Your concept of time is likely to depend on the sort of society you live in. The ancients, living in what we in our arrogance are wont to call a “primitive” or “unsophisticated” society, often regarded time as cyclical. This made sense when you consider the daily round of dawn, noon and night, the annual cycle of the seasons, the ebb and flow of the tides and even the slow march of the constellations across the night sky.
This fatalistic yet comforting notion of time received a rude shock with the invention of history. Whereas the priest emphasized the endless cycles of life where each ending was merely a return to the beginning, the historian emphasized change, differences, uniqueness of events from which flowed still more change. In a “modern” world of careers, deadlines, meetings and anxiety about the future, we no longer have any doubt about the linear nature of time.
But still we wonder. If it is possible to forecast the weather or say at what time the train from Birmingham will arrive, is it possible to know and even alter the future? Although the present seems to be the effect of which the past is the cause, might it still nevertheless be possible to go back into the past, at least as a visitor and even possibly as an agent of change? But if I change the past, how will I ever know because the present is already the result of the past and therefore contains the effects of any interference.
Such speculation leads to a paradox and both philosophers and scientists generally take paradoxes as a sign that the logic is wrong. Suppose there is something I don’t like about the present and I go back in time to effect a change. But the present is already the result of that change having occurred. So I find myself going back in time to do what has already been done, an apparently pointless exercise. Can I then decide not to go back in time? No, because without my visit, the necessary causes of the present would not be in place, but since they are in place, I necessarily have to go back into the past to bring them about. Even though they have already been brought about.
That’s all very interesting (you mutter) but it’s just speculation and couldn’t really happen. Or could it? It all depends on whether or not it will be (or even is now) possible to travel through time. The fact that we haven’t done so yet (as far as we know) doesn’t prove or disprove the theory. After all, it was only recently that men set foot on the moon after countless aeons during which such a thing was thought to be impossible. Now that we have a lunar landing under our belts, perhaps a “temporal landing” is awaiting us in the not-too-distant future.
Could such a thing happen? It all depends on the nature of time. Several models are being considered though none, as far as I know, has a majority vote. One of these, the one that is perhaps easiest to understand, postulates that Einsteinian space-time forms a kind of 4-dimensional cube. I’ll grant that a 4-dimensional cube is hard to imagine but do your best. As an approximation, think of a fish tank. It is possible to specify any position in the fish tank by measuring its distance along any three edges that meet at a common point or “origin”. Now imagine that our fish tank actually has 4 edges, all perpendicular to one another, that meet in a common origin. 3 of these are our familiar space dimensions and the fourth is time. Easy, isn’t it?
With such a 4-D cube, all events can be specified by their distance along each edge (or “axis”) from the origin. We already do something similar when, having come back from our holidays we say “At this time last week I was sitting having a coffee in the station buffet in Penzance.” We have given the event “having coffee” a precise location in both space and time.
If the world can indeed be seen as a space-time cube, then doesn’t this imply that (in principle at least) we can go to any location within it, that is, visit any place at any moment of time? In other words, does it not imply that we could move about in time as much as we do in space?
It might seem that the answer is no because while we can remain still with respect to any of the space dimensions (I can sit in the station buffet in Penzance as long as I like), we cannot do so with respect to the time dimension (by the time I leave the buffet, it will be later than when I arrived – I will have moved inexorably along the time axis).
But just a minute, I hear you say. Quite rightly you point out the famous time dilation effect predicted by Einstein and proved to exist by many observations since. It seems that if a body accelerates greatly with respect to its surroundings, its clock slows down, leading to the famous “twin paradox”, whereby one twin goes on a journey through space and returns home to find he is younger than the twin who stayed at home. It seems that when we travel along a space axis, this slows our rate of travel along the time axis, just as travelling due south slows our rate of movement along the east-west axis.
Could we ever go so fast in space that our progress along the time axis slowed to zero or even went into reverse, opening the possibility of a voyage into the past? Einstein said no but Einstein’s theory is already showing signs of inadequacy especially in extreme conditions, just as Newton’s did before it. Perhaps we are due for an update.
If we could reverse our direction of travel through space-time (e.g. travel back to London from Penzance and reach London before we originally left it), this would obviously have huge implications, not only for physics and for models of the universe but also for such philosophical questions as free will and determinism. And of course, if it turns out possible in the future, then these implications already apply right at this minute whether we realize it or not.