Cherenkov radiation

Cherenkov radiation can be called the physics of the XIX century, accidentally made its way in the XX-th. He could have been predicted (and to some extent the Heaviside physicist did) in the 1880s, but this effect was discovered by chance, perhaps by Maria and Pierre Curie. He was carefully studied by Pavel Cherenkov in the 1930s, and after a few years the effect was explained in detail by Ilya Mikhailovich Frank and Igor Evgenievich Tamm . Three of these physicists were awarded the Nobel Prize in 1958 for studying this phenomenon.

Note transl.: in English-speaking sources, almost always when describing Cherenkov radiation, the authors rush to mention the Curie couple and the fact that they were still observing some blue glow in their experiments with radium at the beginning of the 20th century. Moreover, they usually do not indicate the source of this information; in rare cases, they write that the information was obtained on the basis of a reading of an artistic book, a biography of the Curie couple, written by their daughter, Eva.

And in the biography itself about the blue glow it says only this:

“And among the dark barn glass vessels with precious particles of radium, laid out, in the absence of cabinets, are simply on the tables, on the wooden shelves nailed to the walls, shining with bluish phosphorescent silhouettes, as if hanging in the darkness.” // Pierre and Maria Curie, per. from French S.A. Shukarev, Evgeny Fedorovich Korsh, ed. 1959

What was the observation? Cherenkov studied the blue light that appeared at the moment when radioactive objects (containing atoms, whose nucleus decays into other nuclei, spitting out high-energy particles, among which electrons and positrons are found) were placed next to water and other transparent materials. Now we know that any electrically charged particle, such as an electron moving with sufficiently high energy through water, air or another transparent medium, will emit blue light. This light moves from a particle at a certain angle to the direction of its movement.

What's happening? As understood by Frank and Tamm, this is a photon strike similar to the sonic impact occurring when a supersonic aircraft moves faster than the speed of sound, or the excitement that a ship is going through the water. Light in a transparent medium will move at a speed different from the speed of light in a vacuum due to the interaction between light and charged particles (electrons and atomic nuclei) that make up this medium. For example, in water, light moves about 25% slower than in a vacuum! Therefore, it is easier for the high-energy electron to move faster than the light moves in water, and not to exceed the speed of light in a vacuum. If such a particle goes through the water, it creates an electromagnetic blast wave similar to the blast wave created by a supersonic airplane in dense air. This wave comes from a particle, just as a sound wave comes from an airplane, and carries energy in many forms (wavelengths) of electromagnetic radiation, including visible light. On the violet end of the rainbow, energy is created more than on red, so the light for our eyes and brain looks mostly blue.

Such radiation is extremely useful in particle physics, for it provides an excellent way to detect high-energy particles! Not only can we see the presence of high-energy charged particles due to the light emitted by them, we can comprehend much more by studying the details of this light. The exact radiation pattern can help determine (a) what path a particle follows in the medium, (b) how much energy it carries, and even (c) something about its mass (since electrons will be scattered in the medium, and heavier particles will behave differently). Several very important experiments, including those that later received the Nobel, are based on this radiation. Among them are experiments that played a major role in the study of neutrinos, for example, Super-Kamiokande .

Cherenkov radiation is also very useful in checking the correctness of the description of nature by Einstein's theory of relativity. Cosmic rays - particles flying from deep space (often colliding with something in the atmosphere and generating cascades of particles that can be detected by detectors on earth), in rare cases, can have extremely high energy - 100 million times greater than the energy of protons in Large Hadron Collider. These particles (as far as we know) were created at a distance of many light years from Earth in such powerful astronomical events as supernovae. Suppose that the speed of light would not be a universal speed limit, and these particles would move faster than light in the vacuum of space. Then these high-energy particles would also cause Cherenkov radiation. And since their journey was so long, they would have lost a lot of energy on this radiation. It turns out that this energy loss can occur very quickly, and that in this case, the particles could not overcome astronomical distances and maintain such high energy levels, if only their speed did not remain less than the speed of light.

In short, if ultrahigh-energy cosmic rays could move faster than light, then we could not observe any cosmic rays with such energy, for they would have had to lose all their energy before they reached Earth. But we observe them.

There is a small catch: we are almost sure that most of them have a charge: their properties indicate that they are involved in strong nuclear interaction, and the only stable particles that can travel such distances are protons, and in general, atomic nuclei, and they all have an electric charge. If you even take advantage of this trick, but the restrictions can be slightly relaxed, but they will still remain quite strong.

From this we can conclude: ultrahigh-energy cosmic rays (as well as all low-energy cosmic rays in general) cannot move faster than the speed of light, at least much faster. And if this advance exists, then his estimates, made in the late 1990s by famous physicists Sidney Coleman and Sheldon Glashow , say that this value can be equal to ten parts out of a trillion trillions. Since then, these limitations have probably been improved by experimental data.

In the same way, the fact that we can observe high-energy electrons imposes a limit on their speed relative to the speed of light. One of the latest statements I read about says that from observations of electrons with energies up to 0.5 TeV, it follows that electrons cannot exceed the speed of light by more than one part of a thousand trillion.


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