[neva30 (OP)]

I was wondering whether anybody could explain why it is that somebody travelling close to the speed of light would age far slower than somebody left on earth?
Thanks.

[DoctorBeaver]

Time dilation is not simple for people to grasp. Basically it's to do with the speed of light being constant.

Imagine having 2 boats. At the bottom of the mast of each is a light source shinig up the mast to a mirror. That mirror reflects the light back down the mast to another mirror that reflects the light up the mast again to the 1st mirror, and so on. You now have light travelling up & down the mast. Let's say that both masts are 10m tall. That means that for the light to complete 1 cycle it travels 20m.

1 of the boats remains at anchor, so the light beam on that boat is travelling vertically - still 20m. The other boat, however, starts sailing off. Now, not only is the light travelling up & down the mast, but the boat is moving. By the time the light reaches the top of the mast and is reflected back to the bottom mirror it has travelled 20m plus the distance the boat has travelled in that period of time. That means the light beam has followed the path of an inverted V from the perspective of the stationary boat. That is obviously longer than a simple up-down path; but for those on the moving boat, the beam is still only travelling vertically. We therefore have a discrepancy between the perceived distance that the light has travelled in any given period of time.

The only way to resolve the paradox is time dilation. From the stationary boat's perspective, time must be passing slower on the moving boat.

Here is a page that explains it quite well. Play the little video clip & you'll see what I was trying to say.

[lightarrow]

What Doctor Beaver wrote is correct and I also thank him for the link. I just want to add: it's not completely correct to say that "somebody travelling close to the speed of light would age far slower than somebody left on earth" if you don't specify that  the one who leaves Earth then comes back, and so have to decelerate and accelerate. In theory, he could also stay in his starship without activate the engine, and be the Earth to accelerate and then come back; in this case it would be the Earth and all living or not-living being there to age far slower than the one on the starship; if they both accelerated in the same way, there would be no age difference. Conclusion: is the one who experiences acceleration who age slower.

[neva30 (OP)]

Thanks for the replies. I'm reading through the attached explanation (slowly!) and think I'm making some headway.