# Why the stay-at-home twin is not considered to be accelerating?

An accelerometer attached to B reads 0. Only A is accelerating according to attached accelerometers.

Because accelerometers show that it is not relative.
Thanks. Now getting the point.

Dale
The key point to take away from the twin paradox is that the time difference is due to the moving twin's change of frame.

The acceleration is essentially irrelevant except for the fact that the moving twin must decelerate and accelerate so that they can change their frame.

You can easily set up a twin paradox without any accelerations. I use the values from above.

Set up a stationary observer on earth, another at the turning poiint and another at twice the distance of turning point. All three synchronise their clocks to read 0.

The moving twin_1 accelerates before the experiment starts in such a way that she is travelling at V when she passes earth, and she passes when the earth clock reads 0. She looks at the earth clock and sets her clock to 0.

A similarly moving twin_2 passes the point twice the turning point distance away, travelling at V towards earth.

The two moving travellers meet at the turning point. The stationary person's clock reads 5 but twin_1's clock reads only 4.5.

twin_2 sets her clock to be the same at twin_1, or 4.5. twin_2 continues to earth and arrives at earth when earth says it is 10. However, twin_2's clock reads 9, made up of the 4.5 which she set, plus the 4.5 for her journey time, totalling 9.

So, whereas the earth bound twin says 10 years have elapsed since twin_1 left, the total travel time measured by the two moving twins is only 9 years.

So, we have the same time difference but there have been no accelerations.

Acceleration has nothing to do with the twin paradox other than it is necessary for the moving twin to accelerate and decelerate to change her frame. The twin paradox is about changing frames.
Thanks. A new thought for me.

PAllen
2019 Award
Thanks. Nice explanation of coordinate and proper acceleration. Now I am getting the point. The problem is, in most of the books dealing with SR or GR, that I have read, and as far as I remember, only the term acceleration is used, not proper acceleration. More so, doesn't it mean that, "motion is relative" this statement is not fully applicable in case of motion experiencing proper acceleration?
Actually, Einstein, probably the biggest proponent of ‘motion is relative’, clearly and mathematically specified that if motion involving proper acceleration is compared to motion without, you need to include time dilation due to potential difference in the calculation. This means that the traveling twin must have a period where the stay at home twin’s clock runs faster due to being higher in a potential well.

dayalanand roy
PAllen
2019 Award
All this talk of changing frames leads me to the following analogy. In Euclidean plane geometry we have a simple law, the triangle inequality, closely related to the more general law that between any two points, a path which never changes direction is always shorter than any path with direction changes. These are considered geometric properties of a plane, not related to any coordinates we may place on the plane, nor which of any techniques may be used to measure lengths. The fundamental fact is that the minimizing distance is along a path that doesn’t change direction.

Making a correspondence to special relativity, change of direction corresponds to proper acceration, the latter being exactly change of normalized tangent vector, which can only be change of direction in spacetime. A frame in SR may be said to correspond to using a ruler in a particular orientation in Euclidean geometry.

The claims that acceleration does’t matter, only change of frames matters is like saying the triangle inequality isn’t at all about change in direction (acceleration), instead it is about change of rulers. The triplet form of twin scenario is like claiming that change of direction is irrelevant because instead of changing the direction of one ruler, we can use two rulers in different orientations.

Note that by analogy, the clock hypothesis corresponds to the notion that when measuring the length of a general curve by running a string along it, then measuring the string along a flat ruler, we don’t need to introduce any adjustments for where we had to bend the string. This has nothing to do with the prior, IMO, silly statements about change of rulers and change of reference frames.

Last edited:
vanhees71, A.T. and dayalanand roy
If, as in the twin paradox, the acceleration causes a change in speed then this does does affect the clock rate.

99% of the explanations of the twin paradox say "It is caused by the fact that the travelling twin accelerates" but they never give a calculation showing how it comes about, nor provide an equation relating the age difference to the rate and duration of the acceleration experienced. I suggest the statement is therefore as meaningless as saying "It is caused by the fact that the travelling twin is wearing a bikini".

The twin paradox is explained by the fact that the moving twin changes their frame of reference. That is the essential kernel of the solution - everything else is second order. It is a simple application of the Lorentz transformation equations to get the resultant time difference.

Secondly, no-one citing acceleration as the cause ever shows how acceleration can account for the fact that both twins see each each other age more slowly than themselves during the entire time. This can only be resolved by invoking a frame change. It has nothing to do with acceleration.

Perhaps we could ask the original poster dayalanand roy , who has obviously worked on the subject, whether he found the change of frame explained things to him.
Thanks for your participation and making the debate so illuminating. But, as I told in the very beginning of the post, I am not a physicist. I am a biologist. Relativity and twin paradox is quite naive to me. And my main aim is to understand what is time itself. I am working on this very problem. The current dispute that whether it is just frame change, or acceleration that accounts for the twins gaining different ages can be better solved by you learned physicists itself, and I hope you will do it. I am just here to learn from you people.
Thanks again and regards.

Actually, Einstein, probably the biggest proponent of ‘motion is relative’, clearly and mathematically specified that if motion involving proper acceleration is compared to motion without, you need to include time dilation due to potential difference in the calculation. This means that the traveling twin must have a period where the stay at home twin’s clock runs faster due to being higher in a potential well.
Thanks. But here, the potential difference and potential well are a bit new terms for me. Is the potential difference term used here conveys the same meaning as in case of any electric current?

jbriggs444
Homework Helper
2019 Award
Thanks. But here, the potential difference and potential well are a bit new terms for me. Is the potential difference term used here conveys the same meaning as in case of any electric current?
Similar. They both involve the notion of a mathematical object known as a "scalar field". Which pretty just means associating a number with every point in a space. They both have units of energy per unit charge. It is just that in one case the "charge" is the electrical charge on a test particle and in the other case the "charge" is the mass of the test particle.

But different. The "potential field" associated with a stationary electrical charge is a more or less real thing that is present (in some form) regardless of one's choice of coordinates. The "potential field" associated with an acceleration in special relativity is more of a mathematical fiction -- it reflects a choice of coordinates more than an underlying physical reality.

A.T.
Is the potential difference term used here conveys the same meaning as in case of any electric current?
More like gravitational potential. When a rocket is accelerating you effectively have a gravitational field in the frame of the rocket. Clocks at different potentials in that field run at different rates, even when they are not moving in that frame.

Last edited:
dayalanand roy
Janus
Staff Emeritus
Gold Member
. More so, doesn't it mean that, "motion is relative" this statement is not fully applicable in case of motion experiencing proper acceleration?
No. While you can tell who accelerated ( changed velocities), you can't tell what velocity they began with or ended with, only that they differ by a certain amount. So for example, with two starting ships side by side, and one accelerates by 100 km/sec. There is no way to distinguish between both rockets starting "at rest" and one accelerating to 100 km/sec, and both rockets already moving at 100 km/sec and one accelerating to a stop while the other continues on.

dayalanand roy
No. While you can tell who accelerated ( changed velocities), you can't tell what velocity they began with or ended with, only that they differ by a certain amount. So for example, with two starting ships side by side, and one accelerates by 100 km/sec. There is no way to distinguish between both rockets starting "at rest" and one accelerating to 100 km/sec, and both rockets already moving at 100 km/sec and one accelerating to a stop while the other continues on.
Thanks. Now I am getting the point.

More like gravitational potential. When a rocket is accelerating you effectively have a gravitational field in the frame of the rocket. Clocks at different potentials in that field run at different rates, even when they are not moving in that frame.
Yes. Thanks for a simpler explanation.