Tag Archives: photons

LHC physicists make matter out of light

CERN researchers used the LHC to produce a pair of W bosons from two photons. Credit: CERN.

As another confirmation that we’re living in a quantum mechanical universe, physicists have used the Large Hadron Collider (LHC) in Switzerland to generate matter from energy. Specifically, photons were merged and transformed into W bosons, which are particles that carry a weak force.

For centuries, scientists trained in classical physics lived by an immutable mantra: no matter what happens in the universe, mass is always conserved. What goes in, must always come out. But then came Albert Einstein, whose theory of special and general relativity showed that different observers would disagree about what the energy of a system was, hence mass couldn’t be the only conserved quantity.

Ultimately, this is how we wind up with the most famous equation in physics, E = mc².

I’ll leave it to Einstein himself to explain the equation:

“It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing — a somewhat unfamiliar conception for the average mind.”

From the soundtrack of the film, Atomic Physics. Credit: J. Arthur Rank Organization, Ltd., 1948.

This also means that mass can be converted into energy and vice-versa. At the LHC, scientists regularly smash particles accelerated close to the speed of light together, transforming the particles into energy and then back into different types of particles.

But you can skip a step. In a recent update, CERN researchers working with the ATLAS experiment describe how they were able to collide two photons together, which are massless particles of light. As a result, the interaction led to the formation of W bosons, particles that have both mass and charge, and which play a vital role in nuclear decay.

We’re literally bombarded with countless photons on a daily basis each time the sun rises, so why doesn’t this happen all the time? Einstein’s nifty equation yet again has the best explanation.

If you read the mass-energy equivalence equation from right to left, you’ll clearly see that a small amount of mass can produce a huge amount of energy due to the square of the speed of light (c²). That’s why relatively small hydrogen bombs can wreak devastation across thousands of square kilometers.

When the equation is read from left to right, the interpretation is that you need a boatload of energy to produce mass. One of the few places here on Earth where that kind of energy is possible to generate is at the LHC, the world’s largest and highest-energy particle collider, and the largest machine in the world for that matter.

“If you go back and look at Maxwell’s equations for classical electromagnetism, you’ll see that two colliding waves sum up to a bigger wave,” Simone Pagan Griso, a researcher at the US Department of Energy’s Lawrence Berkeley National Laboratory, told Symmetry Magazine. “We only see these two phenomena recently observed by ATLAS when we put together Maxwell’s equations with special relativity and quantum mechanics in the so-called theory of quantum electrodynamics.”

Along with Z bosons, W bosons are weak nuclear force carriers. The weak force is one of the four fundamental forces, alongside electromagnetism (which holds atoms together), strong nuclear force (which glues atomic nuclei together), and gravity.

According to the laws of electrodynamics, two intersecting light beams should never deflect, absorb, or disrupt one another. You can prove this for yourself at home if you happen to have two lasers handy.

However, quantum electrodynamics allows for light and matter to interact. These new findings actually confirm one of the main predictions of electroweak theory, namely that force carriers like W bosons interact with themselves.

“This observation opens up a new facet of experimental exploration at the LHC using photons in the initial state”, said Karl Jakobs, Spokesperson of the ATLAS Collaboration. “It is unique as it only involves couplings among electroweak force carriers in the strong-interaction dominated environment of the LHC. With larger future datasets it can be used to probe in a clean way the electroweak gauge structure and possible contributions of new physics.”

Image taken during the LightSail 2 sail deployment sequence on 23 July 2019. Credit: The Planetary Society.

Spacecraft that sails on sunlight actually works

Image taken during the LightSail 2 sail deployment sequence on 23 July 2019. Credit: The Planetary Society.

Image taken during the LightSail 2 sail deployment sequence on 23 July 2019. Credit: The Planetary Society.

In 1976, Carl Sagan went on the Tonight Show with Johnny Carson where he presented at exciting and extraordinary technology: solar sailing.

Like a ship’s sails harness the wind to move forward, solar sailing allows a spacecraft to move through space using nothing but the pressure of sunlight. More than four decades later, the Planetary Society, a nonprofit organization that promotes the exploration of space through education, has demonstrated the technology’s potential in the real world. They launched a tiny spacecraft that was able to raise its orbit using the power of the sun alone — a world first.

“We’re thrilled to announce mission success for LightSail 2,” said Bruce Betts, LightSail program manager and the Society’s chief scientist, in a statement.

“Our criteria was to demonstrate controlled solar sailing in a CubeSat by changing the spacecraft’s orbit using only the light pressure of the Sun, something that’s never been done before.”

The Lightsail 2 spacecraft that recently reached this milestone took over a decade and $7 million in crowdfunding to develop. For the past month or so, the spacecraft has been in orbit after it was launched with other payloads aboard a SpaceX Falcon Heavy rocket. Last week it opened its sails for the first time. Since then, LightSail 2 has raised its orbit by 1.7 kilometers (slightly over a mile), solely due to the pressure of photons.

Nothing to do with solar panels — more to do with sailing

Credit: The Planetary Society.

Credit: The Planetary Society.

The LightSail is features a very simple design. Essentially, it looks very much like a kite. The sail is made out of thin Mylar and when it’s fully stretched out, it measures 345 square feet.

Interestingly, LightSail is not a novel concept at all. Nearly 400 years ago, when much of the world thought that reaching North America was a monumental achievement, Johannes Kepler proposed the idea of exploring the galaxy using sails. The astronomer thought the tails of the comets he was observing were actually blown away by some kind of solar breeze, so naturally, this sort of idea of using a solar sail came to mind. This is obviously not true since there’s no “real” wind in space, but we do know that sunlight can exert enough pressure to move objects.

Sunlight pressure offered “tiny push no stronger than the weight of a paperclip” each time the spacecraft completed an orbit around Earth, according to the Planetary Society.

That might not sound like much, but the reality is that many scientists believe that this technology might someday help humans become an interstellar species. Because there’s very little to no resistance in space, light sail-powered spacecraft gain more and more momentum as they travel and an increasing number of photons bounces off the sail. As long as photons hit the sail, the spacecraft will endlessly continue to accelerate — at least in theory. So, even though you still need a rocket to lift such a spacecraft in orbit, a solar sail can reach speeds that a chemical spacecraft could never ever attain.

What’s more, when the sunlight isn’t enough, powerful lasers on Earth pointed onto a sail could accelerate a spacecraft even further. According to NASA researchers, a solar sail pushed by lasers could technically reach Mars in only 3 days.

“It’s counter-intuitive, it’s surprising, and to me it’s very romantic,” Bill Nye, famed science communicator and president of The Planetary Society, told reporters during a a press call Wednesday, “to be sailing on sunbeams.”
This was not the first solar sail. That merit belongs to Ikaros, a solar sail prototype that was launched in 2010 by Japan’s space agency. The Planetary Society’s Cosmos 1 was supposed to reach orbit in 2005 but a rocket explosion destroyed the early prototype. There are no drafts drawn out yet for a LightSail 3, but that’s beside the point for now. What interested the Planetary Society and its generous donors was to prove that the technology works, perhaps inspiring other groups to pick up from here. Mission accomplished!

“LightSail 2 proves the power of public support,” said Planetary Society COO Jennifer Vaughn. “This moment could mark a paradigm shift that opens up space exploration to more players. It amazes me that 50,000 people came together to fly a solar sail. Imagine if that number became 500,000 or 5 million. It’s a thrilling concept.”

Scientists measure all the starlight ever produced in the Universe

Astrophysicists have measured all the photons ever emitted by the trillions and trillions of stars forged during the Universe’s 13.7-billion-long history. Unpeeling the history of star formation is a major breakthrough with ramifications in other areas of research, such as galactic evolution or the search for dark matter. Ultimately, work like this brings scientists closer to understanding the most fundamental processes, all the way back to the Big Bang.

A map of the night’s sky showing the location of 739 blazars used in the study to measure starlight in the universe. Credit: NASA/DOE/Fermi LAT Collaboration.

Scientists estimate that there are about two trillion galaxies, which hold a trillion-trillion stars. In order to measure how much starlight has been emitted, Marco Ajello and colleagues at the Clemson College of Science turned to data from NASA’s Fermi Gamma-ray Space Telescope. Launched in 2008, the Fermi Telescope has so far provided invaluable information pertaining to gamma rays (the most energetic form of electromagnetic radiation) and their interaction with the extragalactic background light (EBL) — a ‘cosmic fog’ composed of all the ultraviolet, visible and infrared light emitted by stars or from dust in their vicinity.

The results suggest that the total number of photons that have been emitted into space by stars is about 4×1084 — that’s 4 followed by 84 zeroes. It’s a staggering number beyond our comprehension, but one that will help scientists greatly to understand how stars and galaxies evolve.

“From data collected by the Fermi telescope, we were able to measure the entire amount of starlight ever emitted. This has never been done before,” said Ajello, who is lead author of the paper. “Most of this light is emitted by stars that live in galaxies. And so, this has allowed us to better understand the stellar-evolution process and gain captivating insights into how the universe produced its luminous content.”

Despite the huge number of photons out there, the Universe is largely dim due to its sheer vastness. According to the researchers, starlight other than that coming from our own and Milky Way is as dim as a 60-watt light bulb viewed from 2.5 miles away.

The study hinged on observations of energetic particles emitted by blazars, which are galaxies with supermassive black holes that are capable of releasing collimated jets of electromagnetic radiation. When such jets happen to be directly pointed at Earth, our instruments are able to detect their source even from billions of light-years away. Gamma rays produced by the jets interact with the cosmic fog, producing an imprint that astrophysicists can use to measure the fog’s density. What’s more, this density can be measured for various times in the history of the universe.

“Gamma-ray photons traveling through a fog of starlight have a large probability of being absorbed,” said Ajello, an assistant professor in the department of physics and astronomy. “By measuring how many photons have been absorbed, we were able to measure how thick the fog was and also measure, as a function of time, how much light there was in the entire range of wavelengths.”

Clemson University astrophysicist Marco Ajello. Credit: Pete Martin / Clemson University.

Clemson University astrophysicist Marco Ajello. Credit: Pete Martin / Clemson University.

By directly measuring the density of this cosmic fog, the authors of the new study were able to eliminate the need to estimate light emissions by far-away galaxies. In total, 739 blazars were included in the study.

“By using blazars at different distances from us, we measured the total starlight at different time periods,” said postdoctoral fellow Vaidehi Paliya. “We measured the total starlight of each epoch—one billion years ago, two billion years ago, six billion years ago, etc. – all the way back to when stars were first formed. This allowed us to reconstruct the EBL and determine the star-formation history of the universe in a more effective manner than had been achieved before.”

“Scientists have tried to measure the EBL for a long time. However, very bright foregrounds like the zodiacal light (which is light scattered by dust in the solar system) rendered this measurement very challenging,” said co-author Abhishek Desai, a graduate research assistant in the department of physics and astronomy. “Our technique is insensitive to any foreground and thus overcame these difficulties all at once.”

According to the study’s results, star formation peaked around 11 billion years ago — but it is still going strong. The Milky Way adds seven new stars every year, for instance.

Understanding star formation is important for astronomy research. For instance, this analysis will enable research in the future to target the earliest days of stellar evolution, using the upcoming James Webb Space Telescope, which should come online in 2021. And because the measurement is very sensitive to the expansion of the Universe, Ajello believes that their data can be used to measure the Hubble Constant more precisely.

“The first billion years of our universe’s history are a very interesting epoch that has not yet been probed by current satellites,” Ajello said in a statement. “Our measurement allows us to peek inside it. Perhaps one day we will find a way to look all the way back to the Big Bang. This is our ultimate goal.”

The findings were reported in the journal Science.


Physicists create a new form of light by binding photons


Credit: Pixabay.

In an MIT lab, physicists have pushed the boundaries of what we thought was possible by creating a new form of light. The scientists were able to coax photons to interact with another, similarly to how atoms attract or repel each other in ordinary matter. This breakthrough could one day enable scientists to fashion so-called “light crystals,” where photons are arranged in fixed patterns and which could be key in producing future quantum computers and communication systems.

The real lightsaber

If you ever tried crossing the beams of two flashlights, you probably don’t remember much about it. That’s because it was rather uneventful, i.e. nothing happens. Typically, photons don’t interact with each other, which is why you don’t see light beam bounce off each other — that would be a weird sight.

However, Sergio Cantu, a Ph.D. candidate at MIT, and colleagues managed the impossible — and they did it with a bit more than flashlights. The team chilled rubidium atoms close to absolute zero, which makes them almost motionless, then activated a laser, resulting in an ultracold cloud. Everything is contained in a small tube that magnetized, thus keeping the rubidium atoms diffuse, slow-moving, and in a highly excited state.

The researchers then fired a very weak laser onto the dense cloud of ultracold rubidium atoms. What happened next was rather than exiting the cloud as single, randomly spaced photons, the photons combined together in pairs or triplets. This means that some sort of interaction between the photons is at play — in this case, attraction.

Surprisingly, the three-photon grouping was even more stable than the pair. “The more you add, the more strongly they are bound,” said co-author  Aditya Venkatramani, a Ph.D. candidate in atomic physics at Harvard University. 

The scientists knew the photons must be interacting, since the pairs and triplets of photons give off different energy signatures or phase shifts, the authors reported in the journal Science

“The phase tells you how strongly they’re interacting, and the larger the phase, the stronger they are bound together,” Venkatramani explained.

Typically, in a vacuum, photons travel at the speed of light (almost 300,000 kilometers/second) and have no mass. However, after they passed through the clouds, the bounded photons were 100,000 times slower than normal non-interacting photons and acquired a fraction of an electron’s mass.

Though not confirmed yet, the physicists think the photon group together by forming a photon-atom hybrid or polariton. When a single photon travels through the rubidium cloud, it hops from one atom to another, like a bee foraging from flower to flower. A photo-atom hybrid forms when one photon briefly binds to an atom and when two such hybrids meet in the cloud, they interact. When the polaritons are about to exit the cloud, the atoms stay behind while the photons lurch forward, still bound together. Triplets form when you add more photons — or so the physicists’ theoretical model goes.

“What’s neat about this is, when photons go through the medium, anything that happens in the medium, they ‘remember’ when they get out,” Cantu said.

“The interaction of individual photons has been a very long dream for decades,” said Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, in a statement.

The findings could put to use in a novel quantum communications system which entangles bound-photons, allowing almost instantaneous transmission of complex quantum information.

“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic said. “With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It’s very uncharted territory.”


Illustration of a photonic boom. Credit: Jinyang Liang and Lihong V. Wang.

Light’s Mach-cone, its ‘sonic boom’ equivalent, filmed for the first time

A fighter jet photographed in the midst of a sonic boom. The plane travels faster than the sound it emits. As it pieces the sound wave, a roaring boom commences. Credit: YouTube capture.

A fighter jet photographed in the midst of a sonic boom. The plane travels faster than the sound it emits. As it pierces the sound wave, a roaring boom commences. Credit: YouTube capture.

Though modern fighter jets are stealthier than ever, they’re hardly inconspicuous once accelerating past the sound barrier. By reaching Mach speeds (faster than 340 meters/second), jets essentially travel faster than sound pressure waves. When the jet crashes into the pressure wave it had just emitted, a nearly deafening ‘sonic boom’ is triggered that can be heard for miles. Though far less spectacular, the same thing essentially also happens with light when it travels faster than the wave it emitted. Now, using the most advanced ultra-speed cameras, scientists have filmed light’s Mach cone for the first time.

Per Einstein’s Theory of Special Relativity, nothing can travel faster than light. But that doesn’t mean light can’t travel slower than c (300,000 kilometers/second). In fact, scientists have previously shown its possible to slow down light to 0.001 percent of c, making it almost look like it’s floating.

Illustration of a photonic boom. Credit: Jinyang Liang and Lihong V. Wang.

Illustration of a photonic boom. Credit: Jinyang Liang and Lihong V. Wang.

The experiment carried out by a team at Washington University was far less extreme and involved the basic knowledge that photons travel slower through some mediums than others. In between two plates made of silicone rubber and powdered aluminum oxide respectively, the researchers sandwiched dry-ice fog. Light was then fired through a channel that pierced the two plates.


Credit: Jinyang Liang

This technique enabled the researchers to witness a photonic boom for the first time in video because the light moves slower through the walls of the tunnel than the fog. The footage we’re seen was recorded with a camera that films at a trillion frames per second. The same camera has been previously used to film light in slow motion, providing a unique glimpse into how rays of light actually look like. 

The Washington University team claims the technique they just demonstrated might prove useful for imaging ultrafast events, even those in the human body.

“Our camera is fast enough to watch neurons fire and image live traffic in the brain,” optical engineer Jinyang Liang from Washington University told Live Science.

“We hope we can use our system to study neural networks to understand how the brain works.”

Illustration of the nanoscale semiconductor structure used for demonstrating the ultralow-threshold nanolaser. A single nanorod is placed on a thin silver film (28 nm thick). The resonant electromagnetic field is concentrated at the 5-nm-thick silicon dioxide gap layer sandwiched by the semiconductor nanorod and the atomically smooth silver film.

Smallest laser is so tiny you can’t see it with the naked eye

Scientists at the University of Texas at Austin, in collaboration with colleagues in Taiwan and China, have developed what’s considered to be the world’s smallest laser; a device so tiny, that it’s invisible to the naked eye. The laser is heralded as a breakthrough in the emerging photonic technology with applications from computing to medicine.

Specialists in photon-based technologies, like ultrafast computer chips, highly sensitive biosensors for detecting, treating and studying disease or the next-generation communication devices, will tell you at any time that the key to pushing the envelope forward in the field is based on two very important parameters: energy and size, and both need to be smaller. You can only get so low until you hit a brick wall, though, or what’s known for physicists as the three-dimensional optical diffraction limit.

Illustration of the nanoscale semiconductor structure used for demonstrating the ultralow-threshold nanolaser. A single nanorod is placed on a thin silver film (28 nm thick). The resonant electromagnetic field is concentrated at the 5-nm-thick silicon dioxide gap layer sandwiched by the semiconductor nanorod and the atomically smooth silver film.

Illustration of the nanoscale semiconductor structure used for demonstrating the ultralow-threshold nanolaser. A single nanorod is placed on a thin silver film (28 nm thick). The resonant electromagnetic field is concentrated at the 5-nm-thick silicon dioxide gap layer sandwiched by the semiconductor nanorod and the atomically smooth silver film.

“We have developed a nanolaser device that operates well below the 3-D diffraction limit,” said Chih-Kang “Ken” Shih, professor of physics at The University of Texas at Austin. “We believe our research could have a large impact on nanoscale technologies.”

In their paper recently published in the journal Science, Shih and colleagues report on the first operation of a continuous-wave, low-threshold laser below the 3-D diffraction limit. When fired, the nanolaser emits a green light. The device is made of a gallium nitride nano-rod , partially filled with indium gallium nitride – both are semiconductor alloys used commonly in LEDs.

The nanorod is the key to the physicists’ success, a material that the Shih lab has been perfecting for more than 15 years. The nanorod is placed on top of a thin insulating layer of silicon that in turn covers a layer of silver film that is smooth at the atomic level. “Atomically smooth plasmonic structures are highly desirable building blocks for applications with low loss of data,” said Shih.

“Size mismatches between electronics and photonics have been a huge barrier to realize on-chip optical communications and computing systems,” said Shangjr Gwo, professor at National Tsing Hua University in Taiwan and a former doctoral student of Shih’s.

The researchers hope this impediment will finally be jumped once “on-chip” communication systems (chips where all processes are contained on the chip) are developed, with the help of the knowledge gained from developing the world’s tiniest laser.


Faster than light Neutrinos FINALLY and OFFICIALLY debunked

This time last year, the whole scientific community was faced with one of the most controversial findings in recent history – namely, that neutrinos could travel at a speed greater than the speed of light, fact which would directly contravene with Einstein’s Theory of Special Relativity and, in consequence, force scientists to rethink the fundamental laws that govern the Universe. Last week, at the XXV International Conference on Neutrino Physics and Astrophysics, or Neutrino 2012 in short, the year’s major findings related to neutrinos was discussed. It was generally agreed by the attending scientists there that neutrinos are extraordinary particles, which exhibit still poorly understood characteristics, and at the same time, maybe more importantly, the scientists gathered there concluded and perfectly illustrated once and for all that neutrinos CAN NOT and WILL NOT ever travel faster than the speed of light.

The announcement which came from OPERA, the team of researchers from CERN which released the controversial claim following thorough experiments at  Italy’s Gran Sasso facility, was met with intense scrutiny even from the get go. A lot of theories were argued, each pointing to a different factor which may have pointed toward the 60 nanoseconds error. And an error it was, indeed, since later investigations confirmed that  the results were the product of an improperly seated optical cable in the OPERA experiment. The time delay introduced by this ill-placed cable was extremely small, but just enough to tamper with results.

Moreover, other scientific teams independentely recreated their own version of the OPERA experiment, most notably the MINOS team, which used protons from the Tevatron’s accelerator chain to produce neutrinos that were detected in a mine in Minnesota. Their results showed that the neutrino arrival time was consistent with the speed of light within experimental error; an experimental error which was about half the size of the original speed difference spotted by OPERA.

Long story short, the faster than light neutrino question has been officially and implacably debunked.

via Ars Technica

Atomic interaction

Atomic-precision heat flow manipulation achieved by scientists

Scientists have described and proven how many of the world’s phenomenae function, from the very fundamental laws of Newtonian mechanics, to the discrete behaviors of quantum physics, eventually peering through some of the Universe’s most well kept secrets. It’s remarkable then, how little we know about how heat is conducted through and between materials. A better understanding of heat transfer, starting from the discrete level, might hold dramatic consequences for the better. Heat exchange and interaction is a constant process which occurs everywhere around us, as thermal balance is sought by nature.

hrough atomic-scale manipulation, researchers at the University of Illinois have demonstrated that a single layer of atoms can disrupt or enhance heat flow across an interface. (c) Mark Losego

hrough atomic-scale manipulation, researchers at the University of Illinois have demonstrated that a single layer of atoms can disrupt or enhance heat flow across an interface. (c) Mark Losego

“Heat travels through electrically insulating material via ‘phonons,’ which are collective vibrations of atoms that travel like waves through a material,” said David Cahill, a Willett Professor and the head of materials science and engineering at University of Illinois. “Compared to our knowledge of how electricity and light travel through materials, scientists’ knowledge of heat flow is rather rudimentary.”

Cahill and colleagues at the University of Illinois used a combination of ultra-fast, sensitive measuring system and atom-size layers of materials arranged in a certain manner to reveal some extremely interesting insights about how heat flows across an interface between two materials.

The main reason why heat flow is so poorly understood is because it’s very difficult to observe and measure at the nanoscale and over extremely short time spans. In an attempt to devise an experimental setup towards heat flow study, the scientists used a laser beam to fire very short bursts lasting only a trilinth of a second to probe through heat flow dispersed over a nanoscale “molecular sandwich”, as described by the paper’s authors.

The researchers first arranged a thin quartz surface, over which they coated it with a single layer of molecules. Again, another thin layer of gold had been placed on top of the latter. They then fired a heat pulse to the gold film and measured how it conducted through the sandwich assembly. The scientists conclude that simply by altering the molecular composition of the atom-sized layer placed between the two materials, a dramatic shift in heat transfer occurs, all depending on how strong the bonds between the gold and intermediate layer molecules are. In various instances, the researchers demonstrated that stronger bonding produced a twofold increase in heat flow.

“This variation in heat flow could be much greater in other systems,” said Mark Losego, who led this research effort as a postdoctoral scholar at Illinois and is now a research professor at North Carolina State University. “If the vibrational modes for the two solids were more similar, we could expect changes of up to a factor of 10 or more.”

The scientists’ findings open a whole new realm of research, one which might rend some extremely fruitful advancements, especially for electronics where integrated circuit heat transfer is of great concern, combustion engines, furnaces, thermal insulating material science and so on.

“For many years, the physical models for heat flow between two materials have ignored the atomic-level features of an interface,” Cahill said. “Now these theories need to be refined. The experimental methods developed here will help quantify the extent to which interfacial structural features contribute to heat flow and will be used to validate these new theories.”

via Eureka Alert