Let's say I have an infinitely long rod which I use to pass information to my friend on the other end. If the rod moves towards my friend, it means I sent a 1 and if the rod moves away from my friend, it means I sent a 0. Using this rod to transfer binary information to my friend, can I send information that travels faster than the speed of light? Or in other words, can I talk with my friend instantly irrespective of the distance between me and my friend?

Is this theoretically possible? My assumption is that push and pull of the rod is instant. Is it really instant or the push and pull also travels at the speed of light limiting the speed of my communication?

  • $\begingroup$ I would think that the speed of transfer would be around the speed of sound of the material, since you are essentially creating a pressure wave pulse when you first jolt the rod to transmit. $\endgroup$ – NaOH Nov 24 '16 at 7:00
  • $\begingroup$ @NaOH so any push or pull on a material traverses the same way sound travels? $\endgroup$ – harun_a Nov 24 '16 at 7:02
  • $\begingroup$ @harunrashid - speed of sound or if you move (or hit) your end of the rod hard enough you could instead generate a supersonic shock wave in your rod. But even that would be a speed less than the speed of light. $\endgroup$ – user93237 Nov 24 '16 at 7:06
  • $\begingroup$ @harunrashid If I move section A of the rod to the right, the only way section B (that is next to A) knows section A has shifted is if the compressed atoms near the boundary between A and B passes this compression information to the right. This compression moves like a sound wave as the atoms in the rod tries to go back to their original equilibrium separation. $\endgroup$ – NaOH Nov 24 '16 at 7:26
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    $\begingroup$ If the rod is infinitely long and your friend is at the other end, your friend will never receive any message from you. $\endgroup$ – RedGrittyBrick Nov 24 '16 at 11:30

I'll give a short answer. 'Is it theoretically possible?' The answer is NO. That's because there can never exist a perfectly rigid body. The actual structure is the crystal lattice, which holds the vibrating atoms in a region of space. In other words, inter-atomic forces act on each atom. Consider some atom A. A is held at the crystal site by inter-atomic forces. A change in the neighbourhood of A, will only have an effect on it, if these inter-atomic forces pass on that information of 'change' to A. So, the information you send(by tapping the rod suppose) will only travel from one point to another at best the speed of light, since in the end, it uses these forces to travel to your friend.


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