Physical Interactions over Great Distances

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Discussion Overview

The discussion revolves around a thought experiment involving a very long rod, constructed of a hypothetical material, and the implications of physical interactions over great distances. Participants explore how the rod behaves when one astronaut pushes it from one end while the other astronaut is at the opposite end, focusing on the speed of signal transmission and the effects of mass and inertia.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the other astronaut would see the rod move instantaneously, suggesting that the answer is no.
  • Another participant clarifies that the "push" would propagate along the rod at the speed of sound in the material, which is significantly slower than the speed of light.
  • A participant asks about the significance of the rod's mass in the context of the thought experiment.
  • One participant reflects on their initial idea and acknowledges a similar concept discussed in a previous thread, expressing humility regarding the originality of their thought.
  • Another participant discusses the implications of mass on inertia, suggesting that a rod with a large mass would be difficult for an astronaut to push.

Areas of Agreement / Disagreement

Participants generally agree that the rod does not move instantaneously and that the speed of the push is limited by the material's properties. However, there are varying perspectives on the implications of the rod's mass and inertia, indicating some unresolved aspects of the discussion.

Contextual Notes

The discussion does not resolve the implications of mass on the rod's behavior or the practicalities of pushing such a long rod, leaving these points open for further exploration.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in physics related to force transmission, inertia, and the behavior of materials over large distances.

SeaDour
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Say you (somehow) constructed a very long, sturdy rod, about a light year in length. Say it was made of unobtanium, so it has the same mass as a relatively short steel rod. Now say there are two astronauts, one at each end, and one of the astronauts gives the rod a firm push in the direction of the other astronaut.

Does the other astronaut instantaneously see the rod move in his direction? If not, how exactly does the rod behave? (Does it move at all when the astronaut pushes on it? Or does it take a really long time to start moving?)

Thanks in advance for your input on this thought problem.
 
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SeaDour said:
Does the other astronaut instantaneously see the rod move in his direction?
No.
If not, how exactly does the rod behave? (Does it move at all when the astronaut pushes on it? Or does it take a really long time to start moving?)
The "push" moves along at the speed of sound in the rod--nowhere near the speed of light, much less instantaneously.

See this thread: https://www.physicsforums.com/showthread.php?p=1245509
 
Can you explain the significance of the mass of the rod in your question?
 
Thanks for the quick re-direct, Doc Al. I had been feeling so proud of myself for thinking up the idea all on my own, but am now humbled to see that the exact same idea was brought up a few short weeks earlier.

country boy -- my thinking was that if it didn't have a small mass, it would have a huge inertia and would be virtually impossible to push by an astronaut anyway.
 
SeaDour said:
country boy -- my thinking was that if it didn't have a small mass, it would have a huge inertia and would be virtually impossible to push by an astronaut anyway.

I see. You might be interested in this thread also, which uses rockets to propel a rod:

https://www.physicsforums.com/showthread.php?t=150905
 

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