"Uncle" Einstein expanded the horizons of human knowledge and at the same time has created serious physical limitations for all physical objects that have a mass. He developed formula, which implies the following:
Any object with mass cannot travel at or faster than speed of light in vacuum
With increasing speed of the object increases its mass. As we can see if the parameter "V" is equal to or exceeds the parameter "C", then in the denominator we get 0 or a square root for a negative number. Thus, the equation loses its meaning for the real object.
Photons, which are essentially constituent particles of light, are able to move with the known velocity of 300,000 km/sec, because they do not have mass. In fact they are not even accelerate, they are born with a constant speed. However, people are able to cheat, in a good sense...
How to overtake a runner who is physically faster than you? That's right, you need to slow him down. How to do this with a photon? The key word in Einstein's restriction is "vacuum".
In the broader physical sense, light is not only an ordered movement of particles (photons), but also an electromagnetic wave, which is subject to such an interesting law as refraction. You can observe the effect of this law at the border of air and water. In transparent media with a higher density than air or vacuum (eg water or glass), light slows down.
The refractive index of glass is equal to 1.49, this means that the phase velocity of light in this media in 1.49 times less. Diamonds have a refractive index of 2.42, i.e. the speed of light inside the diamond is reduced almost 2.5 times! And here other elementary particles have a chance to overtake photons.
The electrons “use” this trick in a nuclear reactor with a liquid cooling system. Inside the thermonuclear reaction, at the very start, our participants have the following indicators: The photon speed is equal to its usual value in a vacuum- "c", the electron velocity close to the speed of light and is 0.89-0.91c. But when both of these particles fall into the liquid, the speed of the photon is reduced by 30-40%, while the electron continues to move without loss, it is not affected by refraction.
In a liquid, a fast electron overtakes photons that were released after collision. Therefore, a light wave moves behind the electron. [source]
Imagine that on the border of water and air, the photon (our first runner) has stumbled, while our second participant (the electron) continues to run further through the crowd of spectators (other elementary particles). Electron touches and pushes them away, in response they angrily begin to emit other photons, which are also slower than our electron-champion, because they are in liquid.
During this process, the fluid in which the electron-champion moves begins to glow with a specific blue or green light. This glow is not a nuclear reaction, this is the direct effect of free electrons (that left the nuclear reaction) on molecules in the neighboring medium. This effect is called Vavilov–Cherenkov radiation (VCR). In contrast to the radiation in vacuum, in this case the photons are left behind fast electrons.