# Relative speed of tachyons greater than c?

1. Aug 9, 2007

### menager31

"Spaceship A sends to spaceship B ( distance between A and B is known)
a tachyon, when B recives signal, it sends back it immediately. B
flies away from A with velocity v, and the tachyon moves away with the
relative velocity V from the spaceship tachyon was sent. What is the
duration between sending and receiving tachyon by spaceship A on his
own clock? V>c, v
I've got problem - how relative velocity can be greater than c?

thanks

2. Aug 9, 2007

3. Aug 9, 2007

4. Aug 9, 2007

### malawi_glenn

tachyons is as much as I know, a hypothetical particle that is supposed to travel faster than light. I do not understand quite really what the question is..

5. Aug 9, 2007

### menager31

we must count the time
but I think that the time is zero

6. Aug 9, 2007

### pervect

Staff Emeritus
I'm pretty sure that the point of the question is intended to demonstrate that the tachyon has to be received before it is emitted.

But as written the question is a bit ambiguous, to make the question unambiguous, some frame specifications have to be made.

The assumptions I made were that the tachyon had some V>c as measured in the frame of the emitter. To make things simple, I made V infinite, and drew a space-time diagram. The emitted signal from spaceship A then was along a "line of simultaneity", a horizontal line. The return signal from spaceship B was than along a line of simultaneity of the emitting spaceship, i.e. of spaceship B.

Drawing a space-time diagram of this situation, we can see in this case that the tachyon must be received before it was emitted.

7. Aug 9, 2007

### menager31

I'm not very advanced. But how to count the time? And why tachyon must be received before it was emitted ? We do not know his history!

8. Aug 10, 2007

### EnumaElish

Just as usual, e.g. in accordance with The train-and-platform thought experiment.
That's because its history is in the future.

Caveat emptor: I am not a physicist.

Last edited: Aug 10, 2007
9. Oct 13, 2007

### einstein_fan

Is this question taken from Polish Physics Olympiad? Because I can find there very similar one :).
There is a hint that you should use laws of adding velocities just like it was a normal less-than-c speed.

10. Oct 13, 2007

### HallsofIvy

Do you understand what a tachyon is? Of course, the velocity of a tachyon, relative to a spaceship (a tardyon) must be greater than c!

11. Oct 13, 2007

### einstein_fan

Firstly, I perfectly understand what does the tachyon mean. I won't put the solution of this problem here till Tuesday, because till Monday we have to send the solutions of problems to the organisers of Polish Physics Olympiad.

12. Oct 13, 2007

### HallsofIvy

Good, I'm glad you know about what "tachyon" means. I guess my question now is, why in the world did you ask "how relative velocity can be greater than c?" That's pretty much the DEFINITION of tachyon isn't it?

13. Oct 13, 2007

### einstein_fan

As I see we have a kind of misunderstanding - I didn't ask this question...