# Thread: Energy transfer faster than speed of light?

1. Hello, new member here.

I know the answer to both questions is no. Just want to know the why.

Assuming a suitable material was invented. A piston ten light minutes long sitting in space.
One end is pushed. Is the transfer of energy from one end to the other faster than light?

Also, a piston swung in an arc. If it was long enough and enough force was applied could the
end exceed the speed of light (like a laser appears to)?

2.

3. In the first case, the transfer will be slower than light, at the speed of sound for the material of the piston. And no, it is not possible to have a material for which the speed of sound is greater than than of light.

The same is true in the second case also. When you swing the piston at your end, it takes time fro the movement to travel the end, the piston will twist into a spiral. You eventually will reach the maximum stress it can handle and it will break apart. Even if you could avoid this, the amount of energy needed to move the end of the piston approaches infinity as the end of the piston approaches the speed of light. That means that the amount of energy you must exert at your end also approaches infinity, and that you will always be short of the energy needed to reach light speed at the end of the piston.

4. Hello Janus,
Thank you for your reply. I understand the second reason, but not the first. I am a bit confused as to where the speed of sound comes into play. For the sake of argument if the piston was steel and 6100 metres long and I push one end, are you saying there will be a delay of one second before the other end moves. (That the transfer of energy is limited to the speed of sound through a given material?) Sorry if I'm talking rubbish.

5. Originally Posted by befuddled
Hello Janus,
Thank you for your reply. I understand the second reason, but not the first. I am a bit confused as to where the speed of sound comes into play. For the sake of argument if the piston was steel and 6100 metres long and I push one end, are you saying there will be a delay of one second before the other end moves. (That the transfer of energy is limited to the speed of sound through a given material?) Sorry if I'm talking rubbish.
There is no perfectly rigid material that will move at one end when you push the other end. What happens is that pushing the rod causes a pressure wave to travel along the rod. The speed of this pressure wave is the speed of sound.

6. Thank you,

I finally get it.

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