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Here's the plan for the proposed system of FTL communication.

A huge pole made of the strongest solid in existence (or whatever solid would work best for this scenario, if no solid exists that would allow for it then assume the pole is made of indestructible Unobtanium) stretches between two space stations on the endpoints of the pole.

The two endpoints are 54.6 million kilometers away from each other (the distance from Earth to Mars), so communication is on a large delay (13 minutes and 48 seconds).

Here's how we get around this:

At Endpoint Two, the end of the pole is suspended in front of a button. The pole is pushed by the station of Endpoint One back and forth to telegraph out a message.

The theory goes that by pushing the pole, the movement of the pole affects the whole pole simultaneously, allowing information to be transferred faster than the speed of light.

Is this in any way plausible as a method of FTL communication? Assume whatever resources and means required are available, and that if need be the pole can be made of indestructible materials.

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  • $\begingroup$ @Gryphon The idea of the molecules being unable to travel faster than light did occur to me, and I was wondering if I'd somehow managed to find some kind of a "Cosmic Loophole" around this. Oh well. If you copypaste your comment into an answer I'd be happy to give you a green tick. $\endgroup$ – Sydney Sleeper May 10 '18 at 0:19
  • $\begingroup$ I do not think that my 3-sentence comment is really worth being an answer, so I`ll let somebody else who knows a lot more about physics than me answer the question in a far better way. $\endgroup$ – Gryphon May 10 '18 at 0:21
  • $\begingroup$ Any real world material would deform or destruct in the process. If you have "indestructible Unobtanium", can I ask if this pole should be able to pierce through the center of a Black Hole? $\endgroup$ – Alexander May 10 '18 at 0:38
  • $\begingroup$ That's going to be one heavy pole! Picture "pushing" a cruise ship so the other end pokes a button. Sounds kind of hard to do. $\endgroup$ – BrettFromLA May 10 '18 at 13:47
  • $\begingroup$ If this worked, you could turn the pole into a FTL catapult. Fit a fulcrum somewhere between the midpoint of the pole and one of the stations, apply thrust, and the second station rockets off to the other end of the universe (or disintegrates, assuming it's not also made of unobtanium). $\endgroup$ – chif-ii May 10 '18 at 19:40
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The answer to your question, regrettably, is no, and the counterargument is a classic one.

  1. The pole is composed of atoms and molecules and particles; it isn't one giant indivisible object.
  2. Those atoms and molecules and particles can move about, to some extent, even if the pole isn't easily deformable.
  3. When you push one end of a pole - any pole - you're only directly pushing on some of the constituent molecules. Those molecules then push on other molecules in the pole via intermolecular forces, including electromagnetic interactions.
  4. Those forces take time to propagate - and they propagate slower than the speed of light. The change in density, in fact, shouldn't propagate faster than the speed of sound in the pole - much less than the speed of light in a vacuum.

Therefore, the pole will be moved at a speed slower than the speed of light.

For more reading, see questions on Physics Stack Exchange (1, 2, 3 and links therein) that talk about the same subject. See also the Ehrenfest paradox, another problem in special relativity about rigid bodies, albeit one with a different resolution.

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  • $\begingroup$ The intermolecular forces themselves are a result of interactions that are bound by light-speed. There are primarily electromagnetic themselves. Even unobtanium materials are usually considered to follow these rules. Things that appear as in-compressible are not, e.g., a lump of plutonium does not seem compressible, but this is how a Pu fission bomb works, the Pu is compressed to make it super critical. Even transmitting a sound wave requires compressibility. $\endgroup$ – Gary Walker May 10 '18 at 3:24
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It's not a way to do FTL signalling. The reason has to do with the details of how materials work.

Basically, when you wiggle a stick (no matter what the stick is made of) the wiggle affects the rest of the stick at the speed of sound in that stick. (This is easily seen using high speed photography.)

The speed of sound in steel is around 5000 meters/second which is about 11,000 mph, so motions propagate very quickly. But they never propagate instantaneously. (Even diamond "only" has a speed of sound of 11,000 meters/second!) Consequently, there is no such thing as a truly rigid rod in nature. Again, this is very easily measured using the right instruments even though to our hands and eyes it seems like it moves as a rigid body.

If unobtanium is made of matter or energy, the speed of sound will be less than that of light.

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  • $\begingroup$ "(T)he speed of sound will be less than that of light." Not always, a paper published in Physical Review, back in the mid-1960s, suggested the speed of sound in ultradense matter could exceed lightspeed. I wish I could post a citation to it. The result surprised me, so it struck in my mind. Ultradense matter would other problems making it unsuitable for a FTL signalling device. Like its mass, for starters. $\endgroup$ – a4android May 10 '18 at 7:08
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    $\begingroup$ I can't check the paper, obviously, but I'll bet on one of two things: Most likely they were talking about the speed of light in the ultradense medium which will, naturally, be slower than the speed of light in vacuum. (The latter is always what's meant in relativity when speaking of the "speed of light".) The speed of light in vacuum is a fundamental feature of nature. The propagation velocity of light in a material is slower because it involves a lot of absorption and re-radiation by the material, which slows light's effective speed.) Alternatively, they were just-plain-wrong. $\endgroup$ – Mark Olson May 10 '18 at 11:52
  • $\begingroup$ I always regarded the idea with amused scepticism. I entertained the idea without accepting it. Ultradense matter would be effectively opaque to light. So no probable propagation. Someone would have written another paper refuting it. But if, they did, I never found it. The first paper was something I found by accident. $\endgroup$ – a4android May 10 '18 at 12:47
  • $\begingroup$ @a4android Slow light is a thing - currently we can only get it down to 9.7km/s, so a mere 28 times the speed of sound in air, or just under twice the speed of sound in steel. I can't find details on what the speed of sound is in the materials used to slow light in those experiments is though. $\endgroup$ – Chronocidal May 11 '18 at 9:59
  • $\begingroup$ @Chronocidal I have seen references to light slowed down to 30 km/hr in a Bose-Einstein condensate. Probably, a news item in New Scientist. The speed of sound rises with density, I presume the paper extrapolated density to the point the speed of sound exceeded light's. Physical Review is refereed journal, so enough physicists thought the paper was credible enough for publication. This doesn't make it right or wrong. A lot of science is about exploring ideas to see whether they can stand or fall over. $\endgroup$ – a4android May 12 '18 at 9:26
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You could shine a very very powerful laser at a distant planet. Maybe it does take quite a long time to get there, but once its there if you were to shine the laser back and forth across the distant planet the change in the lasers position would appear to be happening in real time giving the illusion of faster than light action. Maybe im totally bass-ackwards on my physics because im not sure if the illusion counts in both points of reference on either end of the laser or just the recieving end. If its not just an illusion on the senders end but can also be observed from the recievers end then by jittering the laser on and off a reciever you could theoretically have established FTL morse code. But were getting into frames of reference and time-space wierdness so I dont really know if im fully correct or not.

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