Let's figure it out: what is light?

He is around us and allows us to see the world. But ask any of us, and most will not be able to explain what this light really is. Light helps us understand the world in which we live. Our language reflects this: in the darkness we move by touch, we begin to see the light together with the onset of dawn. And yet we are far from a complete understanding of light. If you bring a ray of light closer, what will be in it? Yes, light moves incredibly fast, but can't it be used for travel? And so on and so forth.

Of course, this should not be the case. Light has puzzled the best minds for centuries, but landmark discoveries over the past 150 years have gradually lifted the veil of mystery over this mystery. Now we more or less understand what it is.

Physicists of our time not only comprehend the nature of light, but also try to control it with unprecedented precision - which means that light can very soon be made to work in the most amazing way. For this reason, the United Nations has designated 2015 as the International Year of Light.

Light can be described in all sorts of ways. But it's worth starting with this: light is a form of radiation (radiation). And this comparison makes sense. We know that excess sunlight can cause skin cancer. We also know that radiation exposure can put you at risk for some forms of cancer; it is not hard to draw parallels.

But not all forms of radiation are created equal. At the end of the 19th century, scientists were able to determine the exact essence of light radiation. And the strangest thing is that this discovery did not come from the study of light, but came out of decades of work on the nature of electricity and magnetism.

Electricity and magnetism seem to be completely different things. But scientists like Hans Christian Oersted and Michael Faraday have found that they are deeply intertwined. Oersted discovered that an electric current passing through a wire deflects the needle of a magnetic compass. Meanwhile, Faraday discovered that moving a magnet near a wire could generate an electric current in the wire.

The mathematicians of that day used these observations to create a theory describing this strange new phenomenon, which they called "electromagnetism." But only James Clerk Maxwell was able to describe the full picture.

Maxwell's contribution to science can hardly be overestimated. Albert Einstein, who was inspired by Maxwell, said that he changed the world forever. Among other things, his calculations helped us understand what light is.

Maxwell showed that electric and magnetic fields travel in waves, and these waves travel at the speed of light. This allowed Maxwell to predict that light itself is carried by electromagnetic waves - which means that light is a form of electromagnetic radiation.

In the late 1880s, several years after Maxwell's death, German physicist Heinrich Hertz was the first to officially demonstrate that Maxwell's theoretical concept of the electromagnetic wave was correct.

“I’m sure that if Maxwell and Hertz lived in the era of the Nobel Prize, they would definitely receive one, ” says Graham Hall of the University of Aberdeen in the UK - where Maxwell worked in the late 1850s.

Maxwell ranks in the annals of the science of light for a different, more practical reason. In 1861, he unveiled the first stable color photography using the tri-color filter system, which laid the foundation for many forms of color photography today.

The very phrase that light is a form of electromagnetic radiation does not say much. But it helps to describe what we all understand: light is a spectrum of colors. This observation goes back to the work of Isaac Newton. We see the color spectrum in all its glory when a rainbow rises in the sky - and these colors are directly related to Maxwell's concept of electromagnetic waves.

The red light at one end of the rainbow is electromagnetic radiation with a wavelength of 620 to 750 nanometers; the violet color at the other end is radiation with a wavelength of 380 to 450 nm. But there is more to electromagnetic radiation than visible colors. Light with a wavelength longer than red is what we call infrared. Light with a wavelength shorter than violet is called ultraviolet light. Many animals can see in ultraviolet light, and some people can see too, says Eleftherios Gulilmakis of the Max Planck Institute for Quantum Optics in Garching, Germany. In some cases, people even see infrared. Perhaps that is why we are not surprised that we call ultraviolet and infrared forms of light.

Curiously, however, if the wavelengths get even shorter or longer, we stop calling them "light." Outside of ultraviolet, electromagnetic waves can be shorter than 100 nm. This is the realm of X-rays and gamma rays. Have you ever heard of X-rays being called a form of light?

"The scientist will not say 'I am shining through the object with X-ray light." He will say “I use X-rays, ” says Gulilmakis.

Meanwhile, beyond infrared and electromagnetic wavelengths, waves stretch up to 1 cm and even thousands of kilometers. Such electromagnetic waves are called microwaves or radio waves. It may seem strange to some to perceive radio waves as light.

“There is not much physical difference between radio waves and visible light from a physics standpoint, ” says Gulilmakis. "You will describe them with the same equations and mathematics." Only our everyday perception distinguishes them.

Thus, we get a different definition of light. This is a very narrow range of electromagnetic radiation that our eyes can see. In other words, light is a subjective label that we only use because of the limitation of our senses.

If you need more detailed proof of how subjective our perception of color is, think of the rainbow. Most people know that the spectrum of light contains seven primary colors: red, orange, yellow, green, cyan, blue, and violet. We even have handy proverbs and sayings about hunters who want to know where a pheasant is. Look at a nice rainbow and try to see all seven. Even Newton did not succeed. Scientists suspect that the scientist divided the rainbow into seven colors, since the number "seven" was very important for the ancient world: seven notes, seven days of the week, etc.

Maxwell's work on electromagnetism took us further and showed that visible light was part of a wide spectrum of radiation. The true nature of light also became clear. For centuries, scientists have tried to understand what form light actually takes on a fundamental scale as it travels from the light source to our eyes.

Some believed that light moves in the form of waves or ripples, through the air or the mysterious "ether". Others thought this wave model was flawed and considered light to be a stream of tiny particles. Newton leaned towards the second opinion, especially after a series of experiments he conducted with light and mirrors.

He realized that the rays of light obey strict geometric rules. A ray of light reflected in a mirror behaves like a ball thrown directly into a mirror. Waves will not necessarily follow these predictable straight lines, Newton suggested, so light must be carried by some form of tiny, massless particles.

The problem is that there has been equally compelling evidence that light is a wave. One of the clearest demonstrations of this was in 1801. The Thomas Young double slit experiment, in principle, can be done independently at home.

Take a sheet of thick cardboard and carefully make two thin vertical cuts in it. Then take a "coherent" light source that will only emit light of a certain wavelength: a laser will do just fine. Then direct the light to two slits so that as it passes through, it falls on the other surface.

You would expect to see two bright vertical lines on the second surface where the light has passed through the slits. But when Jung did the experiment, he saw a sequence of light and dark lines like a barcode.

When light passes through thin slits, it behaves like water waves that pass through a narrow opening: they scatter and spread in the form of hemispherical ripples.

When this light passes through two slits, each wave extinguishes the other, forming dark patches. When the ripples converge, they complement, forming bright vertical lines. Young's experiment literally confirmed the wave model, so Maxwell put the idea into solid mathematical form. Light is a wave.

But then there was a quantum revolution.

In the second half of the nineteenth century, physicists tried to figure out how and why some materials absorb and emit electromagnetic radiation better than others. It is worth noting that the electric light industry was just developing then, so materials that can emit light were a serious thing.

By the end of the nineteenth century, scientists had discovered that the amount of electromagnetic radiation emitted by an object changed with its temperature, and they measured these changes. But no one knew why this was happening. In 1900, Max Planck solved this problem. He found that calculations can explain these changes, but only if we assume that electromagnetic radiation is transmitted in tiny discrete portions. Planck called them "quanta", the plural of the Latin "quantum". A few years later, Einstein took his ideas as a basis and explained another surprising experiment.

Physicists have discovered that a piece of metal becomes positively charged when it is irradiated with visible or ultraviolet light. This effect has been called photovoltaic.

The atoms in the metal lost negatively charged electrons. Apparently, the light delivered enough energy to the metal to release some of the electrons. But why the electrons did this was not clear. They could carry more energy simply by changing the color of the light. In particular, electrons released by a metal irradiated with violet light carried more energy than electrons released by a metal irradiated with red light.

If light were just a wave, it would be ridiculous.

Usually, you change the amount of energy in the wave, making it higher - imagine a high tsunami of destructive power - and not longer or shorter. More broadly, the best way to increase the energy that light transfers to electrons is to make the light wave higher: that is, make the light brighter. Changing the wavelength, and hence the light, shouldn't have made much difference.

Einstein realized that the photoelectric effect is easier to understand if you think of light in terms of Planck quanta.

He suggested that light is carried by tiny quantum chunks. Each quantum carries a portion of discrete energy associated with a wavelength: the shorter the wavelength, the denser the energy. This could explain why the relatively short wavelength portions of violet light carry more energy than the relatively long portions of red light.

It would also explain why simply increasing the brightness of the light doesn't really affect the result.

Brighter light delivers more portions of light to the metal, but this does not change the amount of energy carried by each portion. Roughly speaking, one portion of violet light can transfer more energy to one electron than many portions of red light.

Einstein called these portions of energy photons and are now recognized as fundamental particles. Visible light is carried by photons, and other forms of electromagnetic radiation such as X-rays, microwave and radio waves are also carried. In other words, light is a particle.

With this, physicists decided to end the debate about what light is made of. Both models were so convincing that there was no point in abandoning one. To the surprise of many non-physicists, scientists have decided that light behaves like a particle and a wave at the same time. In other words, light is a paradox.

At the same time, physicists did not have problems with the split personality of light. This, to some extent, made light doubly useful. Today, relying on the work of the luminaries in the literal sense of the word - Maxwell and Einstein - we squeeze everything out of the light.

It turns out that the equations used to describe light-wave and light-particle work equally well, but in some cases one is easier to use than the other. So physicists switch between them, much like we use meters to describe our own height, and move to kilometers to describe a bike ride.

Some physicists are trying to use light to create encrypted communication channels, for example, for money transfers. It makes sense for them to think of light as particles. This is due to the strange nature of quantum physics. Two fundamental particles, like a pair of photons, can be entangled. This means that they will have common properties no matter how far from each other, so they can be used to transfer information between two points on Earth.

Another feature of this entanglement is that the quantum state of the photons changes when they are read. This means that if someone tries to eavesdrop on an encrypted channel, in theory, he will immediately betray his presence.

Others, like Gulilmakis, use light in electronics. They find it more useful to represent light as a series of waves that can be tamed and controlled. Modern devices called "light field synthesizers" can combine light waves in perfect synchronization with each other. As a result, they create pulses of light that are more intense, short-lived, and more directional than light from a conventional lamp.

Over the past 15 years, these devices have learned to be used to tame light to an extreme degree. In 2004, Gulilmakis and his colleagues learned how to produce incredibly short pulses of X-rays. Each pulse lasted only 250 attoseconds, or 250 quintillion seconds.

Using these tiny pulses like a camera flash, they were able to capture images of individual waves of visible light that oscillate much more slowly. They literally took pictures of the moving light.

“Ever since Maxwell's time, we knew that light is an oscillating electromagnetic field, but no one even thought that we could take pictures of oscillating light, ” says Gulilmakis.

Observing these individual waves of light was the first step towards manipulating and modifying light, he says, much like we alter radio waves to carry radio and television signals.

A century ago, the photoelectric effect showed that visible light affects the electrons in a metal. Gulilmakis says it should be possible to precisely control these electrons using visible light waves modified to interact with the metal in a well-defined way. “We can manipulate light and use it to manipulate matter, ” he says.

This could revolutionize electronics, lead to a new generation of optical computers that will be smaller and faster than ours. "We can move electrons as we please, creating electric currents inside solids with the help of light, and not like in conventional electronics."

Here's another way to describe light: it's an instrument.

However, nothing new. Life has been using light ever since the first primitive organisms developed light-sensitive tissues. The eyes of people capture the photons of visible light, we use them to study the world around. Modern technology takes this idea even further. In 2014, the Nobel Prize in Chemistry was awarded to researchers who built a light microscope so powerful that it was considered physically impossible. It turned out that if we try, the light can show us things that we thought we would never see.