Posted by Steve on March 31, 19101 at 15:56:36:

In Reply to: Spin posted by Dan Di Francesco on February 18, 19101 at 15:52:24:

Hi there!

Interesting suggestion... and although I don't think I can shed much light on it, I can at least help with part of it.

There's a great book out by Brian Greene called "The Elegant Universe". In it he suggests that there's a little bit of misunderstanding in why Relativity says that the speed of light is the universal speed limit. It stems around this idea - the 'speed' an object has while moving through space-time is merely a resultant of it's 'speed' in all 4 dimensions (3 spatial and one time). Let me try to describe this on a smaller level.

Let's say you have a simple vector drawn on a graph. Just to put some numbers into the game, we'll say the vector has a length of 10 units and makes a 60 degree angle to the x-axis. According to geometry, this vector is made up of two component vectors that form right angles to each other. The length of the horizontal component can be found by doing (cos60)*10=5 so the horizontal component has a length of 5. the vertical component is found by doing (sin60)*10=8.66. So the vertical component is just about 8.66 units. I'm trying to point out that a component of a vector is never equal to the vector itself (unless one of the components is zero). And the vector is always smaller than the sum of the two components.

The point of this is that Einstein said all matter in the universe was always moving at the speed of light, through space-time. That means that a partical's resultant speed through space-time was the speed of light. This means if you increased your speed in one dimmension, the speed you were going in the other three dimmensions would get smaller. Hense, time dialation - if you increase your speed in the three spatial dimmensions, your speed in the time dimmension slows down. The reason the speed of light is a limit, is because if you were moving with the speed of light in one of the spatial dimmensions, you would 'stop' moving in the other two, AND time. This also suggests a reason why relativity doesn't necessarily subscribe to time travel to the past, because moving backwards in time wouldn't give a particle a resultant speed of 'c'. In fact, I believe you would find that if the time component was negative, all the spatial and time components would add to a number less than 'c', in at least one situation.

Anyhow, I believe that's the reasoning in classical relativity. But I'm not so knowledgeable about how Quantum Mechanics or String Thoery tackle the whole time travel thing. But I believe it has something to do with the fact that Faster-Than-Light travel isn't necessarily faster, it just takes advantage of short-cuts (hense wormholes and whatnot). Besides, I just realized something else... even if we could travel back in time, that would cause a significant change in entropy, the gravitational constant, the cosmological constant... being that more mass would be added to the system... don't know if that would cause a problem or not... hhmm.

Anyhow, hope I didn't bore you... until we meet again, have a very pleasant day!

Steve

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