# No material, real or in theory can be perfectly rigid

## Main Question or Discussion Point

This is a thought experiment, and I am making it as simple as possible. It all takes place out in space. It is necessary to create something that is like a common nail with a large head. It is made of the most ridge material possible, something that is held together by the most potent force between an atomic component. The nail is moving through space and goes through a hole in some massive block of something, no friction. When the head of the nail contacts the surface of the block it will stop. However, the end of the nail must continue to move as the information that it is stopped is limited by light speed. So the nail must stretch. This seems to imply that no perfectly ridge material can be constructed. It would be interesting to know how slow the stretch would move through the nail as that would make the need for additional displacement. Any material parameters or dimensions or motion could be set in a computer simulation of this thought experiment. I think.

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Bandersnatch
It would be interesting to know how slow the stretch would move through the nail as that would make the need for additional displacement.
It moves at the speed of sound in the material.

berkeman
Mentor
This seems to imply that no perfectly ridge material can be constructed.
Yep. Are you familiar with how to calculate the speed of sound in a material? Why would that pertain to your post?

EDIT -- Dang you @Bandersnatch !!

Yep. So what is the point of the rest of your post? Are you familiar with how to calculate the speed of sound in a material? Why would that pertain to your post?

EDIT -- Dang you @Bandersnatch !!
But a perfectly ridge material would not transmit sound, right? If you filp the idea around and stop the far end the material would have to compress but perfectly ridge it would not compress. This would limit the maximum value of how ridge a material could be.

Bandersnatch
A perfectly rigid material would transmit sound - i.e., interactions between its constituent molecules, atoms, etc. - instantaneously. Since every interaction must take finite time, and its speed is limited by the speed of light, there can be no perfectly rigid material.

berkeman
Mentor
But a perfectly ridge material would not transmit sound, right?
We do not discuss questions of the type "This is impossible, but if it happened, what would happen next?" Time to close the thread?

Where do you go to discuss "impossible" ideas? So many elements in our technology were once thought to be impossible but were not. We do not know how "spooky action at a distance" works but I assume it is discussed in these forums, or not?

berkeman
Mentor
I'll send you a PM tomorrow with some alternative forums that allow personal speculation...

We do not discuss questions of the type "This is impossible, but if it happened, what would happen next?" Time to close the thread?
Time to close the thread because someone asked a hypothetical? Because someone uses the word impossible in that context why would you close a thread? It seems like a perfectly legitimate question.

Where do you go to discuss "impossible" ideas? So many elements in our technology were once thought to be impossible but were not. We do not know how "spooky action at a distance" works but I assume it is discussed in these forums, or not?
Theres more questions than answers in our current physics understanding and to close a legitimate thread due to asking a legitimate hypothetical doesn’t make sense.

phinds
Gold Member
2019 Award
Time to close the thread because someone asked a hypothetical? Because someone uses the word impossible in that context why would you close a thread? It seems like a perfectly legitimate question.
Theres more questions than answers in our current physics understanding and to close a legitimate thread due to asking a legitimate hypothetical doesn’t make sense.
You misunderstand the raison d'etre of PF and perhaps have not actually read the rules that you agreed to when you signed up.

You misunderstand the raison d'etre of PF and perhaps have not actually read the rules that you agreed to when you signed up.
With regards to hypothetical questions? Be specific because I have no idea how that statement applies ?

This is a thought experiment, and I am making it as simple as possible. It all takes place out in space. It is necessary to create something that is like a common nail with a large head. It is made of the most ridge material possible, something that is held together by the most potent force between an atomic component. The nail is moving through space and goes through a hole in some massive block of something, no friction. When the head of the nail contacts the surface of the block it will stop. However, the end of the nail must continue to move as the information that it is stopped is limited by light speed. So the nail must stretch. This seems to imply that no perfectly ridge material can be constructed. It would be interesting to know how slow the stretch would move through the nail as that would make the need for additional displacement. Any material parameters or dimensions or motion could be set in a computer simulation of this thought experiment. I think.
So in essence the idea of a rigid body is only an approximation used for theory. Nothing to my knowledge is strictly considered rigid in the sense you are describing. QM describes this and I advise you research “phonons” which may give you some insight into your question.

Thanks

phinds
Gold Member
2019 Award
With regards to hypothetical questions? Be specific because I have no idea how that statement applies ?
Then perhaps you should read the rules more carefully. Speculation is not allowed. This forum is for discussion of established science. You CAN get away with hypotheticals if they do not go against currently understood science. Positing a perfectly rigid substance goes against currently understood science (that, in this case, is VERY unlikely to ever change)

Then perhaps you should read the rules more carefully. Speculation is not allowed. This forum is for discussion of established science. You CAN get away with hypotheticals if they do not go against currently understood science. Positing a perfectly rigid substance goes against currently understood science (that, in this case, is VERY unlikely to ever change)
Established science according to who ? You? Current science has conflicting theories from top to bottom to describe phenomena. QM is not currently understood to some degree. Can we discuss QM? How about string theory? That’s currently understood as well....? OK, got it.

Thanks for the speculation regarding rigid bodies as the last sentence of your post as well.

I'll send you a PM tomorrow with some alternative forums that allow personal speculation...
Looking forward to a place to talk. 73, Bill

berkeman
Mentor
"Perfectly rigid body" is the problem with this hypothetical. If it had been about an "extremely rigid body" that still had a finite modulus, that would be different.

@Suppaman -- will send you that PM in a couple of minutes. 73

russ_watters
Mentor
I realize the thread is locked, but wish to expand on the policy issue:
Established science according to who ? You? Current science has conflicting theories from top to bottom to describe phenomena. QM is not currently understood to some degree. Can we discuss QM? How about string theory? That’s currently understood as well....?
Again, as @phinds said, please read the forum rules. They can be found here: