Transfer of information via a rigid body?

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Discussion Overview

The discussion revolves around the transfer of information via a rigid body, specifically examining the implications of pushing a rigid rod connected to a distant object. Participants explore the speed at which information is transmitted through the rod and the constraints imposed by the principles of physics, particularly regarding the speed of sound and the nature of rigidity in materials.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant proposes that pushing a rigid rod connected to a distant ball would allow for simultaneous acceleration of the ball with their hand, raising questions about the speed of information transfer.
  • Another participant asserts that the speed of information transfer through the rod is limited to the speed of sound in the material, challenging the idea of instantaneous transmission.
  • A later reply emphasizes that all materials will distort when moved, and that a push on one end of the rod travels through the material until it reaches the other end.
  • It is noted that no material can be perfectly rigid, and Einsteinian physics forbids infinitely rigid materials.
  • Participants discuss that denser materials have a faster travel speed, but one participant corrects this by stating that higher density actually means lower speed of sound, while higher rigidity correlates with a higher speed of sound.

Areas of Agreement / Disagreement

Participants express disagreement regarding the nature of rigidity and the implications for speed of information transfer. While some acknowledge the limitations of material rigidity, others debate the relationship between density and speed of sound, indicating that the discussion remains unresolved.

Contextual Notes

Participants highlight the limitations of materials in terms of rigidity and speed of sound, noting that assumptions about perfect rigidity do not hold in reality. The discussion also reflects on the dependence of information transfer speed on the properties of the material used.

Luke Tan
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Let's say I have a ball 299792458m away and i want to apply a force to it.

Evidently any information (for eg if i instantaneously increase my mass to increase the gravitational force) will travel at the speed of light and thus, the increase in gravitational force will only be felt by the ball 1 second later.

What if i connect a rigid rod between the ball and I and push it?

What, then, is the speed at which information is transferred? It makes sense that, for example, once i push against an end of the rod, the relative distance between any two points has to stay the same, so the ball at the end should accelerate simultaneously with my hand. However, this defies the principle that nothing can travel faster than the speed of light.

Thanks!
 
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And here the other trivial faster-than-light ideas:

 
Luke Tan said:
It makes sense that, for example, once i push against an end of the rod, the relative distance between any two points has to stay the same,
No. All materials will distort when moved. A push on one end travels through the material until it reaches the other end.

Denser materials have a faster travel speed, but there is no material so dense as to be perfectly rigid.
Indeed, Einsteinian physics forbids infinitely rigid materials.

Diamond, the hardest material known, has a speed of sound of 12km/s. If this rod of yours were made of diamond, it would take just shy of 7 hours for your push to reach the other end.
 
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DaveC426913 said:
Denser materials have a faster travel speed
High density means lower speed of sound. Higher rigidity means a higher speed of sound. See the table here for some examples. And see the formulas here for details.
 
jbriggs444 said:
High density means lower speed of sound. Higher rigidity means a higher speed of sound.
Thanks. I knew there was something wrong there but couldn't put my finger on it.
Gonna 'stet' it for continuity of thread.
 

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