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Imagine you are one lightyear away from a photon sensitive (light sensitive) switch. So it is obvious that light would take one year to reach to the switch. Now I have a one lightyear long plank. I simply point the plank towards the switch and press it. Now I just did work which light would take 1 year to do in a matter of seconds.

Now the question is, did I break the laws of physics?

Please do not give an answer like " it is impossible to make a plank this long ". Just think about it. Even I am thinking about this question right now.

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Possible duplicates: physics.stackexchange.com/q/2175/2451 and links therein. –  Qmechanic May 31 '13 at 19:42
    
Do you actually say that speed of sound (pointing the pole) is greater than speed of light (pointing light)..? It probably takes centuries and centuries for the mechanical stress (you've applied) to travel ;-) –  Waffle's Crazy Peanut May 31 '13 at 19:44
    
This is an exact duplicate of the link Qmechanic provided. The short answer is that a rod / plank that feels rigid to you isn't and your push takes a long time to propagate through the rod (at the speed of sound in the rod). –  Brandon Enright May 31 '13 at 19:45
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marked as duplicate by Brandon Enright, zhermes, Waffle's Crazy Peanut, dmckee May 31 '13 at 19:56

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1 Answer

when you push the plank, it will take a year for your push to be felt by the other end. In the meantime (assuming you had the strength), the plank will just compress as it moves. The reason is that although it is rigid, it still takes a minimum of a year for the atoms in the plank to communicate down the line (one giant game of telephone) that there was a push at all.

This is not a relativistic effect, it works on short scales too, but the communication speed is near light (assuming a perfectly rigid material in an ideal universe), so you do not notice the compression.

BTW, the physics police probably would pull you over if your trick worked the way you described

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So basically Jim , you mean to say that when I push the plank it would approximately reach the speed of light! You've got to be kidding me. And by the way, you are talking about compressions at the speed of light, which is basically absurd. I do not think that is the answer. But I surely appreciate your thinking... –  Rohinb97 May 31 '13 at 19:52
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no, the plank doesn't reach the speed of light. It's the information that you are pushing it that goes at near light speed. Actually, that travels at the speed of sound for the material, but that is only limited by light speed –  Jim May 31 '13 at 19:54
    
Rohin, you should study a little about relativistic effects and then talk –  Saurabh Raje Jun 1 '13 at 9:46
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