Can a rigid object transmit a signal faster than the speed of light?

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Discussion Overview

The discussion centers around the question of whether a rigid object can transmit a signal faster than the speed of light, exploring concepts related to rigidity, sound transmission, and the implications of physical laws in theoretical scenarios.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants propose that pushing a rigid object would result in instantaneous movement across the object, suggesting a potential for faster-than-light signal transmission.
  • Others argue that the transmission occurs at the speed of sound for the material, as sound is transmitted through particle vibrations, which is always slower than light.
  • One participant notes that no perfectly rigid object exists, and that any compression from a push would propagate at the speed of sound, leading to a time delay in movement at the ends of the rod.
  • A participant introduces a concept related to quantum entanglement, although this is challenged as being outside the main topic of the discussion.
  • Another participant reflects on the complexity of the question, suggesting that applying physical laws complicates the inquiry and that the scenario of a perfectly rigid object lacks physical significance in reality.
  • It is mentioned that in extreme cases, such as neutron stars, the speed of pressure waves may approach the speed of light, but this remains speculative and context-dependent.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of rigidity and the nature of signal transmission, with multiple competing views on the topic remaining unresolved.

Contextual Notes

Limitations include the assumption of perfect rigidity, the dependence on material properties, and the unresolved nature of the relationship between quantum mechanics and relativity in this context.

madness
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If I push a rigid object at one end, the whole rod should move instantaneously. Would this mean that a signal has been transmitted from the end which is pushed to the the far end faster than the speed of light?
 
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Nope, much slower. It's transmitted at the speed of sound for that material, which makes sense, seeing as sound travels merely by movement (well, vibration) of particles.
 
There is no such thing as a perfectly rigid object. The rod will compress when you push it and that compression will travel the length of the rod at the speed of sound for the rod's material (at less than the speed of light), until it reaches the end and uncompresses. Net effect, you push your end, and at sometime later the other end of the rod moves. the time delay between the movement of the ends of the rod will always be longer than it would take for light to traverse the length of the rod.
 
I seem to remember something about splitting up pairs of electrons into opposing paths, and how one of them observed at one end affects the other?
 
This thread is not about entanglement, and even though the thread topic is broad enough to accommodate that, let's stick to the content in the OP.
 
I've seen this "thought experiment" come up on several forums, and it usually generates a lot of argument. I think the problem is that this question doesn't really have an answer, because once you start applying physical laws as restrictions to help you narrow down the possibilities, you have to change the nature of the question. I think it's really designed to get you thinking about what the laws really require of reality, and what the consequences are of the gaps between quantum mechanics and relativity. Not to get an interesting definite answer.

The short answer is that it will take the amount of time required for the eletromagnetic force to transfer your pressure at one end from one atom to the next all the way down the rod. In other words, as someone already stated, the speed of sound is the factor. However, this assumes a certain amount of compressibility at the atomic level. Which means the rod isn't truly rigid. If the rod were perfectly rigid, then the answer to your question would be "yes". However, it would also have no physical significance, as the scenario varies so widely from anything possible in reality. Except maybe a neutron star, which some people think may be regarded as being as close as possible to being incompressible without collapsing to a black hole. It is thought that the speed of pressure waves (and hence sound) through neutron star material is very close to the speed of light.

"Rigid" materials are good for messing around with ideal scenarios of laws which don't deal specifically with phenomena that are pushed to their limits by "rigid" materials.
 
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