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What, really, is the speed of sound?

We know that the speed of light seems to be the upper limit for how fast something can travel in the universe. But there’s a much lower speed limit that we’ve only recently (in the grand scheme of things) managed to overcome here on Earth: the speed of sound.

Vapor cone (or ‘shock collar’) around a fighter as it’s getting near to the speed of sound.
image credits Flickr / Charles Caine.

You’ve heard the term before, and you might even know its exact value or, more accurately, values.

But why exactly does sound have a ‘speed’? Is it the same everywhere? And what happens if you go over the limit? Well, one thing is for sure — sound won’t give you a fine for it. It will cause a mighty boom to mark the occasion, though, because going over the speed of sound isn’t an easy thing to pull off.

In Earth’s atmosphere, sound can travel at around 345 meters per second. Let’s take a look at why this limit exists, what says it should be this way, and just why things go boom when you blast through it. But first, let’s start at with the basics:

What is a sound wave

What we perceive as sound is actually motion. Sound is, fundamentally, a movement or vibration of particles, most commonly those in the atmosphere, where we do most of our talking and sound-making.

In very broad lines, any object in motion will come into contact with the particles in their environment. Let’s take talking as an example. When someone speaks, their lungs collide with and push out air that their vocal cords modulate to create certain sounds. This will push the air in their immediate vicinity, which will make its molecules collide with air molecules farther away, and so on, until the motion reaches the air particles next to you. They will then collide with your eardrums, which ‘translates’ it into the sensation of sound.

File:CPT-sound-physical-manifestation.svg - Wikimedia Commons
Two ways to represent the physics of sound. Areas with more dots (corresponding to peaks) show high air pressure, while whiter areas (corresponding to throughs) show areas of low pressure that interact to create sound. This pressure is generated by moving air.
Image via Wikimedia.

So from a physical point of view, sound behaves quite like waves do on a beach. Its volume is directed by how high the wave goes (amplitude) and its pitch is formed by how often these waves hit the shore (frequency). The farther a wave travels, the less energy it has (so the less pressure it can exert on new particles), which is why eventually sound dies out and we can’t hear something halfway around the world. More on sounds here.

The speed of sound is essentially the speed that these ‘acoustic waves’ can travel at through a substance. Leading us neatly to the role these substances (called “the medium”) play here.

Not all things are equal

The source of sound only plays a limited part in its propagation. Sound propagation is almost entirely dependent on the medium.

Video credits Reddit user renec112.

First off, this means that sound can’t propagate through void, as there is nothing to carry it. One handy example is that in space, nobody can hear you scream; but if you place your visor on another’s astronaut’s visor, they will. Secondly, a medium can’t carry sound unless it has some elasticity, although this is more of an academic point as every material is elastic to some degree. The corollary of this is that the more elastic our medium, the faster sound will travel through it.

Elasticity is the product of two traits: the ability to resist deformation (its ‘elastic modulus’ or rigidity) and how much you can alter it before it stops coming back to its original shape (its ‘elastic limit’ or flexibility). Steel and rubber are both very elastic, but the former is rigid while the later is flexible.

Density has a bit of a more complicated relationship to the speed of sound. Density is basically a measure of how much matter there is in a given space. On the one hand closely-packed, lightweight particles allow for higher speeds of sound as there’s less empty space they need to travel over to hit their neighbors. But if these particles are heavy and more spread apart, they will slow the sound down (as big, heavy particles are harder to move). Sound will also attenuate faster through this last type of material. In general, elastic properties tend to have more of an impact on the speed of sound than density.

A basic example involves hydrogen, oxygen, and iron. Hydrogen and oxygen have nearly the same elastic properties, but hydrogen is much less dense than oxygen. The speed of sound through hydrogen is 1,270 meters per second, but only 326 m/s through oxygen. Iron, although much denser than either of them, is also much more elastic. Sound traveling through an iron bar can reach up to 5,120 m/s.

One other thing to note here is that fluids only carry sound as compression waves (particles bumping into each other in the direction the wave is propagating. Solids carry it both as compression and shear waves (perpendicular to the direction of propagation). This is due to the fact that you can’t cut fluids with a knife (they have a shear modulus of 0). A fluid’s molecules can move too freely from one another for such motions to create such waves.

Sonic boom

So far we’ve seen that sound has a maximum speed it can travel at, based on which material it is propagating through. By ‘travelling’, we mean particles bumping into their neighbors creating wave-like areas of pressure.

So what happens when something moves faster than the speed these particles can reach? Well, you get a sonic boom, of course.

Slow-motion footage of a bullet traveling through ballistic gel. Notice how the gel in the middle is pushed away by the bullet before its edges and corners have time to move. The process is very similar to how airplanes form sonic booms. You can see the metal table buckling under the pressure. That shock corresponds to a by-stander perceiving the sonic boom after the bullet has passed them.
Image via YouTube.

Despite the name, sonic booms are more like sonic yelling. When an object is moving faster than sound can travel in its environment, it generates a thunder-like sound. Depending on how far away the source is, this boom is strong enough to damage structures and break windows.

An airplane moving faster than the speed of sound will compress the air in front of it, as this air can only move at the speed of sound. It can’t physically get out of the way fast enough. Eventually, all this compressed, moving air (which is, in essence, sound) is blasted away from the aircraft’s nose at Mach 1 (the speed of sound through air). If anyone is close enough to be reached by this blast of ultra-pressurized air, they hear the sonic boom.

Although it is perceived as an extremely loud burst of sound by a static observer, the sonic boom is a continuous phenomenon. As long as an object moves faster than sound, it will keep creating this area of ultra-compressed air, and leave a continuous boom in its wake. One nifty fact about sonic booms is that you can’t hear them coming — they move faster than sound, so you can only hear them after they’ve passed you.

Humans have only recently gone above the speed of sound, with the first supersonic flight recorded in 1947. Since then, such flights have been banned above dry land in the US and EU, in order to protect people and property (although they can still be carried out with proper authorization). Faster-than-sound travel, however, is still an alluring goal. One way to allow for supersonic speeds without blasting all the windows in the neighborhood is to travel through a vacuum or low-pressure air — a cornerstone idea of the Hyperloop.

Climate change has already claimed 5 islands in the Pacific

A scientific study confirmed local anecdotes: sea level rise and erosion have claimed five small islands in the Pacific. A further six islands have been heavily eroded and might fall soon. While small, these islands supported rich vegetation — and some are inhabited.

Small islands in the Pacific, such as the Solomon Islands, are at great risk from climate change.

Sea level rise is one of the most direct consequences of global warming. Temperatures rise, then ice starts to melt — it’s pretty straightforward (although the exact way this process unfolds is complex). Another less intuitive process caused by climate change is coastal erosion.

There are two main reasons why this happens: the first is sea-level rise, and the second is an increase in the frequency and magnitude of storm events. However, while this process has been predicted for a long time, we’ve yet to fully observe it take over an entire island — until now.

At least five reef islands in the remote Solomon Islands have been lost completely to sea-level rise and coastal erosion. The islands ranged from one to five hectares, and supported dense vegetation that was over 300 years old.

Vulnerable islands

Nuatambu Island, home to 25 families, has lost more than half of its habitable area, with 11 houses washed into the sea since 2011. Nuatambu is part of six other islands who have been heavily eroded, and are likely to also fall to the waves in the not-too-distant future.

These islands lie very close to the sea level, and any rise — even a sub-centimeter rise — can be devastating. Previous studies analyzing the average sea level rise have concluded that many islands in the Pacific can keep up with the sea level rise as they themselves are slowly rising — but that’s not the case in the Solomon Islands. For the past 20 years, the Solomon Islands have been experiencing a sea level rise of 7-10 mm per year, much higher than the global average of 3 mm per year.

In this new study, researchers used aerial and satellite imagery gathered from 1947 to 2015 to study 33 reef islands. They integrated this information with local and traditional knowledge, sea-level records, wave models, and radiocarbon dating of trees, to tell a compelling story of how sea level rise is impacting these areas.

The team found that rising seas aren’t the only factor at play — waves also have an important role. Twelve islands in a low wave energy area experienced little noticeable change in their shorelines. Meanwhile, 21 other islands, located in similar sea level rise areas, experienced much higher erosion due to higher wave energy. This is an important and often overlooked aspect, researchers say.

People are paying the price

Malaita, Solomon Island.

It’s already reaching the point where people have to move because their homes are disappearing. In some cases, villages dating from 1900 or even earlier were abandoned. Sometimes, villagers are forced to take care of this on their own; in more fortunate cases, they receive support from the state. Communities are being fragmented

Sirilo Sutaroti, the 94-year-old chief of the Paurata tribe, was recently forced to abandon his village.

“The sea has started to come inland, it forced us to move up to the hilltop and rebuild our village there away from the sea,” he told the research team.

It’s a tragedy that’s unfolding before our very eyes, and these islands shouldn’t have to deal with this on their own. The problem is caused by all of us, and the developed world in particular — it’s only natural that the developed world also steps in to help. Melchior Mataki who chairs the Solomon Islands’ National Disaster Council, echoed these feelings, calling for external help:

“This ultimately calls for support from development partners and international financial mechanisms such as the Green Climate Fund. This support should include nationally driven scientific studies to inform adaptation planning to address the impacts of climate change in Solomon Islands.”

It remains to be seen whether the world will rise up to the challenge and take responsibility, or whether these communities will be left to fend for themselves.

The study has been published in Environmental Research Letters.

Credit: Pixabay.

Winds and waves are getting slightly stronger around the globe

Credit: Pixabay.

Credit: Pixabay.

Scientists have become very good at measuring key indicators of climate change such as temperature and sea level rise. However, other important parameters of the global climate system, such as wind strength and wave height, haven’t been given nearly as much attention — until now.

A new study compiled and summarized data from 31 orbiting satellites finding that winds and waves are becoming stronger each year. The year-by-year changes are actually tiny but they still show a clear trend of amplification that might become important in forecasting storms and other extreme weather.

Wind and wave measurements are important for modeling the climate. Both are at the interphase between the atmosphere and ocean, affecting the transfer of energy and matter like carbon. The frustrating part is that it has always been rather tricky to record and analyze historical data on these phenomena.

The problem doesn’t lie in the lack of data itself but rather in reliable measurements across the board. For instance, researchers and the industry have been employing ocean buoys for decades. It’s when you try to compare and normalize their data, which is recorded by different instruments and designs, that you run into trouble.

Ian Young, a professor of marine engineering at the University of Melbourne, and colleagues may have solved the problem by looking to the sky for a solution. In fact, the satellite record, which reliably spans 1985 through 2018, seems like an obvious solution. But shouldn’t the different kinds of instruments aboard satellites be causing the same problems as buoys? Apparently not.

Young and colleagues studied the data collected by altimeters, which can measure both wave height and wind speed, radiometers, which only measure wind speed, and scatterometers, which measure wind speed and direction. After cross-checking the data, the researchers concluded that the satellite records are a reliable source from which you could draw historical trends.

Their results suggest that in the past 30 years global wind speed and wave height have been noticeably getting stronger. These changes were very slight. Wind speeds amplified by only about an inch per second every year south of the equator and about half that in the North Atlantic. Wave height increase wasn’t as uniform as wind speed, but there were large patches were researchers observed an increase of about a tenth of an inch per year. One isolated spot in the North Pacific actually experienced a drop.

Although the changes might look negligible, these trends were much more noticeable in extreme cases. What’s more, these slight up-ticks add up in time to drive global behavior. And, ultimately, any improvements in our understanding of how the climate works are very welcomed.

The findings appeared in the journal Science

Scientists recreate ‘Freak Wave’ in the lab — and it looks like art

The Great Wave off Kanagawa (神奈川沖浪裏 Kanagawa-oki nami ura, “Under a wave off Kanagawa”). Print at the Metropolitan Museum of Art. Artist: Katsushika Hokusai.

On New Year’s Day, 1995, sailors aboard the Draupner oil rig off the coast of Norway were in for quite the surprise. Instead of fireworks, however, the crew was shaken by an ungodly wave measuring 25 meters (80 feet) in height, seemingly coming out of nowhere.

This phenomenon, known as a rogue or freak wave, had been previously predicted theoretically but the Draupner incident — perhaps a one in a century event — was the first evidence to support their existence. Evidence of other such waves soon followed.

The Draupner wave or New Year’s wave was the first rogue wave to be detected by a measuring instrument, occurring at the Draupner platform in the North Sea off the coast of Norway on 1 January 1995. Credit: Wikimedia Commons.

Now, after almost 25 years of investigations, scientists at Oxford and the University of Edinburgh have finally uncovered the formation dynamics of such monstrous waves.

Inside a test tank at the FloWave Ocean Energy Research Facility in the UK, the researchers generated waves of various amplitudes and frequency, tweaking parameters until they found that a giant wave formed when two waves intersect at exactly 120 degrees. The findings appeared in the Journal of Fluid Mechanics.

The lab-made Freak Wave was much smaller than the Draupner wave, measuring less than two meters in height. However, seeing how its relative height was double that of the waves that produced it, researchers are confident that the same dynamics are at play in the open sea.

The freak wave generated by the British researchers not only bears a strong resemblance with photos of real freak waves in the ocean but also to “The Great Wave off Kanagawa’ – also known as “The Great Wave’ – a famous woodblock print published in the early 1800s by the Japanese artist Katsushika Hokusai. The artist may have witnessed a freak wave, serving as inspiration for his masterpiece, although there is no evidence to back up such a claim.

The video below shows the freak wave formation process from start to finish.

“Not only does this laboratory observation shed light on how the famous Draupner wave may have occurred, it also highlights the nature and significance of wave breaking in crossing sea conditions. The latter of these two findings has broad implications, illustrating previously unobserved wave breaking behavior, which differs significantly from current state-of-the-art understanding of ocean wave breaking,” Prof. Ton van den Bremer at the University of Oxford said in a statement

The 1995 freak wave that hit the Draupner oil rig resulted in minimal damage, but others haven’t been that lucky. Freak waves have caused considerable damage to ships in the past and even fatalities. This is why the researchers hope that their research will lay the groundwork for being able to predict potentially catastrophic waves and issue timely warnings accordingly.

Portugal and Spain brace for record-breaking temperatures

Amid a scorching-hot summer spanning almost all of the northern hemisphere, Portugal and Spain are preparing for temperatures that could break not only the national record — but a record for the entire continent.

Forecast via Euronews.

Spain’s current record high is 47.3°C (117.14°F) and Portugal can boast a slightly-higher highest temperature, at 47.4°C. But all that may soon change, as current weather models forecast significantly higher temperatures. It’s not out of the question for Portugal to reach a groundbreaking 50°C, surpassing not only the national record but also the European record, which is currently at 48°C (recorded in Athens, Greece, in July 1977).

The probable maximum is set for Saturday, in the southern parts of Portugal and south-western parts of Spain. Met Office forecaster Sophie Yeomans says that the heatwave is directly connected to “a plume of very dry, hot air from Africa.” Although it’s unlikely for temperatures to go over 50°C, records may very well be broken, Yeomans says.

“There’s an outside chance of hitting 50C,” said Yeomans. “If somewhere gets the right conditions, it could do [it] but that’s a very low likelihood.”

Other forecasters have echoed this prognosis.

“Friday and Saturday are likely to be the hottest days with a very real chance of breaking records,” the forecaster of Meteogroup said.

The Spanish meteorology agency, AEMET, has issued an official warning of extreme temperatures, and authorities are already making emergency preparations for the dramatic heatwave. Some 11,000 firefighters and 56 aircraft have already been deployed and are on standby to tackle forest fires — that are likely to emerge in the searing heat.

Iberia, the peninsula hosting the two countries, is not the only area suffering from extreme heat. Scandinavia, an area known for its frigid temperatures, is reporting record highs, Greece is ravaged by wildfires, and most parts of France and Germany have been scorching for months. Aside from some mountainous areas and northern latitudes, few areas have been spared.

Most of Europe is under a heatwave. It’s hard to say that it’s global warming — but it sure walks and quacks like global warming.

Although it’s very difficult to assign a global trend to individual events, there is already substantial evidence that climate change is connected to these record temperatures. Recent studies have shown that man-made climate change is making heatwaves much more likely and, as was the case in previous years, it’s becoming increasingly unlikely that current temperatures and global warming are not connected.

Although record-breaking temperatures are not the norm yet, it’s becoming increasingly plausible that this will be the case in the very near future. The evidence is indicating that climate change is increasingly affecting our lives, whether we care to admit it or not.

What exactly is a photon? Definition, properties, facts

Imagine a shaft of yellow sunlight beaming through a window. According to quantum physics that beam is made of zillions of tiny packets of light, called photons, streaming through the air. But what exactly is a photon?


Photons are the stuff light is made of. Credit:JFC.


A photon is the smallest discrete amount or quantum of electromagnetic radiation. It is the basic unit of all light.

Photons are always in motion and, in a vacuum, travel at a constant speed to all observers of 2.998 x 108 m/s. This is commonly referred to as the speed of light, denoted by the letter c. 

As per Einstein’s light quantum theory, photons have energy equal to their oscillation frequency times Planck’s constant. Einstein proved that light is a flow of photons, the energy of these photons is the height of their oscillation frequency, and the intensity of the light corresponds to the number of photons. Essentially, he explained how a stream of photons can act both as a wave and particle.

Photon properties

The basic properties of photons are:

  • They have zero mass and rest energy. They only exist as moving particles.
  • They are elementary particles despite lacking rest mass.
  • They have no electric charge.
  • They are stable.
  • They are spin-1 particles which makes them bosons.
  • They carry energy and momentum which are dependent on the frequency.
  • They can have interactions with other particles such as electrons, such as the Compton effect.
  • They can be destroyed or created by many natural processes, for instance when radiation is absorbed or emitted.
  • When in empty space, they travel at the speed of light.


The nature of light — whether you regard it as a particle or a wave — was one of the greatest scientific debates. For centuries philosophers and scientists have argued about the matter that was barely resolved a century ago.

The disciples of a sixth century BC branch of Hindu philosophy called Vaisheshika had a surprising physical intuition about light. Like the ancient Greeks, they used to believe the world was based on ‘atoms’ of earth, air, fire, and water. Light itself was thought to be made of such very fast-moving atoms called tejas. That’s remarkably similar to our modern theory of light and its composing photons, a term coined thousands of years later in 1926 by a chemist named Gilbert Lewis and an optical physicist called Frithiof Wolfers.

Later, around 300 BC, the ancient Greek physicist Euclid made a huge breakthrough when he posited light traveled in straight lines. Euclid also described the laws of reflection and, a century later, Ptolemy complemented with writings about refraction. IT wasn’t until 1021, however, that the laws of refraction were formally established in the seminal work Kitab al-Manazir, or Book of Optics, by Ibn al-Haytham.

The Renaissance would usher in a new age of scientific inquiry into the nature of light. Of note are René Descartes’ incursions in a 1637 essay called La dioptrique, where he argued that light is made of pulses that propagate instantaneously when contacting ‘balls’ in a medium. Later writing in Traité de la lumière published in 1690, Christiaan Huygens treated light as compressible waves in an elastic medium, just like sound pressure waves. Huygens showed how to make reflected, refracted, and screened waves of light and also explained double refraction.

By this time, scientists had split into two entrenched camps. One side believed that light was a wave while the other view was of light as particles or corpuscles. The great champion of the so-called ‘corpuscularists’ was none other than Isaac Newton, widely believed as the greatest scientist ever. Newton wasn’t fond at all of the wave theory since that would mean light would be able to stray too far into the shadow.

For much of the 18th century, corpuscular theory dominated the debate around the nature of light. But then, in May 1801, Thomas Young introduced the world to his now famous two-slit experiment where he demonstrated the interference of light waves.

Young's slit experiment shows how each slit acts as a source of spherical waves, which "interfere" as they move from left to right as shown above. Credit: University of Louisville Department of Physics.

Young’s slit experiment shows how each slit acts as a source of spherical waves, which “interfere” as they move from left to right as shown above. Credit: University of Louisville Department of Physics.

In the first version of the experiment, Young actually didn’t use two slits, but rather a single thin card. The physicist simply covered a window with a piece of paper with a tiny hole in it which served to funnel a thin beam of light. With the card in his hand, Young witnessed how the beam split in two. Light passing on one side of the card interfered with light from the other side of the card to create fringes, which could be observed on the opposite wall. Later, Young used this data to calculate the wavelengths of various colors of light and came remarkably close to modern values. The demonstration would provide solid evidence that light was a wave, not a particle.

Meanwhile, this time in France, the corpuscularist movement was gaining steam after recent developments attributed the polarization of light to some kind of asymmetry among the light corpuscles. They suffered a great defeat at the hand of Augustin Fresnel who in 1821 showed that polarization could be explained if light were a transverse wave with no longitudinal vibration. Previously, Fresnel also came up with a precise wave theory of diffraction.

By this point, there was little stable ground for Newton’s followers to continue the debate. It seemed light is a wave and that’s that. The problem was that the fabled aether — the mysterious medium required to support electromagnetic fields and to yield Fresnel’s laws of propagation — was missing despite everyone’s best efforts to find it. No one ever did, actually.

A huge breakthrough came in 1861 when James Clerk Maxwell condensed experimental and theoretical knowledge about electricity and magnetism in 20 equations. Maxwell predicted an ‘electromagnetic wave’, which can self-sustain, even in a vacuum, in the absence of conventional currents. This means no aether is required for light to propagate! Moreover, he predicted the speed of this wave to be 310,740,000 m s−1 — that’s just a few percent of the exact value of the speed of light.

“The agreement of the results seems to show that light and magnetism are affections of the same substance, and light is an electromagnetic disturbance propagated through the field according to electromagnetic laws”, wrote Maxwell in 1865.

From that day forward, the concept of light was united with those of electricity and magnetism for the first time.

On 14 December 1900, Max Planck demonstrated that heat radiation was emitted and absorbed in discrete packets of energy — quanta.  Later, Albert Einstein showed in 1905 that this also applied to light. Einstein used the term Lichtquant, or quantum of light. Now, at the dawn of the 20th-century, a new revolution in physics would once again hinge on the nature of light. This time, it’s not about whether light is a crepuscule or wave. It’s whether it’s both or not.

Modern theory of light and photons

Einstein believed light is a particle (photon) and the flow of photons is a wave. The German physicist was convinced light had a particle nature following his discovery of the photoelectric effect, in which electrons fly out of a metal surface exposed to light. If light was a wave, that couldn’t have happened. Another puzzling matter is how photoelectrons multiply when strong light is applied. Einstein explained the photoelectric effect by saying that “light itself is a particle,” for which he would later receive the Nobel Prize in Physics.

The main point of Einstein’s light quantum theory is that light’s energy is related to its oscillation frequency. He maintained that photons have energy equal to “Planck’s constant times oscillation frequency,” and this photon energy is the height of the oscillation frequency while the intensity of light corresponds to the number of photons. The various properties of light, which is a type of electromagnetic wave, are due to the behavior of extremely small particles called photons that are invisible to the naked eye.

Einstein speculated that when electrons within matter collide with photons, the former takes the latter’s energy and flies out and that the higher the oscillation frequency of the photons that strike, the greater the electron energy that will come flying out. Some of you have a working proof of this idea in your very own home — it’s the solar panels!  In short, he was saying that light is a flow of photons, the energy of these photons is the height of their oscillation frequency, and the intensity of the light is related to the number of photons.

Einstein was able to prove his theory by deriving Planck’s constant from his experiments on the photoelectric effect. His calculations rendered a Planck’s constant value of 6.6260755 x 10-34  which is exactly what Max Planck obtained in 1900 through his research on electromagnetic waves. Unequivocally, this pointed to an intimate relationship between the properties and the oscillation frequency of light as a wave and the properties and momentum of light as a particle. Later, during the 1920s, Austrian physicist Erwin Schrödinger elaborated on these ideas with his equation for the quantum wave function to describe what a wave looks like.

More than a hundred years since Einstein showed the double nature of light, Swiss physicists at the École Polytechnique Fédérale de Lausanne captured the first-ever snapshot of this dual behavior. The team led by  Fabrizio Carbone performed a clever experiment in 2015 in which a laser was used to fire onto a nanowire, causing electrons to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire. The fired a new beam of electrons to image the standing wave of light, which acts as a fingerprint of the wave-nature of light. The result can be seen below.

The first ever photograph of light as both a particle and wave. Credit: EPFL.

What a photon looks like

Have you ever wondered what shape does a photon have? Scientists have been pondering this question for decades and, finally, in 2016, Polish physicists created the first ever hologram of a single light particle. The team at the University of Warsaw made the hologram by firing two light beams at a beamsplitter, made of calcite crystal, at the same time. The beamsplitter is akin to a traffic light intersection so each photon can either pass straight through or make a turn. When a photon is on its own, each path is equally probable but when more photons are involved they interact and the odds change. If you know the wave function of one of the photons, it’s possible to figure out the shape of the second from the positions of flashes appearing on a detector. The resulting image looks a bit like a Maltese cross, just like the wave function predicted from Schrödinger’s equation.

Hologram of a single photon reconstructed from raw measurements seen in the left-hand side versus the theoretically predicted photon shape on the right-hand side. Credit: FUW

Hologram of a single photon reconstructed from raw measurements seen in the left-hand side versus the theoretically predicted photon shape on the right-hand side. Credit: FUW

Facts about photons

  • Not only is light made up of photons, but all electromagnetic energy (i.e. microwaves, radio waves, X-rays) is made up of photons.
  • The original concept of the photon was developed by Albert Einstein. However, it was scientist Gilbert N. Lewis who first used the word “photon” to describe it.
  • The theory that states that light behaves both like a wave and a particle is called the wave-particle duality theory.
  • Photons are always electrically neutral. They have no electrical charge.
  • Photons do not decay on their own.

What are tsunamis and how they form

Most waves form due to winds or tides, but tsunamis have a different cause altogether. A tsunami is most often formed by an earthquake, but it can also be formed by an underwater landslide, volcano eruption or even meteorite.

The process is fairly complex, so let’s start digging into it.

The Great Wave off Kanagawa, an artistic depiction of a tsunami by Katsushika Hokusai.

What is a tsunami

“Tsunami” is a Japanese word meaning “harbor wave,” but that doesn’t say much about their nature, and tsunamis are not nearly restricted to harbors. A more accurate term would be “seismic sea waves,” and it would describe them more accurately. However, tsunami has stuck and it’s what everyone uses today. People sometimes refer to them as “tidal waves,” but that term is technically incorrect and should be avoided in this context.

Tsunamis are indeed waves, but unlike wind waves, they have a much larger wavelength. Think a bit about waves — in the context of physics, not in the context of sea waves. A defining characteristic of every wave is its wavelength. Wind waves have short wavelengths which can be clearly seen on any shoreline. They come in every few seconds, with a few meters  in between — sometimes, even less. But a tsunami has a huge wavelength, oftentimes longer than a hundred kilometers and this is why they are so dangerous (more on that a bit later). Tsunamis are almost always not singular waves, but come in as train waves.

How tsunamis form – earthquakes

The vast majority of tsunamis form due to earthquakes — specifically tectonic tsunamis. As an earthquake happens, the ground beneath the water is moved up and/or down abruptly and as this movement happens, a mass of water is displaced and starts moving in all directions. This marks the start of a tsunami.

The displaced water starts to move as a wave. At this point, it has a very low amplitude as it is located in deep water (earthquakes on the coastline rarely cause tsunamis). Tsunamis in open water are usually shorter than 0.3 meters (12 inches).


Image by Régis Lachaume. Propagation of a tsunami offshore, showing the variation of wavelength and amplitude as a function of depth.

As the wave starts moving towards the shore, a series of events begin to occur. First of all, water gets shallower and shallower. As a result, the height of the tsunami starts to increase, and can increase dramatically. This is the main reason why these waves are so dangerous: They carry on huge masses of water. When they get closer to the shoreline, the volume of the tsunami remains constant, but because the water gets shallower, their height starts to increase.

The 3D simulation below shows how the process is taking place — note the waterline retreating before the tsunami hits. This is called a drawback.


Also, the shallow water somewhat slows down the waves and the waves start getting closer together. In the deepest parts of the ocean, tsunamis can travel faster than a jet, at 970 kph (600 mph). This means that in only a few hours, it can cross entire oceans.

Tsunamis don’t stop once they hit land. Much of their energy is dissipated and reflected back, but some of it is still maintained and tsunamis will continue to travel inland until all their energy is gone. So don’t think that if you’re a bit farther from the beach, you’re safe. In some rare instances, tsunamis can also travel up river valleys.

How tsunamis form — from other sources

In rare cases, tsunamis can also be caused by landslides, volcano eruptions, and meteorites. In all cases the main principle is the same — a water mass is displaced and as it nears the shoreline it starts growing in height. However, the displacement mechanism differs.


Underwater, landslides are often similar to volcanoes that avalanche into the sea. This process happens as a result of an earthquake, so in a way, the main source is still an earthquake. However, earthquakes can also merely loosen landmass which starts falling at some later point.

Lituya Bay, Alaska, is an area prone to tsunamis (via Wikipedia).


Volcanoes can form tsunamis through two mechanisms. Either they collapse or they eject matter with such strength that they uplift the water. In the first case, land-based volcanoes can also cause tsunamis, if they are very close to the sea.


If you’ve ever thrown a pebble into the water, you’ve seen that it creates ripples. The meteorite works in pretty much the same way, except it creates huge ripples. This kind of tsunamis are really rare, but there is an instance in 1958 where such a wave was created by rockfall in Lituya Bay, Alaska.

Why tsunamis are so dangerous

Tsunamis are not always colossal waves when they come into the shore. According to the USGS, “… most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). Rather, they come in much like very strong and very fast tides (i.e., a rapid, local rise in sea level).”

By now, you should have a pretty clear idea why tsunamis are so dangerous. They can be very long (100 kilometers is a reasonable length), very high (the 2011 Japan tsunami measured over 10 meters) and can travel extremely fast without losing much of their energy. An earthquake far into the ocean can send several devastating tsunamis hundreds or even thousands of kilometers away.

2004 tsunami

A map of the 2004 tsunami with the highlighted epicenter.

In 2004, an earthquake with the epicenter off the west coast of Sumatra, Indonesia struck with a magnitude of 9.1–9.3. The Indian Plate was subducted by the Burma Plate and triggered a series of devastating tsunamis, some over 30 meters high. The tsunamis killed over 230,000 people in 14 countries, being one of the biggest natural disasters in human history. It is just one in many tragic examples highlighting the sheer force of tsunamis.

Safety for tsunamis

  • The first thing to do is to stay informed.

Since science cannot predict when earthquakes will occur, we cannot determine exactly when a tsunami will be generated. However, that doesn’t mean we’re clueless. With the aid of historical records of tsunamis and numerical models of their size and speed, we can get a pretty good idea as to where they’re likely to be generated. You should always know if you’re in a tsunami risk zone. An estimated 85% of all tsunamis have been observed in the Pacific Ocean in the “Ring of Fire,” but other areas can be dangerous as well and as we mentioned above, tsunamis can also travel great distances.

  • If you feel an earthquake in a low-lying, coastal area, keep calm and move away from the coast. Not all earthquakes cause tsunamis, but some do.
  • If you see a large water mass retreating, this is the drawback. It’s a telltale sign that a tsunami is coming. A 10-year-old girl saved many lives in 2004 because she knew this from her geography lessons.
  • Tsunamis are rarely singular waves — they come in packs, so if one hits, don’t think it’s ‘all clear’ – more may be on their way. Earthquakes also often have replicas, which in turn can cause tsunamis.
  • Be on the lookout for tsunami warnings. Tsunamis are fast, but they still take some time to travel. So if you know of an earthquake nearby, check a tsunami forecast and see what it says. Also keep in mind that a small tsunami on one beach can be a big one on a nearby beach. Underwater topography can play a massive role.
  • Buildings are no protection against a tsunami. Going farther away from the beach is the best thing you can do.
  • If you’re somehow on a boat or ship and there’s a tsunami coming your way, it may be smarter to move your ship farther into the ocean where the tsunami is smaller. However, this can be very risky. Stay tuned to your local radio, marine radio, NOAA Weather Radio, or television stations during a tsunami emergency.
  • Whatever you do, don’t purposely go to the beach to see a tsunami. Seriously. It will outrun or outdrive you and it’s not safe at all.


The world’s first image of light as both a particle and a wave

We see light every day, and yet, we don’t truly understand it; it’s either a particle or a wave, or both at the same time… and we don’t really know why. Now, for the first time, researchers have captured an image of light behaving as a particle and a wave at the same time.

Wave-Particle Duality

The wave signature is on the top of the image, while the photons are at the bottom. Image: Fabrizio Carbone/EPFL

Christian Huygens, who was a contemporary of Isaac Newton, suggested that light travels in waves. Isaac Newton, however, thought that light was composed of particles that were too small to detect individually. Strangely enough.. they were both right. In 1801 a physicist in England, Thomas Young, performed experiments which revealed that light is a wave. In the 1890s, Maxwell’s equations described light behavior in such an elegant way that many scientists thought not much was left to say about it.

Then, on Dec. 14, 1900, Max Planck came along and introduced a stunningly simple, yet strangely unsettling, concept: that light must carry energy in discrete, specific quantities. Before you know it, the particle nature came back with a vengeance; to make things even more interesting, Louis De Broglie demonstrated that every physical object actually has wave properties – in a way, everything is both a wave and matter at the same time. This is called the wave-particle duality.

As Einstein wrote:

“It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do”.

This seems to be most intriguing in the case of light – it seems that light behaves selectively depending on the environmental constraints. Sometimes it’s a wave, sometimes it’s a particle… and sometimes it’s both. Scientists have only ever been able to capture an image of light as either a particle or a wave, and never both at the same time… until now.

The first Wave-Particle image

The key to this success lies in the unusual experimental design. The team from the École Polytechnique Fédérale de Lausanne in Switzerland have managed to use electrons to image light by firing a pulse of laser light at a single strand of nanowire suspended on a piece of graphene film. This caused the nanowire to vibrate, and in turn, to send light particles (photons) along two possible directions. When light particles that are travelling on opposite directions meet and overlap on the wire, they form a wave. Known as a ‘standing wave’, this state creates light that radiates around the nanowire.

That’s a pretty creative setup in its own right, but it’s not gonna give you a wave-particle image, so they needed to take it one step further. They sent a stream of electrons into the area nearby the nanowire, forcing an interaction between the electrons and the light that had been confined on the nanowire. This caused the electrons to either speed up or slow down; using an ultrafast electron,  they could visualise the standing wave, “which acts as a fingerprint of the wave-nature of light,” the press release explains. So they were able to capture as  a wave (its fingerprint actually; at the top of the image) and as photons (in the bottom of the image).

“This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” one of the team, physicist  Fabrizio Carbone, said in a press release. “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”

The team in Switzerland also put together the adorable video above, explaining their experiment.

Journal Reference: L Piazza, T.T.A. Lummen, E Quiñonez, Y Murooka, B.W. Reed, B Barwick & F Carbone. Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field. Nature Communications 6, Article number: 6407 doi:10.1038/ncomms7407



The biggest tsunami ever recorded was taller than 500 meters

On the night of July 9, 1958, an earthquake struck Fairweather Fault in the Alaska Panhandle. The result was that about 30.6 million cubic meters of rock were loosened, being thrown from a height of 914 meters down onto the water mass. Here’s a picture so that you can get a perspective on what that means:


The impact generated a tsunami that crashed against the shoreline of Gilbert Inlet. The water hit with such power that it totally destroyed the spur of land that separates Gilbert Inlet from the main body of Lituya Bay and continued its road towards the Gulf of Alaska. It destroyed all vegetation from elevations as high as 500 meters, uprooting millions of trees. It is the biggest wave ever known to man.


We now know the birth place of the biggest guitar in the galaxy

guitarIn case you’re wondering, the biggest ‘guitar’ in our galaxy is in fact a pulsar that was nicknamed The Guitar Pulsar. It’s basically a stellar corpse that emits a beam of electromagnetic radiation that just shreds interstellar gas, creating a wake of hot hydrogen shaped just like a guitar.

Little is known about these remnants, from any point of view. In order to track down it’s birthplace, Nina Tetzlaff at the University of Jena in Germany and her colleagues calculated the location of 140 groups of stars, as they were 5 millions ago.

The pulsar was practically launched from a cluster of massive stars, moving at about 1500 kilometres per second, which is just huge. They were able to pinpoint the exact location it was formed, but why it moved so fast still remains a mystery. Speeds over 1000 km/s are practically not used in current astronomy models, and are considered by many to be borderline impossible.