Time travel explained
As far as actual present-day Physics is concerned, there are only two possible interpretations of time travel.
(Note that the next two paragraphs are a bit technical, but it gets better).
As described by General Relativity, the universe may be regarded as a static 4-dimensional entity - three spatial dimensions and a time dimension. A point object moving in time therefore becomes a line when you imagine it in space-time. General Relativity describes gravity not as a force, but as space-time becoming curved. So a point object moving through deep space with no measurable gravitational influence will move in a straight line, but if it passes near a star then its course will be curved around the star. However, this is not because gravity is "pulling" it towards the star. In Relativity terms the object is not accelerating, has no force acting on it, and is not deviating from its space-time course, it's just that space-time itself is actually curved, and thus an object with no force acting on it follows a curved path.
Now: what happens if space-time is really sharply curved? The answer is that it may be possible for space-time to be so folded back on itself that one point in space-time actually coincides with another point. This is the "wormhole" phenomenon that is much beloved by spaceship-based sci-fi. You fly through one "end" of the wormhole and instantly emerge on the other side without travelling any of the points in between.
Wormholes are respectable Physics, and indeed appear and dissappear all the time - but only on the subatomic scale. The question of whether it is possible to make a wormhole large enough for something the size of a spaceship to pass through is much less clear-cut. Mathematically speaking such a thing could continue to exist - but how it could actually be created is another question. (You should also note that, by definition, there can't be anything "inside" a wormhole. The points at either end are actually the same point in space-time.)
If we could make a smallish wormhole then it's fairly easy to make it into a time machine. Leave one end of it on earth, then take the other end away at very high speeds. In the same way as the so-called "twins paradox" (which is not actually a paradox at all) when you eventually bring the other end back again you will have created a time differential between one side of the wormhole and the other. Step through it and you emerge not just at a different point in space, but a different point in time as well.
The important aspect of all this is that there is only one space-time universe. What that means is that whatever happened in the past happened, and cannot somehow be made to happen another way. Thus, if I somehow acquire a time machine and I go back in time intending to shoot my parents before I was conceived then I already know that I'm not going to succeed. Either they were shot, or they weren't. And it's a matter of direct observation that they weren't. Therefore nothing can change that.
If I go back in time then maybe I won't find them, or maybe my gun will jam - but either way there is no way to alter something that has already happened.
Interestingly, physicists have actually done calculations about the behaviour of a subatomic particle encountering a small wormhole of this sort. If we imagine that the particle collides with another particle in such a way as to get knocked into the wormhole, then emerges from the other end slightly earlier and becomes the particle that knocked its subjectively earlier self into the wormhole in the first place, then you can actually prove mathematically that the only permissible path for the particle emerging from the wormhole is one that preserves the original collision.
Virtually all sci-fi time travel stories fall down if this is how you view time travel.
There is an alternative theory - but it's a much more dubious one.
If you want to talk about "changing history" then time would have to be multidimensional. For example, things happened one way at a time that was 12 o'clock last Tuesday, but at a time that was also 12 o'clock last Tuesday but somehow also not the same 12 o'clock last Tuesday that we were talking about previously, something else happened.
One of the (somewhat doubtful) interpretations of Quantum Mechanics and the whole business with Schroedinger's Cat is that there are actually multiple space-time universes. Each time a quantum event could go two or more different ways, new universes are created for each possible outcome.
For example, if an electron beam is fired a two slots, you might have thought that each electron has to go either through one slot or the other - but what you actually see is a result which makes it look as if the electrons are going through both slots at once in a wave-like fashion and producing interference effects. According to the many-worlds interpretation, the electrons actually do go through one slot or the other, with both possible results happening simultaneously in different universes. Some more eccentric physicists go on to suggest that in fact electrons don't behave like waves at all, but that the different versions of the electrons in nearby universes interact with one another to produce the interference effect.
If you can swallow the idea that all possible outcomes of all possible situations actually have some sort of physical existence, you can then imagine a situation where a time machine crosses from one universe to another at the point where the two branch. For example:
I go back in time and shoot my parents before I'm conceived, then go forward again. We can now imagine two different universes (or "timelines" as Star Trek calls them). In one universe nothing involving shooting happens to my parents, I am eventually given birth to, and then one day step into a time machine and dissappear. In the other universe a mysterious stranger (me) appears in a time machine just after my parents have met and guns them down. The stranger steps back into the time machine and vanishes. NicolasB is never born, and no such individual ever comes into existence.
No contradictions there on either side.
Now, what happens to me when I return to my own time? The answer is that I come back to a reality that is different from the one I left, because I am no longer in the same universe that I set out from. By helping events to turn out as they did, I have moved into a different branch. In the new branch there is no such person as NicolasB and never has been. No one will recognise me or know who I am. Back in the universe where I originally stepped into my time machine, however, nothing has changed. In that timeline there was a NicolasB, who one day climbed into a time machine and dissappeared - but that was the last anyone saw of him. He never reappeared, because he's switched into a different universe.
One could perhaps stretch the idea still farther and imagine a machine which has the ability to move sideways in time as well as forwards and backwards - in other words hop from one timeline to another. Using a machine like that I could perhaps get back to the universe I originally came from - but when I got there, nothing would have changed: the shooting of my parents didn't happen in that timeline, but in another one that I no longer occupy.
No matter how much you play about with this idea, though, most sci-fi time travel stories still don't hold up. Star Trek is particularly bad. According to the Star Trek model, if you watch me getting into my time machine and travelling back and then I "alter" something, then from your perspective suddenly everything will change. It even suggests that somehow you can be protected from the effects of this change (by "chroniton emissions", perhaps) even though the rest of the universe isn't, so that you can actually remember how things were "before" they changed, and recognise the change.
None of this makes any sense at all even with with the "many worlds" interpretation. It might make sense for a person to go back in time to try to "change" things - but all they would be doing would be moving themselves into another timeline. It makes no sense for anyone else to try and stop them from doing whatever they're doing, because the timeline they left cannot be affected by their actions. (And, for that matter, the other timeline in which their actions were significant already exists even before they go back in time, and will continue to exist regardless of what anyone does). Edit: revisited this post on 20/6/2005 and amended some typographical errors.
Last edited by NicolasB; 20-06-2005 at 10:22 AM.
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