lightspeedrod

Can I Send a Signal Faster than Light by Pushing a Rigid Rod?

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One common proposal for achieving faster than light communication is to use a long perfectly rigid object and mechanically send signals to the other end by pushing, pulling, or tapping it. For instance; a hypothetical rigid rod linking two people several lightyears away. The fundamental idea is that when one end is moved the other end is disturbed instantaneously.

However, there is no such thing as a perfectly rigid rod: a mechanical disturbance at one end of any material can only move through the material at a finite speed. This speed is called the speed of sound in that material.

High stiffness materials like metal have a very high speed of sound and low stiffness materials like jello or air have a very low speed of sound. When you push on something made of jello, you can easily see that the disturbance propagates at a finite speed. When you push on something like metal, it is not so easy to see visually, but the disturbance still propagates at the finite speed of sound in the metal. (see e.g. https://www.physicsforums.com/showthread.php?p=4414855#post4414855)

The speed of sound in diamond is about 12000 m/s which is about 25 thousand times slower than the speed of light (299792458 m/s). But what about some hypothetical “unobtainium”? Why couldn’t unobtainium’s speed of sound be faster than the speed of light? The answer is that all materials, even unobtainium, are held together by electromagnetic forces at the molecular level. When one molecule moves then the change in its electromagnetic field propagates to its neighboring molecule at the speed of light. So even in principle it is not possible for any material to have a speed of sound faster than the speed of light.

The following forum members have contributed to this FAQ:
DaleSpam
Ryan_m_b
DrGreg
tiny-tim
with additional review and discussion by several others

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20 replies
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  1. Edriven
    Edriven says:

    Could the speed of light be accelerated by a huge gravitational field?  For example, we know light doesn't escape a black hole and is strong enough to cause lensing, therefore could it accelerate light if it complimented the light's path?

  2. Rx7man
    Rx7man says:

    I think a good example for this would be to look at high speed photography of a golf ball being hit… As far as we can tell, the golf ball is solid, but high speed photography can show us it's really nothing more than jello if you hit it hard enough.

  3. Byron Brubaker
    Byron Brubaker says:

    I’m still waiting on the answer for that “hypothetical rigid rod.” This is how these ‘physics experts’ grind my gears; Q; “Hypothetically?” A; “Here’s a few examples of reality why we can’t hypothetically.”

  4. PeterDonis
    PeterDonis says:

    I’m still waiting on the answer for that “hypothetical rigid rod.”

    The “hypothetical rigid rod” is inconsistent with the laws of physics. There’s no point in asking what happens if the laws of physics are violated–at least not here on PF, where the only basis we have for giving an answer is the laws of physics.

  5. Byron Brubaker
    Byron Brubaker says:

    The “hypothetical rigid rod” is inconsistent with the laws of physics. There’s no point in asking what happens if the laws of physics are violated–at least not here on PF, where the only basis we have for giving an answer is the laws of physics.

    You validated my point, thank you. Nikola Tesla transmitted electrical power longitudinally through the Earth at a velocity of 291,000 miles per second (pi/2*C ‘pi over two times the velocity of light’) Velocity of light is simply an expression of the ratio of energy to mass which is a constant not a limit.

  6. PAllen
    PAllen says:

    You validated my point, thank you.

    What would you want? Once you posit something for which there is no evidence, and which is inconsistent with known physical law, there is no answer that could be given. Make up whatever you like. For example:

    Suppose I could move objects with my mind. What would the weight limit be?

    On what basis is such a question to be answered? Certainly not on the basis of science. Similarly, your hypothetical rigid rod is simply outside of science. Therefore, write any fiction you want.

  7. Byron Brubaker
    Byron Brubaker says:

    What would you want? Once you posit something for which there is no evidence, and which is inconsistent with known physical law, there is no answer that could be given. Make up whatever you like. For example:

    Suppose I could move objects with my mind. What would the weight limit be?

    On what basis is such a question to be answered? Certainly not on the basis of science. Similarly, your hypothetical rigid rod is simply outside of science. Therefore, write any fiction you want.

    I can move objects with my mind. The weight limit is dependent on the maximum weight you can lift after I tell you to “pick that up.”

  8. Michael Lazich
    Michael Lazich says:

    From the Insights post:

    [INDENT]”The speed of sound in diamond is about 12000 m/s which is about 25 thousand times slower than the speed of light (299792458 m/s). But what about some hypothetical “unobtainium”? Why couldn’t unobtainium’s speed of sound be faster than the speed of light?”
    [/INDENT]
    I suppose technically you can say that there *are* materials where the speed of sound, or at least the speed of matter, exceeds the speed of light in that material/medium; resulting of course in Cherenkov radiation. In a periodic material like a crystal you have the Smith-Purcell effect.

    Of course you’d never get the propagation of a signal through the medium to exceed the speed of light in a vacuum.

  9. Byron Brubaker
    Byron Brubaker says:

    The “hypothetical rigid rod” is inconsistent with the laws of physics. There’s no point in asking what happens if the laws of physics are violated–at least not here on PF, where the only basis we have for giving an answer is the laws of physics.

    If that is the case then ‘what-if’ questions should never be asked in the first place. The thing worse than a know-it-all is a know-almost.

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