somebody asked this before, a really long time ago i think

say you had a really long stick(...)

long enough, so that when you rotate it around a pivot(which is somehow suspended in space), the arc of one end would have to travel faster than the speed of light. Like a disproportional seesaw, or the hand of a clock. You would need a tremendous amount of torque, and the material would have to be very strong and long. for the sake of this question, let's say we could achieve both.

would it be possible? if >c speed isn't possible, what would be preventing it?


i hate asking questions like this, because nobody really knows shit. but anyway, speculate, i guess.

This post was edited by zero_infinity on Jan 9 2009 05:36am

and i'm going to bed.... now

I can't quite picture in my mind what you are asking, so I will draw a figure in paint and see if it is the situation you are trying to describe.

and here we go:



This is actually a diagram for a sort of time machine. If as you say the materials are very durable, then one could accelerate the stick to near light speed using huge motors at either end of the stick pointing in opposite directions. Since it is out in space, the drag on the mechanism would be very slight, so once accelerated, it would stay accelerated until something hit it. So what you do is put a put a person in one end of the stick with enough provisions and entertainment for whatever duration he wants, and start the device spinning. Inside, the amount of time the person experienced would be drastically shortened, while those on the outside wouldn't notice much. Of course you'd have to accelerate it slowly, or the "time traveled" person inside would arrive as a mass of jello. And the motors would have to fire exactly evenly or the entire device could wind up spinning uncontrollably through space. The merry-go-round from hell.

As for the arc of one end traveling faster than the speed of light, i don't know if that is possible because you'd have to have motors with an infinite energy source, which would increase the size of the device infinitely, so you'd have to have another infinite energy source to accelerate the infinite energy source, ect ect ect.

This post was edited by Dr_zoidberg71 on Jan 9 2009 06:01am

I guess you could think about it like this:
suppose you didnt have the stick; just the motor and pivot. You wouldn't be able to run the motor at the speed of light at this point.
Then add an short stick to the machine. To achieve the desired speed by the end of the stick, the motor wouldn't need to go as fast, but the energy required would
still be the same as if u wanted to achieve the speed with just the motor by itself, i would assume due to the other factors like the extra mass, inertia etc -
which would all increase as you lengthened the stick.

lol just use a rope swing

That's if you assume a perfectly rigid body but in reality there's no such thing. That means the other end does not start moving instantaneously, likewise if you have a stack of balls in a tube next to each other and you hit a ball on one end, the ball on the other end does not start moving instantaneously.

No material could ever support that. That's how nature prevents faster speeds than the speed of light.

well even if you suppose there's such material, according to general relativity you'd need infinite amount of force to accelerate it to speed of light regardless of how it's moving, in circle or straight line

when you wave a flexible ruler back and forth, it bends and snaps back really quickly(because the length near your rotating wrist is moving faster than the edge of the ruler?). why wouldn't that happen?

also, i can understand when you hit a billiard ball, it takes time to transfer(momentum?) across balls. but if you apply that idea to a sideways moving pole(if that's what you're saying bes), it's just strange, unless you mean like the ruler example.

This post was edited by zero_infinity on Jan 9 2009 06:19pm

well you can simplify this even further, suppose you have a very long rod(a >lightyear long). If you push on one side of the rod will the other side move instantaneously?If the rod was perfectly rigid then yes...however nothing in real life is perfectly rigid, and the propagation time will be equal to the speed of sound in the material. The more rigid the body(atoms packed closer), the higher the speed of sound in the material and the faster it will react. But it's still nowhere near the speed of light.