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peselev t1_j8esdxt wrote

The one who hops into the rocket and leaves, has to accelerate (to gain speed). Because one frame experienced the acceleration, and the other frame didn't, the frames are not symmetrical. The U-turn is not the only element that requires acceleration (and deceleration upon landing)

The paradox itself was not that the twin's experiences are different, rather than the one who traveled more, aged less.

At least, this is what I remember from my university years.

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shimadon t1_j8f5l6w wrote

That's not the paradox. The paradox is that if each twin point of view is relative, then relative to the twin that is on the space ship, it's actually the earth twin that is traveling. So the spaceship twin should be older, not the earth twin. But it's not the case, and this is the paradox.

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wedgebert t1_j8i9ukt wrote

It's only a paradox due to misunderstanding what is happening.

It's not the motion or velocity that causes the time dilation, it's the acceleration. The twin on the ship experiences acceleration (which is absolute, not relative) while the Earth-bound twin does not.

Once you take this into account, the paradox is easily solved.

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shimadon t1_j8iufiu wrote

Acceleration is still not the big picture. Only within the framework of special relativity it is true that an acceleration will always result in a shorter length of space time path. When you're talking about acceleration, you are restricted to special relativity only. But the twin paradox can be formulated in general relativity framework as well. In general relativity, you can have situations in which the accelerating twin is older, because in general relativity, even accelerating objects can have longer spacetime lengths...

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Any-Broccoli-3911 t1_j8mpxqk wrote

Only if the accelerating twin passes through regions of lower space-time curvature (lower gravitational field).

And unless he gets in a region of negative curvature (doesn't exist in our universe as far as we know) his speed must not be too fast.

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PoufPoal OP t1_j8hdt64 wrote

You're right, he has to accelerate to take off too, that's true. Thank you, I didn't even think of that.

> The paradox itself was not that the twin's experiences are different, rather than the one who traveled more, aged less.

No, sorry. The paradox is the fact that depending on which twin you take as a reference frame, each twin would age more than the other.

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