other and arrange themselves in an orderly lattice, forming a crystal. Except that no. Just no. Instead, the particles always adjusted their behavior to what their neighbors were doing, all in a correlated way.

“People discovered all kinds of amazing properties, and the reason was, in a magnetic field, electrons are (classically) frozen in place—all their kinetic energy is switched off, and what’s left is purely interactions,” says Richard Fletcher, assistant professor of physics at MIT. “So, this whole world emerged. But it was extremely hard to observe and understand.”

Fletcher and Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT, wondered if they could replicate this effect in an experiment that makes it easier to see. Electrons in a magnetic field move in very small increments, and the researchers thought that since the motion of atoms under rotation can occur over much larger length scales, they could afford to use ultracold atoms in lieu of electrons to make a nice light show.

For their study, they used lasers to trap around one million sodium atoms, cooling them to around 100 nanokelvins, a hair’s breadth away from absolute zero. A system of electromagnets both further confined the atoms and collectively spun them around like marbles in a bowl at about 100 rotations per second.

Using high-speed precision optical cameras to observe what was happening at the molecular level, the physicists found that the atoms spun into a long thread at around the 100-millisecond mark. This was the threshold at which the atoms’ behavior crossed the quantum barrier.

“In a classical fluid, like cigarette smoke, it would just keep getting thinner,” Zwierlein says. “But in the quantum world, a fluid reaches a limit to how thin it can get.”

“When we saw it had reached this limit, we had good reason to think we were knocking on the door of interesting, quantum physics,” adds Fletcher. “Then the question was, what would this needle-thin fluid do under the influence of purely rotation and interactions?”

As atoms continued to spin, quantum instability kicked in. The thread wavered, then turned the shape of a corkscrew, before finally breaking into a string of rotating blobs resembling tornadoes. The authors called the resulting structure a ‘quantum crystal’, whose shape is purely the result of the interplay between the rotation of the fluid and the forces between the atoms.

“This crystallization is driven purely by interactions, and tells us we’re going from the classical world to the quantum world,” said Fletcher in a statement.

According to the researchers, the evolution of the spinning atoms in the gas broadly mimics how Earth’s rotation creates large-scale weather patterns. A fine example of how the very small and the very large are not that disconnected as the wackiness of quantum physics might lend us to believe.

“The Coriolis effect that explains Earth’s rotational effect is similar to the Lorentz force that explains how charged particles behave in a magnetic field,” Zwierlein notes. “Even in classical physics, this gives rise to intriguing pattern formation, like clouds wrapping around the Earth in beautiful spiral motions. And now we can study this in the quantum world.”

“This evolution connects to the idea of how a butterfly in China can create a storm here, due to instabilities that set off turbulence,” Zwierlein explains. “Here, we have quantum weather: The fluid, just from its quantum instabilities, fragments into this crystalline structure of smaller clouds and vortices. And it’s a breakthrough to be able to see these quantum effects directly.”

The findings appeared in the journal Nature.

Flyboard Air from Zapata.

Hoverboards are now real — and the science behind them is dope

What could be the coolest way of going to work you can imagine? Let me help you out. Flying cars — not here yet. Jetpacks — cool, but not enough pizzaz. No, there’s only one correct answer to this question: a hoverboard.

A whole generation of skateboarders and sci-fi enthusiasts (especially Back to the Future fans) have been waiting for a long time to see an actual levitating hoverboard. Well, the wait is over. The future is here. 

Franky Zapata flying on Flyboard Air. Image credits: Zapata/YouTube.

There were rumors in the 90s that claimed hoverboards had been invented but were not made available in the market because some powerful parent groups are against the idea of flying skateboards being used by children. Well, there was little truth to those rumors — hoverboards haven’t been truly developed until very recently. No longer a fictional piece of technology, levitating boards exist for real and there is a lot of science working behind them.

A hoverboard is basically a skateboard without tires that can fly above the ground while carrying a person on it. As the name implies, it’s a board that hovers — crazy, I know.

The earliest mention of a hoverboard is found in Michael K. Joseph’s The Hole in the Zero, a sci-fi novel that was published in the year 1967. However, before Michael Joseph, American aeronautical engineer Charles Zimmerman had also come up with the idea of a flying platform that looked like a large hoverboard.

Zimmerman’s concept later became the inspiration for a small experimental aircraft called Hiller VZ-1 Pawnee. This bizarre levitating platform was developed by Hiller aircraft for the US military, and it also had a successful flight in 1955. However, only six such platforms were built because the army didn’t find them of any use for military operations. Hoverboards were feasible, but it was still too difficult to build them with the day’s technology.

Hoverboards were largely forgotten for decades and seemed to fall out of favor. Then, came Back to the Future.

A page from the book Back to the Future: The Ultimate Visual History. Image credits: /Film

The hoverboard idea gained huge popularity after the release of Robert Zemeckis’s Back to the Future II in 1989. The film featured a chase sequence in which the lead character Marty McFly is seen flying a pink hoverboard while being followed by a gang of bullies. In the last two decades, many tech companies and experts have attempted to create a flying board that could function like the hoverboard shown in the film.

Funnily enough, Back to the Future II takes place in 2015, and hoverboards were common in the fictional movie. They’re not quite as popular yet, but they’re coming along.

The science behind hoverboards

Real hoverboards work by cleverly exploiting quantum mechanics and magnetic fields. It starts with superconductors — materials that have no electrical resistance and expel magnetic flux fields. Scientists are very excited about superconductors and have been using them in experiments like the Large Hadron Collider.

Because superconductors expel magnetic fields, something weird happens when they interact with magnets. Because magnets must maintain their North-South magnetic field lines, if you place a superconductor on a magnet, it interrupts those field lines, and the magnet lifts the superconductor out of its way, suspending it into the air.

A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Image credits: Mai Linh Doan.

However, there’s a catch: superconductors gain their “superpowers” only at extremely low temperatures, at around -230 degrees Fahrenheit (-145 Celsius) or colder. So real-world hoverboards need to be fueled with supercooled liquid nitrogen around every 30 minutes to maintain their extremely low temperature. 

All existing hoverboards use this approach. While there has been some progress in creating room-temperature superconductors, this technology is not yet ready to be deployed in the real world. But then again, 30 minutes is better than nothing.

Some promising hoverboards and the technology behind them

In 2014, an inventor and entrepreneur Greg Henderson listed a hoverboard prototype Hendo hoverboards on the crowdfunding platform Kickstarter. The Hendo hoverboard could fly 2.5 cm above the ground with 300 lb (140 kg) of weight but just like maglev trains, it required a magnetic track made of non-ferromagnetic metals to function. 

The hoverboard followed magnetic levitation, a principle that allows an object to overcome gravitation and stay suspended in the air in the presence of a magnetic field. However, the hoverboard didn’t go into mass production because Henderson used the gadget only as a means to promote his company Arx Pax Labs.

A year later, another inventor (Cătălin Alexandru Duru) developed a drone-like hoverboard prototype (which is registered under the name omni hoverboard) and using the same approach, he set a Guinness World Record for covering maximum distance with an autonomous hoverboard. During his flight, Alexandru covered a distance of about 276 meters and reached a height of 5 meters. 

ARCA CEO Dumitru Popescu controlling his ArcaBoard through body movement. Image Credits: Dragos Muresan/Wikimedia Commons

In 2015, Japanese auto manufacturer Lexus also came up with a cool liquid-nitrogen-filled hoverboard that could levitate when placed on a special magnetic surface. The Lexus hoverboard consists of yttrium barium copper oxide, a superconductor which if cooled down beyond its critical temperature becomes repulsive to magnetic field lines. The superconductor used both quantum levitation (and quantum locking) to make the hoverboard perfectly fly over a magnetic surface.

The same year in December, Romania-based ARCA Space Corporation introduced an electric hoverboard called ArcaBoard. Being able to fly over any terrain and water, this rechargeable hoverboard was marketed as a new mode of personal transportation. The company website mentions that ArcaBoard is powered by 36 in-built electric fans and can be easily controlled either from your smartphone or through the rider’s body movements.   

Components in an ArcaBoard. Image Credits: ARCA

One of the craziest hoverboard designs is Franky Zapata’s Flyboard Air. This hoverboard came into the limelight in the year 2016 when Zapata broke Cătălin Alexandru Duru’s.Guinness World Record by covering a distance of 2,252.4 meters on his Flyboard Air. This powerful hoverboard is capable of flying at a speed of 124 miles per hour (200 km/h), and can reach as high as 3000 meters (9,842 feet) up in the sky. 

Flyboard Air comes equipped with five jet turbines that run on kerosene and has a maximum load capacity of 264.5 lbs (120 kg). At present, it can stay in the air for only 10 minutes but Zapata and his team of engineers are making efforts to improve the design further and make it more efficient. In 2018, his company Z-AIR received a grant worth $1.5 million from the French Armed Forces. The following year, Zapata crossed the English Channel with EZ-Fly, an improved version of Flyboard Air.

While ArcaBoard really went on sale in 2016 at an initial price of $19,900, Lexus Hoverboard and Flyboard Air are still not available for public purchase. However, in a recent interview with DroneDJ, Cătălin Alexandru Duru revealed that he has plans to launch a commercial version of his omni hoverboard in the coming years.

Physicists make 2-D supersolid that flows without friction, a world first

Credit: IQOQI Innsbruck/Harald Ritsch.

Almost 50 years since scientists imagined what supersolidity — a peculiar quantum state whereby atoms are arranged in a regular pattern but, at the same time, can flow frictionless — might look like, researchers have now demonstrated a two-dimensional supersolid quantum gas in the lab for the first time.

In a gas described by classical physics, you could theoretically label every single constituent atom of the gas and always know its position and momentum. However, you can never know this kind of information for each particle individually in a quantum gas.

At relatively high temperatures, the classical gas model is a good approximation for the behavior of the fluid. After all, engineers have been using classical physics equations for decades and our planes fly nicely and predictably, for instance. However, at very low temperatures approaching absolute zero, atoms and molecules slow to a crawl, and fluid behavior is more accurately described as a quantum gas — a behavior that can be quite challenging to wrap one’s head around.

For instance, at 0.000001 degrees above absolute zero, atoms become so densely packed they behave like one super atom, acting in unison. Atoms can form an exotic form of matter called Bose-Einstein condensate (BEC), also known as the fifth state of matter, in which individual atoms are completely delocalized. This means that the same atom exists at each point within the condensate at any given time, something that makes no sense from a classical physics perspective. It is what it is.

Quantum fluids like BECs tend to exhibit some “quantum” macroscopic behaviors such as superfluidity (fluid flow with zero viscosity) or superconductivity (electrical flow with zero resistance). Apparently, there’s also such a thing as supersolids, bizarre materials whose atoms are arranged in an orderly lattice but which, nevertheless, flow without friction.

In 2019, researchers led by Francesca Ferlaino, a physicist at the University of Innsbruck in Austra, demonstrated a supersolid state in an ultracold quantum gas of magnetic atoms for the first time. However, this effort could only attain supersolid states in a string of one-dimensional droplets. Now, the researchers have pushed the envelope by extending this phenomenon to two dimensions.

 “Normally, you would think that each atom would be found in a specific droplet, with no way to get between them,” says Matthew Norcia of Francesca Ferlaino’s team.

“However, in the supersolid state, each particle is delocalized across all the droplets, existing simultaneously in each droplet. So basically, you have a system with a series of high-density regions (the droplets) that all share the same delocalized atoms.”

The 2-D crystal-like structure is locked in a rigid structure but also delocalized at the same time, a phenomenon that is made possible strong polarity of magnetic ultra-chilled atoms. In doing so, the physicists have created a solid structure with the properties of a superfluid.

Like any respectable quantum physics experiment, this research opens up more questions than it answers. For instance, it’s not clear if it’s possible to make supersolids at a larger scale.

The findings appeared in the journal Nature.

Computer simulation of gravitational wave emissions S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)

What are Gravitational Waves?

Whilst it may not have the snappiest name, the event GW150914 is pretty significant in terms of our understanding of the Universe. This event, with a name that includes ‘GW’ as a prefix which is an abbreviation of ‘Gravitational Wave’ and the date of observation–15/09/14– marked humanity’s first direct detection of gravitational waves.

This was groundbreaking on two fronts; firstly it successfully confirmed a prediction made by Albert Einstein’s theory of general relativity almost a century before. A prediction that stated events occurring in the Universe do not just warp spacetime, but in certain cases, can actually send ripples through this cosmic fabric.

Numerical simulation of two inspiralling black holes that merge to form a new black hole. Shown are the black hole horizons, the strong gravitational field surrounding the black holes, and the gravitational waves produced ( S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project, W. Benger (Airborne Hydro Mapping GmbH)).

The second significant aspect of this observation was the fact that it represented an entirely new way to ‘see’ the Universe, its events and objects. This new method of investigating the cosmos has given rise to an entirely new form of astronomy; multimessenger astronomy. This combines ‘traditional’ observations of the Universe in the electromagnetic spectrum with the detection of gravitational waves, thus allowing us to observe objects that were previously invisible to us.

Thus, the discovery of gravitational waves truly opened up an entirely new window on the cosmos, but what are gravitational waves, what do they reveal about the objects that create them, and how do we detect such tiny tremblings in reality itself?

Gravitational Waves: The Basics

  • Gravitational waves are ripples in the fabric of spacetime.
  • These ripples travel from their source at the speed of light.
  • The passage of gravitational waves squash and stretch space itself.
  • Gravitational waves can be detected by measuring these infinitesimally small changes in the distance between objects.
  • They are created when an object or an event that curves spacetime causes that curvature to change shape.
  • Amongst the causes of gravitational waves are colliding black holes and neutron stars, supernovae, and stars that are undergoing gravitational collapse.

Theoretical Underpinnings

Imagine sitting at the side of a lake, quietly observing the tranquil surface of the water undisturbed by nature, the wind, or even by the slightest breeze. Suddenly a small child runs past hurling a pebble into the lake. The tranquillity is momentarily shattered. But, even as peace returns, you watch ripples spread from the centre of the lake diminishing as they reach the banks, often splitting or reflecting back when they encounter an obstacle.

The surface of the lake is a loose 2D analogy for the fabric of spacetime, the pebble represents an event like the collision of two black holes, and our position on Earth is equivalent to a blade of grass on the bank barely feeling the ripple which has diminished tremendously in its journey to us.

Poincaré and Einstein both saw the possibility of gravitational waves propagating through space-time at the speed of light

Gravitational waves were first predicted by Henri Poincare in 1905 as disturbances in the fabric of spacetime that propagate at the speed of light, but it would take another ten years for the concept to really be seized upon by physicists. This happened when Albert Einstein predicted the same phenomenon as part of his revolutionary 1916 geometric theory of gravity, better known as general relativity.

Whilst this theory is most well-known for suggesting that objects with mass would cause warping of spacetime, it also went a step further positing that an accelerating object should change this curvature and cause a ripple to echo through spacetime. Such disturbances in spacetime would not have been permissible in the Newtonian view of gravity which saw the fabric of space and time as separate entities upon which the events of the Universe simply play out.

But upon Einstein’s dynamic and changing stage of united spacetime, such ripples were permissible.

Gravitational waves arose from the possibility of finding a wave-like solution to the tensor equations at the heart of general relativity. Einstein believed that gravitational waves should be generated en masse by the interaction of massive bodies such as binary systems of super-dense neutron stars and merging black holes.

The truth is that such ripples in spacetime should be generated by any accelerating objects but Earth-bound accelerating objects cause perturbations that are far too small to detect. Hence why our investigations must turn to areas of space where nature provides us with objects that are far more massive.

As these ripples radiate outwards from their source in all directions and at the speed of light, they carry information about the event or object that created them. Not only this, but gravitational waves can tell us a great deal about the nature of spacetime itself.

Where do Gravitational Waves Come From?

There are a number of events that can launch gravitational waves powerful enough for us to detect with incredibly precise equipment here on Earth. These events are some of the most powerful and violent occurrences that the Universe has to offer. For instance, the strongest undulations in spacetime are probably caused by the collision of black holes.

Other collision events are associated with the production of strong gravitational waves; for example the merger between a black hole and a neutron star, or two neutron stars colliding with each other.

But, a cosmic body doesn’t always need a partner to make waves. Stellar collapse through supernova explosion–the process that leaves behind stellar remnants like black holes and neutron stars– also causes the production of gravitational waves.

A simulation of gravitational waves emitted by a binary pulsar consisting of two neutron stars

To understand how gravitational waves are produced, it is useful to look to pulsars–binary systems of two neutron stars that emit regular pulses of electromagnetic radiation in the radio region of the spectrum.

Einstein’s theory suggests that a system such as this should be losing energy by the emission of gravitational waves. This would mean that the system’s orbital period should be decreasing in a very predictable way.

The stars draw together as there is less energy in the system to resist their mutual gravitational attraction, and as a result, their orbit increases in speed, and thus the pulses of radio waves are emitted at shorter intervals. This would mean that the time it takes for the radio wave to be directly facing our line of sight would be reduced; something we can measure.

This is exactly what was observed in the Hulse-Taylor system (PSR B1913±16), discovered in 1974, which is comprised of two rapidly rotating neutron stars. This observation earned Russell A. Hulse and Joseph H. Taylor, Jr, both of Princeton University, the 1993 Nobel Prize in Physics. The reason given by the Nobel Committee was: “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.”

An animation illustrating how gravitational waves are emitted by two neutron stars as they orbit each other eventually colliding (credit: NASA/Goddard Space Flight Center).

Though inarguably an impressive and important scientific achievement, this was still only indirect evidence of gravitational waves. Whilst the effect Einstein predicted of shortening of the pulsar’s spin was definitely present, this wasn’t an actual direct detection.

In fact, though not alive to witness this momentous achievement, Einstein had predicted that this would be the only way we could ever garner any hint of gravitational waves. The great physicist believed those spacetime ripples would be so faint that they would remain impossible to detect by any technological means imaginable at that time.

Fortunately, Einstein was wrong.

How do we Detect Gravitational Waves?

It should come as no surprise that actually detecting a gravitational wave requires a piece of equipment of tremendous sensitivity. Whilst the effect of gravitational waves–the squashing and stretching space itself–sounds like something that should pre-eminently visible, the degree by which this disturbance occurs is so tiny it is totally imperceptible.

Fortunately, there is a branch of physics that is pretty adept at deal with the tiny. To spot gravitational waves, researchers would use an effect called interference, something demonstrated in the most famous quantum physics experiment of all time; the double-slit experiment.

Physicists realised that a laser interferometer could be used to measure the tiny squashing and stretching of space as it would cause the arms of the equipment to shrink by a minute amount. This means when splitting a laser and sending it through the arms of an interferometer the squeezing of space caused by the passage of a gravitational wave would cause one laser to arrive slightly ahead of the other–meaning they are out of phase and causing destructive interference. Thus, this difference in arrival times causes interference that gives an indication that gravitational waves have rippled across one of the arms.

But, not just any laser interferometer would do. Physicists would need an interferometer so large that it constituents a legitimate feat in engineering. Enter the Laser Interferometer Gravitational-wave Observatory (LIGO).

Schematic showing how LIGO works. (Johan Jarnestad/The Royal Swedish Academy of Sciences)

The LIGO detector uses two laser emitters based at the Hanford and Livingstone observatories, separated by thousands of kilometres apart to form an incredibly sensitive interferometer. From these emitters, lasers are sent down the ‘arms’ of the interferometer which are actually 4km long vacuum chambers.

This results in a system that is so sensitive it can measure a deviation in spacetime that is as small as 1/10,000 the size of an atomic nucleus. To put this into an astronomical context; it is equivalent to spotting a star at a distance of 4.2 light-years and pinpointing its location to within the width of a human hair! This constitutes the smallest measurement ever practically attempted in any science experiment.

And in 2015, this painstaking operation paid off.

On 14th September 2015, the LIGO and Virgo collaboration spotted a gravitational wave signal emanating from the spiralling in and eventual merger of two black holes, one 29 times the mass of the Sun, the other 36 times our star’s mass. From changes in the signal received the scientists were also able to observe the resultant single black hole.

The signal, named GW150914, represented not just the first observation of gravitational waves, but also the first time humanity had ‘seen’ a binary stellar-mass black hole system, proving that such mergers could exist in the Universe’s current epoch.

Different Kinds of Gravitational Waves

Since the initial detection of gravitational waves, researchers have made a series of important and revelatory detections. These have allowed scientists to classify different types of gravitational waves and the objects that may produce them.

Continuous Gravitational Waves

A single spinning massive object like a neutron star is believed to cause a continuous gravitational wave signal as a result of imperfections in the spherical shape of this star. if the rate of spin remains constant, so too are the gravitational waves it emits–it is continuously the same frequency and amplitude much like a singer holding a single note. Researchers have created simulations of what an arriving continuous gravitational wave would sound like if the signal LIGO detected was converted into a sound.

The sound of a continuous gravitational wave of the kind produced by a neutron star can be heard below.

(SXS Collaboration)

Compact Binary Inspiral Gravitational Waves

All of the signals detected by LIGO thus far fit into this category as gravitational waves created by pairs of massive orbiting objects like black holes or neutron stars.

The sources fit into three distinct sub-categories:

  • Binary Black Hole (BBH)
  • Binary Neutron Star (BNS)
  • Neutron Star-Black Hole Binary (NSBH)

Each of these types of binary pairing creates its own unique pattern of gravitational waves but shares the same overall mechanism of wave-generation–inspiral generation. This process occurs over millions of years with gravitational waves carrying away energy from the system and causing the objects to spiral closer and closer until they meet. This also results in the objects moving more quickly and thus creating gravitational waves of increasing strength.

The ‘chirp’ of an eventual merger between neutron stars has been translated to sound waves and can be heard below.

Max Planck Institute for Gravitational Physics

Stochastic Gravitational Waves

Small gravitational waves that even LIGO is unable to precisely pinpoint could be passing over Earth from all directions at all times. These are known as stochastic gravitational waves due to their random nature. At least part of this stochastic signal is likely to have originated in the Big Bang.

Should we eventually be able to detect this signal it would allow us to ‘see’ further back into the history of the Universe than any electromagnetic signal could, back to the epoch before photons could freely travel through space.

The simulated sound of this stochastic signal can be heard below.

(R. Williams (STScI), the Hubble Deep Field Team, NASA)

It is extremely likely given the variety of objects and events in the Universe that other types of gravitational wave signals exist. This means that the quest to detect such signals is really an exploration of the unknown. Fortunately, our capacity to explore the cosmos has been boosted tremendously by our ability to detect gravitational waves.

A New Age of Astronomy

GW150914 conformed precisely to the predictions of general relativity, confirming Einstein’s most revolutionary theory almost exactly six decades after his death in 1955. That doesn’t mean that gravitational waves are done teaching us about the Universe. In fact, these ripples in spacetime have given us a whole new way to view the cosmos.

Before the discovery of gravitational waves, astronomers were restricted to a view of the Universe painted in electromagnetic radiation and therefore our observations have been confined to that particular spectrum.

Using the electromagnetic spectrum alone, astronomers have been able to discover astronomical bodies and even the cosmic microwave background (CMB) radiation, a ‘relic’ of one of the very first events in the early universe, the recombination epoch when electrons joined with protons thus allowing photons to begin travelling rather than endlessly scattering. Therefore, the CMB is a marker of the point the universe began to be transparent to light.

Yet despite the strides traditional astronomy has allowed us to make in our understanding of the cosmos, the use of electromagnetic radiation is severely limited. It does not allow us to directly ‘see’ black holes, from which light cannot escape. Nor does it allow us to see non-baryonic, non-luminous dark matter, the predominant form of matter in galaxies–accounting for around 85% of the universe’s total mass. As the term ‘non-luminous’ suggests dark matter does not interact with the electromagnetic spectrum, it neither absorbs nor emits light. This means that observations in the electromagnetic spectrum alone will never allow us to see the majority of the matter in the universe.

Clearly, this is a problem. But one that can be avoided by using the gravitational wave spectrum as both black holes and dark matter do have considerable gravitational effects.

Gravitational waves also have another significant advantage over electromagnetic radiation.

This new form of astronomy measures the amplitude of the travelling wave, whilst electromagnetic wave astronomy measures the energy of the wave, which is proportional to the amplitude of the wave squared.

Therefore the brightness of an object in traditional astronomy is given by 1/distance² whilst ‘gravitational brightness’ falls off by just 1/distance. This means that the visibility of stars persists in gravitational waves for a much greater distance than the same factor persists in the electromagnetic spectrum.

Of course, none of this is to suggest that gravitational wave astronomy will ‘replace’ traditional electromagnetic spectrum astronomy. In fact the two are most powerful when they are unified in an exciting new discipline–multimessenger astronomy

Sources and Further Reading

Maggiore. M., Gravitational Waves: Theory and Experiments, Oxford University Press, [2019]

Maggiore. M., Gravitational Waves: Astrophysics and Cosmology, Oxford University Press, [2019]

Collins. H., Gravity’s Kiss:  The Detection of Gravitational Waves, MIT Press, [2017]

Look Deeper, LIGO, [https://www.ligo.caltech.edu/page/look-deeper]

What is the Standard Model of Particle Physics?

By the mid-20th Century, physicists had begun to understand the fundamental structure of matter to such a degree that a theory was needed to encapsulate the Universe’s particles, the interactions between them and the forces that govern those interactions. That theory was the Standard Model of Particle Physics or just the Standard Model for short.

First devised in the 1970s, the Standard Model would be used to predict a wide variety of phenomena, meet various experimental challenges, before being confirmed by the discovery of the Higgs Boson in 2012. Yet, as successful and fruitful a theory as the Standard Model is, it can’t explain everything. Gravity still evades confinement within the Standard Model, and physicists have caught tantalizing glimpses of physics beyond the theory’s limits.

Before these glimpses can be confirmed and a new chapter in physics is opened, let’s take a trip through the particle zoo and discover the wonders of the Standard Model.

The Matter Particles

The everyday matter that surrounds us is comprised of building blocks called elementary particles. Of these building blocks, there are two main families; fermions and bosons.

Of the fermions, the two mains classes are leptons and quarks. Within each of these groups are six particles that group into three pairings that physicists call generations.

The first generation of leptons and quarks are made up of the lightest and most stable particles. These are the particles responsible for forming the elements of the Universe we are most familiar with–the stars, planets, moons, and us. The second and third generations are made up of increasingly more massive and less stable particles. The greater in mass these particles are, the quicker they decay into their lighter cousins.

Starting with the quarks is an easy way to introduce some of the qualities and values associated with the particles in the Standard Model. One thing you will notice is the interesting naming convention for these qualities. They reflect things we commonly encounter in the everyday macroscopic world such as flavour, colour and spin, but really shouldn’t be confused with those things.

So, let’s make quarks the first stop in our walk through the particle zoo.


The six quarks that make this family of particles are known as up, down quarks, which make up the first generation of quarks, these second comprises of the more massive charm and strange quarks. And the third generation contains the most massive particles, known as the top and bottom quarks.

These are generally known as the ‘flavours’ of quarks, each of which has its own antiquark. Of course, I’ve primed you to realise that this name has nothing to do with how these quarks taste!

Of the four fundamental forces, quarks ‘feel’ electromagnetism, the strong and weak nuclear forces, and gravity, but the latter is too weak to have an effect on quarks’ tiny mass.

The strong nuclear force binds together quarks in nucleons, whilst the weak nuclear force can actually cause quarks to switch flavours something that we’ll look at further when we get to the force-carrying particles.

But these elementary particles don’t just come in flavours–they also come in ‘colours.’

It is this quality–again nothing to do with wavelengths of light, quarks are large enough to reflect light in such a way to have a conventional colour– that determines how quarks come together to form other, more massive, particles.

Incnis Mrsi/CC-by-SA 3.0

Quarks join up to make particles called baryons, the most common of which are the protons and neutrons that come together to form the elements and the matter we interact with on an everyday scale.

Protons are made up of one down quark and two up quarks, whilst neutrons are comprised of two down quarks and an up quark.

A diagram shows how quarks usually fit into our understanding of tiny particles.  (udaix/Shutterstock)

Considering these arrangements and the fact that each flavour of quark has its own charge it’s easy to see why the proton has a positive charge whilst the neutron is neutral. It should also be apparent that when the weak nuclear force causes an up quark to switch to a down quark it also charges the nucleon it is part of from a proton to a neutron.

There are a multitude of other exotic arrangements of quarks like mesons which consist of a quark and its antiquark, and tetra and pentaquarks made up of three and five quarks respectively.

Considering how quarks come together to form particles is important because despite being fundamental particles, quarks are found wandering the particle zoo on their own. They are always found in conglomerations.

There is another important quality of fundamental particles that need to be considered–and yes, just as with ‘flavour’ and ‘colour’ it has a slightly misleading name.–these particles also have ‘spin.’

This shouldn’t be considered as representing a particle constantly revolving. It’s more a description of how a particle reacts when it interacts with a magnetic field.

Quark, like all fermions, are 1/2 spin particles and are described as having ‘up’ or ‘down’ spin. Unlike the spin of a macroscopic object, say a football after it is kicked, the spin of fundamental particles doesn’t change.


Like quarks, leptons are particles with 1/2 spin. They also come in six flavours and across three generations. But, unlike quarks, leptons are freely found wandering the particle zoo alone. The most famous lepton is possibly also the most famous fundamental particle. The electron–a generation I particle possessing a charge of –e.

Leptons can also be sub-divided into two groups; charged, which includes electrons and electron like muons, and chargeless leptons like the neutrinos. The charged leptons also possess a more considerable mass than their uncharged cousins. The reason that uncharged leptons have such smaller masses is not explained by the Standard Model of Particle Physics and doing so requires an extension to the model.

The lack of charge and practical lack of mass of neutrinos has led them to be labelled ‘ghost particles’ and means that 100s of thousands of them can stream through every square inch of your body every second without the slightest interaction with the matter that composes you.

Just like with quarks, each particle has its own anti-particle, including perhaps the most famous example of such symmetry–the antiparticle of the electron, the positron. One possible quirk to this symmetry is the possibility is that neutrinos are their own antiparticles.

Like quarks, leptons interact with gravity and the electromagnetic force, but unlike quarks leptons don’t feel the strong nuclear force.

Leptons obey the Pauli exclusion principle. This means that no two particles can share the same quantum numbers. This is key to the range of chemical elements that exist within the Universe as it forces electrons to occupy increasingly energetic shells around an atomic nucleus. The number of valance electrons in an element’s outer shell determines the chemical properties that the element will have.

The Pauli exclusion principle can be overwhelmed. A neutron star is protected from becoming black holes by this phenomenon, but when it exceeds a certain mass it can no longer rely on this to protect against complete gravitational collapse.

The Force Carriers

There are four fundamental forces in the Universe that we are currently aware of–the strong force, the weak force, the electromagnetic force, and the gravitational force. All of these forces work over different distances with different strengths. For instance, gravity is the weakest of the forces–even though there is actually a force explicitly called the ‘weak force’– but works over a potentially infinite distance. Meanwhile, the electromagnetic force also works over a long-range but is much more powerful than the force of gravity.

The strong and weak nuclear forces work over much shorter ranges; dominating the forces for sub-atomic particles. As the name implies, the strong force is the strongest of all the four forces, whilst the weak force is the weakest barring gravity.

We are certain that three of the four fundamental forces–electromagnetism, the strong and weak forces–are communicated by carrier particles called bosons. Particles exchange these bosons to communicate these forces.

Unlike leptons and quarks–collectively known as Fermions–Bosons have full integer spin. This means that they are not forced to obey the Pauli exclusion principle.

The electromagnetic force is carried by the most familiar of these particles–the photon. The strong force which ‘glues’ quarks together in protons and neutrons is communicated by gluons, and the weak force that influences particles to switch flavours is transmitted by W and Z bosons.

So what about gravity?

Put simply, the force that we are most familiar with and experience every moment of every day isn’t part of the Standard Model. Physicists think that this outsider force is also transmitted by a boson which they have given the provisional name the graviton. As of yet, however, there is no experimental sign of this hypothetical boson. Thus it can’t yet be seen in our particle zoo.

The exclusion of gravity isn’t a massive problem for particle physics, because the model deals with particles that are so small and the fact that gravity is so weak, the force doesn’t really have an effect on this sub-atomic world.

But what this omission does tell us, is that despite its importance and the fact that it has been experimentally verified to an impressive standard, and can now predict that outcome of a wide array of experiments, the Standard Model is by no means a complete description of the physical world.

That means we need extensions to this model to obtain that more accurate description. The problem is, no one can quite agree on what those extensions should look like.

Beyond the Standard Model

The force of gravity isn’t the only element of the Universe that physicists can’t squeeze into the Standard Model at the moment. Despite being a great description of sub-atomic particles, the theory can’t account for dark matter. As this mysterious form of matter that isn’t made up of baryons like protons and neutrons, accounts for around 85% of the mass in the known Universe, this isn’t an insignificant shortfall.

Likewise, the model can’t explain why matter dominates the Universe rather than antimatter. Processes that birth particles produce matter and antimatter in equal amounts. If the Universe had started with these balanced they would have likely met and annihilated each other before large scale structure had the oppotunity to form. That means there must be some reason beyond the Standard Model why the Universe initial favoured matter and allowed an imbalance.

The detection of the Higgs Boson by the CMS detetcor at the LHC was supposed to complete the Standard Model, but the particle isn’t exactly what the theory predicted (CERN)

Another potential issue with the Standard Model could result from the particle that was heralded as marking its completion: the Higgs Boson.

This particle is believed to emerge from the Higgs field and endow mass to most particles. But, the Standard Model isn’t the only theory that posits the existence of the Higgs Boson. The Higgs particle suggested by this theory is the simplest version. The particle that was measured by the CMS detector at the Large Hadron Collider (LHC) certainly conforms to the description given by the Standard Model, but it’s not a perfect fit.

That means that even as we create more Higgs Bosons at the LHC and continue learning more about the particle, the possibility of discovering that it conforms better to another theory remains.

One of the most well-supported extensions to the Standard Model is Supersymmetry (SUSY). This hypothesises a connection between fermions and bosons and suggests that all particles have a superpartner –or sparticle–with the same mass, and quantum numbers but a spin that differs by 1/2.

That means that each 1/2 lepton is a partner ‘slepton’ with a full integer spin–or more simply a boson. So, for the electron, SUSY posits the slepton with the same mass, charge, but with a spin of 1 rather than 1/2 called the selectron. For quarks, there are squarks, and so on.

SUSY could provide a dark matter candidate as the lightest particle suggested by the extension to the Standard Model would, if it existed, be a dead-ringer for dark matter.

Unfortunately, despite some tantalising hints at physics beyond the Standard Model of Particle Physics, experiments have thus far failed to turn up anything substantial. For SUSY specifically, sparticles that should be created in collisions at the LHC have thus far not been detected.

At least until the completion of high-luminosity upgrades at the LHC provide more collisions and thus a greater chance of spotting exotic phenomena, the Standard Model will remain our best, albeit incomplete, description of the sub-atomic world.

Sources and Further Reading

Manton. N., Mee. N., The Physical World, Oxford University Press, [2017].

Martin. B. R., Shaw. G., Particle Physics, Wiley, [1999].

Time Travel Without the Paradoxes

Time Travel Without the Paradoxes

It’s one of the most popular ideas in fiction — travelling back through time to alter the course of history. The idea of travelling through time — more than we do every day that is — isn’t just the remit of science fiction writers though. Many physicists have also considered the plausibility of time travel, especially since Einstein’s theory of special relativity changed our concept of what time actually is. 

Yet, as many science fiction epics warn, such a journey through time could carry with it some heavy consequences. 

Ray Bradbury’s short story ‘A Sound of Thunder’ centres around a group of time travellers who blunder into prehistory, making changes that have horrendous repercussions for their world. In an even more horrific example of a paradox, during an award-winning episode of the animated sci-fi sitcom Futurama, the series’ hapless hero Fry travels back in the past and in the ultimate grandfather paradox, kills his supposed gramps. Then, after ‘encounter’ with his grandmother, Fry realises why he hasn’t faded from reality, he is his own grandfather. 

Many theorists have also considered methods of time travel without the risk of paradox. Techniques that don’t require the rather extreme measure of getting overly friendly with one’s own grandmother Fry. These paradox-escape mechanisms range from aspects of mathematics to interpretations of quantum weirdness. 

ZME’s non-copyright infringing time machine. Any resemblance to existing time travel devices is purely coincidental *cough* (Christopher Braun CC by SA 1.0/ Robert Lea)

Before looking at those paradox escape plans it’s worth examining just how special relativity changed our thinking about time, and why it started theoretical physicists really thinking about time travel. 

Luckily at ZME Science, we have a pleasingly non-copyright infringing time machine to travel back to the past. Let’s step into this strange old phone booth, take a trip to the 80s to pick up Marty and then journey back to 1905, the year Albert Einstein published ‘Zur Elektrodynamik bewegter Körper’ or ‘On the Electrodynamics of Moving Bodies.’ The paper that gave birth to special relativity. 

Don’t worry Marty… You’ll be home before you know it… Probably.

A Trip to 1905: Einstein’s Spacetime is Born

As Marty reads the chronometer and discovers that we have arrived in 1905, he questions why this year is so important? At this point, physics is undergoing a revolution that will give rise to not just a new theory of gravity, but also will reveal the counter-intuitive and somewhat worrisome world of the very small. And a patent clerk in Bern, Switzerland , who will be at the centre of this revolution,  is about to have a very good year. 

The fifth year of the 20th Century will come to be referred to as Albert Einstein’s ‘Annus mirabilis’ — or miracle year — and for good reason. The physicist will publish four papers in 1905, the first describing the photoelectric effect, the second detailing Brownian motion. But, as impressive those achievements are–one will see him awarded the Nobel after all–it’s the third and fourth papers we are interested in. 

1905: young Albert Einstein contenplates the future, unaware he is about to change the way we think about time and space forever. (Original Author Unknown)
1905: young Albert Einstein contemplates the future, unaware he is about to change the way we think about time and space forever. (Original Author Unknown)

In these papers, Einstein will first introduce special relativity and then will describe mass-energy equivalence most famously represented by the reduced equation E=mc². It’s no exaggeration to say that these works will change how we think of reality entirely — especially from a physics standpoint. 

Special relativity takes time — and whereas it had previously been believed to be its own separate entity — unites it with the four known dimensions of space. This creates a single fabric— spacetime. But the changes to the concept of time didn’t end there. Special relativity suggests that time is different depending on how one journeys through it. The faster an object moves the more time ‘dilates’ for that object. 

This idea of time running differently in different reference frames is how relativity gets its name. The most famous example for the time difference is the ‘twin paradox.’

Meet twin sisters Stella and Terra. Stella is about to undertake a mission to a distant star in a craft that is capable of travelling at near the speed of light, leaving her sister, Terra, behind on Earth. 

A spacetime diagram of Terra’s journey through spacetime, against her twin Stella’s. Less ‘proper time’ passes for Stella than Terra meaning when she returnes to Earth Terra has aged more than she has. (Robert Lea)

After travelling away from Earth at near the speed of light, then undertaking a return journey at a similar speed, Stella touches down and exits her craft to be greeter by Terra who has aged more in relation to herself. More time has passed for the ‘static’ Earthbound twin than for her sister who underwent the journey into space.

Thus, one could consider Stella to have travelled forward in time. How else could a pair of twins come to be of considerably different ages? That’s great, but what about moving backwards through time? 

Well, if the faster a particle in a reference frame moves, the ‘slower’ time progresses in that frame, it raises the question, is there a speed at which time stands still? And if so, is there a speed beyond this at which time would move backwards? 

A visualisation of a tachyon. Because a tachyon would always travel faster than light, it would not be possible to see it approaching. After a tachyon has passed nearby, an observer would be able to see two images of it, appearing and departing in opposite directions.
(Wiki CC by SA 3.0)

Tachyons are hypothetical particles that travel faster than the speed of light — roughly considered as the speed at which time would stand still — and thus, would move backwards rather than forwards in time. The existence of tachyons would open up the possibility that our space-bound sister could receive a signal from Terra and send her back a tachyon response. Due to the nature of tachyons, this response could be received by Terra before she sent the initial signal.

Here’s where that becomes dangerous; what if Stella sends a tachyon signal back that says ‘Don’t signal me’? Then the original signal isn’t sent, leading to the question; what is Stella responding to?

Or in an even more extreme example; what if Stella sends a tachyon signal back that is intercepted by herself before she embarks on her journey, and that signal makes her decide not to embark on that journey in the first place? Then she’ll never be in space to send the tachyon signal… but, if that signal isn’t sent then she would have embarked on the journey…

And that’s the nature of the causality violating paradoxes that could arise from even the ability to send a signal back through time. Is there a way out of this paradox?


Interlude. From the Journal of Albert Einstein

27th September 1905

A most astounding thing happened today. A young man in extraordinary attire visited me at the patent office. Introducing himself as ‘Marty’ the youngster proceeded to question me about my paper ‘On the Electrodynamics of Moving Bodies‘– a surprise especially as it was only published yesterday.

In particular, the boy wanted to know about my theory’s implications on time travel! A pure flight of fancy of course… Unless… For another time perhaps.

If this wasn’t already unusual to the extreme, after our talk, I walked Marty to the banks of the Aare river where he told me that his transportation awaited him. I was, of course expecting a boat. I was therefore stunned when the boy stepped into a battered red box, which then simply disappeared.

I would say this was a figment of my overworked imagination, a result of tiredness arising from working the patent office during the day and writing papers at night. That is, were I the only witness!

A young man also saw the box vanish, and his shock must have been more extreme than mine for he stumbled into the river disappearing beneath its surface.

His body has not yet been recovered… I fear the worst.

Present Day: The Self Correcting Universe

As the battered old phone box rematerializes in the present day, Marty is determined to seek out an academic answer to the time travel paradox recounted to him in 1905. 

He pays a visit to the University of Queensland where Bachelor of Advanced Science student Germain Tobar has been investigating the possibility of time travel. Under the supervision of physicist Dr Fabio Costa, Tobar believes that a mathematical ‘out’ from time travel paradoxes may be possible.

“Classical dynamics says if you know the state of a system at a particular time, this can tell us the entire history of the system,” Tobar explains. “For example, if I know the current position and velocity of an object falling under the force of gravity, I can calculate where it will be at any time.

“However, Einstein’s theory of general relativity predicts the existence of time loops or time travel — where an event can be both in the past and future of itself — theoretically turning the study of dynamics on its head.”

Tobar believes that the solution to time travel paradoxes is the fact that the Universe ‘corrects itself’ to remove the causality violation. Events will occur in such a way that paradoxes will be removed.

So, take our twin dilemma. As you recall Stella has sent herself a tachyon message that has persuaded her younger self not to head into space. Tobar’s theory — which he and his supervisor Costa say they arrived at mathematically by squaring the numbers involved in time travel calculations — suggests that one of two things could happen.

Some event would force Stella to head into space, she could accidentally stumble into the capsule perhaps, or receive a better incentive to head out on her journey. Or another event could send out the tachyon signal, perhaps Stella could accidentally receive the signal from her replacement astronomer. 

“No matter what you did, the salient events would just recalibrate around you,” says Tobar. “Try as you might, to create a paradox, the events will always adjust themselves, to avoid any inconsistency.

“The range of mathematical processes we discovered show that time travel with free will is logically possible in our universe without any paradox.”

The Novikov self-consistency principle
The Novikov self-consistency principle (Brightroundircle/ Robert Lea)

Tobar’s solution is similar in many ways to he Novikov self-consistency principle — also known as Niven’s Law of the conservation of history — developed by Russian physicist Igor Dmitriyevich Novikov in the late 1970s. This theory suggested the use of geodesics similar to those used to describe the curvature of space in Einstein’s theory of general relativity to describe the curvature of time. 

These closed time-like curves (CTCs) would prevent the violation of any causally linked events that lie on the same curve. It also suggests that time-travel would only be possible in areas where these CTCs exist, such as in the presence of wormholes as speculated by Kip Thorne and colleagues in the 1988 paper “Wormholes, Time Machines, and the Weak Energy Condition”. The events would cyclical and self-consistent. 

The difference is, whereas Tobar suggests a self-correcting Universe, this idea strongly implies that time-travellers would not be able to change the past, whether this means they are physically prevented or whether they actually lack the ability to chose to do so. In our twin analogy, Stella’s replacement sends out a tachyon signal and travelling along a CTC, it knocks itself off course, meaning Stella receives rather than its intended target.

After listening to Tobar, strolling back to his time machine Marty takes a short cut through the local graveyard. Amongst the gravestones baring unfamiliar dates and names, he notices something worrying–chilling, in fact. There chiselled in ageing stone, his grandfather’s name.

The date of his death reads 27th September 1905. 

Interlude: From the Journal of Albert Einstein

29th September 1905

This morning the Emmenthaler Nachrichten reports that the body of the unfortunate young man who I witnessed fall into the Aare has been recovered. The paper even printed a picture of the young man. 

I had not realised at the time, but the boy bares the most remarkable resemblance to Marty — the unusually dressed youngster who visited with me the very day the boy fell…

Strange I such think of Marty’s attire so frequently, the young man told me his garish armless jacket, flannel shirt and ‘jeans’ were ‘all the rage in the ‘86.’ 

Yet, though I was seven in 1886 and have many vague memories from that year, I certainly do not remember such colourful clothes…

Lost in Time: How Quantum Physics provides an Escape Route From Time Travel Paradoxes

Marty folds the copy of the Emmenthaler Nachrichten up and places it on the floor of the cursed time machine that seems to have condemned him. The local paper has confirmed his worst fears; his trip to the past to visit Einstein doomed his grandfather. 

After confirming his ancestry, he knows he is caught in a paradox. He waits to be wiped from time…

After some time, Marty wonders how it could possibly be that he still lives? Quantum physics, or more specifically one interpretation of it has the answer. A way to escape the (literal) grandfather paradox. 

The double slit experiment (Robert Lea)

The ‘many worlds’ interpretation of quantum mechanics was first suggested by Hugh Everett III in the 1950s as a solution to the problem of wave-function collapse demonstrated in Young’s infamous double-slit experiment.

As the electron is travelling it can be described as a wavefunction with a finite probability of passing through either slit S1 or slit S2. When the electron appears on the screen it isn’t smeared across it as a wave would be. It’s resolved as a particle-like point. We call this the collapse of the wavefunction as wave-like behaviour has disappeared, and it’s a key factor of the so-called Copenhagen interpretation of quantum mechanics.

The question remained, why does the wavefunction collapse? Hugh Everett asked a different question; does the wavefunction collapse at all?

The Many Worlds Interpretation of Quantum Physics (Robert Lea)

Everett imagined a situation in which instead of the wavefunction collapsing it continues to grow exponentially. So much so that eventually the entire universe is encompassed as just one of two possible states. A ‘world’ in which the particle passed through S1, and a world where the particle passed through S2.

Everett also stated the same ‘splitting’ of states would occur for all quantum events, with different outcomes existing in different worlds in a superposition of states. The wavefunction simply looks like it has collapsed to us because we occupy one of these worlds. We are in a superposition of states and are forbidden from seeing the other outcome of the experiment.

Marty realises that when he arrived back in 1905, a worldline split occurred. He is no longer in the world he came from– which he labels World 1. Instead, he has created and occupies a new world. When he travels forward in time to speak to Tobar he travels along the timeline of this world–World 2.

This makes total sense. In the world Marty left, a phone box never appeared on the banks of the Aera on September 27th 1905. This world is intrinsically different than the one he left.

What happens as a result of Marty’s first journey to 1905 according to the Many World’s Interpretation (Robert Lea)

He never existed in this world and in truth he hasn’t actually killed his grandfather. His grandfather exists safe and sound back in 1905 of World 1. If the Many World’s Interpretation of quantum physics is the correct solution to the grandfather paradox, however, then Marty can never return to World 1. It’s intrinsic to this interpretation that superpositioned worlds cannot interact with each other.

With reference to the diagrams above, Marty can only move ‘left and down’ or ‘right’–up is a forbidden direction because it’s his presence at a particular moment that creates the new world. This makes total sense, he has changed history and is in a world in which he appeared in 1905. He can’t change that fact.

The non-interaction rule means no matter what measures he takes, every time he travels back into the past he creates a new state and hops ‘down’ to that state and can then only move forward in time (right) on that line.

Marty’s multiple journey’s to the past create further ‘worlds’ (Robert Lea)

So what happens when Marty travels back to the past in an attempt to rescue his world? He inadvertently creates another state–World 3. This world may resemble World 1 & 2 in almost every conceivable way, but according to the application of the interpretation, it is not the same due to one event–one extra phone box on the banks of the River Aare for each journey back.

As Marty continues to attempt to get back to World 1 — his home — he realises he now lives in a world in which one day in September 1905 on the streets of Bern, hundreds of phone box suddenly appeared on the banks of the Aare, and then simply disappeared.

The sudden appearance of hundreds of red telephone boxes around the banks of the River Aare really started to affect property prices. (Britannica)

He also realises that his predicament answers the question ‘if time travellers exist why do they never appear in our time?’ The truth being, that if a person exists in the world from which these travellers departed they can never ‘get back’ to this primary timeline. 

To someone in World 1, the advent of time travel will just result in the gradual disappearance of daring physicists. That’s the moment it dawns on Marty that as far as World 1 — his world — is concerned, he stepped in a phone box one day and vanished, never to return.

Marty escaped the time travel paradox but doomed himself to wander alternate worlds.

Hey… how do we get our time machine back?


a Representation of the quantum teleport of information from the surface of Earth to space--a sci-fi's fan's dream, almost. ((IMAGE BY CAS))

Quantum Teleportation: Separating Science Fact from Science Fiction

It goes without saying that many terms and concepts from science, and particularly physics, find themselves transported from the pages of journals and reports to the comic book page or TV screen — albeit intrinsically changed. Quantum teleportation is an interesting example of this process working in reverse. 

A recreation of the transporter room from the Enterprise D as featured in the show Star Trek: The Next Generation. Unfortunately, teleportation as made famous by the Star Trek franchise is impossible, but quantum teleportation is no less fascinating. (Konrad Summers/ CC by SA 2.0)

Though physicist and information theorist Charles Bennet took the term ‘teleportation’ from popular culture, quantum teleportation is radically different than the process used by the crew of the Enterprise to ‘beam down’ to an alien vista. But despite this; that image of Kirk, Spock, Bones, and a crew of hapless red shirts travelling down to the surface of a planet and back — albeit minus several of the red shirts — is so ubiquitous it’s hard for even professional physicists to escape.

“Of course, I think of Star Trek where people and things are being ‘beamed-up’,” Ulf Leonhardt from the Department of Physics of Complex Systems at the Weizmann Institute of Science tells me when I ask him what the word ‘teleportation’ means to him. And Leonhardt is no stranger to quantum teleportation either. He is renowned for his work in quantum optics — one the fields in which quantum teleportation is explored, in fact, its no exaggeration to say ‘he wrote the book on it.’

This ubiquity of the pop-culture interpretation of teleportation is a special kind of hindrance when it comes to the understanding of quantum teleportation as the two concepts are so radically different. 

The first, and most radical difference, you won’t be using quantum teleportation to beam to the surface of an alien world or down to the local shops anytime soon. This isn’t because quantum teleportation isn’t up and running; we’ve had the technology in operation since the mid-nineties. It’s because quantum teleportation has nothing to do with the transport of matter.

Mind over Matter–How Quantum Teleportation Shifts Information

The idea of being able to instantly — or almost instantly — relocate matter from one location to another, was granted the name ‘teleportation’ by a purveyor of the weird Charles Fort in his 1931 book ‘Lo!’. But despite this; the idea existed sometime before this.

Many early examples of teleportation were, unsurprisingly, described as being magical in nature, but with the advent of the industrial revolution, remarkable tales of intrigue and suspense began to move away from supernatural explanations to ones of science–albeit none more credible in nature. The first example of matter being transported instantaneously from one location to another being performed ‘scientifically’ occurred in an 1897 novel.

In Fred T. Jane’s ‘To Venus in Five Seconds: An Account of the Strange Disappearance of Thomas Plummer, Pillmaker,’ the titular hero is transported from a pleasant summer house — albeit filled with strange machinery — to the planet Venus where he encounters warring locals and some other displaced Brits. This is the first recorded example of scientific equipment used to transport a hero to the surface of an alien world in fiction, a function which will, of course, become the most infamous use for teleportation.

An illustration from ‘To Venus in Five Seconds: An Account of the Strange Disappearance of Thomas Plummer, Pillmaker,’ by Fred T Jane. Thanks to a teleportation mishap Plummer is menaced by the rather un-PC inhabitants of Venus (A.D. Innes, 1897, the University of Wisconsin – Madison)

But quantum teleportation doesn’t concern the transport of matter — instantly or otherwise — rather it’s about the transmission of information.

“I think for a layman teleportation means instantaneous transport of matter, but a physicist knows that this is impossible,” Leonhardt explains. “Rather, teleportation is the transport of the information of how to assemble matter to make up an object.

“There is zero chance for the instantaneous transport of matter, but a good chance for the transfer of quantum information of not-too-complex systems. Teleporting people is out of the question, though.”

Ulf Leonhardt, author of ‘Essential Quantum Optics’

So if you’re not going to be taking a trip in a teleporter any time soon. With that said, we can investigate the nature of the information that can be sent via quantum teleportation.

From Here to There: State to State Communication

As quantum teleportation doesn’t concern the transmission of matter, but information, it’s more correct to regard it as a form of communication rather than one of transport like its sci-fi counterpart. But that leaves the question, what is being communicated?

“Quantum teleportation is the transport of a quantum state from one object to another,” Leonhardt says. “The quantum state contains all the possible information about the object.”

Thus, quantum teleportation really means transferring the quantum structure of an object from one place to another without the movement of that physical object. This ‘quantum structure’ refers to qualities that a system or a particle can possess, things like momentum, polarization and spin. 

The quantum mechanical counterpart of classical bits can be encoded with a wealth of information (Nicholas Shan)

This information is encoded in qubits — the quantum-mechanical analogue of classical bits. Whereas a bit can only hold the information ‘true’ or ‘false’ a qubit can be encoded with a deep wealth of information. So, quantum teleportation is a mechanism of moving this qubit without moving the particle with which it is associated. This communication requires the system at the starting point and the system representing the endpoint are entangled. 

“The quantum state cannot be measured for an individual system, because an observation may ruin it,” Leonhardt explains.

“Entanglement between the two ports of the quantum teleportation system is an essential ingredient.”

Ulf Leonhardt, author of ‘Essential Quantum Optics’

The first experiments with quantum teleportation dealt with the transfer of state information between single entangled photons, but it has since been realised in a variety of quantum systems — electrons, ions, atoms and even superconducting circuits. 

What is important to note though, is that quantum teleportation isn’t simply creating two copies of the same quantum system. In fact, that is something expressly forbidden by the rules of quantum physics. 

No Cloning Around!

Christopher Nolan’s magical 2006 thriller The Prestige–based upon a 1995 novel by Christopher Priest– focuses on the intensifying rivalry of magicians Robert Angier (Hugh Jackman) and Alfred Borden (Christian Bale). The feud consumes both men costing the lives of both their loved ones and ultimately themselves. 

During the course of this self-destruction, in an attempt to out-do Borden’s most spectacular illusion, Angier seeks the help of a fictionalised version of Nikola Tesla. Telsa provides Angier with a teleportation device but warns him of the machine’s terrible cost. 

The inimitable David Bowie as Nikola Tesla in Christopher Nolan’s The Prestige. In the film, Tesla warns an illusionist with a grudge the terrible cost of his teleporter but is not heeded. (Warner Bros. Pictures 2006)

That cost is that every time the machine is used, it creates a copy of Angier. A clone. Meaning that the illusionist must murder the ‘original him’ each time the trick is performed. He dispatches ‘himself’ in a tank of water hidden beneath the stage where the teleport pod sits. A fitting way for a magician to go.

But, on the quantum scale, there are specific rules in place to prevent the cloning of a system every time an act of quantum prestidigitation is performed. Quantum teleportation has a strict ‘no cloning’ rule. “The no-cloning theorem states that one cannot create two identical copies from the same individual quantum system,” Leonhardt states.

“The quantum state is too fragile and would be compromised in such a process.”

Ulf Leonhardt, author of ‘Essential Quantum Optics’

The Heisenberg uncertainty principle is just one of the rules of quantum physics that the successful cloning of a system would jeopardise. The most well-known version of the uncertainty principle, for example, states that it is impossible to precisely measure the momentum and the position of a quantum system. The more precisely one knows one, the less precisely one can know the other. 

But, if quantum teleportation allowed a system to be cloned, then an enterprising researcher could measure the position of the original system and simultaneously measure the momentum of the ‘clone’, thus violating this rule.

In fact, all sorts of messiness would ensue with a system that could be cloned, including, eventually, the possible violation of causality itself. 

That means that quantum teleportation really ‘moves’ the quantum state from one location to another, destroying that state in the original port… Possibly by drowning it in a tank. 

Quantum Teleportation… Not Instantaneous, But ASAP

Despite the fact teleportation mishaps often became the focus of episodes of Star Trek, initially, it was little more than a Deux ex Machina that allowed the story to flow without characters being tied up by long shuttle journeys. In order for the narrative of a concise 42-minute episode of TV to flow in a pleasing way, it was necessary for characters to be able to instantly move from place to place.

Unfortunately, quantum teleportation differs from its pop-culture ancestor in this respect too. 

Quantum Teleportation. Words and Graphic Innebrooks research team
Quantum Teleportation. Words and Graphic Innebrooks research team

Though instant transmission of information in quantum physics does exist in the form of the instant change in an entangled system when a measurement is made on one part of that system. That measurement and the adoption of a state that it causes results in the partner system adopting the corresponding state instantly — even if it is at the opposite side of the Universe.

This apparent violation of the universal speed limit of the speed of light in a vacuum troubled Einstein so much he referred to it as ‘spooky action at a distance’ and suggested that it demonstrated quantum physics was an incomplete theory with hidden variables yet to be discovered. But, this aspect of quantum physics has been confirmed by decades of research after the Austrian physicist’s death.

Quantum physics is complete, information does travel between entangled quantum systems or particles instantaneously. Yet despite the fact that quantum teleportation functions on the basis of entanglement — passing a state between two entangled particles — that doesn’t mean that quantum teleportation can transfer information instantly too. 

That’s because quantum teleportation isn’t entirely quantum.

When a qubit is transmitted from a sender to a receiver — we’ll call them Alice and Bob respectively as has become standard when describing quantum communication — it’s necessary to transmit two bits of classical information per qubit from Alice to Bob.

This means a classical communication channel has to be created so that Alice and Bob can communicate the results of their measurements. If this isn’t done, Alice and Bob have no way to reconstruct the initial state and the reconstruction will be random. 

Thus, the downside of this is that it limits the speed of information transfer to the speed of classic communication. A qubit can’t be reconstructed before the classical information is received. Researchers can use lasers and photons as the basis of this classical communication system, so even though there is a speed limit, it’s the fastest speed achievable. This also means it can be achieved through ‘open space’ without the need for fiber optic cables. 

Or course, that means not only can Kirk not instantly return to the Enterprise, but he can’t even get a ‘beam me up’ command sent instantly.  

Quantum Teleportation in Practice

The most likely use of quantum teleportation is in the development of quantum computing, quantum networks, and eventually, a quantum internet. Currently, academic debate over this quantum future is focused on which quantum teleportation system is most reliable. 

This image shows crystals which contain photonic information after quantum teleportation. (© GAP, University of Geneva (UNIGE))
This image shows crystals which contain photonic information after quantum teleportation. (© GAP, University of Geneva (UNIGE))

In 2015 paper published in Nature Photonics scientists from the Freie Universität Berlin and the Universities of Tokyo and Toronto, performed a comprehensive review of theory and experiment surrounding quantum teleportation, concluding that no technology in isolation yet provides the perfect solution, meaning hybridisation is needed if quantum computing is ever to be a reality.

This means that many physicists are currently working on improving the distances over which quantum teleportation can be achieved and the kind of quantum systems that states can be communicated between. 

An example of this is the fascinating work of Nicholas Gisin at the University of Geneva (UNIGE). Gisin and his team have consistently been at the cutting edge of pushing the distances across which quantum teleportation can be achieved. In a 2014 study, Gisin’s UNIGE team not only pushed the distance across which a state could be teleported — over 25 meters through an optical cable — but they also managed to communicate the state from a photon to a solid crystal, showing that states can be passed between radically divergent systems. 

Gisin’s research is constantly being improved upon thus the distance across which a quantum state can be transmitted is stretching. And this includes maybe finally reaching the ‘final frontier.’

Space: Probably Not The Final Frontier for Quantum Teleportation

In July this year, scientists finally made teleportation to space a reality , maybe offering some compensation to disappointed sci-fi fans.

a Representation of the quantum teleport of information from the surface of Earth to space--a sci-fi's fan's dream, almost. ((IMAGE BY CAS))
Representation of the quantum teleport of information from the surface of Earth to space–a sci-fi’s fan’s dream, almost. ((IMAGE BY CAS))

In a series of experiments, described by a paper published in the journal Science an international team of researchers described the communication of a quantum state into space and across a distance of up to 870 miles to the Chinese quantum-enabled satellite Micius. The research represented the first meaningful quantum optical experiment to test the fundamental physics existing between quantum theory and gravity. 

Soon, a new Chinese satellite will orbit Earth a distance up to sixty times greater than that between Micius–launched in 2016– and the planet’s surface. This will allow researchers to push the boundaries of quantum teleportation even further.


Ulf Leonhardt believes, however, that our understanding of quantum teleportation and our concept of what is achievable within will eventually become as outmoded as the science described in the escapades of Thomas Plummer.

“I like science fiction as scenarios of social thought experiments, but not so much for technological dreams,” Leonhardt says. “It is amusing to browse through Victorian science fiction. They projected their world of steel and steam into the future, which clearly shows the limits of technological imagination.”

 “Our modern projections will share the same fate.”

Ulf Leonhardt, author of ‘Essential Quantum Optics’

Sources and Further Reading

Leonhardt. L, ‘Essential Quantum Optics,’ Cambridge University Press, [2010].

Bussières. F, Clausen. C, Gisin. N, et al, ‘Quantum teleportation from a telecom-wavelength photon to a solid-state quantum memory,’ Nature Photonics, [2014]

Pirandola. S, Eisert. J, Weedbrook. C, et al, ‘Advances in quantum teleportation,’ Nature Photonics, [2015]

A Journey through Multiverses, Hidden Dimensions, and Many Worlds

A Journey through Multiverses, Hidden Dimensions, and Many Worlds

‘Alternate worlds’ are such a staple of genre television, movies, and fiction and what better challenge to face the hero of a serialised story than to face down an evil doppelganger? How will they overcome a corrupted version of themselves, identical in every way barring their lack of moral fortitude… and sometimes with a beard? 

No platform has embraced the idea of the alternate world more than the superhero comic book. Since the Silver Age of comics during the 1950s and 60s, Marvel and DC have thrilled their readers with tales of alternate worlds and altered heroes and villains. DC’s ‘Infinite Earths’ and ‘Elseworlds’ grew so complicated and convoluted decades after the Flash took a trip to the rather dismissively named ‘Earth-2’ to meet his predecessor, the Golden Age Flash, that in 1986 they had to hold a ‘clearance event’ to get rid of some of this excess baggage… A situation the publisher has had to repeat several times since. 

Most of us won’t be surprised to learn that the idea of ‘alternative universes’ is a facet of science, particularly of physics. But actually, the idea of ‘many worlds’ and that of a ‘multiverse’ of alternative universes arise from very different and disparate concepts.

A Journey through Multiverses, Hidden Dimensions, and Many Worlds
A Journey through Multiverses, Hidden Dimensions, and Many Worlds. (Robert Lea)

The former is an idea born of what is known as the wave-function collapse or measurement problem of quantum mechanics, whilst the latter is a proposition born from cosmology and the question of what existed ‘before’ our Universe began its process of rapid inflation and what exists outside of it now. Likewise, these parallel worlds are often referred to as ‘alternative dimensions’ — another phrase that can be found in the physics textbook, but with a radically different meaning than presented in sci-fi. 

These ideas, whilst suffering from some conflation in the minds of some science fiction writers and fans, could not be more different; one suggests an infinite number of almost identical Universes, whilst another suggests a finite set of Universes existing in their own bubbles. Some of which are anything but similar. And third, refers to hidden ‘directions’ curled up within the familiar 3 -D space that we inhabit. 

So, sit down with your evil twin, and whilst admiring their impressive goatee, take a journey with ZME Science through these hidden dimensions, many worlds and bubble-universes. And where better to begin our journey than at the beginning. 

Meeting the Multiverse

“For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there is an infinite number of universes, credibility reaches a limit.” 

Paul Davies, A Brief History of the Multiverse.

There was a time when the word ‘universe’ referred to everything is existence, but modern cosmology has changed this concept irrevokably. There is now the possibility of being ‘outside’ the Universe. In fact, our Universe maybe just a small part of of a much larger patchwork.

As Paul Davies states above, one of the most dangerous things about the concept of a ‘multiverse’ — a stack of Universes placed alongside each other, is how close it veers towards mysticism. This becomes even more of an issue when considering that even many proponents of this hypothetical idea doubt that it could ever really be experimentally tested.

For others, however; the question is fundamental to science, and the closer we come to a complete picture of our Universe we come to, the more tempting it is to consider others.

Bubble boy: Some iterations of multiverse theory suggest that universes inflate side by side in seperate ‘bubbles’ each possing different physical laws. (Robert Lea)

Fred Adams, an American astrophysicist and Ta-You Wu Collegiate Professor of Physics at the University of Michigan, sees the need for a series of alternative or parallel universes as a necessary extension of the fact that our’s is just too convenient. Why is the Universe ‘fine-tuned’ for life? “The laws of physics are described by a collection of fundamental constants that could, in principle, take on different values,” Adams explains. “Determining the range of constants that allows for a working universe helps quantify the degree to which our Universe is special — or not.”

Adams suggests that our Universe has just the right parameters to support the formation of structure, stars, planets, and even biological systems, but there may be a multitude of ‘empty’ Universes where the conditions were not quite so favourable. And, on the other hand, Adams suggests that there could be universes alongside ours even more favourable to the development of such objects. Universes that are, therefore, even more, favourable to life. That is as much as one could expect to a hypothesised set of over 10⁵⁰⁰ universes. 

But, with even such a large set of ‘alternate Universes’ the chances of finding another ‘you’ is still pretty slim. Especially as the laws of physics in these worlds are likely to be radically different, some even precluding the clustering of fundamental particles and the formation of large scale bodies like stars and planets.

One of the more popular ideas for how a series of Universes could grow and co-exist is the inflationary multiverse theory. Introduced by Paul Steinhardt, Albert Einstein Professor in Science at Princeton University, in 1983 and adapted and advanced by such luminaries in physics as Alan Guth, this theory suggests the idea that inflation doesn’t end with our Universe. It could be eternal with the totality of space broken up into bubbles or patches. Each of these bubbles could possess different physical laws, just as Adams puts forward. 

This idea of eternal inflation does run into the problem that it may well be untestable and thus, unfalsifiable, a key aspect of a scientific theory according to one of history’s most important philosophers of science, Karl Popper. However, this doesn’t deter supporters of the theory, with Alan Guth, in particular arguing that a multiverse is simply a logical extension of the fact we have found our own Universe to be undergoing inflation. 

“It’s hard to build models of inflation that don’t lead to a multiverse. It’s not impossible, so I think there’s still certainly research that needs to be done,” Guth remarked during a news conference in 2014. “But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking the idea of a multiverse seriously.”

Another interesting concept for the structure and arrangement of this multiverse is American theoretical physicist, mathematician, and string theorist, Brian Greene’s ‘Brane theory.’ This posits that our Universe and all others sit on a vast membrane located in a higher dimension. Alongside it reside all other universes.

As these universes move around this ‘brane’ they occasionally collide, with each other. These bumps release vast amounts of energy causing ‘big bangs’ to occur and lead to the birth of further universes. 

Greene’s theory is classified as a superstring theory, a hypothetical concept that underlies all physics and unites quantum physics and general relativity — putting forward a theory of quantum gravity. But, superstring theories are in need of an added element, with this need dictating where our trip must head next — in search of hidden dimensions. 

‘I Need Some Space.’ Exploring Hidden Dimensions

“If string theory is right, the microscopic fabric of our universe is a richly intertwined multidimensional labyrinth within which the strings of the universe endlessly twist and vibrate, rhythmically beating out the laws of the cosmos.”

Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory

The statement that string theory–which suggests that fundamental particles are string-like loops vibrating in space–is in need of ‘hidden dimensions’ may initially summon images of alternate universes occupied by all manner of strange creatures, maybe even an alternative version of you, but, your evil beard wearing doppelganger may find the kind of ‘alternate dimensions’ discussed in string theory a bit of a tight squeeze.

An artist’s concept of hidden dimensions curled-up within the three spatial dimensions of spacetime. (World Science Festival)

One of the exciting things about superstring theories is that, unlike other theories in physics, this class of explanations are able to predict the number of dimensions that the spacetime platform in which they play out possesses. To explain this a little better; general relativity — the geometrical theory of gravity developed by Einstein — plays out in four dimensions, x, y, and z — the spatial dimensions, and time. 

But, this dimensional prediction alludes that string theory requires ten — possibly eleven, maybe even 26 — dimensions to be consistent. This fact leaves a very pressing and pertinent question; where the heck are these other six or seven (or 22!) dimensions? Why do we only perceive the world in four dimensions?

The most simple and straightforward way of answering these questions is to suggest that these added dimensions are ‘curled up’ hidden within the three spatial dimensions of which we are aware. Physicists define this as these dimensions being ‘compacted on an internal manifold’ but for our purposes, it’s just as easy as thinking of them as being very small.

This idea referred to as ‘compactification’ actually predates string theory. It was first put forward by Theodor Kaluza and Oskar Klein in the 1920s, the eponymous theory which introduced compactification was a suggestion to unify gravity and electromagnetism. It’s perhaps ironic it now finds itself employed to unite gravity and quantum mechanics.

The idea of dimensions being hidden due to size isn’t as counter-intuitive and extraordinary as it may initially appear. Think about a cylinder. When held close the object appears 3 dimensional, but take that cylinder to a sufficient distance and it will appear two-dimensional.

Analogously, at low energies and the scale at which we view the Universe, space appears 3-dimensional with us aware of that four dimension — time. At sufficiently high energies, however, these hidden dimensions may become observable. Thus, the search for such hidden dimensions is now focused on particle accelerators such as the Large Hadron Collider (LHC). 

That’s now two disciplines within physics scoured and no evil-doppelgangers to be found. Can quantum physics rescue this much-loved sci-fi trope? 

Many Worlds, Many Yous? 

“I am about to say something that might sound lunatic…” 

Erwin Schrodinger about to discuss in public the idea of many worlds existing simultaneously for the first time, 1952, (possibly apocryphal).

It’s a well-established rule of quantum physics that things are always found in the last place you look*; so it is fitting that the last realm of physics we search for our counterparts is the quantum realm. 

The ‘Many Worlds’ interpretation of quantum physics, first suggested by Hugh Everett III in the mid-1950s, suggests a solution to the problem of wave-function collapse in quantum mechanics. It was decades, however, before physcists began to take it seriously.

Many Worlds, Many Earths, Many yous. (Robert Lea)

Here’s the problem it attempts to answer; whilst conducting the famous double-slit experiment researchers find that electrons propagate as waves yet interact with other systems as particles — appearing as a single spot on a fluorescent detector. Likewise, when given a binary choice between two slits an electron will pass through as a wave unless a detector is placed on the side of the slit. The attempt to detect which slit an electron passed through causes it to ‘choose’ either slit A or slit B.

The Copenhagen interpretation of quantum mechanics suggests this choice arises from the collapse of the wavefunction–wave-like behaviour being destroyed and giving way to particle-like action. The only problem; there is no solid answer to what causes this collapse.

The Many Worlds interpretation suggests another way around the collapse issue; maybe there is no collapse. Everett suggested that instead of collapsing, the wave function grows exponentially, quickly engulfing the researchers, their lab, planet, galaxy, and then their entire Universe.

Therefore, whereas in the Copenhagen interpretation the electron goes through either slit A or slit B, the Many Worlds interpretation says the electron goes through both and when the researchers examine which slit the electron passed through, what they are actually discovering is if they are in a universe in which the electron went through slit A, or if they are in a universe in which it went through slit B. 

So, how does this reflect on the chances of finding your doppelganger? Well, it makes it a certainty. In fact, one of the problems that many physicists have with the the ‘Many Worlds’ interpretation is the fact that it creates the need for infinite worlds. If you consider just the act of turning on a lightbulb, the photons streaming everywhere, there should be a world for every outcome of every interaction. 

And if that hasn’t boggled your mind, consider it in light of the multiverse and hidden dimensions. Every one of these worlds that branches out has its own hidden dimensions curled up within it AND carries with it its own version of the multiverse starting with one difference: slit A not slit B. 

So, how does this reflect on the chances of finding your doppelganger? Well, it makes it a certainty. In fact, one of the problems that many physicists have with the the ‘Many Worlds’ interpretation is the fact that it creates the need for infinite worlds.

If you consider just the act of turning on a lightbulb, the photons streaming everywhere, there should be a world for every outcome of every interaction. 

And rather than starting from the bottom-up as a universe inflating in a bubble would, this new world has a head-start, everything that already exists is there present and correct. The physical laws are identical, large-scale structure exists and so do you.

And if that hasn’t boggled your mind, consider it in light of the multiverse and hidden dimensions. Every one of these worlds that branches out has its own hidden dimensions curled up within it AND carries with it its own version of the multiverse starting with one difference: slit A not slit B. 

Me, is that you?

“Penny, while I subscribe to the many-worlds theory which posits the existence of an infinite number of Sheldons in an infinite number of universes, I assure you in none of them am I dancing.”

Sheldon Cooper, The Big Bang Theory

Even with all this in mind and us determining that if the Many Worlds interpretation of quantum physics is true there almost infinite versions of ‘you’ out there, what are the chances of finding one that is *cues ominous music* PURE EVIL… possibly, with a beard…

He or she is out there… Exactly the same as you, just evil… Plus beard. (Robert Lea)

Just like the electron faced with the ‘choice’ of which slit to pass through, every time you are faced with a choice, no matter how minute, neurons fire in your brain corresponding to the decision you make. Thus, it’s quite possible that there is a version of you out there who always made the wrong choice. In fact, if there are infinite worlds, it’s a certainty. 

The rotter.

The main issue with the Many Worlds interpretation is the idea of its testability. One of the rules of the Many Worlds interpretation is the inability of these worlds to interact. 

Suggestions have been made regards falsifying Many Worlds but they all require placing a macroscopic object into a quantum ‘superposition.’ This is something that is currently beyond experimental limits, though researchers are constantly finding quantum effects in increasingly larger collections of atoms.

Likewise, the idea of the Multiverse is currently untestable. Doing so would probably require viewing the edge of our Universal bubble, and as this is accelerating away from us, possibly faster than light, as the Universe expands that isn’t likely to happen.

At the moment, the most likely of the ideas discussed above to be evidenced is that of hidden dimensions. These could ‘unfurl’ from the 3 spatial dimensions of our visible Universe at high energies. Energy levels that were present in the early universe and could conceivably be reached at the LHC after it’s high luminosity upgrades. 

Just don’t expect to be faced with your bearded, evil, but otherwise exact duplicate from another world any time soon… Probably. 

*There’s probably a universe where this is true, anyway.

Heisenberg’s uncertainty principle is more than a mathematical quirk, a handy guiding principle, or the inspiration for some really nerdy t-shirts. It is intrinsic to nature, weaved into the fabric of all matter. Together we take a trip to ZME labs to use some everyday objects to demonstrate how nature tells us “you can’t have it all.”

Certainly Uncertain: What’s Heisenberg’s Uncertainty Principle

At the beginning of the 20th Century, physicists were developing the field of quantum physics, discovering in the process that the rules they had grown comfortable with no longer applied at the smallest scales. For example; the argument about the nature of light — was it particle or wave — that had raged for decades  could be answered only by concluding it is neither but has properties of both. Furthermore, they found that this particle/wave duality applies to matter particles like electrons too.  

German theoretical physicist Werner Heisenberg was about to make his own shocking discovery, he was about to find that nature imposes a fundamental limit on what even the most aspiring physicists could know.

He would formulate this concept into the uncertainty principle.

A portrait of Werner Heisenberg taken in 1933. Ironically the author of the image is unknown (CC by SA)

In 1925, Heisenberg would publish a paper that informed physicists that nature has a way of telling you that you can’t have your cake and eat it too. Something intrinsic and built into the fabric of the very Universe that reminds you that no matter how smart you are, no matter how sophisticated your experimental method, how sensitive your equipment, you can’t ‘know’ everything about a system. An idea that contradicts the principles that classical physics was built upon.

Assuming the name Heisenberg’s uncertainty principle, the Heisenberg uncertainty principle, or simply, the uncertainty principle, the concept would become arguably the second most commonly recognised element of quantum physics, outside of Schrodinger’s eponymous feline. Eventually, this idea would find itself absorbed into pop-culture, making its way to jokes, newspaper strips, t-shirts, and cartoons.

“The uncertainty principle ‘protects’ quantum mechanics,” said legendary physicist Richard Feynman of the utility of Heisenberg’s breakthrough. “Heisenberg recognized that if it were possible to measure both the momentum and the position simultaneously with greater accuracy, quantum mechanics would collapse. So he proposed that must be impossible.”

What is the Uncertainty Principle?

The most generalised version of Heisenberg’s uncertainty principle says that if you measure the momentum of a particle with uncertainty Δp, then you are limited in how precisely you can ‘know’ its position. You can’t know it any more precisely than Δx ≥ ℏ/2Δp, where ℏ (or H-bar) is a value known as the reduced Planck’s constant and is extremely small, a fact that will become important when we ask why macroscopic objects like cars and balls don’t seem to be affected by the uncertainty principle. 

Rearranging the equation above gives the most common version of Heisenberg’s uncertainty principle, and perhaps the most famous equation in physics outside of E=mc^2. This equation tells us that when the uncertainty in position is multiplied by the uncertainty in momentum its value can’t be greater than the reduced Planck’s constant divided by two. 

The equation above also applies to several other variables, most notably energy and time, it can also be adapted to any suitable pairs of operators in a system.

The momentum and position version of the uncertainty principle may well be the most familiar but it is by no means the only version, nor should the other versions be considered less important. In fact, the energy/time variation of the uncertainty principle gives rise to one of the most striking and counter-intuitive elements of reality — the idea that virtual pairs of particles can pop in and out of existence. 

If you consider an infinitesimal isolated area of spacetime observed for a precisely ‘known’ period of time, then the uncertainty principle for energy and time (ΔE Δt = ≥ ℏ/2Δ) says that you can’t precisely know the energy content of that area. Meaning that particles must be popping in and out of existence in that box.

This concept, wittily named ‘nature’s overdraft facility’ by some waggish physicists, is a phenomenon that sounds unlikely, impossible even, but has been experimentally verified. The Heisenberg uncertainty principle limits just how long the Universe will allow itself to go ‘overdrawn’ before the particles annihilate and that energy loan is paid back.

In order to get aspiring-physicists to accept the radical ideas birthed from the uncertainty principle, and that concept that there is a fundamental limit to what can be known about a system — something contrary to classical physics —thus meaning that everything classical physics imparts about the ‘knowability’ of a system is wrong,  a ‘semi-classical’ version was first presented to the scientific community.

We approach it now with some trepidation and the warning that it barely scratches the surface of the uncertainty principle and somewhat downplays how intrinsic it is in nature. 

The Semi-Classical Uncertainty Principle

You’re asked to take part in a quantum physics experiment at the ZME labs. You arrive, are immediately handed a tennis racquet and asked to step into an extremely dark room. Once in there, a voice announces that your task is simply to find the tennis balls in the room with the racquet. 

“Sounds simple enough,” you think. That is until a tennis ball strikes your leg at high-speed. You realise that the tennis balls are being fired into the room at completely random angles. Eventually, after some failing around in the dark, you’re racquet hits a ball. “Got one!” you exclaim. 

“That’s great,” comes the voice over the intercom. “Where is it now?”

Of course, the problem with that crude little analogy is that the very act of ‘measuring’ the ball’s position or momentum, intrinsically changes the state of the system and essentially puts you back at square one. It’s a little like that every time we try to take a quantum mechanical measurement. 

In order to ‘see’ an electron, researchers have to fire photons at it. The problem is that photons carry with them momentum. And as electrons are so small, the wavelengths of the photons have to be of a similar scale. The issue is, the shorter the wavelength, the higher the energy and, in turn, the greater the momentum. 

Thus, bombarding an electron with photons imparts this moment to them, changing the very state of the system. 

The reason that the semiclassic description of the Heisenberg uncertainty principle is that it gives the impression that if there was some incredibly sensitive measuring technique, it could, perhaps, be ‘worked around.’ This isn’t true. No matter how sensitive, this relationship is something that can’t really be avoided. It’s ‘built-in’ to nature. 

To see why this is the case it’s necessary to investigate one of the founding principles of quantum mechanics, the ubiquitousness of waves. 

Wave Certainty Goodbye

You receive a call from ZME labs. “We know the last experiment didn’t go so well, and we really hope the bruises are healing,” says a painfully familiar voice. “Look, we’ve got another test and this one will really demonstrate Heisenberg’s uncertainty principle… no tennis balls.”

You reluctantly agree to attend. 

Upon your arrival, you are handed a skipping rope and asked to wave it up and down rhythmically. The opposite end is held by a nervous-looking lab assistant who you notice is covered in tennis ball sized welts. 

Below is what results from your frantic, yet rhythmically waving. A steady wave shape. But, here comes the voice through the loudspeaker again: “Ok, now tell us, where on the x-axis [which marks position] is the wave?”

As you can see, the wave has no well-defined position, and here is how that is analogous to a particle in quantum mechanics. In the mathematics used to describe a quantum system, the spread of the wave is momentum, the square of the amplitude is the probability of the particle being located in a particular position.

Thus, in the above image what we actually have is a very well-known momentum. And as Heisenberg’s uncertainty principle primes us to believe, we can see that we can say nothing about the position as the wave can’t be said to possess a single position on the x-axis. The square of the amplitude is the same everywhere.

Back to ZME labs. You’ve had just about enough of these cryptic unanswerable questions and bizarre sports-equipment related experiments. So to teach the researchers a lesson, you give the rope one sudden ‘whip’ — Indiana Jones style. 

The wave is suddenly localised, as you can see, the amplitude and the thus the square of the amplitude is zero everywhere but in one spot. A position can be assigned to the wave, but as you can see, there is no spread anymore — the wavefunction is destroyed. 

This is analogous to having exact knowledge of a particle’s location. As the wavefunction spread is destroyed and this was the representation of the particle’s momentum, you suddenly have no knowledge of momentum. 

All this shows that Heisenberg’s uncertainty principle really arises from the fact that matter can be described as waves on the quantum level.

You are on your way out of ZME labs, for what you hope is the final time and nursing serious whiplash in your wrist when the lead researcher hands you a tennis ball. “As a memento,” he says chirpily.

You thank him, but mentally vow to throw it over the tallest wall you can find on the way to your car and home. 

Little do you know, your rage against the ball will reveal how without the phenomena described by Heisenberg’s uncertainty principle  the Universe would be a much colder, and darker place. 

Quantum tunnelling: Quantum balls and tall walls

One of the most remarkable features of the quantum realm is the phenomenon of quantum tunnelling, without which the nuclear fusion processes that power the stars and create the Universe’s heavier elements would not be able to take place. 

Tunnelling allows protons in the core of the sun to overcome mutual repulsion caused by their positive charges, a potential barrier that even under extreme pressure, they do not have the kinetic energy to overcome. This allows the formation of deuterium from hydrogen nuclei and begins the nuclear fusion process in the star’s core which leads to the formation of helium from hydrogen and powers its immense energy output.

You’re thinking about quantum tunnelling on your way to your car when you feel the tennis ball you received as a ‘memento’ and stuffed in your pocket pressing into your thigh. Remembering your promise, you look at the nearest wall, noting that it’s probably higher than you can throw the ball. 

You resolve to give it a few tries anyway. 

You throw the ball a few times, each with exactly the same force against the same resistance provided by gravity and air resistance, realising you can’t give it enough kinetic energy to get it over the top of the wall. In fact, you’re falling considerably short. But, this is a special ball. The researchers at ZME labs have found a way to imbue it with the qualities of a quantum particle. 

On your 47th throw of the quantum ball with the same kinetic energy, the ball approaches its usual limit and simply disappears. You inspect the wall seeing no holes, and you know there is no way the ball could have broke through the wall… then you hear a loud cry from the over the side of the wall scream: “My flowers… Whose ball is this?” You decide discretion is the better part of valour, and flee the scene. 

So, how can Heisenberg’s uncertainty principle be responsible for the ball travelling to the other side of the wall, an area that in physics we would describe as ‘classically forbidden’?

The key is, that as we precisely know this quantum ball’s momentum, we can’t be sure of its position. This means that there is a tiny probability that the ball can be found in a region it should be impossible to reach. 

Below, you can see a simulation of what happens when a particle of certain energy approaches an energy barrier that exceeds its energy. It should be noted here that the ‘wider’ or ‘taller’ the barrier, ie. the greater the energy demand, the less likely a particle is to clear it. 

You can think about tunnelling like this. A particle of energy <E> approaches a barrier of <2E>. Clearly it doesn’t have enough energy to ‘jump’ this barrier. Yet, in quantum physics, we find a small probability that transmission occurs. Obviously, that means that in circumstances where you have a lot of particles, like in the core of a star, the law of large numbers suggests that this kind of rare event still happens a lot.

As you muse on this, you have a worrying thought: “I know the exact momentum of my car. Does that mean I can’t know its position?”

You quicken your step considerably. 

Dude where’s my car? Why Heisenberg’s uncertainty principle doesn’t apply to everyday objects

Heisenberg and Schrödinger get pulled over for speeding. The cop asks Heisenberg: “Do you know how fast you were going?”
Heisenberg replies: “No, but we know exactly where we are!”
The officer looks at him confused and says: “you were going 108 miles per hour!”
Heisenberg throws his arms up and cries: “Great! Now we’re lost!”

We’ve thus far had a little fun describing macroscopic objects like tennis balls and skipping ropes displaying quantum behaviour, so it’s probably an idea to explain why this isn’t something we actually see every day.

The key is the very small value of the reduced Planck’s constant (ℏ). This means that the lower limit in the uncertainty of measuring the position and momentum of large objects is negligible when compared to massive objects like tennis balls, skipping ropes or cars. 

All matter has a de Broglie wave (λdb) but that wave has to be a comparable size to the Planck’s reduced constant for Heisenberg’s uncertainty principle to have a considerable effect. The de Broglie wave of a tennis ball is way too small to be subject to the uncertainty principle in any significant way.

It’s for much the same reason that moving objects don’t diffract around trees. Their de Broglie wave is way too small.

Sorry, you’re not getting off with that speeding ticket so easily. 

Sources and further reading

Griffiths. D. J, ‘Introduction to Quantum Mechanics,’ Cambridge University Press, [2017].

Feynman. R, Leighton. R. B, Sands. M, ‘The Feynman Lectures on Physics. Volume III: Quantum Mechanics,’ California Institute of Technology, [1965].

Bolton. J, Lambourne. R, ‘Wave Mechanics,’ The Open University, [2007].

The tale of physics’ most famous cat is one that is familiar to many, but what is the inside story of the feline so demanding it requires its own Universe, and how does it illustrate the 'weirdness' of the quantum world?

Superposition! The strange tale of Schrödinger’s cat

The tale of physics’ most famous cat is one that is familiar to many, but what is the inside story of the feline so demanding it requires its own Universe, and how does it illustrate the 'weirdness' of the quantum world?
The tale of physics’ most famous cat is one that is familiar to many, but what is the inside story of the feline so demanding it requires its own Universe, and how does it illustrate the ‘weirdness’ of the quantum world? (Robert Lea)

Of all the counter-intuitive elements of quantum physics introduced to the public since its inception in the early days of the twentieth century, it is quite possible that the idea that a system can be two (or more) contradictory things at once, could be the most challenging.

As well as defying a well-known aspect of logic — the law of non-contradiction — thus irritating logisticians, this idea of the coexistence of states, or superposition, was even a challenge to the fathers of quantum physics. Chief amongst them Erwin Schrödinger, who suggested a diabolical thought experiment that would show what he believed to the ludicrous nature of a system existing in contradictory states. 

The thought experiment would go on to become perhaps the most well-known in the history of physics, weaving its way on to witty t-shirts, hats, bags and badges, infiltrating pop-culture, TV and film. This is the strange tale of Schrödinger’s cat, and what it can teach us about quantum physics and the nature of reality itself. 

Before delving into the experiment that Schrödinger devised, it is worth examining the circumstances that led him to consider the absurd situation of a cat that is both living and dead at the same time. 

Wanted: Dead or Alive! How the cat got put in the box

In many ways, Erwin Schrödinger’s place in the history of quantum mechanics is overshadowed by his feline-based thought experiment. The Austrian physicist was responsible for laying the foundation of a theoretical understanding of quantum physics with the introduction of his eponymous wave equation in 1926. As Joy Manners describes in the book ‘Quantum Physics: An Introduction’:

“The Schrödinger equation did for quantum mechanics what Newton’s laws of motion had done for classical mechanics 250 years before.”

Joy Manners, Quantum Physics: An Introduction

What Schrödinger’s equation shows is that the state of a system — the collection of all of its measurable qualities — can be described as a wavefunction — represented by the Greek letter Psi (Ψ). This wavefunction contains all the information of a system that it is possible to hold. Each wavefunction is a solution to Schrödinger’s equation, but here’s the crazy part; two wavefunctions can be combined to form a third, and this resultant wavefunction can contain completely contradictory information.

When the wavefunctions of a system are combined it is in a ‘superposition’ state. There is also no limit no how many of these wavefunctions cam be combined to form a superposition. 

Yet, infinite though a wavefunction can be, eternal it is not. The act of taking a measurement on the system in question seems to cause the wavefunction to collapse — something there is as yet no physical or mathematical description for. There are, however, interpretations of what happens, which go to the very heart of reality.

Before tackling these interpretations, first, we should get to our cat in the box before he gets too impatient. 

A most diabolical device 

It was in 1935, whilst living in Oxford fleeing the rise of the Nazis, that Schrödinger first published an article that expressed his concern with the idea of measurement, wave function collapse, and contradictory states in quantum mechanics. Little would he know, it would lead to him becoming history’s most infamous theoretical-cat-assassin. 

A common illustration of the Schrodinger’s Cat thought experiment (Dhatfield / CCbySA 3.0)

Below Schrödinger describes the terrible predicament that his unfortunate moggy finds himself in. 

“A cat is placed in a steel chamber with the following hellish contraption… In a Gieger counter a tiny amount of a radioactive substance, so that maybe within an hour one of the atoms decays, but equally probable is that no atom decays…”

So, there is a 1/2 chance that an atom of the substances decays and causes the Gieger to tick over the hour duration of the experiment. 

“If one decays the counter triggers a little hammer which breaks a container of cyanide.” 

So, if the atom decays over the hour, the cat is killed. If it doesn’t, the cat survives. Treating the box and the cat as a quantum system how would we describe its wavefunction (Ψ)?

The wavefunction of the system now exists in a superposition of the individual wavefunction that describes the cat as being alive, and the one that declares it dead. According to the rules of quantum physics, the cat is currently both dead and alive.

Our unfortunate feline isn’t doomed to live out its existence as some bizarre quantum zombie, though. A quick peek inside the box constitutes a measurement of the system. Thus, by opening the box we collapse the wavefunction and determine the fate of Schrödinger’s cat. It really is curiosity that kills the cat, in this case.

Let’s end our analogy on a happy note. We open our box and fortunately the substance has not undergone decay. The cyanide bottle remains intact. Our moggy survives, unscathed if irritated. The wavefunction collapsed leaving the blue sub-wavefunction intact, but what actually just happened here? How was the cat’s fate determined? 

The short answer is, we don’t know, but we have some interpretations. Next, we compare the two most prominent. 

Way more than nine lives. The many-worlds interpretation 

What we have discussed thus far consists of a very rough description of the Copenhagen interpretation of quantum mechanics. The reason it’s common sense to present this first is that it is generally the interpretation that is most widely accepted and taught.

As you’ve seen, the Copenhagen interpretation describes a system with no established values until a measurement occurs or is taken and a value — in our case ‘alive’ — emerges. If this sounds deeply unsatisfactory, well, it is. One of the questions it leaves open is ‘why does the wavefunction collapse?’

In 1957, an American physicist Hugh Everett III, asked a different question: ‘What if the wavefunction doesn’t collapse at all? What if it grows?’ From this emerged Everett’s ‘relative state formulation’, better known to fans of science fiction, comic books and fantasy as the ‘Many Worlds Hypothesis/interpretation’.

Below we see what happens to the wavefunction in the Copenhagen interpretation. The box is opened and the wavefunction collapses. 

So what happens in the ‘many worlds’ interpretation? Rather than collapsing, as the box is opened the wavefunction expands. The cat does not cease to be in a superposition, but that superposition now includes the researchers and the very universe they inhabit. We become part of the system.

In the many-worlds interpretation, the researchers do not open the box to discover if the cat is dead or alive, they open the box to see if they are in the universe where the cat survived or the universe in which it was dispatched. They and their world have become part of the wavefunction. An entirely new universe in superposition with the old. The only difference. 

One less cat.

Schrodinger’s Kittens: Some words of caution

Again, as with the Copenhagen interpretation, there is no real experimental evidence of many worlds concept. In many ways, any interpretation of quantum mechanics is really more a realm of philosophy than science. Also, when considering ‘many worlds’ it’s worth noting that this is a different concept than the idea of a ‘multiverse’ of different universes created at the beginning of time. 

Further to this, there are some real problems with considering the ‘cat in a box’ as a quantum system. Researchers are constantly finding quantum effects in larger and larger systems, the current record seems to be 2,000 atoms placed in a superposition. To put that into perspective; a humble cat treat contains around 10²² atoms!

Many physicists have suggested reasons why larger systems fail to display quantum effects, with Roger Penrose suggesting that any system that has enough mass to affect space-time via Einstein’s theory of general relativity can’t be isolated. Via the influence of gravity, it is constantly having ‘measurements’ taken. This would definitely apply to even the most minuscule moggy. 

It is worth noting here that the general description of the thought experiment and the opening of the box has led some to speculate that it is the addition of a ‘consciousness’ that actually causes the wavefunction collapse. 

This is an idea that has sold a million or so books on ‘quantum woo’ and it arises from the unfortunate nomenclature of quantum physics. The use of the words ‘measure’ and ‘observe’ imply the intervention of a conscious observer. The truth is that any interaction with another system is enough to collapse a quantum wavefunction, as they tend to exist in incredibly delicate, easily disturbed states. 

Sources and further reading

Schrödinger. E,

Griffiths. D. J, ‘Introduction to Quantum Mechanics,’ [2017], Cambridge University Press.

Broadhurst. D, Capper. D, Dubin. D, et al, ‘Quantum Physics: An Introduction,’ [2008], Open University Press.

Nomura. Y, Poirer. B, Terning. J, ‘Quantum Physics, Mini Black Holes, and the Multiverse,’

Orzel. C, ‘How to Teach Quantum Physics to your Dog,’ [2009], Simon & Schuster. 

Physicists solved decades-old puzzle with huge implications for quantum computers

Artist impression of a nano-scale electrode that locally controls the quantum state of a single nucleus inside a silicon chip. CREDIT: Picture: UNSW/Tony Melov.

Researchers at the University of South Wales in Australia have controlled the nucleus of a single atom using only electric fields for the first time. This idea was first proposed in 1961, but the practicalities have eluded scientists since then. To make things especially interesting, the discovery of the method was made completely by accident.

“This discovery means that we now have a pathway to build quantum computers using single-atom spins without the need for any oscillating magnetic field for their operation,” said UNSW’s Scientia Professor of Quantum Engineering Andrea Morello. “Moreover, we can use these nuclei as exquisitely precise sensors of electric and magnetic fields, or to answer fundamental questions in quantum science.”

Nicolaas Bloembergen, the pioneer of magnetic resonance and Nobel Laureate, first suggested in 1961 that nuclear spin could theoretically be controlled with electric fields. How exactly we might go about that has escaped scientists ever since — until now.

“I have worked on spin resonance for 20 years of my life, but honestly, I had never heard of this idea of nuclear electric resonance,” Prof Morello says. “We ‘rediscovered’ this effect by complete accident – it would never have occurred to me to look for it. The whole field of nuclear electric resonance has been almost dormant for more than half a century, after the first attempts to demonstrate it proved too challenging.”

Magnetic resonance is one of the most practical and widespread techniques in modern physics and medicine. It’s what allows doctors to peer inside the tissue of patients in exquisite detail or what allows mining and oil companies to analyze mineral samples.

However, the efficacy of magnetic resonance is very poor for certain applications that work with objects at a very small scale, such as atoms.

To explain the difference between controlling nuclear spins with magnetic and electric fields, Morello uses an analogy of a billiard table.

“Performing magnetic resonance is like trying to move a particular ball on a billiard table by lifting and shaking the whole table,” he says. “We’ll move the intended ball, but we’ll also move all the others.”

“The breakthrough of electric resonance is like being handed an actual billiards stick to hit the ball exactly where you want it.”

Morello and colleagues originally planned to perform nuclear magnetic resonance on a single atom of antimony, an element with a large nuclear spin. This was a curiosity-driven project with no practical goal in mind — the physicists were simply interested in exploring the weird boundary between the quantum and classical worlds.

The experiment proved more than they bargain for, though. At one point, the researchers found that the nucleus refused to respond to certain frequencies, but showed a strong response at other frequencies. Imagine the surprise they had when they realized they were actually performing electric resonance instead of magnetic resonance!

“What happened is that we fabricated a device containing an antimony atom and a special antenna, optimized to create a high-frequency magnetic field to control the nucleus of the atom. Our experiment demands this magnetic field to be quite strong, so we applied a lot of power to the antenna, and we blew it up!” said Dr. Serwan Asaad, one of the lead authors of the study.

A melted wire turned the device into an electric field resonator instead of a magnetic field one. Credit: Nature, Assad et al.

When working with small nuclei like phosphorus, a broken antenna usually ends the experiment. However, the researchers could still perform measurements because of the large size of the antimony nucleus. What’s more, in this case, the antenna still worked — it’s just that the damage caused it to create a strong electric field instead of a magnetic field.

Later, the researchers in Australia performed computer modeling that showed that nuclear electric resonance was truly a local phenomenon. The electric field distorted the atomic bonds around the nucleus, causing the nucleus to change orientation.

“This landmark result will open up a treasure trove of discoveries and applications,” says Prof Morello. “The system we created has enough complexity to study how the classical world we experience every day emerges from the quantum realm. Moreover, we can use its quantum complexity to build sensors of electromagnetic fields with vastly improved sensitivity. And all this, in a simple electronic device made in silicon, controlled with small voltages applied to a metal electrode!”

The findings appeared in the journal Nature.

Two different quantum optomechanical systems used to demonstrate novel dynamics in backaction-evading measurements. Left (yellow): silicon nanobeam supporting both an optical and a 5 GHz mechanical mode, operated in a helium-3 cryostat at 4 Kelvin and probed using a laser sent in an optical fibre. Right (purple): microwave superconducting circuit coupled to a 6 MHz mechanically-compliant capacitor, operated in a dilution refrigerator at 15 milli-Kelvin. (I. Shomroni, EPFL.)

Side stepping Heisenberg’s Uncertainty Principle isn’t easy

Two different quantum optomechanical systems used to demonstrate novel dynamics in backaction-evading measurements. Left (yellow): silicon nanobeam supporting both an optical and a 5 GHz mechanical mode, operated in a helium-3 cryostat at 4 Kelvin and probed using a laser sent in an optical fibre. Right (purple): microwave superconducting circuit coupled to a 6 MHz mechanically-compliant capacitor, operated in a dilution refrigerator at 15 milli-Kelvin. (I. Shomroni, EPFL.)

Recent developments in science such as the detection of gravitation waves by way of the minute displacement of mirrors at LIGO and the development of atomic and magnetic force microscopes to reveal atomic structure and spins of single atoms have pushed the boundaries of what can be defined as measurable. 

Yet, as scientists push the limits of mechanical measurements the spectre of Heisenberg’s Uncertainty principle remains to remind that no matter how accurate their equipment and procedures become, nature has an intrinsic, in-built limit to what they can ‘know’. 

One of the main results of early investigations in quantum physics, the uncertainty principle says that even as the sensitivity of our measuring equipment improves — these conventional measures are limited by a “measurement backaction”. The most common and easiest to explain example of the uncertainty principle is the idea that knowledge of a particle’s exact location immediately destroys knowledge of its momentum — and by extension, the ability to predict its location in the future. 

Sense and sensitivity in laser interferometers

Despite this seeming hinderance, researchers are hard at work developing potential methods to help them ‘sidestep’ Heisenberg’s uncertainty principle. Thes techniques hinge on the careful collection of only certain information about a system, whilst intentionally omitting other aspects.

So, for example, waves and wavefunctions are of vital importance in quantum mechanics. Using this selective method researchers would attempt to take the measurement of the wave’s amplitude, whilst simultaneously ignoring its phase. 

These methods could, in principle at least, have unlimited sensitivity with the drawback of only being able to gauge half of the information about a system. That is the aim of Tobias Kippenberg at Ecole Polytechnique Federale De Lausanne (EPFL). In conjunction with scientists at the University of Cambridge and IBM Research, Zurich, Kippenberg has uncovered new dynamics that place further unexpected constraints on such systems and just what levels of sensitivity are achievable.

An aerial view of LIGO. The laser interferometer that runs through these massive kilometre scale arms must be incredibly sensitive to detect gravitational waves. But new research suggests another hindrance to such sensitivity. (LIGO)

The team’s work shows particular interest to the interferometers that are used to measure gravitational waves. The sensitivity of these instruments is of vital importance as gravitational waves are incredibly difficult to detect. As these pieces of equipment use disturbances in laser beams shined down their massive, kilometre-scale arms, improving their sensitivity means trying to avoid backaction in electromagnetic waves. 

The team’s study — published in the journal Physical Review X — demonstrates that small deviations optical frequency, coupled with deviations in mechanical frequency can lead to mechanical oscillations being amplified out of control. This mimics the physics displayed in a state physics refer to as “degenerate parametric oscillator”.

This behaviour was found by Kippenberg and his team in two radically different systems — one operating with optical radiation, the other operating with microwave radiation. This is a fairly disastrous discovery as it implies that the dynamics are not unique to any particular system, but rather, are common across many such systems. 

The researchers from EPFL investigated these dynamics further — tuning the frequencies and demonstrating a perfect match with pre-existing theories. EPFL scientist Itay Shomroni, the paper’s first author, explains: “Other dynamical instabilities have been known for decades and shown to plague gravitational wave sensors. 

“Now, these new results will have to be taken into account in the design of future quantum sensors and in related applications such as backaction-free quantum amplification.”

Original research: Shomroni, A. Youssefi, N. Sauerwein, L.Qiu, P. Seidler, D. Malz, A. Nunnenkamp, T. J. Kippenberg. Two-tone optomechanical instability and its fundamental implications for backaction-evading measurements. Physical Review X 9, 041022; 30 October 2019. DOI:10.1103/PhysRevX.9.041022

Artistic illustration of the delocalization of the massive molecules used in the experiment. © Yaakov Fein/University of Vienna/ Universität Wien

Super-Superposition: 2,000 atoms in ‘two places at once’

Artistic illustration of the delocalization of the massive molecules used in the experiment. © Yaakov Fein/University of Vienna/ Universität Wien

A research team of scientists from the University of Vienna and the University of Basel have tested the principle of quantum superposition on an unprecedented scale. The team brought hot, complex molecules comprised of approximately 2,000 atoms into quantum superposition and caused them to interfere. 

This confirms quantum phenomena can occur on a mass-scale never achieved before, resulting in new constraints being placed on alternative theories to quantum mechanics. 

Markus Arndt is a Professor of Quantum Nanophysics at the University of Vienna and led the research team. Their results are published in the journal Nature Physics. He explains the principle of superposition:

“While in classical mechanics you describe bodies with momentum and position, quantum mechanics tells us that matter has to be described by a wave function.

“The amplitude squared of this wave function tells us where to find a particle.”

The principle of superposition emerges from one of the fundamental elements of quantum mechanics — the Schrodinger equation. Drawing an analogy to water waves, this means that these ‘quantum waves’ — or de Broglie Waves named after French Physicist Louis De Broglie — can exhibit constructive and destructive interference effects. The major difference is, whilst the water wave contains a multitude of particles, the de Broglie wave describes a single particle.

Arndt continues the analogy: “Like water waves, the quantum matter-waves can fill large areas of space, this means that a particle has no well-defined position anymore.

“Colloquially we say that the particle can be in two or more places at once. Whilst in free propagation the wave function collects information about places that a classical billiard ball could never know. ”

When a measurement is made the particle can only be measured in one place, a behaviour described as the collapse of the wavefunction. The most famous and familiar demonstration of this effect is the double-slit experiment. 

Light allowed to pass through both slits displays a wave-like pattern

The experiment was initially run with photons — unveiling that they possess both wave-like and particle-like properties [you’ll often see this described as light ‘being both a particle and a wave’ which is incorrect. It’s actually neither]. 

This particle-wave duality nature was later discovered in matter particles by re-running the experiment with particles of increasing mass — first electrons — right up to carbon-60 molecules, also known as Bucky Balls.

This wave-like aspect of nature is clearly an aspect of quantum mechanics and the very small — but not something we see in the world around us. This leads researchers to an interesting question: where does the boundary between quantum and classical mechanics lie? 

Or alternatively, how large does a billiard ball have to be before it can’t be described with the mathematics of waves anymore?

Where is the quantum/classical boundary?

Arndt and the team demonstrated quantum interference with larger objects than ever attempted before. The previous largest molecule used for such experiments weighed in at 10.123 atomic mass units (amu), whilst the molecules that Arndt and his team used were over 2.5 x 10⁴ amu — greater by a magnitude of 100 — the researcher tells me. 

One of the largest molecules the team sent through their interferometer —  C707H260F908N16S53Zn4 — consists of more than 4.0 x 10⁴ protons, neutrons and electrons. Its de Broglie wavelength is a thousand times smaller than the diameter of a hydrogen atom. 

These molecules were specially created for the experiment by Marcel Mayor and his team at the University of Basel. Their technique makes the molecules stable enough to form a beam of molecules in an ultra-high vacuum. 

The matter-wave interferometer in Vienna that the team used was specially designed with a two-metre-long baseline, in order to make it adept at highlighting the quantum nature of the particles in question. They also have exciting potential future applications. 

“These interferometers that we build for these foundational questions, are exquisite force sensors,” Arndt explains. “Applied to biomolecules or cluster we use them to learn about the internal properties of these particles, even though quantum mechanics forbids us to know where they are.”

The team calls this matter-wave interference assisted metrology and are still the only group in the world working on it, Arndt says.

Living on the edge. Probing quantum physics boundaries

By showing that a superposition can be maintained for a massive particle, Arndt and his team have effectively placed important boundary conditions on a class of models aimed to define that transition from quantum to classical mechanics. 

Arndt explains why models that tie mass to the collapse of superposition — legitimate mathematical extensions to Schrodinger’s equation — are appealing:

“Continuous spontaneous localization models need this to explain why small things behave quantum mechanically and big ones don’t.

“ The Schrödinger equation depends on derivatives by space and time. If a particle curves space-time in different places, how can one still have a consistent Schrödinger equation?”

As for where that boundary lies, Arndt believes that the researchers aren’t quite at that limit just yet. 

“It’s hard to say. It’s a matter of experimental control above all,” he explains. “There is good reason to believe that if we do it right, we will see quantum effects for quite a while yet.”

The crux of the challenge of finding where quantum effects cease could lie in the discovery of a quantum theory of gravity. 

Arndt elaborates: “There is a well-founded suspicion that something may change at high masses because gravity deforms space-time and the particles themselves become a source for that. 

“When exactly this may happen, no one can say with certainty. There is no quantum gravity theory yet.”

Original research: https://www.nature.com/articles/s41567-019-0663-9

The Micius satellite--the first experiment to test quantum physics in space

Quantum satellite investigates the gap between Quantum Mechanics and General Relativity

Experimental diagram of testing gravity-induced decoherence of entanglement (provided by University of Science and Technology of China)

Quantum mechanics and general relativity represent the two most successful theories in 20th-century physics. But despite almost 100 years of continued experimental verification and practical application, researchers remain unable to unite the disciplines. 

As general relativity describes the effects of gravity on Einstein’s four-dimensional spacetime — three dimensions of space and time — this means that a quantum theory of gravity continues to evade detection. 

As the problem of unification remains unsolved, physicists put forward various models that require experimental verification. 

A team of international researchers has developed a framework to test a model which may account for the breakdown of general relativity’s rules on the quantum scale. They tested this framework using the quantum satellite — Micius — a Chinese project which tests quantum phenomena in space. 

The research — documented in a paper published in the journal Science — represents the first meaningful quantum optical experiment testing fundamental physics between quantum theory an gravity, says Jian-Wei Pan, director of the CAS centre for Excellence in Quantum Information and Quantum Physics at the University of Science and Technology of China.

Pan and his team wanted to test the event formalism model of quantum fields model — a theory that suggests that the correlation between entangled particles would collapse — a phenomenon known as decoherence — as they pass through the gravitational well of Earth. The idea is that the differences in the gravitational force would force decoherence as the particle experiencing less gravity would be able to travel with less constraint than its counterpart in an area of stronger gravity.

Pan suggests that event formalism presents a description of quantum fields existing in spacetime as described by general relativity — consisting of curvature caused by the presence of mass. Thus if the team can observe this model’s effects, they can imply the presence of quantum phenomena on a larger scale as described by general relativity.

Pan says: “If we did observe the deviation, it would mean that event formalism is correct, and we must substantially revise our understanding of the interplay between quantum theory and gravity theory.”

In their test, the team used pairs of particles described as ‘time-energy entangled’ — a recently discovered type of entanglement which photons are entangled in terms of their energies and the times they are detected. 

The team was unable to detect the particles deviating from standard behaviour expected in quantum mechanics, but they plan to retest a version of their theory that is more flexible. 

“We ruled out the strong version of event formalism, but there are other versions to test,” Pan says. “A modified model remains an open question.”

To put this revised version to the test a new satellite will be launched that will orbit up to sixty-times higher than Micius — enabling it to test a wider variation in gravitational field strength. 

Original research: https://science.sciencemag.org/content/early/2019/09/18/science.aay5820

Physicists are a step closer to a theory of quantum gravity

New research centering around the Unruh effect has created a set of necessary conditions that theories of quantum gravity must meet.

Quantum physics has, since its development in the early years of the 20th century, become one of the most successful and well-evidenced areas of science. But, despite all of its successes and experimental triumphs, there is a shadow that hangs over it. 

Despite successfully integrating electromagnetic, the weak and strong nuclear forces — three of the four fundamental forces — quantum physics is yet to find a place for gravity. 

As such, it cannot link with one of physics other great triumphs, that of Einstein’s theory of general relativity. Thus, physicists are currently working hard to develop a quantum theory of gravity. 

Now researchers led by the SISSA (Scuola Internazionale Superiore di Studi Avanzati), the Complutense University of Madrid and the University of Waterloo — have identified the sufficient and necessary conditions that the low-energy limit of quantum gravity theories must satisfy to preserve the main features of the Unruh effect.

Any new theory of physics must factor in this effect — this means quantum gravity theories too, must have a place for the Unruh effect and its predictions (which are detailed below). 

The new study — published in the journal Physical Review Letters — provides a solid theoretical framework to discuss modifications to the Unruh effect caused by the microstructure of space-time.

Eduardo Martin-Martinez, an assistant professor in Waterloo’s Department of Applied Mathematics, elaborates on the team’s work: “What we’ve done is analyzed the conditions to have Unruh effect and found that contrary to an extended belief in a big part of the community thermal response for particle detectors can happen without a thermal state.”

The team’s findings of importance because the Unruh effect exists in the boundary between quantum field theory and general relativity, and quantum gravity, which we are yet to understand.

“So, if someone wants to develop a theory of what’s going on beyond what we know of quantum field theory and relativity, they need to guarantee they satisfy the conditions we identify in their low energy limits.”

What is the Unruh effect?

The Unruh effect was first described by Stephen Fulling in 1973, followed by Paul Davies in 1975 and William G Unruh — after whom it was named — in 1976.

W.G Unruh, one of the developers of the Unruh effect, after whom it was named.

 It predicts that an observer in a non-inertial reference frame — one that is accelerating — would observe photons and other particles in a seemingly empty space while another person who is inertial would see a vacuum in that same area.

In other words; a consequence of the Unruh effect is that the nature of a vacuum in the universe is dependant on the path taken through it.

As an analogy, consider a universe with a constant temperature of zero and in which, no heat arises from the effects of friction or kinetic energy contributions. A still thermometer would have its mercury-level sat permanently at zero.

But the Unruh effect posits that if that thermometer was waved from side-to-side, the temperature measured would no longer be zero. The temperature measured would be proportional to the acceleration that the thermometer undergoes.

Raúl Carballo-Rubio, a postdoctoral researcher at SISSA, Italy, explains further: “Inertial and accelerated observers do not agree on the meaning of ‘empty space.

 “What an inertial observer carrying a particle detector identifies as a vacuum is not experienced as such by an observer accelerating through that same vacuum. The accelerated detector will find particles in thermal equilibrium, like a hot gas.”

He further explains that as a result of this, it is reasonable to expect that any new physics that modifies the structure of quantum field theory at short distances, would induce deviations from this law. 

Carballo-Rubio continues: “While probably anyone would agree that these deviations must be present, there is no consensus on whether these deviations would be large or small in a given theoretical framework. 

“This is precisely the issue that we wanted to understand.”

Defining the conditions theories of quantum gravity must satisfy

The researchers analyzed the mathematical structure of the correlations of a quantum field in frameworks beyond standard quantum field theory. 

The result of this analysis was then used to identify the three necessary conditions that are sufficient to preserve the Unruh effect. 

Low-energy predictions of quantum gravity theories can be constructed from the results. The findings of this research provide the tools necessary to make these predictions in a broad spectrum of situations.

Having been able to determine how the Unruh effect is modified by alterations of the structure of quantum field theory, as well as the relative importance of these modifications, the researchers believe the study provides a solid theoretical framework to discuss and perhaps test this particular aspect as one of the possible phenomenological manifestations of quantum gravity. 

This is particularly important and appropriates even if the effect has not yet been measured experimentally, as it is expected to be verified in the not so distant future.

Original research: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.041601

Purdue researchers have modified a popular theorem for identifying quantum entanglement and applied it to chemical reactions. This quantum simulation of a chemical reaction yielding deuterium hydride validated the new method. ( Purdue University image/Junxu Li)

Physicists measure quantum entanglement in chemical reactions

Quantum entanglement and other quantum phenomena have long been suspected by scientists to play a role in chemical reactions like photosynthesis. But, until now, their presence has been hard to identify.

Purdue researchers have modified a popular theorem for identifying quantum entanglement and applied it to chemical reactions. This quantum simulation of a chemical reaction yielding deuterium hydride validated the new method. ( Purdue University image/Junxu Li)

Purdue researchers have modified a popular theorem for identifying quantum entanglement and applied it to chemical reactions. This quantum simulation of a chemical reaction yielding deuterium hydride validated the new method. ( Purdue University image/Junxu Li)

Researchers at Purdue University have unveiled a new method that enables them to measure entanglement — the correlation between the properties of two separated particles — in chemical reactions.

Discovering just what role entanglement play in chemical reactions has implications for the improvement of technologies like solar energy systems if we can learn to replicate them.

The study — published in the journal Science Advances — takes the theorem ‘Bell’s Inequality’ and generalises it to identify entanglement in chemical reactions. In addition to theoretical arguments, they also performed a series of quantum simulations to verify this generalized inequality.

Sabre Kais, a professor of chemistry at Purdue, explains further: “No one has experimentally shown entanglement in chemical reactions yet because we haven’t had a way to measure it. For the first time, we have a practical way to measure it.

“The question now is, can we use entanglement to our advantage to predict and control the outcome of chemical reactions?”

Bell’s Inequality — identifying entanglement.

John S. Bell designed an experiment to prove if quantum mechanics is complete (CERN)

John S. Bell designed an experiment to prove if quantum mechanics is complete (CERN)

Since its development in 1964, Bell’s Inequality has been validated as the go-to test that physicists use to identify entanglement in particles. The theorem uses discrete measurements of properties of particles such as the orientation in their spin — nothing to do with angular momentum in the quantum world — to find if the particles are correlated.

The problem is, discovering entanglement in chemical reactions requires that measurements are continuous. This means measuring aspects such as the angles of beams which scatter reactants forcing them into contact and transform into products.

To combat this, Kai’s team generalised Bell’s Inequality to include continuous measurements in chemical reactions, in a similar way to how the theorem had previously been generalised to examine light — photonic systems.

The team then tested their generalised Bell’s inequality using a quantum simulation of a chemical reaction yielding the molecule deuterium hydride.

The process was built on a foundation established in a 2018 experiment developed by Stanford University researchers that aimed to study the quantum states of molecular interactions.

Because the simulations validated the Bells’s theorem and showed that entanglement can be classified in chemical reactions, Kais’ team proposes to further test the method on deuterium hydride in an experiment.

Kais says: “We don’t yet know what outputs we can control by taking advantage of entanglement in a chemical reaction — just that these outputs will be different.

 “Making entanglement measurable in these systems is an important first step.”

Close-up of the 'quantum compass'. Credit: Imperial College London

‘Quantum compass’ can locate objects without GPS

It’s difficult to imagine modern life without GPS — we use it for everything from personal and in-car navigation, to drones and self-driving cars. Sometimes, however, a GPS signal might not be available due to factors such as tall buildings, lack of signal, or deliberate jamming by a third party. Now, British researchers at Imperial College London (ICL) have presented an alternative to the time-honored GPS, a quantum “compass” that can pinpoint its location on the globe without having to rely on satellites or any other external reference.

Close-up of the 'quantum compass'. Credit: Imperial College London

Close-up of the ‘quantum compass’. Credit: Imperial College London

The remarkable device was recently unveiled at the National Quantum Technologies Showcase, an event dedicated to presenting the technical progress achieved by projects funded by the UK National Quantum Technologies Programme, worth £270m.

Credit: Imperial College London.

Credit: Imperial College London.

The researchers who built the device call it a “standalone quantum accelerometer”. Today, accelerometers are commonly embedded in all sorts of common technologies, including airbags or your phone. As the name implies, accelerometers are electromechanical devices that sense either static or dynamic forces of acceleration to keep track of the movement and orientation. Using an accelerometer, it’s easy to determine the position of an object knowing its original position and velocity. However, over time, a regular accelerometer loses precision without an external reference, such as a GPS signal, to calibrate it.

This new quantum accelerometer doesn’t have this problem, being able to accurately measure its position without any external reference. It is, for all intents and purposes, self-contained.

What sets the ICL technology apart from other accelerometers is that it measures the properties of supercooled atoms. Close to absolute zero, the movement of atoms starts exhibiting quantum behavior, acting as both particles and waves. Because the wave properties are affected by acceleration, an ‘optical ruler’ can measure extremely minute changes with accuracy. The atoms are both cooled and measured by a laser system.

“When the atoms are ultra-cold we have to use quantum mechanics to describe how they move, and this allows us to make what we call an atom interferometer,” Dr. Joseph Cotter, from the Centre for Cold Matter at Imperial, said in a statement.

As it stands today, the quantum compass is far too large to fit inside a phone, but it could prove useful for the navigation of ships or trains. However, since the principles are the same, the device could also be suitable for research in fundamental science such as the search for dark energy or detecting gravitational waves. Military applications will also find quantum compasses incredibly appealing since a nuclear submarine or airplane can now precisely track its position without having to rely on GPS, whose signal is both detectable and jammable.

Having an alternative to GPS will certainly come in handy. According to an estimate, if the satellite network was denied for a single day, the UK would stand to lose about a million pounds. This is just the beginning, though — we should all expect more interesting things to come out of such a technology.

“I think it’s tremendously exciting that this quantum technology is now moving out of the basic science lab and being applied to problems in the wider world, all from the fantastic sensitivity and reliability that you can only get from these quantum systems,” Professor Ed Hinds, from the Centre for Cold Matter at Imperial, said in a statement.

The results are promising, but are still in an early stage, and have not been peer-reviewed. It remains to be seen whether the results really stand true, and whether the technology could be incorporated into practical situations. At least for now, the jury is still out on this one.

Credit: Public Domain.

Physicists may have solved ‘chicken or the egg’ paradox — both can come first

Credit: Public Domain.

Credit: Public Domain.

All chickens hatch from eggs and all chicken eggs are laid by chickens — and here lies the baffling conundrum. The paradox is typically used to illustrate how in certain situations it’s not clear which of two events is the cause and which is the effect. But the ancient Greek philosophers who first proposed this dilemma had no notion of quantum physics — that’s the domain of physicists at the University of Queensland, Australia and the NÉEL Institute, Frace who claim that the chicken and egg can both come first.

According to Dr. Jacqui Romero, a physicist at the University of Queensland (UQ), cause and effect are not always as straightforward as they are in the macroscopic, ‘real’ world.

“The weirdness of quantum mechanics means that events can happen without a set order,” Dr. Romero said. “Take the example of your daily trip to work, where you travel partly by bus and partly by train. Normally, you would take the bus then the train, or the other way round. In our experiment, both of these events can happen first.

“This is called ‘indefinite causal order’ and it isn’t something that we can observe in our everyday life.”

This Indefinite causal order was observed in the laboratory when the physicists measured the polarisation of photons at the output of a quantum switch. This measurement suggests that the order of transformations on the shape of light was not set, the authors reported in the journal Physical Reviews Letters. 

“This is just a first proof of principle, but on a larger scale indefinite causal order can have real practical applications, like making computers more efficient or improving communication,” said UQ’s Dr. Fabio Costa.

Is this the last word on the matter? Perhaps, perhaps not. Quantum physics is often weird and confusing, but there is some more ‘concrete’ research that investigated the chicken or egg dilemma.

In 2010, researchers from Scotland and England used a supercomputer called HECToR to analyze in high detail a chicken eggshell, determining the vital role of a protein used to kick-start the egg’s formation. The protein was known to be linked to egg formation, but the analysis showed it is an important catalyst. It’s also a protein only found in a chicken, which some see literally as scientific proof that the chicken definitely came before the egg.



Illustration of the 15-micrometre-wide drumheads prepared on silicon chips used in the experiment. The drumheads vibrate at a high ultrasound frequency, and the peculiar quantum state predicted by Einstein was created from the vibrations. Credit: Aalto University.

Scientists demonstrate quantum entanglement with objects big enough to see

Illustration of the 15-micrometre-wide drumheads prepared on silicon chips used in the experiment. The drumheads vibrate at a high ultrasound frequency, and the peculiar quantum state predicted by Einstein was created from the vibrations. Credit: Aalto University.

Illustration of the 15-micrometer-wide drumheads prepared on silicon chips used in the experiment. The drumheads vibrate at a high ultrasonic frequency, and the peculiar quantum state predicted by Einstein was created from the vibrations. Credit: Aalto University.

Physics deals with its fair share of strangeness and — let’s be honest here — that’s especially abundant in quantum theory. However, you’ll be challenged to find a weirder, more counter-intuitive concept than quantum entanglement, the phenomenon in which the quantum states of two or more objects are intertwined such that they have to be described with reference to each other, even though the individual objects may be spatially separated.

Even if you separate entangled particles by billions of miles, changing one particle will induce a change in the other. This information appears to be transmitted instantaneously, with no violation of the classical speed of light because there’s no “movement” through space.

When Albert Einstein first spoke about quantum entanglement in 1935, he famously called it “spooky action at a distance.” But although quantum entanglement is still very strange, at least scientists are no longer strangers to it. Physicists have so far shown how quantum entanglement works over various distances both on land and in space. Most recently, a Chinese satellite entangled particles over 1,200 km apart, paving the way for the future’s quantum communication and quantum encryption networks.

The problem with entanglement is that it’s very fragile, as the conditions necessary to maintain the quantum state can be challenging. What’s more, entanglement has only been demonstrated in microscopic systems such as light or atoms — that’s, until recently, when an international team of researchers showed how to generate and detect entanglement of macroscopic objects.

The researchers led by Prof. Mika Sillanpää at Aalto University in Finland entangled two individual vibrating drumheads made from metallic aluminum. Each drumhead had a diameter the size of a human hair, making it huge by quantum standards.

“The biggest challenge was the theoretical understanding of the data. We measured the data exactly two years ago, but it is published not until now! This is because it was so demanding to develop the proper theoretical model. When we took the data, we had no idea if we are entangled or not. It was a great moment later to find out that all features of the data were explained by the new modeling,” Sillanpää told ZME Science.

In order to entangle the two bodies, the researchers forced them to interact via a superconductive microwave circuit, whose magnetic fields absorb all the thermal disturbances thus removing noise — and leave behind only the quantum mechanical vibrations. It’s with these high ultrasonic vibrations that the scientists entangled the drumheads. All of this occurred at a temperature near absolute zero, -273 °C, where molecular motion almost grinds to a halt.

Quantum entanglement between the large bodies was maintained for up to half an hour. When asked if there is any limit to how large any two objects can be for them to become entangled, Sillanpää said that there are no fundamental limits but, practically, gravity might “collapse” the quantum properties of objects that are too large.

“How large, is totally unclear. There are, however, limits that in practice are fundamental: Let us consider for example, a cat-size object that is in a quantum state of being at the same time in two places that are spaced by centimeters. Now, the neutrino irradiation (which is impossible to avoid because even the Earth is transparent to neutrinos) from the Sun will destroy such a quantum state in about a pico-second (one millionth of a micro-second), so such states do not appear in nature.”

This exciting research will prove very important in the forthcoming quantum technology revolution. The results show that it is possible to generate and stabilize exotic quantum states in large mechanical objects, enabling all sorts of new opportunities like novel quantum technologies and sensors that might revolutionize research in fundamental physics. Perhaps research in the future attempting to teleport mechanical vibrations might offer some very interesting surprises.

“To build a quantum internet that will connect distant quantum computers, one needs entangled particles or objects. The vibrating drumheads can serve as such components that convert the quantum bits in a processor into the flying quantum information. In fundamental research, one could improve the sensitivity of the existing gravitational wave detection systems,” Sillanpää explained.

“Nature can be more awesome than you could ever imagine!” the physicist concluded.

The findings appeared in the journal Nature.

Quantum technology of spinning black holes

This is a guest post by Ovidiu Racorean, who studies quantum physics and financial economics.

As I’m sitting on a bench, a coffee in one hand and a cigarette in the other, I contemplate the black and starry sky. Dragging smoke from the cigarette and drinking some coffee, I think of the light that crosses the Universe all the way from the farthest stars.

A strange idea does not give me peace.

Could black holes be powering natural quantum computers? Image credits: NASA / JPL.

Like a mosquito, it irritates me with its perpetual buzz. Does the light that comes to us from far away contain quantum information, just like the one I produce in the laboratory, with the help of mirrors, beam splitters and prisms?

We used to encode quantum information in photons by manipulating the polarization and orbital angular momentum (OAM). It’s tedious work, but in the end, twisting the polarization and creating OAM, using beam splitters and prisms, a quantum message is stored in a tiny single photon.

More smoke, more coffee.

Surprisingly, dead stars can provide a plausible solution to that question. Remarkably, our laborious work in the is easily done, naturally, by the gravity around rotating black holes.

Black holes are changing the “face” of the surrounding spacetime, curving and twisting it. While traveling through this distorted spacetime, the photons twist the polarization and create OAM. Finally, if they finally manage to escape the gravity of the black hole, they have a quantum message stored. A message that we might be able to read. If this is the case then…

Unbelievably, a quantum computer already exists in nature!

More smoke, more coffee.

Still, what an irony to think that gravity is creating the “spooky action at a distance”. I wonder what Einstein would have said about this, as I finish my last drop of coffee.

I finish the cigarette and it’s time to put on paper all these thoughts. You can find them in my newly published research in the New Astronomy journal (Creation of single-photon entangled states around rotating black holeshttps://doi.org/10.1016/j.newast.2017.09.001).

Now, all that remains is to decipher the quantum message that the black holes send us. But that’s a story for another coffee.