## Physics Questions People Ask Fermilab

**What are tachyons?**

**You Wrote:**

I'm an italian student and I'd like to know if there are any proofs of the existence of the tachyons. Please answer me.

Yours faithfully

Angelo Colbertaldo

Hi Angelo,

Tachyons are hypothetical particles that can only travel faster than the speed of light. As you probably know, objects with a real number for mass can never travel at the speed of light because of Einstein's theory of relativity. As a consequence of this theory, as a objects velocity increases its mass increases. As is it can be seen by the following formula mass=rest_mass*1/sqrt(1-v^2/c^2). At the speed of light the mass becomes infinite. So, it would take an infinite amount of energy for a massive particle to reach the speed of light. These objects are sometimes called tardyons. Photons can travel at the speed of light because they have no mass and their energy is E=planck's constant * nu(frequency of the photon).

In order for something to travel at the speed of light it would have to have an imaginary number for its mass. An imaginary number is a number that is a multiple of the square root of a negative number. As a particle travels faster than the speed of light the denominator of mass=rest_mass*1/sqrt(1-v^2/c^2) becomes imaginary, the imaginary mass would counteract this and we (in the rest frame) would see something that had real mass in the rest frame but something that always traveled faster than the speed of light.

There have been a few experiments to find tachyons using a detector called a cerenkov detector. This detector is able to measure the speed of a particle traveling through a medium. Photons travel at a slower speed inside a medium. If a particle travels though a medium at a speed that is greater than light for that medium cerenkov radiation occurs. This is analogous to the sonic boom produced when an airplane travels faster than the speed of sound in air or the shock wave at the bow of a ship.

If tachyons existed you would be able to see cerenkov radiation in a vacuum. A few cerenkov experiments were conducted in a vacuum and no radiation was found, so it is generally accepted that tachyons do no exist.

I hope this helped you.

Sincerely,

Christina L. Hebert Graduate Student at Fermilab

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- 04/28/2014
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