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

• Suppaman
In summary, 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.

#### Suppaman

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.

Suppaman said:
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.

Suppaman said:
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 !

berkeman said:
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.

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.

Suppaman
Suppaman said:
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?

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

Suppaman
berkeman said:
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.

Suppaman said:
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.

waves and change said:
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.

waves and change said:
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.

phinds said:
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 ?

Suppaman said:
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

waves and change said:
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)

russ_watters
phinds said:
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.

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

waves and change said:
"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

I realize the thread is locked, but wish to expand on the policy issue:
waves and change said:
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:

Specifically, the first bullet in the summary:
• We wish to discuss mainstream science. That means only topics that can be found in textbooks or that have been published in reputable journals.
So in short, if you can find it in a textbook or reputable journal, you can discuss it. If you are speculating beyond the published discussion with your own idea about an open issue, that is out of bounds.

Even more specifically, the issue in this thread gets asked about once a month and is thoroughly covered in a forum search and its Insight article without the need to open a new thread.

And back to broad, since what the OP proposed is slightly different: the problem with creating a hypothetical about a known broken law is that you have to define what laws to break and how. And when you're done, the hypothetical yields whatever answer you chose by your definitions, which tells you nothing of value. Such speculation is a waste of time.

weirdoguy, Asymptotic, Bystander and 2 others

## What does it mean for a material to be perfectly rigid?

A material is considered perfectly rigid if it is able to maintain its shape and size when subjected to external forces, without any deformation. This means that the material does not experience any change in its internal structure or volume.

## Why is it impossible for a material to be perfectly rigid?

According to the laws of physics, all materials have some degree of flexibility and can undergo deformation when subjected to external forces. This is due to the fact that all materials have some level of intermolecular interactions and can undergo changes in their atomic or molecular structure.

## Can any material be close to being perfectly rigid?

No, it is not possible for any material to be completely rigid. However, some materials can exhibit a high level of stiffness and resistance to deformation, making them appear to be almost perfectly rigid. This is often seen in materials such as diamond or carbon nanotubes.

## What are the consequences of a material being perfectly rigid?

If a material were to be perfectly rigid, it would be unable to bend, stretch, or compress. This would have significant implications in various engineering and construction applications, as it would limit the ability to design structures that can withstand different types of external forces.

## How does the concept of perfect rigidity relate to the field of materials science?

The concept of perfect rigidity is important in materials science as it helps to define the mechanical properties of different materials. By understanding the limitations of a material's rigidity, scientists and engineers can develop new materials with specific properties to meet the needs of various applications.