[Tim_Lou]

ok, lets say there is a twin on earth, a twin in a space ship.

the space ship travels near the speed of light, which the local time is slower than the "earth" time in the frame of the twin on earth.

after years as they meet on the earth, the twin on the space ship aged less than that on the earth.

i understand this part.

but what if according to the frame of the twin in the space ship?
relative to that frame, it is the earth that is moving away.
so, it is right to say that the twin on the earth aged slower than the twin on the space ship according to the space ship frame? so, after a couple years, the twin on earth appear to have aged less than the twin on the space ship when the twin on the space ship comes back to earth???

but, doesnt it contradict the first part??
im confused.

[Tormod]

I'll see if I can explain it, but I'm sure others are more apt at this than me.

I think that even though you can say the Earth is also moving away from the spaceship, we have to keep in mind that what is actually moving at (near) light speed in reference to something, is the spaceship. The Earth is not moving at a relativistic speed because it is still moving in orbit around the Sun, which orbits the galaxy, which is a part of the larger universe.

But the spacesgip is in an isolated reference frame. It does not only move relative to the Earth but also relative to everything else.

Here is a thorough explanation of the twin paradox:

http://www.phys.unsw.edu.au/~jw/twin.html

There is a nifty animation on that page, but it takes some reading to really get what it is illustrating.

[Freethinker]

Yes Tormod and good question Tim. Perhaps the confusion is point of reference. Tim you want to compare the RELATIVE speed of the ship to the Earth. But what is important for the "local frame" is it's relative speed to "a photon traveling in the same direction".

[DivineNathicana]

Also, it is important to remember that the twin on the space ship has to accelerate in order to lift off, penetrate the atmosphere, and escape Earth's gravitational pull, as well decelerate in order to near Earth and eventually land. We can only say that either of the twins is moving away from the other as long as both are undergoing uniform motion - which in this case, they are not.

- Alisa

[Tim_Lou]

i guess a better phrase to that question would be what would happen if the twins are moving relative to each other in space at a constant speed... they start from very far away, moving close to the speed of light, and meet at a certain point in space.

would the twin paradox be gone this way?

[Tim_Lou]

i think the problem is, why does length contraction applies only to the space ship in the frame of space ship, but not the opposite?

[DivineNathicana]

Tim_Lou,

It applies to both. Both twins would think that the other is younger, as long as we have uniform motion. However, when the twin in the spaceship returns to Earth, it is obvious that HE had been the one to accelerate, and thus the symmetry is broken.

I suggest Brian Greene's "The Elegant Universe" for a clear and simple explanantion. Way too much to type, sorry. = ) He provides actual examples using some weirdos named Gracie, George, Slim, and Jim. Heh.

- Alisa

[BlameTheEx]

Is there evidence to eliminate this hypothesis?

For this location of space there is a velocity which is absolute zero*. The Earth is, in relativistic terms, close to that velocity. When our travelling twin zips off in his space ship he is time dilated because he has a relativistic (close to C) velocity when measured against the local absolute zero.

*The universe is expanding, or perhaps just appearing to expand. The local absolute zero will be such that velocities of distant galaxies balance out, as appearing at that point. That is zero velocity with respect to the contents of the universe in general, and presumably thus the universe itself.

[Freethinker]

Originally Posted by BlameTheEx

Is there evidence to eliminate this hypothesis?

When our travelling twin zips off in his space ship he is time dilated because he has a relativistic (close to C) velocity when measured against the local absolute zero.

But the Earth twin would have a "relativistic (close to C) velocity when measured against the local absolute zero" of the frame for the space ship twin.

The different time dilations can not be based on comparisons to the other as the difference is the same while all that is changed is the frame of reference wtih each being equally valid and each being equally valid as a PoR (Point of Reference). It is only to some common external PoR that each's dilation would be based and measured and cause a difertiation between the two's special frame.

[DivineNathicana]

Originally Posted by BlameTheEx

Is there evidence to eliminate this hypothesis?

I don't understand where that question comes from... Regarding my latter post? It's simple relativistic properties:

1.) Any frame of reference in uniform motion is as credibe as any other frame in uniform motion.
2.) Acceleration of any sort by either of the two parties breaks the symmetry between them.

If you were asking for experimental data, I don't believe so. This is due to the fact that any signal sent out by either of the two parties would not travel instantaneouly, and would also be subjected to relativistic weirdness. The end result would be that upon receiving the respective signals, both parties would conclude that they are aging faster than the other.

However, once either breaks the symmetry, we clearly see who is aging slower and who is aging faster.

- Alisa