# Information Faster Than Light - Thought Experiment

Assuming it were possible to build a sturdy enough rod like structure, say equal the length it would take light to travel in two minutes.

If you were to push one side of said rod and move it, would the far side of the rod move before light were able to travel the length of the rod?

If so does this mean the energy used to move the rod traveled faster than light, if not then why not?

Staff Emeritus
This is a very common misunderstanding.

The molecules and atoms in the rod are connected together by electromagnetic forces, the same type of description that resulted in light or any EM radiation. So these interactions between atoms travel at the speed of light. When you hit one end, the vibrations gets transmitted to the next atom via such interactions. So in fact, it travels significantly SLOWER than the speed of light.

Zz.

1 person
So as someone on the other end of the rod would measure the light beam before the rod movement?

siddharth5129
A push is a compression wave. Travels at the speed of sound in that medium. At any rate, the forces between the atoms are fundamentally electromagnetic in nature, so a 'push' cannot be transmitted at a speed greater than that of light.

So as someone on the other end of the rod would measure the light beam before the rod movement?

yes. they would.

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Mentor
Assuming it were possible to build a sturdy enough rod like structure, say equal the length it would take light to travel in two minutes.

If you were to push one side of said rod and move it, would the far side of the rod move before light were able to travel the length of the rod?

If so does this mean the energy used to move the rod traveled faster than light, if not then why not?
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1 person
Gold Member
Yes, assuming the clocks are calibrated [which is an issue in itself]. The time between movement of the near and opposite ends of a rigid body is limited by the compression wave velocity [i.e., speed of sound] in the material from which it is constructed, and is much less than the speed of light for any known material. In theory, only a perfectly rigid body could react instantaneously - which relativity tells us cannot exist. For a more detailed explanation, see http://physics.stackexchange.com/questions/48392/extended-rigid-bodies-in-special-relativity.

1 person
Actually, you can sidestep clock synch here. Imagine you put a contact explosive on the end of a long diamond rod, with a mirror a little off center near the explosive end, aimed at a detector on the other end of the rod. You hit the explosive end of the rod with a hammer. Now the rod is pushed both by the hammer and the explosive. You ask which happens first at the other end: movement of the end or arrival of the reflected light from the explosive. The reflected light arrives first (even though it has a few extra inches to travel).

Experiments effectively equivalent to this, using ceramic rather than diamond, with rods as short as one meter, and modern ultra-high speed photography have been done. They show light wins the race easily.

cosmosmike
Interesting thought experiment, as the speed of light is slower in a medium, it is only c in free space. For example, the speed of light in a metal is zero. Seems like we could theoretically create a material where the speed of light is very slow. You might think about how that would be done.

Mentor
Interesting thought experiment, as the speed of light is slower in a medium, it is only c in free space. For example, the speed of light in a metal is zero.
Depends on the wavelength of the light in question...

Seems like we could theoretically create a material where the speed of light is very slow. You might think about how that would be done.
Of course there is nothing wrong with traveling faster than the speed of light in a medium - the universal speed limit is the speed of light in vacuum. You might try googling for "Cherenkov radation".

Interesting thought experiment, as the speed of light is slower in a medium, it is only c in free space. For example, the speed of light in a metal is zero. Seems like we could theoretically create a material where the speed of light is very slow. You might think about how that would be done.

My proposed experiment did not have the light traveling through the material. That's what the off center mirror was all about - to reflect light from the explosion to the other end of the rod directly through vacuum.