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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Can you see Cherenkov radiation in empty space?

This radiation occurs if a particle tries to travel faster than the speed of light through the medium it is in. In nuclear physics, physicists construct detectors made of plastic, or even use water, in which the speed of light is lower than the 300,000 kilometers/sec we see in empty space, typically 150,000 - 250,000 kilometers/sec. When an energetic particle enters the plastic or water traveling faster than 150,000 or 250,000 kilometers/sec, it will produce Cherenkov radiation which can be detected by sensitive light meters and photomultiplier tubes that surround the tank of liquid, or are attached to the plastic 'wave guide'. This, by the way, is different than synchrotron radiation which is produced by charged particles, usually electrons, as they travel through strong magnetic fields at speeds close to the speed of light in a vacuum.

I believe some physicists have searched for Cherenkov radiation appearing in a vacuum, as a method for detecting hypothetical particles that travel faster than the speed of light ( 300,000 kilometers/sec). These particles, called tachyons, may actually be real but they would also be devilishly difficult to detect. It is as impossible to slow a tachyon to the speed of light as it is to accelerate matter to faster than the speed of light. The most common tachyons would have speeds millions of times faster than light because as they loose energy, they speed up!

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.