A thought experiment for surpassing the speed of light

[k4ff3]

You have a straight, light, hard, inelastic rod at hand. It's pretty long, long enough so that you can hold it out of your window and into your friends window at the other side of the street.

The way you and your friend communicate is through morse code. You move the rod a distance x so that it hits your friend's wall and makes a sound. *Bump--BumpBump-Bump* The morse code is easily heard and interpreted.

Extrapolate.

Make your rod reach from your house to another galaxy, and place your friend there. Continue communicating as usual. Will the information now travel faster than the speed of light?

 

[Argonil]

This is a known thought experiment, but I have never heard an answer for it.

My answer would simply be Yes. It doesn't actually travel there however.

 

[AlephZero]

No. The information will travel along the rod at the speed of sound in the material. For most solids the speed of sound between 1 and 10 km per second, which is much slower than the speed of light.

 

[QuantumPion]

You have a straight, light, hard, inelastic rod at hand. It's pretty long, long enough so that you can hold it out of your window and into your friends window at the other side of the street.

The way you and your friend communicate is through morse code. You move the rod a distance x so that it hits your friend's wall and makes a sound. *Bump--BumpBump-Bump* The morse code is easily heard and interpreted.

Extrapolate.

Make your rod reach from your house to another galaxy, and place your friend there. Continue communicating as usual. Will the information now travel faster than the speed of light?

No, because there is no such thing as a perfectly inelastic material. The force of pushing one end will propagate at the speed of sound through the material to the other end.

 

[Doc Al]

This thought experiment comes up about every two weeks. Here's a recent discussion: https://www.physicsforums.com/showthread.php?t=489667

 

[k4ff3]

Doc-Al: Sorry, I did not know that.

However, it seems like the you overlook the fact that the rod is inelastic. The same thing goes for the answers in the thread you linked to.

And even if the rod was elastic, I do not buy your answer that the information/displacement will propagate as a wave pulse with the speed of sound in the medium. I am displacing the whole rod, so that the displacement is much higher that the wavelength of the pulse induced.

 

[berkeman]

Doc-Al: Sorry, I did not know that.

However, it seems like the you overlook the fact that the rod is inelastic. The same thing goes for the answers in the thread you linked to.

And even if the rod was elastic, I do not buy your answer that the information/displacement will propagate as a wave pulse with the speed of sound in the medium. I am displacing the whole rod, so that the displacement is much higher that the wavelength of the pulse induced.

Nobody is overlooking anything. Your question has been answered.

 

[ZapperZ]

Doc-Al: Sorry, I did not know that.

However, it seems like the you overlook the fact that the rod is inelastic. The same thing goes for the answers in the thread you linked to.

And even if the rod was elastic, I do not buy your answer that the information/displacement will propagate as a wave pulse with the speed of sound in the medium. I am displacing the whole rod, so that the displacement is much higher that the wavelength of the pulse induced.

Er.. hello? What do you think holds the material that make up the rod?

If you've learned any amount of solid state physics, you'd know that these are linked by various types of BONDS, and they interact with the lattice ions via electromagnetic interactions! Light is a form of electromagnetic interaction! This means that anything done to the rod can't transmit something faster than what light can do!

Zz.