Imagine that you have a straight, rigid body, with virtually no internal elasticity, connecting two points A and B. Imagine that it is very long, like 300,000 km long, just to make the math easier. At time t, you shine a light from A towards B, and at the same time you push the body towards the same direction (assume that you can apply sufficient force to cause it to move, by whatever infinitesimal amount).
What would an observer in B observe first, the light shining (at time t + 1sec) or the end of the body moving?
My intuition, most certainly wrong, is that the observer in B would observe the moment first. If that is not the case, why is that? What other factors other than internal elasticity would delay the movement of the far end of the body?
OTOH, if my intuition is not wrong, then this would effectively be a way to achieve FTL communication. Other then being extremely impractical, what are the laws of physics that would make the construction of such a device impossible in the real world?
I understand that perfect rigidity is an ideal, so let’s forget about the real world and focus on the thought experiment. Would the compression wave still propagate at the speed of sound if the material was perfectly rigid? Or is its speed some inverse function of the elasticity?
By definition, the effects would propagate down the rod at the speed of sound. That’s what sound is: A propagation of a longitudinal movement of a material.
I intuitively understand that, but why is it the case? Does it have to do with the fact that any molecular arrangement at temperatures above 0K has a lot of empty space between and within atoms that can be compressed or is there some other fundamental force at work?
Ultimately, c is the (maximum) speed of propagation of information, regardless whether that information is carried via photons travelling through a perfect vacuum, or a field imposing itself on another field - so the atoms in the physical material cannot propagate forces to their neighbours faster than the universal speed of propagation of information.
The maximum possible speed of sound in the theoretically most rigid material that could possibly exist, is c. You push on the atoms at the end of the super-rigid rod, and they cannot communicate that force to their neighbours any faster than the speed of light in a vacuum.
In practice, of course, it’s nowhere near that fast in any real material.
This question is in the same category as the one about closing a really long pair of scissors, or what a rotating disc looks like, or whether a guy walking really fast and carrying a ladder would fall down a hole.
I think it’s a bit different to those. The ‘point of closure’ of a pair of scissors can (appear to) move at any arbitrary speed, because it’s not a physical object - it’s little more than a concept
The trouble with this question is that it really amounts to: ‘When you disregard the fundamental means by which any meaningful answer to question X could ever be given, what’s the answer to question X?’
It’s not the temperature, though you could never have anything at 0K.
It’s not really about the spacing either. If you “pushed” on one side of a neutron star, the effect wouldn’t propagate faster than the speed of light.
It’s just that that’s the fastest that information can travel. If you push on one end of your stick, then the material next to your finger has to communicate that it’s been moved to the material next to it, and so on.
In any normal material, this would be mediated by the electromagnetic force, the repulsion between electrons. But, even in a more exotic material that is far more compressed to where the strong force is dominant, it would still be transmitted by a force, and that force cannot communicate faster than light.
It’s the relativistic physics version of “assuming a wizard who can create a local exemption to the laws of Newtonian physics has magically forbidden the plane from taking off, will the plane on the treadmill take off?”
Instead of looking at a collection of neutrons, which will distract us into talking about how closely those neutrons are spaced, let’s talk about one single solitary neutron. If something pushes a neutron, how long does it take for that information to get to the other side of that neutron? If you tell me that it is not quite instantaneous, then what is happening meanwhile? Is the neutron squeezed flat to some degree?
Come on, cut me some slack I had stated in the OP that I understood that it was not physically possible. Yet asking this question made me understand why perfect rigidity is not possible, so at the end of the day a little bit of ignorance has been fought, yay!
I had stated in the OP that I understood that it was not physically possible. Yet asking this question made me understand why perfect rigidity is not possible, so at the end of the day a little bit of ignorance has been fought, yay! 