The first relativity theory is the Lorentz Ether Theory : https://en.wikipedia.org/wiki/Lorentz_ether_theory
Only this theory correctly uses Lorentz transformations in accelerated frames of reference, Einstein's relativity only works in inertial frames of reference. Explain to me what happens according to Einstein's theory of relativity in accelerated frames of reference.
If you observe an object that is accelerating, you see that the Doppler effect it perceives is due to its change in speed relative to light. If we place ourselves in the reference frame of the accelerating object, what explanation do you find for the Doppler effect other than the same thing, a change in the speed of light?
During inertial phases, it is impossible to know who is moving and who is stationary relative to the ether, but during acceleration it becomes visible, because the one who accelerates is the one who changes speed relative to the ether and who produces the Doppler effect.
In the twin paradox, the traveling twin ages less from the Earth's point of view during the outward and return journeys, and from the traveling twin's point of view it is the Earth that ages less during the outward and return journeys. This is because each assumes that it is the other who is moving and undergoing time dilation. But this assumption is arbitrary, it could very well be the observer who is moving and the observed who is at rest, the observed Doppler effect would be the same because the relativistic Doppler effect is symmetric. In fact, once the traveling twin accelerates to leave Earth he undergoes time dilation, but due to the symmetry of the relativistic Doppler effect, he can assume that it is Earth that is moving and undergoing the Doppler effect. As I have already explained, the fact that he perceives the effect instantaneously shows that he is the one who is moving and undergoing time dilation. All that I am saying here is nothing more than Lorentz's theory, which claims that the symmetry of Lorentz transformations is only observational and not physical.
In fact, Einstein's theory does not work in accelerations, it predicts physical changes in simultaneity that do not exist experimentally. It predicts that the Earth ages suddenly at the time of the U-turn, but this is false because the light signals emanating from the Earth do not undergo a jump into the future, they are entirely predicted by kinematic Doppler effects. The Earth's jump into the future comes only from resetting the clocks. After the U-turn, the clocks are resynchronized because the speed of light has changed and they need to be recalibrated. The estimated time of the Earth is then projected into the future, but the light signals from the Earth are not projected into the future, it is only the hands of the clocks that change. There is no physical jump.
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Look at this paper : https://www.researchgate.net/publication/228609140_The_twin_paradox_in_special_relativity_and_in_Lorentz_ether_theory