A Thought Experiment for FTL communication

A Thought Experiment for FTL communication

Special Relativity theory forbids FTL (Faster Than Light or Superluminal) communication, but I have a thought experiment that would seem to violate that notion. For the purpose of this experiment, I’ll define communication as “The exchange of information between two or more parties”.

Imagine an apparatus that consists of tube with a ¼ inner diameter and is completely filled with incompressible marbles of ¼ inch diameter. A cup hangs from each end of the tube. If one were to insert another marble into one end of the tube, all the marbles in the tube will be pushed along the tube ¼ inch and the marble at the other end will fall into the cup.

Alice and Bob each have a laboratory. One end of the tube is in Alice’s laboratory and the other is in Bob’s. Bob is expecting Charlie to visit him at his laboratory. Alice wants to know when Charlie arrives. Alice and Bob have previously agreed that when Charlie arrives, Bob will insert a marble into his end of the tube. Alice will immediately notice a marble falling into the cup on her end and will acknowledge this to Bob by inserting the marble back into the tube so that one falls out at Bob’s end. At this point, communication has been achieved – a message has been sent, received and acknowledged all in a matter of a few seconds.

The length of the tube does not matter as long as it reaches between the two laboratories. It would appear that communication occurs instantaneously no matter how far apart the locations are.

Now imagine that the two laboratories are separated by a distance of one light-year when the experiment is performed. Alice and Bob have seemingly exchanged information much faster than the speed of light. Note that no part of the apparatus has moved anywhere near light-speed so no physical laws have been broken.

If FTL communication is impossible, then what part of this thought experiment is faulty?

originally posted by: Shadoefax
... what part of this thought experiment is faulty?

The incompressible marbles.

originally posted by: Shadoefax
then what part of this thought experiment is faulty?

It's the form of communication

You're trying to compare one form of communication with another

It's like comparing Posting a letter with smoke signals with Fiber Optic line, they're all mediums for communication but all incredibly different

It would transmit information at the speed of sound.

We had a very similar thread a few days ago, complete with moving illustrations, that said more or less the same thing using an iron rod instead of a bunch of marbles. The response was that the propagation of the "pushed marbles" (in this case) would travel at the speed of sound. The energy has to travel clear through the tube; it's not instantaneous.

Although this next is an imperfect analogy (that's the nature of analogies.), it's kind of like a line of cars at a stop light. When the light turns green, why don't all the cars in the line start up at once, assuming all drivers are paying strict attention? Because the information of the green light has to be somehow transmitted along the line of cars.

It's the same with the marbles. Each marble has to receive the information that the marble behind it has moved before it can move and transmit that information to the next marble in line. That takes time.

originally posted by: Shadoefax
A Thought Experiment for FTL communication

Imagine an apparatus that consists of tube with a ¼ inner diameter and is completely filled with incompressible marbles of ¼ inch diameter. A cup hangs from each end of the tube. If one were to insert another marble into one end of the tube, all the marbles in the tube will be pushed along the tube ¼ inch and the marble at the other end will fall into the cup.

If FTL communication is impossible, then what part of this thought experiment is faulty?

The part that says the change at the tube's one end will be seen instantly at the tube's other end.

Motion in the tube will happen in sublight speeds, and therefore the message will not be transmitted with superluminal speeds.

A very similar thought experiment was discussed here a couple of weeks ago (on the space exploration forum). In that thought experiment, an "incompressible rod" was used.

However, as moebius and others pointed out above, there is no incompressible material. Even if the most incompressible material was used (a diamond rod maybe? Water is very incompressible, too) the "signal to push" from one atom to the next to the next to the next, and so on, does not propagate instantaneously.

Here is that other thread:
www.abovetopsecret.com...

a reply to: Shadoefax

As far as thought experiments go,that one isn't too bad.I enjoyed visualising it.

It works.Sadly,the physics police will stop such shenanigans in real life.😃

a reply to: Shadoefax

Quantum Communications. You have a simple voice system (or complex video system) that works off of "Quantum Entanglement" between two (of if complex 256) particles. This system could in theory be used between two parties located anywhere in the universe (or theoretically even in time and dimensions).


We have some basic (very basic) quantum computers, and we have conducted some successful quantum entanglement tests. With these studies being done the idea of FTL communications isn't very far off.


So with that in mind, the issue with the marble tube is mass. At the point you have a enough marbles to fill the tube you will have to use a massive amount of force to move the marble down the tube. While the idea does seem feasible, the infrastructure needed to push a LY of marbles would make this a very impractical system.

a reply to: Guyfriday


Nobody has been able to get past the problem that the quantum states (spins) at both ends would be random. If my friend and I were 10 LY apart, and I set the spin of a particle at my end that is entangled with a particle at he end, the particle at his end would react instantaneously, but the effect would be random. There would be ne exchange on information, just randomness.

Here is a part of a longer YouTube where the idea of FTL communication through entangled particles is discussed. The entire video is worth watching, but the part I included in the link below is his summation:

a reply to: Shadoefax

..what part of this thought experiment is faulty?

Its not the same marble that falls out at either 'end'.

Sound transmission in the atmosphere works the way you propose. Light is stretched end to end from its source to our eye (like starlight). But you see the star light the star emitted x number of light years ago, not the light it just emitted.

Is there some way to transmit directly across that distance from the star to your eye, right now?

Wouldn't that be neat.

originally posted by: intrptr
a reply to: Shadoefax

..what part of this thought experiment is faulty?

Its not the same marble that falls out at either 'end'.

It doesn't matter if it's the same marble. If a marble inserted at one end instantaneously caused a marble at the other end to fall out, then you could conceivable send Morse code messages faster than light across many light years by adjusting the times between marble falls (e.g., one marble in two seconds = "dot"; two marbles in two seconds = "dash".

However, as mentioned above, the marbles (or even a solid rod) in not incompressible, and the marble at the far end will fall instantaneously by inserting a marble at this end. The time it takes for the marble at the far end to fall out after inserting a marble at this end would actually be slower than light speed.

a reply to: bananashooter

I'm not completely convinced.

Let's re-scale the apparatus. Instead of one light-year long, it is only 100 miles long. And let's let the tube and marble's diameter be 1 millimeter (although I don't think the size really matters, it just makes it easier to visualize).

If my calculations are right, it would take light a little more than 536 microseconds to traverse 100 miles. Sound would take 469 seconds (about 7 minutes, 49 seconds). Are you implying that it would take at least 7 minutes, 49 seconds for the other marble to drop (no matter what speed the marble was inserted)? If Bob had a device on his end that could slam the tiny marble completely into the tube in less less than 500 microseconds and the drop was instantaneous, it would be faster than light.

a reply to: Shadoefax

I don't think the speed of sound is the limitation (nor do I think the speed of sound is relevant, but I might be wrong). The limiting factor is that the marbles are NOT 100% incompressible, and the slower-than-light speed at which the propagation of the movement of the marble would occur due to that compression.


Your marble idea has too many variables to trip over. In the other similar thread on this, the who person posted suggested a more simplified apparatus -- a solid metal rod instead of a tube full of marbles. He would move one end of the rod, and (as per his claim) the other end of the rod would instantaneously move.

However, the problem with his though experiment (and yours) is that the rod is not incompressible. Once I push on my end of the rod, that "push" needs to propagate atom-by-atom through the rod (one atom telling the next atom that it was pushed, then onto the next atom, and so on). That atom-by-atom propagation of my "push" is not instantaneous, and would be somewhere below light speed.

So the movement of the rod at the other end would not occur instantly after I pushed my end. The time it took for the push to get to the other end would occur (at the very fastest) at light speed, and probably slower.

a reply to: Guyfriday

.

But entanglement is not really Faster Than Light communication even if it is instantaneous since it have no speed. To have a speed you have to have the information go from A to B thru a medium between A and B. Entanglement have no real speed since it does not go thru the medium from A to B but instantaneous appear at B as the wormhole phenomena it is.

You change the particle at A and the particle is instantaneous changed at B.

a reply to: Shadoefax


Didn't you try this theory last week and it was shown to be a poor example of trying to prove your case?

You can't deem a sort of marble "incompressible" that is a violation of physics. BTW, Bob and Alice both failed their mid-terms and were kicked out of the school's lab. Bob sells shoes now and Alice works on the street.

Soooo long rods and marbles talk to each other now. Ok Einstein 😊

In all seriousness though, thanks for the explanation. The OP (here and previous idea with rods) actually had my rusty gears talking to each other. Thank you for the "like I'm speaking to DFC" explanation. That makes sense.



a reply to: Soylent Green Is People

originally posted by: Aliensun
a reply to: Shadoefax

Didn't you try this theory last week and it was shown to be a poor example of trying to prove your case?

No, that wasn't me. But it is curious that we both had the same idea and posted it to ATS. I thought I came up with an original thought but it appears that it occurred to someone else also.

please add further discussion to the existing thread:

www.abovetopsecret.com...

Thread closed

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