Tapping a message over lightyears in an instant?

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Discussion Overview

The discussion revolves around the hypothetical scenario of transmitting a message over a 100 light-year-long rigid rod using Morse code taps. Participants explore the implications of such a setup in the context of physics, particularly focusing on the speed of signal propagation and the limitations imposed by the laws of relativity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant proposes a scenario involving a rigid rod and questions what prevents instantaneous message delivery across vast distances.
  • Another participant explains that disturbances in the rod propagate at the speed of sound in the medium, which is less than the speed of light, thus negating the idea of instantaneous communication.
  • A later reply reiterates that the tap will propagate through the rod at the speed of sound, emphasizing that this speed is slower than light.
  • One participant questions the utility of the rod concept, arguing that even if atoms could travel at the speed of light, it would still take 100 years for the message to be transmitted due to the distance involved.
  • Another participant mentions that moving a shorter rod could result in a response faster than the speed of sound, but clarifies that this does not apply to the speed of sound through the material of the rod.
  • There is a challenge regarding the method of checking the speed of response in a physical rod, indicating a need for clarification on the claim made.

Areas of Agreement / Disagreement

Participants generally disagree on the feasibility of instantaneous communication through the rod, with some asserting that it is impossible due to physical laws, while others explore the implications of the scenario without reaching a consensus.

Contextual Notes

The discussion highlights limitations related to the assumptions of rigidity and the nature of signal propagation in materials, as well as the implications of distance on communication speed.

airbourne
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I have another question (I asked one a month ago, thanks to those who helped clarify it for it). I have not studied physics formally, I am just avidly interested.

Situation:

You have a very rigid rod, 100 light years long. You, and a friend, at the other end of the rod, 100 light years away, know morse code and can easily understand messages by being tapped into your leg. So you set the rod up so it will tap your leg if the other person pushes it forward a little and vice versa.

What prevents the message being delivered in real time (getting the message across 100 light years of space-time in an instant)?

I understand a huge amount of energy would be required to move the rod, let's forget that (unless it is somehow relevant to this hypothetical question) and imagine it's no harder to push than a matchstick.
 
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Atoms in the rod push on the next atom, and so on. This disturbance propagates at less than the speed of light. It's called the speed of sound in the medium. It's much less than the speed of light. So your idea of an instantaneous pushing idea doesn't hold up. Relativity still works. Sorry about that. There are no rigid rods; they don't exist.
 
airbourne said:
I have another question (I asked one a month ago, thanks to those who helped clarify it for it). I have not studied physics formally, I am just avidly interested.

Situation:

You have a very rigid rod, 100 light years long. You, and a friend, at the other end of the rod, 100 light years away, know morse code and can easily understand messages by being tapped into your leg. So you set the rod up so it will tap your leg if the other person pushes it forward a little and vice versa.

What prevents the message being delivered in real time (getting the message across 100 light years of space-time in an instant)?

I understand a huge amount of energy would be required to move the rod, let's forget that (unless it is somehow relevant to this hypothetical question) and imagine it's no harder to push than a matchstick.
The "tap" will propagate through the rod at the speed of sound of the rod, which is slower than light.
 
Thanks, I had wondered if there was a simple law I wasn't thinking of, and thanks ghswllsjr for the link, I can gain a better understanding this this. Sorry for asking something that's been covered, I didn't find anything when I searched.
 
I don't understand the point of the rod idea anyway. Suppose the atoms in the rod DID travel at the speed of light? If you and your friend were 100 light years apart, it would still take 100 of your years to get the message transmitted. We can send messages to each other today at the speed of light, but that doesn't mean that the receipient gets the message instantaneously.
 
Last time I checked you could move a rod that was only a foot long and the other end would respond faster than the speed of sound.
 
John232 said:
Last time I checked you could move a rod that was only a foot long and the other end would respond faster than the speed of sound.

Not faster than the speed of sound through that rod. For an iron rod this will be the speed of sound through iron (About 14654mph), for a wooden rod it's the speed of sound through wood, etc.

Just ot of curiosity; how did you "check"?
 

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