Clocks in a Spaceship

When our spaceship speeds up, it gains in size and time slows down. However, there is a much more direct thing going on when it comes to time.

As seen by an observer from behind the spacecraft, time is not only slowing down due to increase in size. It is slowing down because the spaceship becomes ever more distant. Between each tick of the clock, the spaceship is farther away. If the spaceship sends a flash of photons towards the observer every second, the flashes will be spaced out more than a second. This is true even if the spaceship did not grow.

An observer in front of the spaceship on the other hand will see the opposite. The flashes will come more frequently than every second. For every flash, the spaceship is closer to the observer, so the most recent flash will have shorter to travel than the older ones.

It is only when viewed from the side that the observer sees the time effect solely due to the spaceship increased size. Viewed from the front and the back, the spaceship's clock appears to go faster or slower respectively.

By Mettness, CC BY-SA 3.0, Link