My high school physics teacher used the train example to explain relativistic time dilation back in the 1970s. 40 years later relativity still gives me a headache.
Let’s use the old twins example. One twin stays home in mission control and the other twin leaves on a rocket going at close to the speed of light. The twin on the rocket ages at a slower rate. When he arrives at Alpha Centauri he will be younger than his twin back on Earth.
Here is where I get lost. Speed and motion are entirely relative and not absolute. To the twin on the rocket, the twin on Earth is moving at close to the speed of light and the twin on the rocket is stationary. Why doesn’t the twin on Earth age at a slower rate relative to the twin on the rocket?
To really complicate things, use triplets with two going off in rockets in opposite directions from Earth each moving at just under the speed of light. To the brother on Earth, each of his brothers is moving away at the speed of light. To the brothers in the rockets, the brother on earth is moving away at the speed of light. What about the brother in the other rocket? He cannot move away at a relative speed of greater than the speed of light. So what speed are the brothers in the rockets moving relative to each other?
Like I said, relativity gives me a headache.
The "relative" nature of it collapses when the traveling twin returns to the same reference frame of the earth bound one. And to change ones inertia state of motion (constant speed, constant direction), an outside force must be applied. In other words, for these effects to become permanent energy must be applied. A lot of it. In order for the traveling twin to have stopped, he had to have applied a force, exerted energy. Once acceleration (or deceleration) enter into it, the relative aspects of it go away.