Time

Is time an independent physical dimension or a human construct designed to compare events to each other ? If it is a physical entity why can we move only in one direction and at an inexorable pace? Is it theoretically possible for a time machine (Hot Tub or any other sort) could exist?

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Time is a physical dimension. The dimension in which something exists is just the minimum number of co-ordinates that are needed to locate the point at which it exists. So three co-ordinates are needed to specify a point in Euclidean space, and accordingly Euclidean space has a dimension of 3. In the physics of relativity theory space and time are not 'independent', as you put it. On a relational or Leibnizian view, such as relativity theory, a space is merely the order of the space occupants.

Time, on the other hand time, is one-dimensional. All we need to do to locate an event is to specify one time, say Tuesday: 'The murder happened on Tuesday.' (Some philosophers have discussed the question whether time itself could fork, and whether there could be disjoint times, as distinct from distinct possibilities within time.)

Psychological events are also scaled in time. The horrified reaction to Tuesday's murder might take place on Wednesday, say, as one reads the morning newspaper.

Accordingly time is not a 'physical entity', and that is why it goes in only one direction. This is essentially the same problem as why length only goes in one direction. We have a series: 1ft, 2ft, 3 ft, 4 ft . . . , always getting bigger, just as times always get later. Why do the series keep going up, and never down?

Time itself is however not 'a human construct', in the sense that it is no human construct whether the the Earth is older than the Moon, for example. According to the current 'impactor' theory, it is, and no 'human construction', whatever exactly that is, can change this fact. The units of time (femtoseconds, nanoseconds, milliseconds, seconds, minutes, hours, days, months and years) are another matter. These are divisions which, though they may correspond to natural events or cycles, are 'freely chosen' - they are not unfreely chosen, of course.

If all this is right, then time does not move in a direction. The propositions that it moves at all, and has a speed, have been debated since D.C. Williams' article on 'The Myth of Passage'. Williams' worry about the idea that time has a speed is that speed is change (in some dimension) over time, so the speed of time would be the change in time over time; which makes no sense. There have been recent criticisms of Williams argument that are very interesting.

I am with Aristotle. 'Time is the measure of change', he writes in the Physics. There are interesting things that happen, such as volcanic eruptions, earthquakes, murders, the having of emotions, and thoughts. If we call them collectively 'changes', then the (one-dimesnional) dimension in which they can be scaled is time.

Just a footnote on Jonathan's reply on the matter of direction. Length is a measure of a property of things, and it has a natural 'direction' from shorter to longer. As Jonathan suggests, it wouldn't make any sense to say that the difference between one direction and another on the length scale is just a matter of convention. But it may be that position coordinates are closer to your worry. We can assign position coordinate so that heading north from my desk gives us bigger numbers. Or we could do it the other way: going north fives us smaller coordinates. And in this case, we'd say that the choice really is just convention. Nature doesn't favor one direction in space over another. But time seems to be different. There seems to be a real difference between the direction we label with increasing numbers and the opposite direction. As it turns out, physicists and philosophers have written a great deal about this asymmetry. As it also turns out, there isn't a consensus about the best way to think of it. Cups of coffee left to themselves get cooler and never warmer. People get visibly older, not younger. Or, as the physicists say, entropy increases with increasing time. (We'd need to add some technical bits to get that exactly right.) The basic laws of physics, however, are "time reversible"; the reverse processes are all physically possible. What we still don't fully understand is where the asymmetry comes from.

I would like to continue the discussion by saying something about Allen's response. I agree that there might be a question about why position coordinates get bigger and bigger. Answer: we are heading North. But then we are moving. My concern is that if we think in this way, we are already thinking of time as something in which we move and travel. So why not backwards as well as forwards? My orthodox and perhaps crude belief is that time travel is impossible because of the grandmother paradox: if a time machine is possible, then I could use it to travel back two generations, and then kill my maternal grandmother. In that case, my mother would not have existed. But then nor would I. So then I couldn't go back in a time machine and kill my grandmother. So I both would go back in time and kill my grandmother and I would not go back in time and kill my grandmother. This is impossible.

The logician Kurt Gödel has a nice version of one response to this paradox. A time machine is possible, but as a matter of fact no persons will ever kill their own grandmothers. Gödel's view was that we know this a priori, so the a priori (what is independent of experience) is very powerful. There perhaps is a slight danger of determinism here, but on the other hand the inability to build a particular type of machine, or the mere fact that some machine will never be built, no more threatens freewill than the inability of ordinary persons to walk on water.

I am left with the thought that that when people get older, this is not because time is moving in an anti-entropic direction, but because as a matter of empirical fact certain physical processes are. This is a matter of physics, and needs explanation, no doubt, but it is not a question about time. Why do people get older? It's not because time is attacking them - the ravages of time - but because of the behaviour of cells. Time does nothing. In particular it doesn't move. It is a dimension, not a force.

Just to continue the conversation - Jonathan and I agree that time is a dimension and not a force. It's just that this still leaves room for interesting questions about the relationship between thermodynamics and fundamental physical laws.

We don't agree, it seems, about time travel, but we may agree for "all practical purposes." I'm quite satisfied with David Lewis's treatment of the grandparent paradox. Lewis agrees: you can't travel back in time and kill grandpa or grandma. Any such story is inconsistent, and inconsistent stories are guaranteed to be false. In fact, loosely put, you can't go back in time period unless it's actually a part of the world's history that it happened. Unless it's "already" a fact about the world that an adult Allen Stairs was wondering around the streets of his boyhood home in the 50s and 60s, then we are guaranteed that I will never do any such thing. Lewis's point was that in spite of this, we can tell consistent time travel stories. They just require very careful bookkeeping. Neither logic nor, as it happens, the laws of physics rule such stories out. The discussion is in his classic and entertaining paper "The Paradoxes of Time Travel."

That said, the physical situations that could produce a trajectory in space-time with the right properties are -- so the physicists tell us -- exotic to say the least. They aren't the kinds of things we're able to produce at will, calling as they do for arrangement involving the right kinds of black holes, for instance. And so it's pretty safe to say that we'll never be able to create conditions that would allow these backward-bending trajectories. But whether nature herself has made such arrangements somewhere in the universe is something on which Jonathan and I may have a residual disagreement, though I'm not sure. I say: we can't rule this out a priori. I wasn't sure whether his view was that we can or not. In any case, we do agree that certain kinds of time travel stories can be ruled out a priori. They can be ruled out because they are inconsistent. Where we may (or may not) still disagree is over which cases get caught by this stricture.

I agree with everything that Allen writes in his last comment. Some time travel scenarios are ruled out a priori : these are the inconsistent ones, and there may be others, for all I know. Are the consistent ones ruled out by anything? I can't see that they are, as the only reason I am clear about for thinking time travel is possible is the grandfather paradox. But it may only rule out the inconsistent cases. So I am in agreement with Allen here too, and in the dark as to whether anything in physics allows or rules out non-contradictory time travel. Time is a dimension, and dimensions are things that allow you to scale. A direction in the structure of the dimension itself seems a slightly incoherent idea to me, as opposed to the direction of the thing moving in the dimension, e.g. a place moving through colour space, such as the sky going from blue to red, or a bullet moving from there to here.

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