They would say, of course, that because nothing can be allowed to transmit faster than light, it would take 8.20 minutes for the fellow at the other end of the rod to feel the push.

But wouldn't the work have already been done? The rod is there. It's the same concept for why gravity can 'escape' a black hole.

If we set up this experiment, when the rod was pushed forward, it seems to me the fellow at the other end would immediately feel it.

Do all the atoms in the rod really have to compress and transfer all the way to the other end? To avoid faster-than-light information transference?

EDIT @planetmaker has just pointed out it is far, far worse than I thought. The push would actually take an entire year to feel since apparently it would move at the speed of sound.

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    $\begingroup$ I'd politely suggest to perhaps revise your writing style to be a bit less aggressive/editorial, you might get some better responses. $\endgroup$
    – user438383
    Sep 9, 2022 at 12:10
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    $\begingroup$ @user438383 I don't at all mean to come across as aggressive, for what it's worth. $\endgroup$ Sep 9, 2022 at 12:29
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    $\begingroup$ "You genuinely believe all the atoms in the rod have to compress and transfer all the way to the other end?" Are you proposing some alternative to that? Atoms move according to the laws of physics, not magic. $\endgroup$
    – PM 2Ring
    Sep 10, 2022 at 9:38
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    $\begingroup$ @PM2Ring Do you genuinely think it would an entire year for the fellow at the other end to feel/see the rod move? $\endgroup$ Sep 10, 2022 at 9:41
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    $\begingroup$ @WhitePrime Of course. Why do you ask if I genuinely think that? $\endgroup$
    – PM 2Ring
    Sep 10, 2022 at 9:47

2 Answers 2


It would take MUCH longer. A push is communicated at the speed of sound inside the medium. Within solid materials that's typically a few km/s, not the speed of light at 300.000km/s. Thus it is about 100.000 times slower, and it would take about a year for this sound wave to travel through this rod from here to the Sun.

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    $\begingroup$ Has this been experimentally confirmed in any way, though? $\endgroup$ Sep 9, 2022 at 9:26
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    $\begingroup$ Are you asking whether sound waves and their propagation speed have been measured? $\endgroup$ Sep 9, 2022 at 9:32
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    $\begingroup$ You know very well that's not what I'm asking, mate. $\endgroup$ Sep 9, 2022 at 11:42
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    $\begingroup$ Honestly, no, I don't. I'm really not sure what you are asking for in your comment, other than speed-of-sound measurements. Because here on Earth we have these kind of measurements daily: an earthquake is nothing different than something pushing on Earth. And we do feel the quake in Japan only many minutes or few hours later here in Europe $\endgroup$ Sep 9, 2022 at 13:39
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    $\begingroup$ @WhitePrime - The speed of sound is, by definition, the speed at which a pressure change at one end of a medium (a push) transfers through the medium. The speed of sound in various solid objects has been measured a number of times; the largest such object is probably the earth, where the speed of sound through various parts of the earth has been measured seismologically. $\endgroup$
    – antlersoft
    Sep 11, 2022 at 17:21

When I hang a basket on a rope of one AU long to get my groceries up in my air-castle, the rope can be momentarily one meter longer when I pull the rope one meter up. Likewise, the stick gets compressed when I push it one meter ahead of me. A wave of increased mass travels along the stick with the speed of sound and when that arrives on the other end, the stick jumps to its original length, as the rope does when I pull it up one meter. So it can take some time to pull in my groceries. The fruit in my basket will be wasted before it arrives... no matter how hard I pull the rope in.


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