Suppose you have a hypothetical rod long enough to reach Mercury from the Earth. You begin pushing the rod from here in such a manner that the rod reaches mercury in some time. Once the rod reaches Mercury, it'll take us some finite amount of time for us to know that the rod can't be pushed any longer. If the information travels at the speed of light and assuming that Earth and Mercury are closest to each other at that time of the year, it'll take the information around 4 minutes to reach us. During those 4 minutes, what happens to the rod? Can we keep pushing it? If we do the same thing on earth but from one building to an adjacent one, without seeing one can tell that the rod has reached the building because information has travelled that fast. What about here?
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Qmechanic
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1The motion of the rod travels at the speed of sound in the material, not the speed of light. Pushing on one end of the rod leads to a compression wave that travels at this speed. It will not move as a rigid body. – Marius Ladegård Meyer Nov 06 '21 at 04:48
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@MariusLadegårdMeyer, det er godt du Marius- +1 mange tak -NN – niels nielsen Nov 06 '21 at 06:06
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Possible duplicates: https://physics.stackexchange.com/q/2175/2451 and links therein. – Qmechanic Nov 06 '21 at 09:24
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@nielsnielsen Hjertelig takk! – Marius Ladegård Meyer Nov 06 '21 at 12:11
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From our end nothing will change about the rod for those 4 minutes - obviously that would violate the idea of information traveling at/faster than the speed of light.
Upon impact with mercury, some slight compression will take place in such a way that the speed of light will never be violated for parts of the rod farther away than $c\Delta t$.
This compression - or possibly compression waves - will actually never even travel faster than the speed of sound in whatever material the rod is made of.
Señor O
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