Classically communicate information faster then light?

[Philip Land]

Where in this though-experiment do I get it wrong?

Even though no mass can travel faster then c, maybe information can? And I'm not talking about quantum entanglement etc.

Consider a pipe, filled with balls that are very tightly arranged. If I push the outermost ball on one side of the pipe, the ball on the outermost other side of the pipe will fall out, outside the pipe.

Now let's mak this pipe longer, and disregard friction.

This mean, that if I want to move the ball on the other end of the pipe. I can simply push the ball on the side of the pipe close to me.

So my push has now been propagated throughout the balls in the pipe. Like a longitudinell wave if you like.

Now, If we make this pipe $6*10^8$ m It will take light 2 s to travel through the pipe, but the ball in the end of the pipe gets moved instantaneously as I push the ball in the start of the pipe?

Is this correctly thought or where does relativity prevent this from happening? Because we actually don't have quick velocities here, just propagation (I know that sounded strange).

Or is this a cheat thought-experiment because the pipe with the balls inside acts like an extension of my pushing finger?

 

[jbriggs444]

https://www.physicsforums.com/insights/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod/

 

[Philip Land]

https://www.physicsforums.com/insights/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod/

Thanks! Didn't see that :)

 

[A.T.]

So my push has now been propagated throughout the balls in the pipe. Like a longitudinal wave if you like.

... the end of the pipe gets moved instantaneously as I push the ball in the start of the pipe?

You kind of answered your own question, before you asked it.

 

[Dale]

Like a longitudinell wave if you like.

And what do we call the speed of longitudinal pressure waves through a material?