Relativity

[ATMLVE]

I've decided to ask this here because it seems just as good as any science forum. I understand a lot about the theories of relativity but the basic concept of the whole thing still has me confused.

One of the things Einstein first studied was relative motion. If you don't know what that is, stop reading now.

Well one of the concepts of relativity is time dilation. I'll use a ship for examples. As a ship gets closer and closer to the speed of light:
-relative to the ship, time passes normally, but time for the rest of the universe passes more quickly.
-relative to the rest of the universe, time passes normally, but time for the ship slows down.

What bugs the heck out of me though is the concept of relative motion. Einstein said that with only two reference frames, if at least one is motion and there is no acceleration, it is impossible to tell which is in motion and which is at rest; and indeed, if there truly are only two reference frames and no acceleration, then there is no correct answer.

So going back to the ship in the universe. Given relativity, shouldn't the passage of time of the rest of the universe, relative to the ship, appear to move more slowly since, relative to the ship, the ship is standing still and the rest of the universe is approaching the speed of light? Why is it that time for the ship slows down, since it appears, from the ship, that the ship is sitting still and everything else is approaching light speed?

And then if that were the case, why wouldn't it be the way it actually is now... And thus a paradox. This does apply to the other effects of relativity, but I find time dilation the most intriguing.

[Drashner1]

Well one of the concepts of relativity is time dilation. I'll use a ship for examples. As a ship gets closer and closer to the speed of light:
-relative to the ship, time passes normally, but time for the rest of the universe passes more quickly.
-relative to the rest of the universe, time passes normally, but time for the ship slows down.

This relates to the Theory of Special Relativity, which deals with the structure of space-time.

What bugs the heck out of me though is the concept of relative motion. Einstein said that with only two reference frames, if at least one is motion and there is no acceleration, it is impossible to tell which is in motion and which is at rest; and indeed, if there truly are only two reference frames and no acceleration, then there is no correct answer.

This relates to the Theory of General Relativity, which is described as a theory of gravitation - so, for starters it's somewhat of a 'comparing apples and oranges' situation.

Also, General Relativity doesn't quite say what you've said here. It's sounds like what you're describing is the equivalence principle. From Wikipedia:

The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical.

Also:

The upshot of this is that free fall is inertial motion: an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.

So going back to the ship in the universe. Given relativity, shouldn't the passage of time of the rest of the universe, relative to the ship, appear to move more slowly since, relative to the ship, the ship is standing still and the rest of the universe is approaching the speed of light? Why is it that time for the ship slows down, since it appears, from the ship, that the ship is sitting still and everything else is approaching light speed?

See above - time dilation is a consequence of special relativity, while the equivalent principle relates to general relativity. Two different aspects of reality, and two different theories, each dealing with one of them.

Combing these two aspects of reality may mean getting into some kind of unified field theory, which has so far eluded us.

At least I think that's how it works. One of our resident physics experts will hopefully see this and weigh in with more detail.

Todd

[iancampbell]

IIRC the twin "paradox" is eliminated by noting that the situation is only symmetrical if neither of the observers is accelerating. Which isn't the case, in the usual formulation of the problem. Earth is following a geodesic (the curved space equivalent of a straight line), but the spaceship is accelerating throughout and that's why the time spans are different in the direction they are.

[ATMLVE]

Thank you Todd, your reply helped me see it all in a different way.

IIRC the twin "paradox" is eliminated by noting that the situation is only symmetrical if neither of the observers is accelerating. Which isn't the case, in the usual formulation of the problem. Earth is following a geodesic (the curved space equivalent of a straight line), but the spaceship is accelerating throughout and that's why the time spans are different in the direction they are.

While that does make sense and it did cross my mind, what of a ship travelling at constant velocity? It still experiences the effects, doesn't it?

[iancampbell]

Thank you Todd, your reply helped me see it all in a different way.

IIRC the twin "paradox" is eliminated by noting that the situation is only symmetrical if neither of the observers is accelerating. Which isn't the case, in the usual formulation of the problem. Earth is following a geodesic (the curved space equivalent of a straight line), but the spaceship is accelerating throughout and that's why the time spans are different in the direction they are.

While that does make sense and it did cross my mind, what of a ship travelling at constant velocity? It still experiences the effects, doesn't it?

Yes, but the two parties never meet again to compare notes.

[Bear]

In the case of two (relativistic) ships that meet in the Big Night, as they pass each other there is perhaps a period of time for them to get in contact and exchange news.

But syncing up the signals will force both sets of people to compensate for an interesting fact. Why does time on the other ship seem to be moving more slowly than time on their own ship?