Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Possible Duplicate:
Is it possible for information to be transmitted faster than light?

I've thought about this since I was a little kid. I know it isn't exactly feasible, but it still bothers me.

I hand you a really long wire, and we agree that "a long tug means 1, two short tugs mean 0", then you move off into the galaxy, a few light-years away from me. I proceed to give you information by tugging on the wire.

With a really tight wire, couldn't I talk to you faster than the speed of light?

share|improve this question
Even a "really tight" wire, is not "infinitely" tight, which is what you'd need. –  Lagerbaer Apr 10 '12 at 3:28
In my humble opinion, I do not agree with closing the question. For example, we all know that perpetuum mobile cannot be created because this breaks laws of thermodynamics. However, if one proposes perpetuum mobile mechanism, it is instructive to crack it down - this way we can all learn something about physics. Proposed larger than speed of light travel was based on principle on transverse wave speed and replying to this question requires explaining the difference between the speed of crests (theoretically arbitrary) and speed of information (limited). Regards. –  Pygmalion Apr 15 '12 at 13:27
add comment

marked as duplicate by Qmechanic, Colin K, Manishearth, David Z Apr 10 '12 at 4:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2 Answers

In theory you could not: information can't go faster than the speed of the light.

The movement along the wire would not happen at the same time along it. It would start from the side of the mover, and would propagate all around its source. At which speed? well .. it depends on the elasticity of the material the wire is made of.

A funny note is that when you film your moving wire and then replay the video at a very slow speed, you will notice that all types of matter appear to be somehow elastic.

share|improve this answer
add comment

I think the question was about transverse and not longitudinal waves. In case of transverse waves the wave velocity - speed of crests - goes with square root of tension/linear density ($v^2 = \frac{T}{\mu}$) and is therefore theoretically possible to be infinity for every material (large $T$, small $\mu$). However, velocity of information is still smaller than velocity of light.

In particular, imagine that you started wave at one end of the resting wire. The beginning of disturbance would surely go slower than velocity of speed, but speed of individual crests could go faster than that. It would seem like if crests keep bumping into the start of disturbance and disappear. Hard to imagine, but I've tried to explain.

share|improve this answer
Both T and lamda depend on the electromagnetic properties of the medium. When the density is small there can be no elasticity , no?. So there must be a limit to the formula, no wire, due to reality. –  anna v Apr 10 '12 at 4:32
You've missed my point. Imagine that we stuck with a specific material, let's say steel. Elasticity and voluminous density are fixed for steel, so the longitudinal wave speed is fixed. However, you can make steel wire thinner and put a larger tension to it, so you can actually change transverse wave speed quite easily. This is the principle of musical strings. –  Pygmalion Apr 10 '12 at 6:58
add comment

Not the answer you're looking for? Browse other questions tagged or ask your own question.