Corona688

OK, we've all heard the sci-fi scenario... travel fast enough, and time slows down for you. Approach the speed of light and it slows to a crawl, and you end up returning millions of years later to find the planet's been taken over by those damn dirty apes. But something about that confuses me...

according to relativity, couldn't it be seen as, rather than YOU going .9c and planet earth staying relatively still, couldn't it be interpreted as YOU sitting still and the *PLANET* doing .9c? Relatively, at least? And if so, why's time slowing down for YOU, but not the planet?

Undercurrent

The planet doesn't accelerate. Up until the space traveller turns back, the situation is symmetric between the planet and the astronaut.

As soon as the astronaut accelerates to turn back to the planet, the situation is no longer symmetric.

Demosthenes

Who said anything about time slowing down for you? From your viewpoint, time flows normally for you, it's the rest of the universe, including earth that has slowed down. To earth, it's you that has slowed down.


That isn't quite correct, the situation is still symmetric when the astronaut turns back to earth. To him, earth is now moving towards him at .9c

NialScorva

It's not symmetric because the astronaut has undergone acceleration. It's all about the acceleration-- different inertial frames of reference.

Abacus

No, the symmetry is broken when the astronaut turns back to earth. The astronaut undergoes an acceleration. The earth does not.

People on the earth would say that the astronaut's time has slowed down, and the astronaut would say that earth time has slowed down. Both would be correct.

Corona688

Why does that matter? It's not the ACCELERATION that causes the time differential.

How does the acceleration matter? It's not the accelleration that's producing the time differential, it's the VELOCITY...

Corona688

They can't BOTH be right, that's an outright contradiction. Wouldn't the astronaut's relative time have to *SPEED UP* to produce the time differential?

Bialar Crais

The FAQ for sci.physics.-relativity has a detailed section on the twin paradox:

Here!

Might clear some things up.

Abacus

Nope. They'd both be right.

Suppose we have two twins. Suppose that one joins NASA and becomes and astronaut, while the other stays at home and becomes an astronomer. Suppose that on their 30th birthday, the astronaut twin boards a rocket and blasts away from earth at 0.8c (for simplicity sake, we'll assume the acceleration time to be instantaneous).

Now, from the perspective of the astronomer twin, the day that he celebrates his 40th birthday, his twin brother will be celebrating his 36th birthday aboard his rocket ship.

From the perspective of the astronaut twin, the day that he celebrates his 36th birthday, his brother will be 33.6 years old and between birthdays.

Sound crazy? Well, you have to understand that simultaneity is not absolute. While the astronaut's 36th birthday and the astronomer's 40th are simultaneous events in a reference frame in which the earth is stationary, they are most definitely not simultaneous events in any other reference frame.

ex-xian

They're both right because there is no absolute frame of reference. It's just as correct to say that the astronaut went .9c away from earth as it is to say the earth went at .9c away from the astronaut.