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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52 replies
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  1. russ_watters
    russ_watters says:

    [QUOTE=”Byron Brubaker, post: 5239707, member: 562156″]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.[/QUOTE]
    I think you are missing the point. Many people who ask the question [B]don’t know[/B] that the question itself contains an error. So answering the question as-is (yes: a hypothetical perfectly rigid rod could be used to send a signal FTL) might inadvertently confirm their erroneous understanding of how reality works. That’s why one should always correct the question before answering it.

  2. PeterDonis
    PeterDonis says:

    [QUOTE=”Byron Brubaker, post: 5239707, member: 562156″]If that is the case then ‘what-if’ questions should never be asked in the first place.[/QUOTE]

    There are many what-if questions that don’t hypothesize something that violates the laws of physics. Those can certainly be asked here.

  3. B
    Byron Brubaker says:

    “Hi Byron Brubaker,

    In regards to your thread/post: [URL=’https://www.physicsforums.com/posts/5239472/’]Can I Send a Signal Faster than Light by Pushing a Rigid Rod? – Comments[/URL]

    While reviewing your post we’ve found you are stubbornly promoting content that is false or misguided.

    Please view our [URL=’https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/’][B]forum rules[/B][/URL] guidelines for more information.

    Thanks for your understanding and participation at Physics Forums!

    Orodruin”

    Would you mind being specific in your claims? The thread does not show what I am being chastised for nor does your claim hold any validity. Show me proof that I “stubbornly promoted content that is false or misguided.” If you think you have absolute proof please, by all means back up what you say. My email is [EMAIL]mx6maximus@gmail.com[/EMAIL]

  4. B
    Byron Brubaker says:

    [QUOTE=”russ_watters, post: 5239722, member: 142″]I think you are missing the point. Many people who ask the question [B]don’t know[/B] that the question itself contains an error. So answering the question as-is (yes: a hypothetical perfectly rigid rod could be used to send a signal FTL) might inadvertently confirm their erroneous understanding of how reality works. That’s why one should always correct the question before answering it.[/QUOTE]
    I can deal with that statement. When a person points it out all h*ll breaks loose. I’d like to see the math from anyone proving the improbability of the hypothetical question rather than just being told ‘well, it’s impossible.’

  5. russ_watters
    russ_watters says:

    [QUOTE=”Byron Brubaker, post: 5239730, member: 562156″]I can deal with that statement. When a person points it out all h*ll breaks loose. I’d like to see the math from anyone proving the improbability of the hypothetical question rather than just being told ‘well, it’s impossible.'[/QUOTE]
    What math, exactly, would you like to see? Did you read the insight article and the thread linked in it? The math for calculating the speed of sound in a material is shown in the thread.

  6. B
    Byron Brubaker says:

    [misinformation deleted by mod]
    [QUOTE=”russ_watters, post: 5239736, member: 142″]What math, exactly, would you like to see? Did you read the insight article and the thread linked in it? The math for calculating the speed of sound in a material is shown in the thread.[/QUOTE]
    The question was dealing with ‘faster-than-light’ now we’re on ‘speed of sound?’ I’ll rebut with my own question to get away from constants and ‘not violate the almighty thread rules’; What’s the limit to/of “SPEED?”

  7. russ_watters
    russ_watters says:

    [QUOTE=”Byron Brubaker, post: 5239742, member: 562156″]The question was dealing with ‘faster-than-light’ now we’re on ‘speed of sound?'[/QUOTE]
    Yes, that’s still the wrong/a poor question. The right question is: what is the maximum speed of a signal propagating through an object like a rod? And the answer is: the speed of sound.
    [quote] What’s the limit to/of “SPEED?”[/quote]
    The maximum speed of an object with mass is just under (asymptotically) the speed of light. Or, if one prefers, the speed of light is the speed limit of the universe.

  8. J
    jbriggs444 says:

    [QUOTE=”Byron Brubaker, post: 5239742, member: 562156″]Speed of light in space vacuum is a CONSTANT, NOT A LIMIT TO SPEED! Anyone have a problem with that statement?

    The question was dealing with ‘faster-than-light’ now we’re on ‘speed of sound?’ I’ll rebut with my own question to get away from constants and ‘not violate the almighty thread rules’; What’s the limit to/of “SPEED?”[/QUOTE]

    The speed, for instance of the intersection of a pair of scissors? Of the illuminated spot of a laser pointer being swept across the moon? Or of a signal originated over here, and being transmitted somewhere over there?

  9. M
    m4r35n357 says:

    [QUOTE=”Byron Brubaker, post: 5239742, member: 562156″]
    The question was dealing with ‘faster-than-light’ now we’re on ‘speed of sound?’ I’ll rebut with my own question to get away from constants and ‘not violate the almighty thread rules’; What’s the limit to/of “SPEED?”[/QUOTE]
    Rather than stamping your feet and insulting people who are trying to help you, have you considered doing some basic research yourself? Your question is not rocket science.

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