http://www.alternativephysics.org/book/ ... llenge.htm

I have one clock here, and one clock at the other end of the Universe. And I move my clock a little, I go to the mall. Now my clock has a speed in relation to the other clock. This means mysterious forces will act on my clock, and it will go slower, than the other clock at the end of the Universe, with which it was synchronized. Isn't this instantaneous action at a distance? My clock slows down - but only in relation to another, does this not mean there is a physical phenomenon linking them, no matter the distance?If at the points A and B of [the coordinate system] K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by ½ t v2/c2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B.

Some say what's going on is going on only "from the point of view of the other clock", and no real "slowing down" happens. I agree with that. Then comes the "twins paradox" called a paradox because this reciprocity is broken, and one twin really remains younger.... Because he "changed the reference frame" (this is the mainstream accepted solution). And the instantaneous action at a distance is real here.

Twins A, B, they are equal. B goes on a journey, while he goes away he sees A clock slowing down, and A sees B clock slowing down. But nothing real happens. Then, at the moment B changes course and returns, real time dilation kicks in and he arrives younger....That's what the laws of physics say...

What if at the point of course change, when B returns, there is twin C, C = A = B? While B goes from A to C, these three remain equal. Then B leaves C and returns to A, and remains younger than A at arrival.... But in the same way as when B was leaving A and the time dilation was "not real", now B leaves C, and the dilation is not real because no course change or reference frame change happened. This means B = C. And C = A, this means B = A. Else we live in Harry Potter land.

https://en.wikipedia.org/wiki/Twin_paradox

Hafele Keating proved nothing. Even the atomic clock creator disagreed with them.The paradoxical aspect of the twins' situation arises from the fact that at any given moment the travelling twin's clock is running slow in the earthbound twin's inertial frame, but based on the relativity principle one could equally argue that the earthbound twin's clock is running slow in the travelling twin's inertial frame.[16][17][18] One proposed resolution is that the earthbound twin is at rest in the same inertial frame throughout the journey, while the travelling twin is not: in the simplest version of the thought-experiment, the travelling twin switches at the midpoint of the trip from being at rest in an inertial frame with velocity in one direction (away from the earth) to being at rest in an inertial frame with velocity in the opposite direction (towards the earth).

https://debunkingrelativity.com/twin-flight-experiment/