Time Travel Troubles

Time travel has always been a popular subject in science fiction, but in the last two decades it has entered the realm of serious science following a number of theoretical physics papers outlining the details of how to construct a time machine. Of course this also requires a serious investigation into several paradoxes inherent in time travel - what happens if you kill your own grandfather before you are born, and therefore make it impossible for you to go back and kill him? If you go back in time and give a copy of Shakespeare's plays to him before he writes them, who actually wrote them? And my favourite, an orphaned boy grows up, goes back in time, fathers himself, then gets a sex change, goes back in time, and is his own mother as well...

So could these happen? There is no answer yet from the scientific community. One solution is to argue that some undiscovered law of physics prevents time machines (or at least hides them). For example, if light falls in a time machine, goes back in time, and then comes out and re-enters the time machine it doubles the energy in the machine. Now that doubled light does the same thing and you have four times as much energy. Obviously with this argument you can quickly argue that as soon as a time machine forms the feedback it generates destroys it! A similiar argument would have the huge energy form an event horizon (black hole) around the time machine so it cannot influence anything else.

It is the other solution which provides an interesting philosophical problem. It has been demonstrated that quantum mechanics can also solve the problems for simple systems, by ALWAYS going to a non-paradoxical solution. The standard example is of a bomb rolled into a time machine in such a way that it goes back in time and destroys itself before being launched. It turns out that the bomb will always come out of the machine slightly off course, and therefore does not destroy its original self but instead knocks it off course, and then the off-course original bomb comes out of the machine off course. No paradox!
So in the grandfather paradox, the solution is that the gun jams, or the grandfather somehow survives the killing attempt, or some other unexpected contigency.

As for the creation paradoxes, such as the plays of Shakespeare, it is possible that the same method prevents the transfer of information. But we also know in quantum mechanics that particles can suddenly appear, travel for a while and then bounce back and travel back in time until they destroy themselves. Perhaps the plays are a large scale version of this effect, where they are spontaneously created? (This actually violates several other laws of physics, and is not much of an answer anyway, but it is interesting...)

There is a third solution which is also quite interesting - multiple universes. There is an idea in quantum mechanics that every time a decision is made, a new universe is created with each universe containing one outcome of the decision. Flip a coin, and in one universe it is heads while another universe is exactly the same except in contains tails. Then time travel is easy! By arriving at a previous time, you have automatically landed in a different copy of the universe (because one copy you arrive, one copy you don't) and if you then kill someone or change something, you go into yet another version of the Universe. And it doesn't matter, because you are born in your original universe, so what you do in the other one has no effect on you or your world!