Originally Posted by

**David Mellor**
My thanks to you all for attempting to answer my question. I looked at the suggested articles but they do not seem to deal with my specific question or if they do I lack comprehension.

In my scenario there is only one observer and he is the spaceship pilot who sets the two clocks to exactly the same time on the launch pad. One clock is left on the launch pad. The pilot boards the spaceship with the second clock and blasts off into space returning to earth after a one hour flight at near the speed of light. On landing he compares the two clocks and finds that the spaceship clock has lost time such that it appears that the clocks have been apart for less than one hour.

My difficulty is understanding what mechanical, physical, gravatational force or whatever caused the spaceship clock to tick more slowly than the clock that remained on earth.

It's really simple, much simpler than you think, but to understand it you have to go back to basics. Instead of employing mechanical clocks, use parallel-mirror light clocks. Now look at the

Simple inference of time dilation due to relative velocity. The hypotenuse is the light path, the base is the speed as a fraction of c, and the height gives the Lorentz factor. It's basically just Pythagoras' theorem, and the bottom line is that the second clock shows a lower reading because it did a lot of moving out and back through space. The light-path between its parallel mirrors was like this /\/\/\/\/\/\/\ instead of this

**|**, so it "ticked" at a slower rate.

The next step is to appreciate the relevance of

pair production where we make subatomic particles out of light. There's also annihilation, which is the inverse:

You're made out of subatomic particles, which have spin. Think of them as "light going round and round". When you move a particle, the light path is no longer circular, but more like a stretched-out spring. Look at it from the side and it's like this: /\/\/\/\/\/\/\/\/\. So matter is affected just like the light between the parallel mirrors.