Quote:
Originally Posted by
Zoerawrr 
Alex raised his hand. "I was just wondering, Professor, about what you said about the faster you are, and the slower time ticks thing. I think I heard about that once to do with a space shuttle, and being on it for three years and getting off and having aged slower than their friends and spouses. So..." the boy said slowly.
He was thinking. Something was going down.
"Say you had a set of twins and at birth one was taken away to live on a train. Let's call this train!twin 'Blaine'. Now... the riddle of Blaine iiiiiis... on the land!twin's 30th birthday... Blaine wouldn't be 30, would he? But... I dunno... would it be a big-ish difference? A year? Would there even be a conceivable difference? I don't know... I'm not well versed on this stuff. Or trains."
...
"Or... twins."
Risu unearthed his impressed face again. "Have you been sneakily reading up on relativity, Mr Miner?" he asked, looking at the boy shrewdly. "Twins on a spacecraft feature in one of the most famous thought experiments of relativity, and we will get to that experiment in a moment. But first..."
He hoped the rest of the class was still listening. "Regarding your example. You're correct. Blaine on the, er, train would age slower than his twin - let's call her Patricia - would just sitting around. Now, in reality, the age difference would be absolutely tiny, as trains aren't moving very fast at all. On a, you know, relativistic scale. Even travelling for 30 years would only lead to an age difference of a fraction of a second. For time to slow down noticeably, Blaine's train needs to be moving at an appreciable fraction of the speed of light. For example, if you somehow managed to get the train moving at around 90 percent the speed of light, time would only tick half as fast for Blaine. In
that case, getting off the train at 30, his twin Patricia would be 60 years old already."
He looked at Alex questioningly. "Clear so far?"