Faster Than Lightspeed?

[Adonis1x23]

My new physics professor (former professor had a massive breakdown) went on a rant recently on how the speed of light could be broken *theorhetically* in a disk system.

His rationale was that if the center of a disk was brought up to lightspeed, then the outer rim would have to be traveling faster than the speed of light.

Frankly, I thought that this was such a overly simple concept that someone must have already thought it and disproven it. Someone please tell me why this is or isn't true.

[Dr. Mobius]

I want to say the answer is no, but I don't remember. I do know, though, that the answer has to do with special relativity... I think.

[eriador]

Well... it depends of the viewpoint of the observer. If the observer is at system center, they just see a ring rotating. If the observer is on the ring, they just see the ring rotating around the center. If the observer is traveling towards system center (or vice versa, it doesn't matter) time dilation effectively slows down the rotation of the ring, because as v -> c, everything takes longer, so speed decreases. The dilation of time is enough that the edge of the ring doesn't break c.

What's even weirder is that if you're a beam of light and I'm a beam of light and we're both zipping towards Dr. Mobius from opposite directions he sees us each moving at c towards him, and we each see each other moving at c towards each other. Even though Mobius would think I'd see you moving at 2c, in fact the time dilation slows that down.


However, I don't know whether I'd see Mobius approaching at c, .5c, or some weirder value. Probably .5c, 'cause that makes sense (i'm approaching him at .5c, you're approaching me at c, so we're doing the same speed relative to him)

One thing to keep in mind: motion can only be measured relative to the observer, by special relativity. You can't say "x is moving at c relative to y and y is moving at c relative to me, so x is moving at 2c" (even if the vectors line up) because you're changing viewpoints, which aren't comparable. You can only get away with that usually because you don't have to account for relativistic speeds, but even a first-year physics course will force you to confront them. It sucks

[Slim]

If you are traveling at the speed of light, your clock has stopped, so it is pointless to talk about how fast others are going relative to yourself.

But, yeah, you are right though. Say you replace c with .6c, then one might expect the sum to be 1.2c, however, it really only comes to .88c. (as shown on The Mechanical Universe, best physics show ever)

Anyway, as far as the rotating disk, I'm not sure on that one. However, I think it is a flaw to assume that a disk can spin while the center travels the speed of light. Maybe they can, and I'm wrong, but my guess is no. It seems he is assuming what needs to be proven.

Then he says "brought up to lightspeed" instead of something more scientifically sound.

[zeroguy]

Anyway, as far as the rotating disk, I'm not sure on that one. However, I think it is a flaw to assume that a disk can spin while the center travels the speed of light. Maybe they can, and I'm wrong, but my guess is no. It seems he is assuming what needs to be proven.

Then he says "brought up to lightspeed" instead of something more scientifically sound.

Yes, I'm pretty it's that last part that's what makes this impossible. If you assume you can accelerate an object to lightspeed, you just assumed away relativity. And if you have the disk moving close to lightspeed, but not actually at it, then it's easy to explain the motion of the outer rim of the disk with the relativistic velocity composition law(s). The outer rim will just appear to be moving faster than the center, but still slower than lightspeed.

It'd probably be more assuring if jota or Steve got their asses in here, though.

[Bean_wannabe]

The point is moot, really, because as soon as you break the light barrier you become infinitely small and infinitely heavy, destroying the entire universe. So it isn't likely that anyone is going to be allowed to research making it happen.