# Fermilab

## Inquiring Minds

All About Light Main Page  |  Classical  |  Relativistic  |  Quantum

 Relativistic - c-the Ultimate Speed-But Not In Matter! c and Einstein  |  c Top Speed  |  Speed & Speed  |  Faster than c? Mass Gain  |  E=m*c^2  |  Age Slower  |  Time Dilation  |  Twin Paradox Dopper Shift  |  Ticketed  |  Simultaneity  |  Competition  |  Fast Strudel Paradox? No!  |  Confusions  |
We at Fermilab deal everyday with particles moving faster than the speed of light! In fact, we use them to detect some fast particles moving through our detectors.

You might say: "What?? In the previous 12 pages you convinced me that there is no such thing as an object moving faster than the speed of light!".

Don't worry: you are right. The trick is that I carefully left out in the first paragraph the words IN A VACUUM when I said "particles moving faster than the speed of light!" Got it? Great, but let's take a closer look at this issue.

When I discussed the propagation of the light in certain media I said, that light in a medium slows down due to their interaction. All particles entering a medium interact with the medium, but not the same way as light. In fact particles might slow down at a different rate than light.

but light slows down a lot. These particles then travel faster than the speed of light in that particular medium. (Just as a fighter jet might fly faster than the speed of sound in air).

When any charged particle moves faster than the light, it radiates electromagnetic waves (called Cerenkov radiation). Sometimes this radiation lies in the visible spectrum. Here at Fermilab, we collect this light and use it to identify very high energy particles flying through our detectors.

Conclusion: There is no contradiction with Einstein's theory of relativity. Nothing moves faster than the light in a vacuum.

On the next page I will discuss why it is dangerous to move with relativistic speeds.

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