2018-02-28 14:24:54 UTC
This is universally taught but is non sequitur - it doesn't follow from Einstein's 1905 postulates.
The following two conclusions do follow from the postulates:
Moving clocks tick more slowly than stationary ones, as judged from the stationary system.
Moving clocks tick FASTER than stationary ones, as judged from the moving system.
The two conclusions that do follow from the postulates are sterile - they involve no prediction. In contrast, the non sequitur predicts the miracle that converted Einstein into a deity (and killed physics):
Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this striking example of time dilation proves that time travel (towards the future) is possible. As a gedanken experiment (if we neglect practicalities such as the technology needed for reaching velocities comparable to the velocity of light, the cost of the fuel and the capacity of the traveller to sustain high accelerations), it shows that a sentient being can jump, "within a minute" (of his experienced time) arbitrarily far in the future, say sixty million years ahead, and see, and be part of, what (will) happen then on Earth. This is a clear way of realizing that the future "already exists" (as we can experience it "in a minute")." http://www.bourbaphy.fr/damourtemps.pdf