Thread: Physics problem

Hi, I wanted to know if there is a flaw in an analogy from the book "elegant universe" by Brian Greene. When he explains relativity, he uses a carnival ride where you spin in a cylinder as an example to explain how space warps. He says that while the ride is spinning, if one person were to measure the circumference of the ride with a ruler, and compare it with the radius of the ride (measured while the ride is spinning), you would get a result different from the 2 pi r rule. He says that the ratio between the circumference and the radius will be greater than 2 pi r.

This is because the ruler contracts when you are measuring the circumference, while as you measure the radius it does not.

The problem is, wouldn't the ride itself contract as well to the same degree as the ruler contracts? So when you measure the circumference of the ride, the outside should measure to be the same whether you are moving or not, since any contraction of the ruler should also apply to the ride itself. And when you measure the radius, you would not feel any contraction, and you would get the same result moving or standing anyway.

What is wrong?

Yeah I read that book and noticed that too, but I don't have an answer =[.

Still a great book though, I love reading that stuff

It's likely just another one of those physics assumptions that the ruler itself doesn't contract, like ignoring air resistance and such. You just assume that you have a ruler that isn't subject to relativity.

EDIT: Hmm, I read over the part that says it's the ruler itself that contracts while the ride does not. If that's indeed what the book says, I do not have a solution for you, though I'll certainly check my copy once I get home and see if I can figure it out.

EDIT2: I have consulted my copy and figured out your problem. He explains it fairly thoroughly in endnote number 5. The answer is that we took our measurement of the already spinning ride with a stationary ruler. Then we toss it (the ruler) to the guy on the ride so the ruler contracts (from our point of view) when he takes his measurement. The ride was already contracted when we took our measurement so when we gave to ruler to the guy on the ride, only it contracted. I hope that helps.

oh I see now, thanks!

No probalo, Elizagerth...

Physics is a problem that needs fixing.
EDIT: I'm sorry physics. I know we have our disagreements but in the end we know we belong to each other.