If I have a very long rigid rod which is of rectangular cross section and small mass. Let it's length be 1 light year and total mass be 1 kg now at one end if I rotate that rod, since the rod is rigid it should rotate as a body about the axis parallel to its length and normal to its cross sectional area. If this event happens instantaneously, does it mean that the information of rotating the rigid rod has been sent instantaneously (more than the speed of light)? If the assumption is false what actually happens there?

  • $\begingroup$ Also this and probably others $\endgroup$
    – Chris
    Aug 10, 2018 at 17:19
  • $\begingroup$ Ridig rods and solid bodies may exist in non-relativistic classical mechanics but not in relativistic theories. $\endgroup$
    – Frobenius
    Aug 10, 2018 at 17:29
  • 1
    $\begingroup$ The question is well phrased and doesn't deserve a downvote. The OP is asking where the flaw in his logic is, so why is he punished by downvotes? People are asking questions here, because they don't know the answer. So not knowing the answer is not a reason for a downvote. If everyone knew everything, this site would not even exist. So please stop this corrupt practice of punishing people for asking honest questions. Besides, the people who downvote new users are the veterans here who got their overblown reputation unfairly when this site was new and every question and answer was highly rated. $\endgroup$
    – safesphere
    Aug 10, 2018 at 21:27
  • $\begingroup$ @safesphere You're quite possibly wrong when you blame the down votes on the veterans (and they did work for their reputation scores). The downvotes are more likely to be from people who can't vote to close (i.e. low reputation users) or from that minority on the internet who downvote things they don't personally like, regardless of rules (an internet wide phenomenon ). I agree that this question probably didn't derserve downvotes (but was a valid VTC duplicate) but not with your statements about who is doing the downvoting. IF you want a discussion of this try opening a topic on Meta. $\endgroup$ Aug 10, 2018 at 21:57

1 Answer 1


The solution to this "paradox" is that such a rod simply cannot exist. Relativity prohibits the existence of perfect rigidity.

If you rotated one end, that rotation would propagate from one end to the other at approximately the speed of sound in that rod. Since the speed of sound in any object is slower than the speed of light (c, that is in vacuum), your paradox is averted.

  • $\begingroup$ " Since the speed of sound in any object is slower than the speed of light in that object, your paradix is averted." I do not think that is correct. $\endgroup$ Aug 10, 2018 at 23:29
  • $\begingroup$ Whoops, that was definitely a typo. Fixed. $\endgroup$ Aug 11, 2018 at 1:35
  • $\begingroup$ It is not right still. The speed of photons in the Sun is less then the speed of sound. $\endgroup$ Aug 11, 2018 at 5:14
  • $\begingroup$ @ÁrpádSzendrei that's not what it is about. It's just about the speed of sound being lower than c, the vacuum light-velocity. And this is true in any case, and it is what the answer states (now). $\endgroup$ Aug 11, 2018 at 8:20

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