Cherenkov radiation faster than the speed of light?

Cherenkov radiation is a type of electromagnetic radiation emitted when a charged particle (usually an electron) moves faster than the speed of light in a specific dielectric medium.

Speed of Light in Different Mediums

The speed of light we commonly refer to is the speed of light in a vacuum, meaning light does not collide with any matter while it travels. This speed is abbreviated as C and has a relatively precise value of 299,792,458 m/s.

In fact, this is the speed at which any massless particle moves (light is a stream of photon particles. Photons are massless particles).

In non-vacuum environments, including air, light takes a longer time to travel the same distance compared to when it moves through a vacuum. This reduced speed is commonly referred to as the phase velocity of light.

In material environments, photons do not move completely freely as they continuously collide with the atoms and molecules of the medium. During such collisions, they are absorbed and re-emitted. The time interval between absorption and emission increases the total time light requires to travel compared to when it moves smoothly without collisions.

Cherenkov radiation is a type of electromagnetic radiation emitted when a charged particle moves.

This means that if you could accurately measure the time it takes for light to travel through a transparent medium, you would find that the speed of light is less than C.

However, this is the average speed over a distance and not the actual speed of the photon, as photons do not change speed. This is similar to driving on a straight road but encountering many intersections and roundabouts that force you to navigate around them, which takes more time even if you maintain a constant speed.

The average reduction in the speed of light in different environments depends on a familiar index known as the refractive index of the medium. The speed of light in any given medium is calculated by dividing the speed of light in a vacuum by the refractive index of that medium (with a vacuum conventionally having a refractive index of 1).

Clean air (Earth’s atmosphere) has a refractive index of 1.000293, meaning the speed of light in air is approximately 299,704,645 m/s, which is almost unchanged (for polluted air or high humidity, the reduction will be slightly higher).

In contrast, water has a refractive index of 1.333, meaning the speed of light in water is only 75% of its speed in a vacuum. This allows certain types of particles under specific conditions to exceed the speed of light when traveling through water.

Origin of Cherenkov Radiation

When a charged particle (such as an electron) moves, it disrupts the local electromagnetic field of the surrounding medium and causes polarization. If the particle moves slowly, specifically slower than the electromagnetic wave, then there is enough time for the electromagnetic waves to return to equilibrium after the particle passes through.

However, when the particle moves faster than the speed of light in that medium, the disturbance it causes remains even after it has passed through, while subsequent disturbances continue to be generated. As the particle continues to move, it creates a boundary of disturbed waves.

The energy overlap of these disturbances causes the release of electromagnetic energy in the form of a shockwave similar to the sonic booms produced when aircraft exceed the speed of sound in air.

The electromagnetic radiation generated by a charged particle moving through a dielectric faster than the speed of light in that medium is called Cherenkov radiation (named after the Russian scientist Pavel Alekseyevich Cherenkov who discovered it).

Electrons released from nuclear reactions or accelerated can reach speeds exceeding 75% of C, which is faster than the speed of light in water. For this reason, Cherenkov radiation is produced in nuclear reactors submerged in water, observable as blue light.

With this foundation, we can address the questions many people have about this radiation.

1. Does Cherenkov radiation travel faster than light?

The answer is no. Many people misunderstand that Cherenkov radiation travels faster than the speed of light in water when reading materials that are not very detailed. In reality, Cherenkov radiation is emitted when a charged particle moves faster than the speed of light in the dielectric medium (for example, an electron moving in water).

Cherenkov radiation is distinct from the charged particle itself. This radiation is electromagnetic and travels at the speed of light in the same medium. The charged particle that causes this radiation can never reach the speed of light in a vacuum under any circumstances.

2. Can Cherenkov radiation be generated in a vacuum or in air?

This radiation is only generated when a charged particle moves faster than the speed of light in the dielectric medium it traverses. Regardless of how it is accelerated, a charged particle like an electron cannot exceed the speed of light in a vacuum.

Therefore, theoretically, Cherenkov radiation cannot be generated in a vacuum or even in Earth’s atmosphere (since the speed of light in air is only slightly less than the absolute speed).

However, there is a hypothesis that this radiation could still be generated in a vacuum in structures that slow down waves, meaning the speed of light is reduced even though it is not absorbed and re-emitted by the medium. But this has never been observed or confirmed.

3. What can move faster than the speed of light?

It is important to note that when there is no additional phrase indicating the speed of light in any particular medium (such as in water, in glass, etc.), the term “speed of light” is understood to refer to the speed of light in a vacuum. This speed has been clearly proven to be the maximum speed in nature.

Articles with titles claiming that the motion of electrons generates Cherenkov radiation faster than light are merely attention-grabbing and hold no scientific significance. Some other opinions suggest that there are cases that could exceed the speed of light, such as cosmic inflation or wormholes. However, these do not violate the principle of the absolute speed of light.