How does movement through solid objects work?

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Discussion Overview

The discussion centers on the theoretical implications of movement through solid objects, particularly focusing on a hypothetical scenario involving a metal bar extending from Earth to a distant star. Participants explore concepts related to rigidity, the propagation of motion, and relativistic effects, while considering the limitations imposed by the laws of physics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether movement at one end of a rigid bar would be observed instantaneously at the other end or if it would be limited by the speed of light.
  • Another participant argues that an infinitely stiff bar cannot exist, as it would imply surpassing the speed of light.
  • It is noted that disturbances in the bar would propagate at the speed of sound in the material, which is significantly slower than the speed of light.
  • Some participants assert that Einstein's relativity excludes perfectly rigid materials and that no material can transmit motion at or faster than the speed of light.
  • Concerns are raised about the physical feasibility of a bar-shaped object of significant mass, with one participant suggesting that gravitational forces would deform it into a sphere.
  • There is a discussion about the effects of accelerating a long rod to relativistic speeds, with some participants questioning whether it would split apart due to length contraction and acceleration forces.
  • Several participants express confusion over the assumption that the other end of the bar would move instantaneously, emphasizing that even light does not travel instantaneously.
  • One participant highlights the importance of recognizing that a rod is made of atoms that move, which affects how forces propagate through the material.
  • There is a mention of a previous discussion involving the time it takes for forces to travel through large objects, such as a supertanker, to illustrate the concept of non-instantaneous movement.

Areas of Agreement / Disagreement

Participants express a range of views, with some agreeing on the impossibility of instantaneous movement through solid objects, while others challenge the assumptions made about rigidity and the behavior of materials under relativistic conditions. The discussion remains unresolved regarding the implications of these theoretical scenarios.

Contextual Notes

Limitations include the dependence on hypothetical scenarios that ignore physical realities, such as the structural integrity of materials and the effects of gravity on large masses. The discussion also reflects varying interpretations of relativistic effects on solid objects.

gatzke
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If there was a metal bar:
- with no flexibility
- one end is at earth, on a pivot
- the other end is at the nearest star and free to move​

If you moved the bar up and down at the star end, would you see the bar pivot here on Earth immediately or would it take the same time as the speed of light to see any movement?

Earth end (on a pivot)... Star end (moves up and down)^v
O-----------------------------------------------------------*

What if the bar was free to move on both ends and you shoved it towards earth; would the Earth end move immediately?

Earth end (moves left to right) <-----> Star end (moves left to right)
O-----------------------------------------------------------*
 
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If the bar was infinitley stiff then it would beat the speed of light
You can't beat the speed of light therefore you can't have an infinitely stiff bar
 
A longitudinal disturbance in the bar will propagate at the speed of sound in that material, which is much less than the speed of light.
 
To sum: nobody's and bc's posts:

1] Einstein's relativity excludes perfectly rigid materials. They cannot exist.

2] Even if it didn't, no material made of matter could transmit motion through itself at the speed of light, let alone faster (afterall, it's made of atoms).

3] In reality, a material made of matter will transmit motion far, far slower than the speed of light; it will transmit it at the speed of sound in that material. Diamond's speed of sound is a glacial 12km/s (1/10,000th of c).
 
In reality, no piece of mass even a tenth of the size of the Earth could ever be in the shape of a bar. It's own gravitational attraction on itself would deform it into a sphere, just like all the other massive objects in the universe.

But, we can ignore reality, just like we do in all our other thought experiments and talk about what happens when you accelerate one end of a bar-shaped object like a rocket. We know that if it attains a speed that is a significant fraction of the speed of light, the length of the rocket will become contracted. So if we had a very long rod and accelerated one end, wouldn't it split apart?
 
ghwellsjr said:
In reality, no piece of mass even a tenth of the size of the Earth could ever be in the shape of a bar. It's own gravitational attraction on itself would deform it into a sphere, just like all the other massive objects in the universe.

I am not convinced this is true.

Natural objects will collapse into a sphere because of entropic effects, and it take a long time. Artificial objects can be consructed as needed, and may only need to last as long as the experiment. Apples and oranges.


ghwellsjr said:
But, we can ignore reality, just like we do in all our other thought experiments and talk about what happens when you accelerate one end of a bar-shaped object like a rocket. We know that if it attains a speed that is a significant fraction of the speed of light, the length of the rocket will become contracted. So if we had a very long rod and accelerated one end, wouldn't it split apart?
No. Like all objects, the force will be translated along its length, it is pliable.
 
This "bar" thread has been used many times in this forum, and the thing that I can't understand is this. Even if the atoms in the bar COULD move at the speed of light through the bar, it would STILL require the same length of time to get the other end to move as it would if one just sent a light beam from one star to another. Why is there a supposition that it would do it instantaneously? Light itself doesn't move instantaneously, so why should the other end of the bar do so?
 
I'm just asking for someone who understands these things to explain what would happen if you tried to accelerate a very long rod to relativistic speeds and to ignore the phyiscal problems that we ignore in all our other explanations of various thought experiments. Can't this experiment be analyzed by just SR?
 
ghwellsjr said:
I'm just asking for someone who understands these things to explain what would happen if you tried to accelerate a very long rod to relativistic speeds and to ignore the phyiscal problems that we ignore in all our other explanations of various thought experiments. Can't this experiment be analyzed by just SR?
Nothing untoward would happen. If the rod were going to break, it would break because of excessive acceleration, not because of excessive velocity.

As the rod accelerated to relativistic velocities, in addition to being compressed by the accelerative force, it would indeed get contracted more at its faster (back) end" than its slower (front) end.

Since we are dealing with a real material made of atoms, you can simply look at how they behave in the material. Atoms push on other atoms. If the accelerative force is within the failure limit of the material, it holds together, if the accelerative force exceeds the materials' strength, it will crush.

But relativistic length contraction does not change this.
 
  • #10
LBrandt said:
This "bar" thread has been used many times in this forum, and the thing that I can't understand is this. Even if the atoms in the bar COULD move at the speed of light through the bar, it would STILL require the same length of time to get the other end to move as it would if one just sent a light beam from one star to another. Why is there a supposition that it would do it instantaneously?

When this experiment is brought forth, invariably the poster has not made the connection that a rigid rod is subject to the same laws that prevent spaceships from thrusting to > c. They assume a rigid rod is indeed perfectly rigid - move one end, the other moves instantly.

Once they see that a rod is made of plain old atoms that move, they immediately see how SR applies to them as much as to a rocket.
 
  • #11
DaveC426913 said:
When this experiment is brought forth, invariably the poster has not made the connection that a rigid rod is subject to the same laws that prevent spaceships from thrusting to > c. They assume a rigid rod is indeed perfectly rigid - move one end, the other moves instantly.

Once they see that a rod is made of plain old atoms that move, they immediately see how SR applies to them as much as to a rocket.

Dave, I agree with you. I can't understand why some posters assume that the rod would move instantaneously, since even light doesn't move instantaneously.
 
  • #12
LBrandt said:
Dave, I agree with you. I can't understand why some posters assume that the rod would move instantaneously, since even light doesn't move instantaneously.

People are used to seeing "speed of light" questions revolving around small, solid objects being moved through space, and issues about how to get a small solid object up to a certain speed with a rocket.

It is a leap in insight to realize that movement through a solid object is not instant. I recall as discussion where it took some time to convince one poster that a large wave hitting the bow of a 1000 foot supertanker does not move the tanker all at once, it takes time for the force to travel the length of the ship (the "shudder"). It's even harder to visualize when we think about something as small as a "rod", which we are used to thinking of as a perfectly rigid, immutable object.

Of course, once they get it, they do a face palm (well, except the aforementioned supertanker guy).
 

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