Tapping a message over lightyears in an instant?

[airbourne]

I have another question (I asked one a month ago, thanks to those who helped clarify it for it). I have not studied physics formally, I am just avidly interested.

Situation:

You have a very rigid rod, 100 light years long. You, and a friend, at the other end of the rod, 100 light years away, know morse code and can easily understand messages by being tapped into your leg. So you set the rod up so it will tap your leg if the other person pushes it forward a little and vice versa.

What prevents the message being delivered in real time (getting the message across 100 light years of space-time in an instant)?

I understand a huge amount of energy would be required to move the rod, let's forget that (unless it is somehow relevant to this hypothetical question) and imagine it's no harder to push than a matchstick.

 

[Phrak]

Atoms in the rod push on the next atom, and so on. This disturbance propagates at less than the speed of light. It's called the speed of sound in the medium. It's much less than the speed of light. So your idea of an instantaneous pushing idea doesn't hold up. Relativity still works. Sorry about that. There are no rigid rods; they don't exist.

 

[Al68]

I have another question (I asked one a month ago, thanks to those who helped clarify it for it). I have not studied physics formally, I am just avidly interested.

Situation:

You have a very rigid rod, 100 light years long. You, and a friend, at the other end of the rod, 100 light years away, know morse code and can easily understand messages by being tapped into your leg. So you set the rod up so it will tap your leg if the other person pushes it forward a little and vice versa.

What prevents the message being delivered in real time (getting the message across 100 light years of space-time in an instant)?

I understand a huge amount of energy would be required to move the rod, let's forget that (unless it is somehow relevant to this hypothetical question) and imagine it's no harder to push than a matchstick.

The "tap" will propagate through the rod at the speed of sound of the rod, which is slower than light.

 

[ghwellsjr]

See here:

https://www.physicsforums.com/showthread.php?t=386687

 

[airbourne]

Thanks, I had wondered if there was a simple law I wasn't thinking of, and thanks ghswllsjr for the link, I can gain a better understanding this this. Sorry for asking something that's been covered, I didn't find anything when I searched.

 

[LBrandt]

I don't understand the point of the rod idea anyway. Suppose the atoms in the rod DID travel at the speed of light? If you and your friend were 100 light years apart, it would still take 100 of your years to get the message transmitted. We can send messages to each other today at the speed of light, but that doesn't mean that the receipient gets the message instantaneously.

 

[John232]

Last time I checked you could move a rod that was only a foot long and the other end would respond faster than the speed of sound.

 

[LURCH]

Last time I checked you could move a rod that was only a foot long and the other end would respond faster than the speed of sound.

Not faster than the speed of sound through that rod. For an iron rod this will be the speed of sound through iron (About 14654mph), for a wooden rod it's the speed of sound through wood, etc.

Just ot of curiosity; how did you "check"?