A spring attached to the other side of the galaxy

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Discussion Overview

The discussion revolves around the hypothetical scenario of pulling on a spring attached to the other side of the galaxy, exploring the implications of mechanical wave propagation in such a context. Participants examine the nature of wave transmission in materials, comparing it to similar scenarios involving rigid rods and discussing the limits of mechanical disturbances.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that a decompression wave would travel along the spring at the speed of sound in the material, which is significantly slower than the speed of light.
  • Others question why the wave would only travel at the speed of sound, suggesting that this is the maximum speed for mechanical disturbances in a substance.
  • A participant draws a parallel between the spring scenario and pushing on a rigid rod, asserting that the other end would not move immediately due to the limitations of wave propagation.
  • Some participants mention that using a weak spring, like a slinky, can serve as a demonstration of these principles, noting that in stronger springs, the propagation velocity is faster but still limited.
  • A question is raised about the behavior of a rigid and brittle rod when pulled suddenly, with some asserting it would break if the force exceeded its strength.
  • Another participant clarifies that the outcome of pulling on a rod depends on the force applied relative to the rod's strength, indicating that a wave would travel at the speed of sound if the force is below the breaking point.

Areas of Agreement / Disagreement

Participants generally agree that mechanical disturbances travel at the speed of sound in a material, but there is ongoing debate about the implications of pulling on different types of materials and the conditions under which they might break. The discussion remains unresolved regarding the specific outcomes of the hypothetical scenarios presented.

Contextual Notes

Limitations include the assumptions about the properties of the spring and rod materials, the definitions of rigidity and brittleness, and the conditions under which the wave propagation is analyzed. There are unresolved mathematical steps regarding the exact nature of the forces involved.

malignant
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If you had a spring attached to the other side of the galaxy and pulled on it from one end, it can't instantly stretch on the other side, right? Even though that's what springs do? So what would happen?
 
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A compression wave (well, in this case a "decompression wave" I suppose) would travel along the spring at the speed of sound in the spring. The speed will be significantly slower than light. If the galaxy is taken to be about 120,000 light years in diameter, the receiving end won't notice the tug for well over 120,000 years.
 
gneill said:
A compression wave (well, in this case a "decompression wave" I suppose) would travel along the spring at the speed of sound in the spring. The speed will be significantly slower than light. If the galaxy is taken to be about 120,000 light years in diameter, the receiving end won't notice the tug for well over 120,000 years.

How come only at the speed of sound?
 
malignant said:
How come only at the speed of sound?

That's as fast as a mechanical disturbance will move through a substance (without "breaking it" in some way).
 
malignant said:
If you had a spring attached to the other side of the galaxy and pulled on it from one end, it can't instantly stretch on the other side, right? Even though that's what springs do? So what would happen?

This is basically the same question as asking what happens when you push on one end of rigid rod many ligh-tyears long - does the other end move immediately because the rod is rigid? The answer is no, and there's a pretty decent explanation of why in the FAQ for the relativity forum: https://www.physicsforums.com/showthread.php?t=536289
 
Last edited by a moderator:
malignant said:
If you had a spring attached to the other side of the galaxy and pulled on it from one end, it can't instantly stretch on the other side, right? Even though that's what springs do? So what would happen?

This is actually a pretty fun demo to do. All you need is a very weak spring. In other words, a slinky...



In a strong spring, the propagation velocity will be much faster, so it isn't as obvious on normal scales, but effectively, the same thing will happen.
 
Last edited by a moderator:
It's the speed of sound because that's what sound is. Sound is a mechanical disturbance moving through an object or medium.
 
cjl said:
This is actually a pretty fun demo to do. All you need is a very weak spring. In other words, a slinky...



In a strong spring, the propagation velocity will be much faster, so it isn't as obvious on normal scales, but effectively, the same thing will happen.


What would happen if you had a very rigid and strong, but also brittle rod and you would suddenly pull it very hard? Would it break?
 
Last edited by a moderator:
luitzen said:
What would happen if you had a very rigid and strong, but also brittle rod and you would suddenly pull it very hard? Would it break?

yes, if you pull on it hard enough and suddenly enough.
 
  • #10
luitzen said:
What would happen if you had a very rigid and strong, but also brittle rod and you would suddenly pull it very hard? Would it break?

That depends on how hard you pulled it. If your pull force was less than the strength of the rod, you would get a wave traveling through the rod at the speed of sound in the rod. If your pull force was higher than the strength of the rod, it would break.
 

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