B Could information travel faster than C

[victorhugo]

I've heard a few times that information can't travel faster than C, but what if you have an extremely dense metal rod that extends for 1 light year in length, the moment you move it here, wouldn't it instantaneously move at the other end 1 light year away? Or would the electrons that make it up cause it to take time?

 

[Samy_A]

I've heard a few times that information can't travel faster than C, but what if you have an extremely dense metal rod that extends for 1 light year in length, the moment you move it here, wouldn't it instantaneously move at the other end 1 light year away? Or would the electrons that make it up cause it to take time?

We have a great FAQ section on this forum.

https://www.physicsforums.com/insights/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod/

 

[DennisN]

wouldn't it instantaneously move at the other end 1 light year away?

No. See e.g. : http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4

EDIT: Samy managed to sneak in a reply before mine...

 

[victorhugo]

We have a great FAQ section on this forum.

https://www.physicsforums.com/insights/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod/

Awesome.

 

[pixel]

You have asked about a specific example using a rigid rod. But it can be shown in general that if a signal could travel faster than c, there would be frames of reference in which causality would be violated i.e. the effect of the signal would occur before the signal was generated.

 

[PFfan01]

According to a PRL letter, superluminal communications have been well demonstrated experimentally:

“We note that such a superluminal phenomenon ... should consist with the causality and special relativity. Our experiments show that the group velocity of the signal pulse does exceed c ... provides a new way of opening up superluminal communications via optical fibers."

L. Zhang, L. Zhan, K. Qian, J. Liu, Q. Shen, X. Hu, S. Luo, Superluminal propagation at negative group velocity in optical fibers based on Brillouin lasing oscillation, Phys. Rev. Lett. 107 (2011) 093903).

 

[Dale]

According to a PRL letter, superluminal communications have been well demonstrated experimentally:

“We note that such a superluminal phenomenon ... should consist with the causality and special relativity. Our experiments show that the group velocity of the signal pulse does exceed c ... provides a new way of opening up superluminal communications via optical fibers."

L. Zhang, L. Zhan, K. Qian, J. Liu, Q. Shen, X. Hu, S. Luo, Superluminal propagation at negative group velocity in optical fibers based on Brillouin lasing oscillation, Phys. Rev. Lett. 107 (2011) 093903).

First, this paper is not relevant to the OP who is asking about superluminal information transfer in a rigid rod.

Second, Zhang is infamous for performing rather mundane experiments and then grossly mis-representing the results of those experiments, usually with the clear goal to disprove something about relativity. He is a highly biased author and, to my knowledge, none of his outlandish claims have ever been reproduced. This paper demonstrates a clear misunderstanding of basic signal properties. Specifically, he reports the group velocity as -0.151 c, decidedly sub-luminal. The group velocity is the velocity that carries the signal pulse and is useful for information transfer. A superluminal phase velocity is well-known and also known to not carry information.

 

[vanhees71]

Well, well, well... It's a PRL ;-)) SCNR.

 

[Svein]

I've heard a few times that information can't travel faster than C, but what if you have an extremely dense metal rod that extends for 1 light year in length, the moment you move it here, wouldn't it instantaneously move at the other end 1 light year away? Or would the electrons that make it up cause it to take time?

You would set up a longitudinal wave in the rod. The speed of that wave front is given by v = √(λ/ρ), where λ is the stiffness constant in Hooke's law and ρ is the linear density of the rod.