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Suppose there's a very long bar in space, which is about to be hit by a space hammer. At the exact same time that the bar receives the hit, a photon cannon releases a photon at the speed of light in a vacuum. The extremity of the bar which receives the hit and the exit of the cannon are placed at the exact same distance from a vertical red line. Which cross the red line first, the bar or the photon? Or both cross it at the same time? Can physics solve this?
Pic related.
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>Is the speed of light in a vacuum greater or less than the speed of sound in the bar?
Gee, can physics solve this?
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Of course it can. The impulse you give to the bar travels at the speed of sound in this material, meanwhile the photon travels at the "speed of light" The photon crosses the line first... BY FAR
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>>8819187
Think of the bar as being a bunch of smaller parts - atoms, if you want - all stacked together, but spaced apart slightly. (To be slightly more accurate, you can imagine they're connected by springs. )
In order for the atom at the very end to move, it has to be hit by the atom behind it. Before that atom can move, it has to be hit by the atom behind *that*. All the way down the line.
This means that the space blow from the space hammer will travel all the way down the line, one atom at a time, at the same speed any other kind of space shockwave space would.
Sound, in fact, is the exact same kind of phenomenon, and travels through materials in the exact same way - the difference is only that sound waves follow the push forward with another pull backwards, shaking the material back and forth instead of just forth.
And so, the push on one end of the rod will travel only as fast as the speed of sound in whatever material the rod is made of. The more rigid the material is - the less slop there is in the lattice of molecular bonds connecting the atoms - the faster it will be. But because the bonds connecting the atoms are made of electromagnetic fields, and disturbances propagate through those no faster than light, even the most perfectly rigid material would have a speed of sound no faster than light.
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>>8819247
Yhea. That's the long version
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>But because the bonds connecting the atoms are made of electromagnetic fields, and disturbances propagate through those no faster than light, even the most perfectly rigid material would have a speed of sound no faster than light.
Thanks, I thought it was something along these lines.
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>and disturbances propagate through those no faster than light
Shouldn't changes in eletromagnetic fields propagate at the speed of light? Wouldn't a perfectly rigid material have speed of sound equal to the speed of light? And what a about an hypothetical material with no lattices in its structure?
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>>8819555
The thing about "a hypothetical rigid material not made of smaller components" is that that's not even really a thing that makes sense under known physics.
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While what the other people have written so far is no doubt true, the really technically correct answer is that is arrives at the same time.
As soon as the very first particle is hit by the hammer, its gravitational field will experience a wave that propagates at the speed of light and influences all the other particles of the rod it passes. This is at least as long as what LIGO is actually correct.
But yes the traditional answer would be that the impulse travels at the speed of sound.
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>the really technically correct answer is that is arrives at the same time
> its gravitational field will experience a wave that propagates at the speed of light and
No.
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>>8820842
I second this.
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>>8820842
>>8820914
explain?
Or maybe I should clarify myself: It doesnt have to be a gravitational wave.
The particles of the rod dont actually have to touch to pass on their impulse, but only have to get closer, so that the electromagnetic force that one excerts on the other gets stronger, say from particle As electrons on Bs electrons, so that B gets impulse away from A.
You can hopefully agree with me that changes in fields happen at the speed of light, so the very last particle in the rod will, completely regardless of how many particles are between them, experience a blueshift in the electromagnetic field emmitted from particle As electrons, as A is moved towards it by the hammer.
This blueshift will excert a force on that last particle pushing it over the red line.
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>>8819207
First off, the stick is in a vacuum so its the speed of sound in a vacuum.
Second, space is black so the photon doesnt exist.
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>>8822517
the stick would just fall anyway
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>>8819187
anyone who thinks causality is related to the speed of light is fucking retarded.
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