# Fermilab

## Inquiring Minds

 Questions About Physics

Speed of Light

I heard threw a third party the some ofthe particals in the colision were clocked at a speed over the speed of light. Is there any truth to this?

Hey, NO and YES!

It would be great if we discovered a particle going faster than the speed of light but as far as we are aware that has not yet happened. None of the particles detected in the collisions are known to have a speed greater than that of light. The speed of light is special in Einstein's theories and so far noone has uscceeded in finding a Tachyon although the fact that we already have a name for such a beast means that we have contemplated such a thing. In fact sometime in the last 20 years there was quite a bit of activity because an Australian Cosmic Ray group thought they were seeing a signal consistent with Tachyons. This is the NO answer.

Now the speed of light referred to above is the speed of light in the absence of any material.. however when light travels through matter, a gas for instance, its observed speed from one end to the other of the gas volume, is less than the "speed in vacuum". IF a particle passes through such a gas volume it can indeed go faster than the speed of light in that gas... if that happens there is a shock wave phenomenon analogous to the Sonic Boom we hear when an aircraft passes overhead at a speed greater than the speed of sound (Mach 1!). In the case of our particle the shock wave is not of sound but of light and the light pulse which results can be observed. For our particles this phenomenon is called the Cherenkov after a Russian physicist who was the first to understand the effect. Getting particles to go so fast is easier for lighter particles so we often use this phenomenon to decide whether a particle is light or heavy, whether it is an electron, a pion or a proton. This is the YES answer.

Unfortunately it is not as exciting as you rightly imagined it would be if we had observed a Tachyon. Nevertheless the phenomenon is a beautiful example of a situation where physicists have understood a rather complex phenomenon and have then put it to use in a detector.

I hope this helps to answer your question.

Sincerely,
Judy Jackson, Fermilab Office of Public Affairs
Hugh Montgomery, Fermilab D0 Experiment

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 last modified 1/16/1997   physicsquestions@fnal.gov

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