Hi Gregger,

It's a fact in the sense that two synchronised clocks can be compared after one has gone up on a plane. The one that stayed on the ground shows a shorter time has passed. The amount is miniscule and it is necessary to use accurate atomic clocks or something like that but it has been measured. Now, as you say, I've only read that in a book and haven't performed the experiment myself but it makes sense. It's the same with your brother, you don't see the difference because it is miniscule. If your brother's clock was running 0.00000000005% less than yours your not going to notice. You have to get close to the speed of light to actually see the effect. I have read many accounts of Relativity and while I cannot do the maths I don't think you have to in order to understand the concepts, even though they are tricky.

I'm afraid my posting rambled a bit but perhaps you can agree that if a person travels to a star five light years away at half the speed of light it should take him ten years, in theory. Now if you on Earth look through your telescope after ten years to see him reach the star you won't see it happen. He will still be only part of the way through his journey. If it does take ten years for the trip then once the astronaut has reached the star, the light from that event has to take five years to return to our telescope. Ten plus five years = 15 years for a ten year journey. Einstein's formula for time dilation (don't ask

but it's the same one involved with the plane above and it has been tested thousands of times) will give a reading on the clock on board the rocket that is less than ten years when it arrives at the star, say eight years for the trip. So you on Earth see the rocket reaching the star after 15 years when you know that it has taken 10, and you see on the rocket's clock that it reads eight years!

The bottom line here is that as two clocks travel away from each other the time taken for the time information (light) to travel between the two progressively increases so something has got to give. The light from your clock reaches your eye immediately but the light from the other can take weeks or years. It is impossible for them to look the same or tell the same time.

Regards,