|MadSci Network: Physics|
Travelling forward in time is much easier to explain -- and is also far more likely!
You could "travel forward in time" relative to everyone on Earth if you jumped into a spaceship that took you nearly up to lightspeed, flew around in a big circle, and then came back to Earth. When you return, everyone else would have aged more than you had; in effect, you will have travelled forward in time.
You might describe this by comparing a wristwatch on the "time-traveller" to a clock on Earth. For someone watching from Earth, the watch would seem to move much slower than the clock; although the time-traveller wouldn't notice anything strange about her own watch. She'd just step off the spaceship and would find that even though (perhaps) she had aged a year during the trip, everyone else had aged a hundred years. Effectively, she travels into the future.
As for going backwards in time... Yes, the equations make it look like you could travel backwards in time by going faster than light, but the same equations show that going faster than light is impossible! I suppose one way to describe this would be to say that your watch slows down more and more the closer you get to lightspeed, and if you could ever get to lightspeed the watch would stop completely. Going faster than that (even though it's impossible) would theoretically take you back in time.
Of course, nothing can help wrap young minds around tough science concepts than good science fiction. Look for the book "Danny Dunn and the Time Machine", for a story that got me thinking about time travel when I was 11. And there are also movies, which usually aren't consistent when it comes to time travel, but can still stimulate discussion. Back to the Future II wasn't a great movie, but it was more sophisticated than most when dealing w/ time travel paradoxes. I suppose your cousins aren't quite ready for the Terminator movies, but I always use those two movies to discuss the two different philosophies of time travel. The first movie is the T1 theory, where there is only one universe, and anything that goes backwards in time has to be consistent with what has already happened. In T2, consistency is no longer needed, and now you can change the future, branch off into different possible universes.
Good luck! Here are some more resources.
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