Is the Speed of Sound the Limit of Cause and Effect in a Medium?

[1MileCrash]

Lately I've been thinking of c as the maximum speed of cause and effect in the universe, and the speed of sound as the maximum speed of cause and effect within/for that specific medium.

Is it correct to say that if i had an iron bar as a lever, and i threw on end down, the other end would not travel up immediatelly, that there would be a lag between the two actions, the speed of sound through iron?

 

[bp_psy]

You can send signals that travel faster then sound in pretty much any medium. So no the speed of sound in some medium does not set any causality limitations in that medium.

 

[K^2]

If you only consider mechanical interactions, then yes, you can consider speed of sound as the limiting speed for cause-effect propagation. Though something like piezoelectric effect might throw a bar into that logic.

 

[1MileCrash]

You can send signals that travel faster then sound in pretty much any medium. So no the speed of sound in some medium does not set any causality limitations in that medium.

Signals that differ from the medium itself though?

Could i send a movement through a medium and it reach its destination faster than the speed of sound through that medium? If i swing a wooden bat, the part closest to my hands must start moving before the end of the bat, or else we have information traveling faster than the speed of light. So what speed does the "information" of me swinging the bat pass through the bat itself? I presumed it to be the speed of sound through the bat.

 

[russ_watters]

We have supersonic bullets and airplanes, for example...

 

[1MileCrash]

We have supersonic bullets and airplanes, for example...

That is not movement of the medium itself. Mechanical changes exhibited in the air itself are limited by the speed of sound through air. That's why we get a sonic boom at mach 1.

 

[Dr Lots-o'watts]

Lately I've been thinking of c as the maximum speed of cause and effect in the universe

I agree.

, and the speed of sound as the maximum speed of cause and effect within/for that specific medium.

Ehh... Not so sure. An example is a laser crystal, in which an emitted photon on one end could stimulate an emission on the other end, such that the time interval between both events would be determined by the speed of light within the medium, not the speed of sound. Now it would be a good idea for someone to verify this with standard solid state and laser emission theory, because at some point there is a relation between speed of sound and speed of light in a medium. I just don't recall at the moment what it is. Remember also that frequencies travel at different speeds, so the "speed of sound" is not a unique speed, but some distribution or function.

Is it correct to say that if i had an iron bar as a lever, and i threw on end down, the other end would not travel up immediatelly, that there would be a lag between the two actions, the speed of sound through iron?

Yes.

 

[K^2]

That is not movement of the medium itself. Mechanical changes exhibited in the air itself are limited by the speed of sound through air. That's why we get a sonic boom at mach 1.

The example's off, of course. The airplane doesn't move through the medium at supersonic speeds. It creates a shock-wave, so that local air speed is sub-sonic, but he brings up a good point. Shock-wave itself travels at supersonic speeds through the medium, even without the object that started it. Think shock-wave from the explosion. It propagates at speeds faster than Mach 1, so it can cause destructive effect from the cause that was too recent for your hypothesis.

Seems that we were both wrong.