#### Bose

Why do most explanations of the Twins Paradox claim that the twin on the space ship ages less because he is the one who undergoes acceleration and/or changes direction, causing asymmetry between the points of view of each twin? It seems clear to me that the twin on the space ship would age less even if we ignore acceleration, or if we use a variation of the Twin Paradox where there is no acceleration of either twin during the experiment.

The Twins Paradox is asymmetrical in a very important way that has nothing to do with acceleration, and that is rarely even mentioned. The turnaround point is stipulated to be a certain distance from earth, as measured from earth, typically a distant star assumed to be at rest relative to earth. Each twin uses the distance between earth and this distant star, and the elapsed time of the trip between them in their calculations, then they compare them to each other.

Importantly, these two objects (earth and the distant star) are both at rest relative to the twin on earth, and they are both in motion relative to the twin on the space ship. Without showing all the math, this fact alone accounts for the asymmetry between the respective twins.

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#### JesseM

Why do most explanations of the Twins Paradox claim that the twin on the space ship ages less because he is the one who undergoes acceleration and/or changes direction, causing asymmetry between the points of view of each twin? It seems clear to me that the twin on the space ship would age less even if we ignore acceleration, or if we use a variation of the Twin Paradox where there is no acceleration of either twin during the experiment.
How could you "ignore acceleration", or create a scenario where there is "no acceleration", and still have a twin paradox where the two twins depart from a common location and reunite later to compare clocks locally?
Bose said:
The Twins Paradox is asymmetrical in a very important way that has nothing to do with acceleration, and that is rarely even mentioned. The turnaround point is stipulated to be a certain distance from earth, as measured from earth, typically a distant star assumed to be at rest relative to earth.
This is irrelevant, you could equally well state the turnaround point and the distance in some other frame, it would still always be the twin that accelerates that ages left. You could even imagine that the twin departs Earth, then after the twin has reached a certain distance from the Earth, the Earth accelerates to catch up with the traveling twin, who continues to move inertially; in this case, when they reunite, it will be the Earth twin who has aged less, because the Earth accelerated.
Bose said:
Importantly, these two objects (earth and the distant star) are both at rest relative to the twin on earth, and they are both in motion relative to the twin on the space ship. Without showing all the math, this fact alone accounts for the asymmetry between the respective twins.
If you actually do the math you will see that you are wrong--it doesn't matter what inertial frame you use, it will always be true that the twin that accelerates has aged less when they reunite.

#### matheinste

Hello Bose.

I believe that it is generally accepted that the asymmetry in the twin "paradox" is due to the different spacetime intervals traversed by the twins. The acceleration is not the direct cause of the ageing difference but is necessary to make the paths travelled, that is the intervals, different. A clock ( and a person ) travelling inertially between two events, departure from and return to earth, experiences the greatest time difference and so ages more than the travelling twin who must experience acceleration at some points of the journey.

In other words the path difference causes the asymmetry and the acceleration causes the path difference.

Matheinste.

#### JustinLevy

This is irrelevant, you could equally well state the turnaround point and the distance in some other frame, it would still always be the twin that accelerates that ages left. You could even imagine that the twin departs Earth, then after the twin has reached a certain distance from the Earth, the Earth accelerates to catch up with the traveling twin, who continues to move inertially; in this case, when they reunite, it will be the Earth twin who has aged less, because the Earth accelerated.
I agree, assigning a "cause" due to this is very problematic.

How could you "ignore acceleration", or create a scenario where there is "no acceleration", and still have a twin paradox where the two twins depart from a common location and reunite later to compare clocks locally?
I've seen examples that do this two ways. One involves three inertially moving observers, all in line to cross each other once... comparing clocks when they pass.

Another involves inertial paths that cross more than once (so needs either the curved space of GR, or SR on a closed universe). Imagine the surface of a cylinder (a closed universe), all parallel lines remain parallel, and thus spacetime is flat... but straight lines can cross multiple times. Or if we allow curved space, just have the twins move inertially along two different orbits around the sun and which intersect multiple times.

#### Phy6explorer

A doubt

Acceleration.

Can someone please tell me the literal meaning of acceleration that has been used in this discussion! Because I love thinking and discussing about the Twins Paradox but since I am just an Eighth Grader I would like to clear this doubt.Hey, don't think this is a silly message. I am new to the forum. But you can check out my post in the "Time Dilation question" thread.

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