There are some basic facts about time travel, because i've always found it fascinating. First, we need to understand that there is only one conception of time in which time travel makes sense - the ??tenseless? theory of time. The common-sense view of time is the ??tensed? view which holds that time, unlike space, flows or becomes. On this view, time, like a river, is a dynamic or changing entity. According to the tensed theory of time, the future is undetermined, unreal, and open. The past is set, but long gone. Only the present is real on this view, and the present is where the past and future meet. The tensed view of time is not only the common sense view, at one time it was also widely held by philosophers. Take this passage from Augustine(spellin?): ??How can the past and future be when the past no longer is and the future is not yet? As for the present, if it were always present and never moved on to become the past, it would not be time but eternity?.

By contrast, the tenseless theory of time holds that the ??past,? ??present,? and ??future? are all equally real. On this view there is no becoming, no change, and the future is not open. In addition to the three spatial dimensions, time is conceived as a fourth dimension that is very much like just another spatial dimension. Events such as your birth, graduation, and death are all equally real (though not equally present) and we can plot them on a space-time diagram just as we would plot a point on a regular map. On this ??block? or ??static? picture of time the universe is a ??four-dimensional space-time continuum.? The notion of ??now? or ??the present? has no fixed position on this view. Indeed, past, present, and future are relative notions, relative to where you are on the space-time block. The events of your birth and death, just like Paris and Hong Kong, are equally real, they just exist at different space-time points. The temporal relations among all four-dimensional objects are fixed forever. As evidenced by the following quotation Einstein himself held the tenseless view of time because he believed, as do many philosophers and physicists today, that the relativity of simultaneity implies it: ??[t]he distinction between past, present and future is only an illusion, even if a stubborn one".

From a four-dimensional perspective there is no such thing as change. The universe is like a still-born space-time jewel with many facest (e.g., space-time points), hence the name ??blockk world.? The best way to conceive of change in such a world is by analogy with the illusion of change created by still film frames moving through a projector.

It should now be easy to see why time travel only makes sense on the tenseless view of time. On the tensed picture of time neither the past nor the future exist; only the present is real. Therefore there is no past or future to travel to on the tensed view! On the other hand, the tenseless view makes time travel possible, at least in principle, because all space-time points are equally real. In the block universe ??past? and ??future? (relative to our frame of reference) are fixed forever. One interesting consequence of all this is that even if time travel is possible, it is not possible to change the past or the future. Thus all time travel stories in which people go back in time and change events that have already happened are inconsistent. Of course one can always try to render such stories consistent by claiming that the ??new? past or future is really a distinct branch of time, different time line, different world, parallel universe, etc. But it is very different to travel ??across? possible worlds as they do in the TV show Sliders and to travel in time as they do in the film Twelve Monkeys. For example, the film Back to the Future is a consistent story about possible worlds travel but an inconsistent time travel story because Marty changes things that have already occurred, such as the past and present events of his parent??s lives. There is nothing wrong with this but such stories are better dubbed possibility travel than time travel, because one is moving through possibility space and not time within our universe. To put it another way, if you go back in time and perform some action, according to block world, this means you always went back in time and performed that action. It cannot be that the past event unfolded the first time without you and then you go back in time and the past event unfolds for the second time with you! For example, if you go back in time and assassinate Hitler before the start of WW II then you were always the one to shoot Hitler; which means that you should not bother trying to do this, because it never happened! You may ask: but what prevents me from shooting Hitler if time travel is possible? People differ about how best to answer this question but since time travel is only possible in the block universe and since Hitler was not shot before the war, then we know that you cannot go back and shoot him now. It would be a contradiction if both Hitler was not shot before the war started and Hitler was shot before the war started; and logical contradictions just cannot happen. In short, the only time travel events that are possible (that can exist) are those that do not entail a contradiction.