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. R
    RockyMarciano says:

    [QUOTE="Battlemage!, post: 5629734, member: 135434"]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).

    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?[/QUOTE]

    [QUOTE="Ibix, post: 5629839, member: 365269"]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.

    [/QUOTE]

    All this is clear enough and I don't know what the controversy could be. Everyone agrees there are no rigid rods.

    Perhaps people gets confused because there are some contradictory messages by regulars that claim that this conclusion that is agreed here as the only that makes sense is not correct. I found some such postings in a search of the last year only. Disregarding the context of the discussion wich I haven't read completely this is an example: https://www.physicsforums.com/threads/why-arent-standard-rods-and-clocks-affected-by-lc-and-td.854826/page-2 posts 24# and 39# where it seems a ban was given for claiming what is claimed in this insights article.

  2. russ_watters
    russ_watters says:

    [QUOTE="RockyMarciano, post: 5632345, member: 585697"]All this is clear enough and I don't know what the controversy could be. Everyone agrees there are no rigid rods.[/QUOTE]

    The problems generally occur when someone insists on applying the impossible assumption of an infinitely rigid rod and won't let it go.  You can't give a meaningful answer to a question based on a nonsensical premise and even if you just dropped all connection to facts and logic and answered "yes", it still isn't a useful or meaningful answer and is beyond the scope of the forum anyway.  It's fantasy pretending to be science.

    At the risk of drawing the ire of the other moderators, I'm going to quote a post of mine that was deleted due to it being a response to just such a member:

    "Many people who ask the question don't know 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."

    And often people who refuse to drop the assumption end up later proving to be first-order crackpots.

  3. Ibix
    Ibix says:

    [QUOTE="RockyMarciano, post: 5632345, member: 585697"]All this is clear enough and I don't know what the controversy could be. Everyone agrees there are no rigid rods.

    Perhaps people gets confused because there are some contradictory messages by regulars that claim that this conclusion that is agreed here as the only that makes sense is not correct. I found some such postings in a search of the last year only. Disregarding the context of the discussion wich I haven't read completely this is an example: https://www.physicsforums.com/threads/why-arent-standard-rods-and-clocks-affected-by-lc-and-td.854826/page-2 posts 24# and 39# where it seems a ban was given for claiming what is claimed in this insights article.[/QUOTE]I think it's worth reading the context before taking any statement at face value. Especially from that thread. It's a very confusing thread.

    I think that Peter was pointing out that it is perfectly possible to have objects that behave rigidly by Born's definition under steady acceleration. That's enough for Einstein's purposes (building a grid of unaccelerated rods to use as a coordinate system), but isn't enough to transmit faster than light. That would require rigidity under varying acceleration as well.

  4. C
    Chris Miller says:

    Probably nothing in which a compression wave is faster than light. But I read somewhere that the ends of long rod pointing downward will move simultaneously when the rod is released.

    EDIT

    Although, a diamond is pretty fluffy compared to the primordial universe (quark-gluon plasma) or even a neutron star. Wonder what the speed of sound in those would be…

  5. Nugatory
    Nugatory says:

    [QUOTE="Chris Miller, post: 5633007, member: 608318"]Probably nothing in which a compression wave is faster than light. But I read somewhere that the ends of long rod pointing downward will move simultaneously when the rod is released.

    [/QUOTE]

    Will move simultaneously in which frame?  If the rod is being held at the top, what exactly does "when the rod is released" mean for the bottom end?

    But even setting aside the relativistic problems with stating exactly what is happening….  Check some of the youtube videos you'll find under the topic "slinky drop".

  6. C
    Chris Miller says:

    [QUOTE="Nugatory, post: 5633021, member: 382138"]

    But even setting aside the relativistic problems with stating exactly what is happening….  Check some of the youtube videos you'll find under the topic "slinky drop".[/QUOTE]

    Thanks. No need. The term "slinky drop" clears up my thinking.

  7. D
    Dale says:

    [QUOTE="Ibix, post: 5629839, member: 365269"]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.[/QUOTE]This was bobc2 here:

    https://www.physicsforums.com/showthread.php?p=4414855#post4414855

    I have a link to that post in the body of the insights article.

  8. russ_watters
    russ_watters says:

    [QUOTE="A.T., post: 5632925, member: 85613"]Well said. I wish the other Russ Watters would have the same insight:

    https://www.physicsforums.com/threads/relativistic-tips-of-a-propeller.861028/#post-5403578[/QUOTE]

    You and I really weren't very far apart there, A.T.  I think we both recognize that making appropriate impossible assumptions is a critical component of problem (even thought problem) solving, we just disagreed on where to draw the line on what assumptions were acceptable and what aren't.  Your position (and you are welcome to it) was that any non-physical assumption falling under the header "Special Relativity" should be unacceptable in a problem where the answer depends on SR.  My position is that SR is a broad theory, with lots of components that can be addressed separately.  It's a judgement call and we'll just have to agree to disagree.

    [edit] Also, one component of dealing with assumptions is recognizing if they are relevant or even necessary and constructing thought experiments in such a way as to avoid tripping over them.  Most of these "what if…" thought experiments have multiple points of failure and you can construct the helicopter one in such a way as to avoid the use of the "infinitely rigid" assumption.  That is part of the reason I put it on the other side of that line.

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