The meaning of James R has been clearly presented. The dilation of time does not only depend on the speed of A and B, but also depends on the acceleration of A and B. James R, is my understanding correct?

Yes.

James R told us that the initial velocity of A and B is very important, and the acceleration of A and B is also very important. James R, when you talk about velocity and acceleration, do you consider reference objects?

Reference

*frames* have to be specified, obviously. They can be defined with reference to particular objects, if you like.

James R From your description, we can see that you take the earth as a reference object, and you endow A and B with an initial velocity of 0. Do you agree with me?

You told me A and B started from Earth. If that's correct, then I agree with you.

When we observe A and B, we must understand the acceleration and deceleration process of A and B from the beginning to the present.

If we're going to compare their ages, yes. If they are the same age at the start (on Earth, in this case), then we need to know how they travelled to find their ages at some later time.

Only in this way can we know who will be younger between A and B.

Correct.

If A comes from the earth, and B comes from a planet that James R does not know, does that mean that James R can never know the initial velocity of B, nor can he know the acceleration process of B, nor can he know the total time dilation on B? ?

Of course. If we have no idea where the unknown planet is or what it is doing, how could we possibly guarantee that A and B started off at the same age?

For the two brothers in the twin paradox experiment, the elder brother left the earth in a spaceship, and the elder brother has acceleration and deceleration processes, so according to James R's point of view, the elder brother will be younger.

The twin paradox starts off with twins

*of the same age* on Earth. Twin A stays on Earth. Twin B makes a round-trip journey to a (known) distant planet. Twin A does not accelerate. Twin B accelerates three times: once when he leaves Earth, once when he turns around at the distant planet, and once when we arrives back at Earth (to slow down to a stop). Clearly, the accelerations are not symmetrical.

Don't put words in my mouth, Tony. I have said nothing about any "elder brother". I have said nothing about any elder brother being younger. You shouldn't tell lies.

But did James R notice that our earth has been accelerating relative to the sun?

From one point of view, that's true. From another, it is false.

In what I just wrote above about the twin paradox, I assumed, for the sake of simplicity, that the Earth was not accelerating. That is the usual assumption in that hypothetical scenario. However, if you want to calculate for a more complex scenario, that is certainly possible.

This acceleration is obviously different from that of his brother.

Correct.

How does James R think about the impact of the acceleration of the earth on his brother's time dilation?

Any acceleration of the Earth, while one brother is journeying, will obviously affect the relative ages of the twins when they meet up again.

As the great James R who came from the Middle Ages and was proficient in SRT, did you teach Einstein the great SRT?

Yes, Tony. I taught Einstein everything he knew. I was 700 years old, back then. Ah, those were the days, when I was younger! Being the modest guy I am, I let Einstein take all the credit for the theory of relativity.

Or are you and Einstein the twins?

That's a secret, Tony.

Is there anything else I can help you with?