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...