Cherenkov Effect: What Happens When a Particle Moves Faster Than Light?

If you have ever seen pictures of a nuclear reactor, you might observe a blue glow surrounding the core. This is called Cherenkov radiation, an electromagnetic radiation that appears as a shockwave of light.

Exception to the Rule

The scientific community agrees that the speed of light is the absolute speed limit of the cosmos. According to Albert Einstein, nothing will go faster than 186,000 miles per second (300,000 kilometers per second) since it will require an infinite amount of energy to do so.

When Einstein proposed his theory of relativity, he hypothesized that only massless particles could ever attain the ultimate cosmic speed limit. For massive particles, they can only approach the speed of light but would never reach it.

However, this postulate actually has a limitation. This can only happen in the vacuum of perfectly empty space. Through a medium like air, water, glass, or acrylic, light travels at a slower speed.

On the other hand, energetic particles can only travel slower than light in a vacuum, not light in a medium. This leads to an amazing prediction: anything can move faster than light as long as it is in a medium where the speed of light is below the speed of light in a vacuum.

Many nuclear processes emit charged particles like electrons through radioactive decay, fusion or fission. If these particles pass through a medium as simple as water, it can move faster than the speed of light and emit a special form of radiation in a phenomenon known as Cherenkov Effect.

What Is the Cherenkov Effect?

The Cherenkov Effect happens when a particle carrying an electric charge passes through a transparent medium. If this particle travels faster than light in a medium, it causes a short flash of blue light called Cherenkov light.

In 1934, Soviet physicist Pavel Alekseyevich Cherenkov observed what happens when it does. After bombarding water with radiation, he noticed that a blue light was emitted.

With the help of his colleagues Il´ja Mikhailovich Frank and Igor Yevgenyevich Tamm, Cherenkov discovered what was causing the strange glow. They found out that charged subatomic particles that move faster than the speed of light produce an effect which is similar to a sonic boom, where a plane travels faster than the speed of sound. For their work, Cherenkov and his colleagues were awarded the Nobel Prize in Physics in 1958.

High amounts of energy are involved during Cherenkov radiation. The photons travel as waves with high frequencies and short wavelengths, the typical characteristic of violet and blue colors. As the light obtains higher frequencies and shorter wavelengths, the bluer or more violet it appears. Since ultraviolet light is not visible to the human eye, only blue light can be seen from the phenomenon.

A strange thing about Cherenkov radiation is that it can occur in a person's head. For several decades, cancer patients undergoing therapy have reported seeing flashes of blue light even with their eyes closed.

In 2020, a team of researchers conducted a study where they used a CDose camera imaging system to capture an image of the light emitted from biological systems. This confirmed the reports, and provided evidence that the light was indeed Cherenkov radiation.