lightspeedrod

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

[Total: 7    Average: 4.4/5]

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. V
    Vanadium 50 says:

    The actual experiment [URL=’https://www.physicsforums.com/threads/question-about-instantaneous-travel-of-information-on-a-solid.696067/#post-4414855′]was done and the data shown[/URL]. This is linked in the thread. It shows that when you push a rigid rod, electrical impulses (which travel at about 2/3 of the speed of light) travel faster than the opposite end of the rod. Ignoring the experimental data when making your argument is un-scientific.

    In the past, discussions of this nature were not allowed on PF – how can you have a scientific discussion when one person refuses to consider experiment? It’s not like there is a complicated chain of inference here – it’s the actual experiment: push one end of the rod and see when the other end moves.

  2. A
    A.T. 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]
    No, it just your question. If the premise of a question is making any possible answer irrelevant, then it is indeed a pointless question.

  3. T
    thhog says:

    I do hope that hypotethical questions are allowed. So my hypotethical rod is made of some exotic stuff (a bunch of entangled particles or …?) and therefore perfectly rigid. Obviously you will be able to transmit a tap directly over any distance. Then the question will be; can such a rod be inserted/created in our universe? What specific laws of nature will be violated and in what way? Do we have to choose between the universe and the rod? As I understand it, the rod can't be described within the current set of geometrical/mathematical statements describing our universe. Does that mean that the description of such an object will make all current statements logically false and prohibit any logical matematica/geometrical consistent description of our universe?

  4. PeterDonis
    PeterDonis says:

    [QUOTE="thhog, post: 5628794, member: 609795"]I do hope that hypotethical questions are allowed.[/QUOTE]

    Not ones that violate the laws of physics. A "perfectly rigid" rod violates the laws of physics (and the Insights article explains why).

    [QUOTE="thhog, post: 5628794, member: 609795"]What specific laws of nature will be violated and in what way?[/QUOTE]

    The law that causal influences can't propagate faster than light. (QM entanglement does not violate this law, btw.)

  5. D
    Dale says:

    After some discussion, this thread is reopened. Please review the forum rules before posting. There have been 23 posts deleted, one full ban, and one thread ban. The forum rules apply here and they apply to you!

  6. B
    Battlemage! says:

    Question: wouldn't the assumption of a perfectly rigid rod automatically change the rules anyway? Since how can energy transfer through the rod if the atoms inside of it don't move back and forth? If the atoms vibrate doesn't that cause absurdly tiny changes in the size of the rod? (I wouldn't know: I am not sure how macroscopic shape arises from microscopic arrangement of atoms).

    Another question: Since all atoms are held together by electromagnetic forces, wouldn't the speed limit of a signal through the rod already be automatically capped by the vacuum speed of light?

    So I guess basically it seems that the only way for a rod to be perfectly rigid would be for it to be made out of some non-physical material, right? And if we're going that far why even keep a pretense of physics in the first place?

  7. Ibix
    Ibix says:

    Exactly. The matter in the rod has to be held together by some force, and they all propagate at or below the speed of light. So a "but what if…" question boils down to "what if magic happens?" In which case "the rod turns into a flock of unicorns" is an equally reasonable answer.

    I'm sure someone on this forum actually did this experiment a year or two back. He set up a metal bar with a couple of strain gauges along it then whacked one end with a hammer and showed that the other end didn't move for a couple of milliseconds. My search-fu is failing me, though.

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