• Bose
In summary, the twin on the spaceship 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.
Bose
Why do most explanations of the Twins Paradox claim that the twin on the spaceship 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 spaceship 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.

Bose said:
Why do most explanations of the Twins Paradox claim that the twin on the spaceship 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 spaceship 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.

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 ) traveling inertially between two events, departure from and return to earth, experiences the greatest time difference and so ages more than the traveling 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.

JesseM said:
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.

JesseM said:
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.

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.

## 1. What is the Twins Paradox?

The Twins Paradox is a thought experiment in special relativity that explores the effects of time dilation on the aging of twins in different frames of reference. It involves one twin traveling at high speeds and returning to Earth, while the other twin remains stationary on Earth. Due to time dilation, the traveling twin will age slower than the stationary twin, resulting in a perceived paradox when they are reunited.

## 2. How does acceleration affect the Twins Paradox?

Acceleration plays a crucial role in the Twins Paradox. It is the acceleration and deceleration of the traveling twin that causes the effects of time dilation. When the twin is accelerating, they experience a different frame of reference, leading to a difference in their perceived passage of time compared to the stationary twin.

## 3. Can the Twins Paradox be explained by the Equivalence Principle?

No, the Equivalence Principle, which states that the effects of gravity and acceleration are indistinguishable, cannot fully explain the Twins Paradox. While both involve acceleration, the Twins Paradox also involves a difference in frames of reference and the effects of time dilation.

## 4. Are there any real-life examples of the Twins Paradox?

There are no real-life examples of the Twins Paradox as it is a thought experiment. However, the effects of time dilation have been observed in experiments with atomic clocks on airplanes and satellites traveling at high speeds.

## 5. How does the resolution of the Twins Paradox affect our understanding of time and space?

The resolution of the Twins Paradox through the theories of special and general relativity has greatly expanded our understanding of time and space. It has shown that time is not absolute and can be affected by factors such as speed and gravity, and that space and time are interconnected in the fabric of the universe. This has led to advancements in fields such as cosmology and GPS technology.

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