Consider a pair of twins, born on Earth. One of them boards the starship Enterprise, which quickly accelerates to a velocity of 0.9c relative to the Earth. This twin travels for what appears to him to be 10 years, then turns around and returns to Earth, where he decelerates and stops. To the traveling twin only 20 years have passed, while the prediction of relativity is that 46 years will have passed to the twin who stayed at rest on the Earth.
One might argue that we could have considered a reference frame in which the spaceship remained at rest, while the Earth moved away and came back to the spaceship. This should be an equivalent physical situation, so why isn't the twin in the spaceship the older one? This argument is incorrect. Only the Earth twin stayed in the same inertial frame. The spaceship accelerated upon leaving the Earth, at the turnaround point, and when stopping at the end, so the spaceship's frame is not inertial and hence Postulate 1 does not apply.
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