The astronaut at the bottom of the spaceship is going to send a pulse of light to the astronaut at the top each time his clock ticks. The astronaut at the top can then compare the arrival of these pulses with the ticking of her clock. Without acceleration or gravity, the arrival of the pulses and the ticking of the clocks will be in synch. However, let me now accelerate the spaceship in the direction of the top of the spaceship. A pulse is emitted at the bottom of the spaceship, and, as the spaceship accelerates away, the light has further to travel each time, so that it takes longer and
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Wow what an explanation!
