Liquidrage

You can't have one twin younger when they meet back up.
Anything done to bring them back would make them the same age again.
Else what would decide which twin was actually the one to be younger since both have the right to claim it is the other that was in motion?

Abacus

But only one twin underwent an acceleration. That is the difference.

Liquidrage

That is irrelevant because in this case bringing the twin back would require a deceleration.

We can make a measure and state that at a certain time to an observer then was a dialation.
We can record it and be sure of it just like we have with atomic clocks. But when we bring the clocks back, sure enough, the clock are once again in unison.

Friar Bellows

Liquidrage, the two situations are not identical. Both can claim to have been in motion, but both can't claim to have remained in a single inertial reference frame for the entire duration of the experiment. It is quite easy to construct an experiment to tell whether or not you are in an inertial frame. A simple pendulum and timer would do. The twin that stays on Earth remains in an inertial frame for the entire duration of the experiment. The twin that leaves the Earth, even if we start the experiment after the initial acceleration, exists in two inertial frames, the one leaving the Earth, and the one returning to the Earth. Even if you made the deceleration at the turnaround point of infinitesimally small duration, it wouldn't matter: you'd still have two seperate inertial frames for the space travelling twin. That's why you can't consider the experiences of the twins to be identical.

But don't take my word for it: experiments have been done which have measured this effect. Check out the following link, section 5:

<a href="http://www.math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html" target="_blank">Experimental Basis of Special Relativity</a>

Abacus

Ah, screw it. That was as clear as mud. Maybe I'll write something when I'm a little more cogent.

[ September 05, 2002: Message edited by: Abacus ]</p>

Liquidrage

Friar,

I'm not saying the time dialation doesn't occur.
I'm simply pointing out the fallacy in the twins story. When the twins are bought back to the same spot they are the same age. It is while they are appart they appear at different ages to an observer.
Obviously, I'm not capable of making up my own theories in physics so this viewpoint is something I've come accross several times in the reading I've done. I'll look for a reference later when I have more time.

Abacus

Why do you say this?

ohwilleke

No they aren't. They actually are different ages. The return does not "balance" the ledgers. Put another way, in special relatvity, the passage of time is "path dependent". Two people who take different paths involving different amounts of accelleration, go through different amounts of time on the way, even though they end up back in the same place at the same time.

wade-w

I've never like the word "paradox" in this context. There is nothing paradoxical about it. Its a situtaion that is predicted by the Lorentz transformations.

Now that would be a true paradox. Since the twins have experienced different durations, they are now different ages.

Jesse

As long as the twin in the rocket is moving at a constant velocity away from earth (staying in a single inertial reference frame), the situation is totally symmetrical. The twin on earth would say the rocket twin's clock was running slow, but the twin in the rocket would say the earth-twin's clock was the one slowing down. It is only because the rocket-twin breaks the symmetry by turning around to return to earth that when they meet up again they can both agree that the rocket-twin aged less. If the symmetry were broken the other way, with the rocket staying in a single inertial reference frame but with someone strapping huge rockets onto planet earth so it can move out to catch up with the rocket, then it would be the earth-twin who would have aged less when they met up again. From the point of view of the rocket-twin, it would look like he had been standing still the whole time, with the earth initially moving away at a constant velocity, then accelerating, then moving towards him at a constant velocity until they met again--a perfect mirror image of what the earth-twin sees in the normal version of the twin paradox.