Faster than light travel implications

[Cobalt101]

A couple of questions around faster than light travel.
1. I don't understand why this implies traveling back in time (as is sometimes suggested). For example : A tachyon traveling at 150% c travels between Point A and Point B and back again. While it will arrive at Point B before it is observed from there leaving Point A, but not before it actually left. Similarly it arrives back at Point A some time after it left (not before). So no (backward) time travel is involved.

2. But what would an observer looking at the tachyon's clock see ? I understand that at c (e.g. for a photon) time will appear to have stopped from the point of view of the observer), but at faster than c the equations as I understand it mean that time doesn't become negative, but rather becomes imaginary (due to the √ impact). What meaning can be applied to this ?

 

[mfb]

1. I don't understand why this implies traveling back in time (as is sometimes suggested).

It allows going back in time if superluminal signals are possible in every reference frame (=if the fundamental principle of the same laws of physics in every inertial frame is still valid). Then there are observers where your 150% c tachyon is going backwards in time.

 

[phinds]

I understand that at c (e.g. for a photon) time will appear to have stopped from the point of view of the observer),

It is unfortunate that you "understand" this, since it is wrong. It is a common misconception that it is meaningful to talk about a photons point of view or what the passage of time is for a photon. It is not. There is a FAQ on this somewhere on the forum.

 

[DaveC426913]

faster than light travel ... implies traveling back in time.

This is a common misconception. True, when talking about the hypothetical tachyon particle, it seems that it would move backward in time,.

Here's an article that explains the paradox.
https://en.wikipedia.org/wiki/Tachyonic_antitelephone

 

[PeterDonis]

It is a common misconception that it is meaningful to talk about a photons point of view or what the passage of time is for a photon. It is not. There is a FAQ on this somewhere on the forum.

The FAQ is here:

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

 

[DaveC426913]

I think correcting the OP on the rest frame of a photon does not address the larger question here about tachyons.

 

[PeterDonis]

what would an observer looking at the tachyon's clock see ?

A tachyon can't even have a "clock" in the usual sense; it is following a spacelike worldline, not a timelike one, so the concept of "proper time" does not make sense for it.

at faster than c the equations as I understand it mean that time doesn't become negative, but rather becomes imaginary

No, it doesn't mean either of those things. You can't change a timelike object to a spacelike object via a Lorentz transformation; you can't "switch frames" from one movlng slower than c to one moving faster than c. Any theory that allows tachyons can't work that way.

Basically, it looks to me like you need to take some time and think through more carefully how a theory that allowed tachyons would have to work. As the above comments show, you can't just think of it as working like ordinary SR but with velocities faster than c. It's more complicated than that.

You might want to read this Usenet Physics FAQ article for some discussion of the complexities involved:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html

 

[PeterDonis]

A tachyon traveling at 150% c travels between Point A and Point B and back again.

The question is, 150% c according to what frame? If it's the same frame both ways, yes, the tachyon won't arrive at Point A before it left.

But, as mfb pointed out in post #2, assuming it's the same frame both ways violates the principle of relativity. A tachyon that obeyed the principle of relativity would have to travel at 150% c, or whatever the "standard tachyon speed" is, according to the frame of whatever material object it last interacted with. So going from Point A to Point B, it would travel at 150% c relative to whatever emitted it at Point A, and going from Point B back to Point A, it would travel at 150% c relative to whatever reflected it at Point B. If the emitter and reflector are in relative motion, it's possible for this kind of scenario to result in the tachyon arriving back at Point A before it actually left.

 

[Nugatory]

I don't understand why this implies traveling back in time (as is sometimes suggested).

Google for "Tachyonic anti-telephone" to see what is involved in getting backwards time travel (actually, backwards causality, where an effect happens on a timelike worldine before its cause) out of faster than light travel.

 

[Cobalt101]

The FAQ is here:

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

thanks

 

[Cobalt101]

A tachyon can't even have a "clock" in the usual sense; it is following a spacelike worldline, not a timelike one, so the concept of "proper time" does not make sense for it.
No, it doesn't mean either of those things. You can't change a timelike object to a spacelike object via a Lorentz transformation; you can't "switch frames" from one movlng slower than c to one moving faster than c. Any theory that allows tachyons can't work that way.

Basically, it looks to me like you need to take some time and think through more carefully how a theory that allowed tachyons would have to work. As the above comments show, you can't just think of it as working like ordinary SR but with velocities faster than c. It's more complicated than that.

You might want to read this Usenet Physics FAQ article for some discussion of the complexities involved:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html

thanks for the link

 

[DaveC426913]

Google for "Tachyonic anti-telephone"

...or just click the link in post 4...

 

[Nugatory]

...or just click the link in post 4...

ooops - sorry about that - somehow missed it on my first pass through.

 

[Fuinne]

This is a common misconception. True, when talking about the hypothetical tachyon particle, it seems that it would move backward in time,.

Here's an article that explains the paradox.
https://en.wikipedia.org/wiki/Tachyonic_antitelephone

In uttermost respect, the idea of a particle being able to arrive at a destination before leaving seems rather pseudoscientific. I don't see how that can happen?

 

[Nugatory]

In uttermost respect, the idea of a particle being able to arrive at a destination before leaving seems rather pseudoscientific. I don't see how that can happen?

Right, and since faster than light travel implies that it would happen, we conclude that faster than light travel is also impossible.

 

[Jonathan Scott]

It is only if signals can travel a finite amount faster than light in every frame of reference that causality problems arise.

If it were hypothetically possible to send signals which were faster than light (or even instantaneous, that is purely spacelike) relative only to some preferred frame of reference, then that would not violate causality, provided that the signal propagation transformed between frames of reference in the usual way. (This would make it barely faster than light in some frames and propagating backwards in time a little faster than light in others). This would of course be in violation of Special Relativity, but suggestions have been made that some mechanism like this (effectively a form of explicit non-locality) could explain entanglement in Quantum Mechanics.

The concepts of "Subspace", "Hyperspace" and so on in Science Fiction usually assume something like this.