View Full Version : A VERY long string

2004-Dec-04, 01:09 AM
If we have a extremely long string(Yes this was mentioned in the gravity field propogation question (http://www.badastronomy.com/phpBB/viewtopic.php?p=371977#371977)) say the string is 1ly long had no mass and no internal friction... a "perfect physics string." Now say you are swinging it about(Don't ask me how!) now if you cut the string, how long would it take for the ball at the end of the string to go flying off tangentially... it would be instantaniously set in a tangential motion from its current spot..

Let me try to rephrase this..
A perfect physics world is assumed and the orb is traveling in UCM(Uniform Circular Motion)

1 Light Year in Length
(orb) O--------------------------------(+) Pivot point

Oh no!

1 Light Year in Length The string has been cut
(orb) O--------------------------------x x(+) Pivot point

Now the orb at the end would go straight forwards ^ That way? right? or would it take one year for the orb to obtain a tangential velocity?

EDIT: Edited for URL to Origonal Post

2004-Dec-04, 01:17 AM
If we have a extremely long string(Yes this was mentioned in the gravity field propogation question) say the string is 1ly long had no mass and no internal friction...
I think that given your starting assumptions about the qualities of the string, you may assume anything you want about how the system performs.

I guess I'm no good at thought experiments.

2004-Dec-04, 01:18 AM
One year. Someone riding on the end of the string would gain information about the other end being cut when they saw their trajectory change. You can't transmit information faster than c.

2004-Dec-04, 01:18 AM
Ok, but WHY? What force travels at the speed of light that governs that?

2004-Dec-04, 01:38 AM
Because thats just how it is? :o This stuff confuses me... I don't get it yet. :-?

2004-Dec-04, 01:54 AM
I mean, if there is no internal friction and the string is infinitly thin, what could possibly hold it back from instantanous change? What enforces the global speed limit of everything?

2004-Dec-04, 04:14 AM
Since you are postulating a massless "object" with no friction and it is transmitting a force to a mass one light year distant it can be thought of exactly the same as moving that mass (a light sail perhaps?) with a massless laser beam. It will take one year to get there before it can impart momentum to the object.

More to the point, Einstein thought experiments deal with idealized cases of real hypothetically possible circumstances. This is not one. An infinitely thin massless, frictionless string that can transmit a physical force based on the electroweak and strong forces is not an idealized case of reality. It can't exist within our present understanding of physics.

When creating a thought experiment it is necessary to use what is known and extrapolate from there.

In this case given the parameters the closest approximation to what you propose is a beam of light.

2004-Dec-04, 04:31 AM
if all the electrostatic forces in the string were all stretched to their greatest length when one of them gets broken, what stops the object from flying off instantly?

2004-Dec-04, 04:58 AM
Electrostatic forces travel at the speed of light.

Iti, are you proposing the string is made of matter?