Puzzling thing about Speed of Light/Speed of Causality

[polymath257]

What happens when you 'use the stick to turn off the sun'? You push on it. That produces a compression wave that travels down the stick at the speed of sound in that stick. This is much, much slower than the speed of light. So, in fact, it would take much longer than 8 minutes for the compression wave from your push to get to the sun. Once it does, and the sun goes out, the effects of that move outward at the speed of light.

The point is that in relativity, there are no 'solids' that instantaneously transmit information from one end to the other. Such transmission happens as a compression wave along the solid which travels at much less than the speed of light.

Wow. Thanks for the info. But still it seems paradoxical to me for this reason: Let's say that the side of the stick that is near the switch is only 3cm away from it, but on earth, when I thrust the stick I thrust it 10 cm forward. If it takes it so long for my thrust to "register" on the other side of the stick, will I not feel the resistance of the other end striking the switch on my end?

I suppose, for sake of this second example, we could replace the lightswitch with a brick wall. Obviously, I can't push the stick with enough force to push through a brick wall. But how does that work if the stick is only 3 cm away from the wall and I thrust the stick 10 cm?

The signal travels down the stick at the speed of sound in that stick: the speed of a compression wave. This is fast enough that for 3cm, we don't notice the difference. But if the stick was a few kilometers long, we would.

So, in your scenario, you push on the stick with a certain force. This accelerates the near end of the stick, starting a compression wave in that stick that travels down the length. Typically, for 'short' sticks in your room, the speed of that compression wave is fast enough  that it looks like the whole stick is moving as one piece, but a slow motion camera will show otherwise. When the far end of the stick hits the wall, there is a reverse wave that is created, that also travels down the stick (in the opposite direction), eventually reaching your hand, producing a force on it that you notice.

There are signals in both directions.

The amount of compression for a given force is determined by the type of material in the stick, which also determines the speed of sound in that stick.

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You might want to look at this:

https://en.wikipedia.org/wiki/Young%27s_modulus