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Rigid Poles and Faster Than Light Communication

We recently received an interesting question:

I have an interesting idea that could in theory break the famous law that states nothing can travel faster than the speed of light. More specifically, a method of communicating that could instantly travel across space. Suppose a really long pole similar to the space elevator made out of carbon nanotubes could be constructed in space. Suppose this pole was one light year long. Assuming that all this was possible and ignoring all the orbits and gravity from everything near it. Couldn’t you push and pull this pole to create coherent binary code that would be expressed instantly at the other end? I am very sceptical about this idea as I know nothing can break the light speed barrier. What am I missing here?

Thanks all, keep up the good work.
Paul   Manchester, UK

 Hi Paul,
Thanks for the question
Your proposal sounds plausible and I remember thinking along similar lines a while back. It just seems to make such good sense that pushing on one end of a pole instantly moves the other end, no matter what its length is.

Can you think of anything in your experience that would lead you to the opposite conclusion?
Unfortunately, common experience is often of little help when discussing such over-the-top scenarios.

As you may have guessed..pushing on one end of a pole (even a nanotube pole) would unfortunately not cause the opposite end to move in any way that would permit instantaneous or even incredibly fast communication.

The bottom line is that because the pole is so long and atoms cannot influence each other instantaneously, the push would necessarily propagate along its length at a sub-light speed that can’t be overcome.

In fact, the push would necessarily propagate at the speed that sound travels within the material the pole is made of. The speed is determined by the elasticity and density of the pole.

Sure, if you had an infinitely rigid pole (…ahem) you might have something but Relativity limits the potential rigidity of any object in such a way that the speed of sound could never exceed the speed of light.

3 comments to Rigid Poles and Faster Than Light Communication

  • Fergus Gallagher

    I am reminded of an old “paradox” where a pole-vaulter runs at near light speed into a garage. When the far end of the pole hits the back of the garage, the door will automatically close.

    In the runner’s rest frame, the garage will be relativistically contracted and it seems he will never get in on time. But in the garage’s rest-frame, the pole is really short and will fit in easily.

  • Mark

    This very question reminds me about the ‘troll physics’ meme. This problem popped up in the 2nd link.

    I really adore these comics. Sometimes they take you off guard.

    Someone even showed it to a prof about it.

    My favorite is of course winning the lottery by looping two black holes :p (last link)

    1. http://knowyourmeme.com/memes/troll-science-troll-physics

    2. http://knowyourmeme.com/photos/74360

    3. http://www.urlesque.com/2010/10/25/i-showed-troll-physics-comics-to-a-physics-professor/

    4. http://i.imgur.com/Hj0va.png

  • klox

    I used to enjoy solving those simple kinds of relativity problems in my undergrad physics classes. My professor used to frame it as ovens and relativistic pizzas.

    As far as faster than light communication is concerned, wikipedia has some nice summaries. I particularly like the idea of shining a laser across the sky. If you go from one horizon to the other, the laser is clearly moving faster than the speed of light. But you can’t communicate by moving that spot with someone. An observer would have to simultaneously be at both spots in order to get faster than light communications.

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