h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

You forgot the 'you can't go back before your time machine was built' theory.

It is quite possible that a time machine, like your telephone, requires a device at both ends. That would mean that the time machine would have to exist at the point you wanted to go back to, and the result of this is that we won't see visitors from the future until someone invents a time machine.

There is always the alternate universe theory as well. I believe that Einstein was a supporter of this one. So, you can go back and kill your mother, just, when you return to the present, you can't nip down the bank and take out a tenner for the pub, as you don't exist to have a bank account.

As far as the "if you go back in time you alter an alternate universe" theory, my response is, "How, exactly, are you going to go into the past to begin with?" If you can answer that, then maybe that will work.

But about parallel universes. They can't exist. After all, if all parallel universes started out the same, and the universes don't interact (the above theory goes against this, hence it's potentiality), then the universes will stay the same, all the way up to the point where somone goes back in time and kills their mother. (Which will, of course, destroy the universein which it happenes and, since this will happen in each universe, the entire multiverse.)

Sorry about my spelling of uinverse in the third-from-bottom line.

I just added a new part to my entry about this theory. Read it!

This is an interesting idea. However, this does not answer any of the key questions necessary to be, in my eyes, a real theory. Two examples of these questions are 'Why?' and 'How?'. Nice idea, though. If I fine more info or public interest, I'll add a section.

P.S. The thing about the lack of visitors from the future being evidence against time travel was just an interesting way to start the article. Alone, it proves nothing, as you said. However, it is not alone. I feel the rest of the article denounces the major theories quite well.