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

From the Insights post:

"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?"
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.
 
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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.
 
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Evidently it is well nigh impossible for people to respond to this Insights article without going out of bounds. Therefore, this thread is closed.
 
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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!
 
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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?
 

Ibix

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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.
 

vanhees71

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Well, this signal propagates with the speed of sound in that medium, which is way smaller than the speed of light anyway.
 
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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?
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.
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 <link to irrelevant and closed thread>
 
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russ_watters

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

A.T.

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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...
 
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Nugatory

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

russ_watters

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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
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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russ_watters

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Thanks. No need. The term "slinky drop" clears up my thinking.
Since you asked earlier, I'm going to use you as an example of why I advocated for leaving this thread open. It isn't always easy to tell from the first post whether someone legitimately doesn't understand the mechanics behind what happens - the fact that a long metal rod behaves very similarly to a slack spring. But it is almost always clear from the second post, when either the person gets combative or in your case, the light bulb goes on. On this particular topic it takes an awful lot of effort to keep the noise down so we can help people like you are are trying to learn. So thanks for saying thanks.
 
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Don't remember, it was long ago, and the slinky example has cleared up my thinking. And even if the two ends did respond to letting go and gravity "simultaneously" I still don't think it would qualify as FTL.
 

Chronos

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Consider how would you test the propagation velocity in a 'perfectly rigid' material? You would need a signal device to tell you when the distant end of your hypothetical rod moved, was pushed, etc. The receiving end of your detector would need to acknowledge the signal before the near end of the rod moved. The only possible non null result would be If the near end of the rod moved before the signal registered, which would only prove the reaction time [hysteresis] of your signaling system was slower than the propagation speed of your 'perfectly rigid' material. This is reminiscent of Galileo's attempt to measure the speed of light using lanterns on distant hills. He concluded he was only measuring the reaction times of his students and the speed of light was very much faster [possibly infinite], which was entirely logical given the measurement precision possible in his test setup.
 

Meir Achuz

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How do you explain the Mossbauer effect where there is no sound wave?
 

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