So I was watching PBS last night, and there was this program on Time Travel. I didn't watch too much of it, but there were some interesting theories. One physicist mathematically "proved" that if time travel is possible, then you will not be able to change the past. This is a closed-system problem, where if you try to change the past then there will be a force that stops you, in order to satisfy some equation. That one's a bit weird, although mathematics have proven some weird things before.

Now, the show goes on to interview Stephen Hawkings and some other physicist. Both claim that time travel is impossible. Hawkings' simple rationale for this is that if time travel were possible, then there should have been real evidence by now. Another "scientist" claimed that time travelers would have the technology to keep themselves hidden, and that perhaps we have seen time travelers and mistaken them for UFOs and other such phenomena. To which Hawkings scoffs that time travelers must be rather addled to show themselves only to wackos and nutcases.

Later on in the show we learn that Hawkins has mathematically proven that it is impossible to time travel. As your machine gets to the point of time travelling, there is an energy feed- back that destroys it. Well, that's not quite true. Standard Einsteinian physics say that the time machine will explode. But, at the time that a time machine is fully activated, quantum mechanics takes over and at that point Hawkings is not too sure what will happen. Well, I wasn't paying full attention to the show.

Yet another theory (and all these theories have some basis on scientific principles) is that any time machine will not be able to go farther back than it's creation. That prevents you from going back in time to kill your father, although how that prevents you from waiting a year and then going back to tell yourself the stock market performance, I don't know.

The last theory that I can remember is for having multiple universes. It goes something like you can't precisely determine the speed AND location of a particle. There is some uncertainty. If you take a laser, pass it through two slits, you get an interference pattern. If you filter the laser beam such that only one or so photons are going through at once, the random splashes on the screen, when taken together over time, sum up to an interference pattern. How can single photons interfere to create an interference pattern? The answer is that they interfere with photons from parallel universes. The infinite possibilities all exist at the same time in a multitude of universes. This is all to say that this guys point is that when you time travel, you always end up in a different universe. Therefore you can't have any causality dilemmas. Whatever.

What I take out of this is that scientists are people too. They have their beliefs and they'll doggedly find evidence to support those beliefs. And with something as speculative as time travel, or anything at the frontiers, you will get many wildly different theories about what things are possible. It's interesting and makes some great story ideas.