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Suppose that there's a button at the other end of the universe, and a 'stick' that goes all the way there.

If I push the stick, will the button get pushed instantly, or there is some latency that I am missing?

(Suppose that the stick is made from whatever strong enough material that can stand whatever physical or practical constraints over there.)

I want to use this mechanism to build a real-time communication system that spans multi-light-years areas. So is this a good idea?

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This has been asked and answered many times on physics.stackcxchange.com. The short answer is no, it won't work. The latency that you're missing is that when you push on one end of a stick, you're pushing on a bunch of atoms. Those atoms push against other atoms, which push against other atoms, and so on - but they don't move instantaneously, and the influence of your push travels as a wave, somewhat slower than light. This will be true no matter what the stick is made of, for fundamental reasons having to do with relativity. – Nathaniel Feb 28 at 11:39
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The pressure allowing the stick to move when pushed at one end will move at the speed of sound in that material.

Imagine the rod is a slinky. Understand springs (Hooke's Law, etc.) and you understand solids in general: they are just orders of magnitude stiffer. It might be more philosophically correct to note that a spring is just like any solid rod, just wimpier. A wall resists pushing because it compresses by some invisible amount.

Imagine a short rod of steel the size of a pencil. On this scale you find it hard and unbending. But, it's really a sample of a product sold as rope! It comes in spools 10 feet in diameter. For a length of a bridge span, it wobbles and obviously ropelike.

If you have a steel I-beam a mile long, and you push one end hard enough to shove it, you will note a delay before the far end moves, and the length shortens like a piece of rubber, compressing first, and then the wave of compression moving to the other end where that eventually causes the far end to move.

Radio would be far faster than mechanical linkages.

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+1 Great explanation! I didn't know that before – Youstay Igo Feb 28 at 9:55
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"If you have a steel I-beam a mile long, and you push one end hard enough to shove it, you will note a delay before the far end moves" - OK, I want to see that in a video. – Jan Dvorak Feb 28 at 12:13
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That's going to take a lot of energy. And steel. And a huge warehouse. I'd watch that too. – Polyducks Feb 28 at 13:14
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@JörgWMittag can we adopt the term "speed of causality" already? :-) – Jan Dvorak Feb 28 at 19:36
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@technosaurus How exactly would quantum entanglement help you here? – immibis Feb 28 at 22:33

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