travelling time

I had a class earlier in the week where something was said that I just didn't understand.  It was the Christianity and Modern Science class - which in itself may be considered a strange choice for me to have taken given that I didn't take GCSE physics and was kicked off the chemistry course before the exam (to save embarassing the school).  Nevertheless, it looked like an interesting course (which it is), and so I went for it.

On Monday's class the lecturer was speaking about time, and told the following story;

There were two twins.  On their 20th birthday one of them gets in a rocket and takes off: For the first year the rocket accelerates until it is travelling just slower than the speed of light and then for the second year it maintains that speed.  After two years it turns around and spends the first year picking up its speed again until it is just short of the speed of light and then spends the second year continuing at that speed.  The rocket lands back on earth exactly four years after it left, and he goes to find his brother to celebrate their 24th birthday.

But he can't find him, because his twin has died - of old age.  In fact, in the four years that he has been travelling, hundreds of years have passed on earth.

He had been wearing a watch which had gone round 24 times a day for the past 1461 days; he had ticked each day off on his calendar; the annually-flowering-flower that he had taken to brighten up his space ship had flowered four times; and he had aged only by four years.  But hundreds of years had passed on earth.

I don't get that.  Apparently time isn't constant.  I really don't get that.  Feel free to explain...

Comments

[michelle]

Hi Nick,

I've been asked this a couple of times recently, so I've started to answer on my (almost entirely unused-don't hold your breath for any new posts) blog. Check it out for the beginnings of an explanation

[Graham C]

Hey Nick...If your still confused after you've read michelle's explanation (which is rather awesome). Then i have some notes which explain it another way. Gimi a shout if you need them :)

[Susan]

Jurgen can explain it to you like I cant. He's very clever!!

[Aged Chris]

There are dozens of science fiction stories written round this phenomenon - you're not alone in finding it baffling. Time is elastic, as we know from daily experience. Time as a dimension in space is relative. I haven't yet read Michelle's text, and of course I'm a linguist, not a scientist, but I have read a lot of science fiction. Get stuck in and be amazed! (Just steer away from all that fantasy. Star Wars isn't science fiction; Isaac Asimov is. Take it from an old bigot.)

[Simon Cox]

Maybe his watch was slow?

[Dave Slater]

I have to agree that Michelle's explanation is great, but can also offer you a loan of Robert Heinlein's 'Time for the Stars', a book written around just this premise, if you want. It's one of his 'juvenile' books in that it's written for a teenage market but I still find it eminently readable aged 32 and it doesn't talk down to the reader. Plus Heinlein is my fav SF author which probably helps!