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View Full Version : long stiff stick, faster then light?

10001
21st February 2006, 07:32 AM
when an end of a stick is pushed. the other end, seemingly, responds imediately.

Then, if we connect a very long stick from earth to the moon. this stick will have to be very hard and stiff. And push it. will the other end of the stick react imediately? as in instantly. faster then the light to get to the moon from earth?

Terry
21st February 2006, 07:39 AM
in fact, when you push on a stick, that propogates at the speed of sound in the stick, not instantaneously. Speed of sound in any practical material is *way* slower that the speed of light.

ImaginalDisc
21st February 2006, 07:40 AM
when an end of a stick is pushed. the other end, seemingly, responds imediately.

Then, if we connect a very long stick from earth to the moon. this stick will have to be very hard and stiff. And push it. will the other end of the stick react imediately? as in instantly. faster then the light to get to the moon from earth?

Not that I'm an expert, so what follows is speculation, but I don't think that works. The stick is made of, say, wood. Pushing on the end of the stick causes the enegry of that motion to be tranfered through the material of the stick. If the acceleration is very great, the material will fail, and break. I don't see how moving around a long, solid stick is much different from the wave propogation speed of a fluid. If you exceed the acceleration tolerence of the material, it breaks, and the acceleration you meant to impart to the far end does not make it.

Pragmatist
21st February 2006, 07:41 AM
when an end of a stick is pushed. the other end, seemingly, responds imediately.

Then, if we connect a very long stick from earth to the moon. this stick will have to be very hard and stiff. And push it. will the other end of the stick react imediately? as in instantly. faster then the light to get to the moon from earth?

No. This has been discussed many times on here, check some of the old threads.

The stick consists of molecules, when you push, you push one set of molecules, which in turn push another set and so on. The stick undergoes compression and all that you achieve is a wave of force which travels at a maximum of the speed of light through the body of the stick. In other words, the far end of the stick doesn't move instantaneously.

Ziggurat
21st February 2006, 07:44 AM
in fact, when you push on a stick, that propogates at the speed of sound in the stick, not instantaneously. Speed of sound in any practical material is *way* slower that the speed of light.

Yes, in every known material the speed of sound in much less than c, but it's more than that. Relativity dictates that it's not physically possible for the speed of sound in any material to exceed c, regardless of what it's made of.

Ryokan
21st February 2006, 02:59 PM
This topic brought me a lot of headaches back in my early teens.

epepke
21st February 2006, 05:38 PM
Yes, in every known material the speed of sound in much less than c, but it's more than that. Relativity dictates that it's not physically possible for the speed of sound in any material to exceed c, regardless of what it's made of.

Even without relativity explicitly, the atoms in the material push on each other with photons, which go at c.

Soapy Sam
22nd February 2006, 06:46 AM
10001