# A spring attached to the other side of the galaxy

## Main Question or Discussion Point

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?

## Answers and Replies

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gneill
Mentor
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.

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?

gneill
Mentor
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).

Nugatory
Mentor
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 [Broken]

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cjl
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.

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Khashishi
It's the speed of sound because that's what sound is. Sound is a mechanical disturbance moving through an object or medium.

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?

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Nugatory
Mentor
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.

cjl