FTL Communication: The Long Pole Experiment & Modification

[aademarco]

Hello!
It was some time ago on one of my visits to this forum where I saw a thought experiment about achieving faster than light communication using an impractically long pole of sorts. For example, practicality aside, if we were to construct a pole with the length of 1 light year, and have an observer at each end of said pole, could they send a form of push and pull Morse Code communication instantaneously regardless of the time it took to set up the system? When the pole is pushed from one end, 1 light year away at the other end of the pole, will they see the corresponding push instantaneously?

The answer to the above question after some research I found to be: No. You Cannot. The reasoning is that when you push on your end of the pole, you are actually pushing the atoms on your end of the pole against the rest of the atoms in the pole towards its other end, which subsequently push the atoms adjacent to them and so on, and that pushing occurs at the speed of light or slower. Therefore even though you pushed the pole now, it would take exactly one light year or less for the other person at the other end to see the corresponding push. (If they would see the motion in exactly one year I did not confirm, but I did confirm they would NOT see it transmitted faster than light).

Revisiting this for a moment, I came to this question: What if we take a single atom, albeit having an extremely small diameter, and played the same game of push/pull Morse Code? if we have a detector/observer on each side of an atom, and we push the atom, will the other observer on the opposite side see movement faster then the time it took light to travel from one side of the atom to the other (after all, they won't see the other observer make his 'push' until the light makes it over to the other side, but shouldn't the atom move on his end instantly?) Does the same argument which makes the answer for the question about the pole NO apply here? If so, is it because the smaller elements of the atom are being pushed up against each other at no faster than the speed of light? Furthermore, if that is the case, would that imply that there always needs to be something 'smaller' that an element is comprised of to prevent this FTL communication?

 

[Orodruin]

Realize that at the atomic level you do not have material packed up and the concept of a "push" is not very relevant. Instead, interatomic forces are carried by fields (such as the electromagnetic field) and changes in the fields are also limited to propagate at light speed or slower.

I would also like to point out that in your example of the pole, the signal would travel significantly slower than light speed - it would travel at the speed of sound in the pole material.

 

[aademarco]

I see, so if we consider the atom in terms of movement derived from a force being applied to it, it cannot be imagined as something like a baseball moving back and forth, but propagation of information throughout the field which still occurs at sub-luminal speeds. While indeed frustrating when trying to 'visualize' the situation, it surely makes sense that changes in the field are limited to the speed of light/information. I'm sure any experienced physicist went through the breaking of bad habits such as thinking of atomic structures as 'material' :) Thank you for the answer.

 

[Orodruin]

I'm sure any experienced physicist went through the breaking of bad habits such as thinking of atomic structures as 'material' :)

We have all been through some phases before learning more. Back in compulsory school I hypothesized that electrons were like miniature planets orbiting a nucleus central "star" - which was quite natural based on my knowledge at that stage but utterly wrong... I think it is healthy to make such questions as long as you are willing to accept that a preconceived notion regarding the answer may be wrong.