Calamities of Nature Comic :: Tachyons

Greg Bernhardt

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MathematicalPhysicist

How can Tachyons not contradict SR?
?-)

cristo

They have imaginary mass. The speed of light is a speed 'barrier' in the sense that a particle travelling less than c cannot travel faster than c, but also a particle travelling faster than c (like a tachyon) cannot slow down to travel less than c.

MathematicalPhysicist

How can you measure imaginary mass?

tpiro

Cristo is exactly correct. SR doesn't allow a particle moving slower than the speed of light to subsequently be accelerated past c, but there can still be particles that always travel faster than light and cannot go below c.

As far as I know, most physicists are doubtful that tachyons exist, because of the causality problems that I refer to in the comic. But of course, nature is final judge of what is and isn't true, and I don't know of any experiments that conclusively rule out the existence of tachyons.

MathematicalPhysicist

No sweat I can grasp a lot of wacky ideas, but imaginary mass, I would like to know how would you measure it?

I don't think that SR deals with particles with imaginary mass, how would such particles interact with ordinary particles, and more perplexing why can't they be slowed down below c if they do exist, what stops them from slowing down?

I got to say this type of thinking is so fascinating in theoretical physics.
(and mind boggling).
:-)

D H

Since most physicists imagine that tachyons don't exist, most physicists don't bother dealing with this imaginary problem.

There's nothing wrong per se with something with non-zero mass going faster than the speed of light. What special relativity does say is that something with non-zero mass cannot go at the speed of light. Since the only way to get from speed v₀ to some other speed v₁ is to go through all of the speeds in between, this alone pretty much precludes slow-moving particles from exceeding the speed of light -- and it also pretty much precludes fast-moving particles from dropping below the speed of light.

There's a bigger problem, and that is energy. Adding energy to ordinary matter increases speed, but by an ever decreasing amount as v→c from below. Reducing energy of ordinary matter reduces speed, eventually reaching the rest state (the minimum energy state) of ordinary matter at v=0. Adding energy to a tachyon decreases speed, but by an ever decreasing amount as v→c from above. Reducing energy of tachyonic matter increases speed, eventually reaching the rest state (minimum energy) of tachyons at v=∞.


The reason most physicists don't think tachyons exist is that they violate causality, big time. If only I could send a tachyon message back to myself in August, 2004:

"Buy Google stock, you fool! Buy as much as you possibly can afford, and then buy some more!"

Borek

Welcome to PF, Tony

tpiro

Thanks! I hope to drop by from time to time to see what's going on here. But if people start asking me to do path integrals, I'm outta here! ;)