How does movement through solid objects work?

[gatzke]

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-----------------------------------------------------------*

 

[NobodySpecial]

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

 

[bcrowell]

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.

 

[DaveC426913]

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).

 

[ghwellsjr]

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?

 

[DaveC426913]

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.

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.

 

[LBrandt]

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?

 

[ghwellsjr]

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?

 

[DaveC426913]

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.

 

[DaveC426913]

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.

 

[LBrandt]

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.

 

[DaveC426913]

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).