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Question: How is our everyday notion of time flawed?

David Albert: Well, in a number of ways. First of all, you might say that the sort of gesture with which physics gets under way in the first place is a gesture that lots of philosophers in the Continental tradition have come to call the spatialization of time. That is, the way time appears in physics is as another component, a fourth component, of the address of an event, okay? You want to say -- what there is to say about events is where they were located and how they were distributed, blah blah blah, in the X dimension, in the Y dimension, in the Z dimension and in the T dimension. And what physics aspires to tell us -- and all physics aspires to tell us, when you get right down to it -- is how matter and energy gets distributed over this four-dimensional block of X, Y, Z and T, okay? You tell me where all the particles are in X, Y, Z and T; I'll tell you the history of the world, okay? That's supposed to be, according to the model we've been dealing with in physics from the word go, all there is to say about the world.

From the word go, then, something is being done with time that does enormous violence to our prescientific intuitions about time. Prescientifically, it seems like time is as different from space as you could possibly imagine. And the way discussions of this often go is that people will immediately say, well, time moves or flows; space doesn't move or flow. I can move around at will in space; I can't move around at will in time. All these kinds of things. Time seems just as different as it could be from space, and indeed there's a tradition in Continental philosophy, in European philosophy -- you find this in people like Heidegger or Deleuze or people like that -- that it is the fundamental mistake of physics, this spatialization of time. It's from that moment on that physics had already doomed itself to not being able to say anything deep or interesting about what time was.

Physics, on the other hand, has this interesting retort that it can apparently explain why you say all these things you say about how time is different from space on a model in which time is just another parameter in the address of an event. So there is this very interesting conversation, there is this very interesting dialog, where on the one hand it looks like physics has made from the word go a profound error about what time is. Physics, on the other hand, has been up to now always in a position to come back and say, you want me to explain to you, based on physics, why you say time passes even though that doesn't actually mean anything, and why you're talking the way you do about our capacity to move around in space but not in time, so on and so forth? I can do all that. It's like -- it's very much like when Newtonian mechanics was first proposed, and it was widely objected that if the earth was indeed moving through space at the enormous velocities that it's predicted to be by Newtonian mechanics, everybody would fall off, okay? And the beauty of Newtonian mechanics was to be able to say not merely that I have an explanation of why we don't fall off, but that the very same theory that predicts that the earth is going to be moving so fast is also going to predict that we wouldn’t fall off. Indeed, it's also going to predict that we would think it isn't moving, even though it is. One wants to play this game with the scientific conception of time as well.

Maybe I should go a bit further. Specifically, there is within physics the following problem that's been sitting around for about a hundred years now, at the foundations of physics, which goes like this: imagine watching a film of two billiard balls collide. So in the first frame you just see a billiard ball sitting at the center of the frame. Then another billiard ball comes in, hits the one that's at rest. The one that was moving is now at rest, and the one that was previously at rest goes off and exits the frame on the other side. Imagine that you were shown this film in reverse, so what you're going to see is the second billiard ball coming in here, hitting the first one, which has now been at rest up until this point in the reversed movie, and now the first one goes out and leaves the frame on the other side.

Note that the set of events depicted by the movie being shown in reverse is just as much in accord with everything we believe about the laws governing collisions between billiard balls as is the movie being shown in the correct direction. That is, if you were shown a movie like this and asked to guess -- just based on your familiarity with the laws of physics, just based on your familiarity with how billiard balls behave when they collide -- if you were shown a film like this and asked to guess whether it was being shown forward or in reverse, you wouldn't be able to tell. Physicists express this by saying that the laws governing collisions between billiard balls are symmetric under time reversal, okay? And what that means more concretely is -- a law is said to be symmetric under time reversal if it's the case that for any process which is in accord with that law, the same process going in reverse -- that is, the same process as it would appear in a film going backwards -- is also in accord with that law. So we say that the laws governing collisions between pairs of billiard balls are time-reversal symmetric. Good.

It's an astonishing thing that all of the serious candidates that anybody has entertained for fundamental laws of physics, from Newton onward -- so by this I mean Newton's laws, I mean Maxwell's equations, I mean the Schrödinger equation, I mean general relativity, I mean string theory, everything -- it's an astonishing fact about all of these theories, even though they differ wildly from one another in all kinds of other respects, and even though they straddle different sides of enormous scientific revolutions, it's an astonishing fact about every single one of these theories that all of them are perfectly symmetric under time reversal. Every single one of them has the feature that for any process which is in accord with those laws, the process going in reverse is in accord with those laws as well. But this poses a problem. And once again, just as with the measurement problem, the structure of this problem is a clash between the laws of physics that we have developed by carefully observing the behaviors of microscopic systems and our everyday macroscopic experience of the world.

Okay, what's the problem? The problem is that if we get more sophisticated than collisions between billiard balls, okay -- if we imagine a film of someone walking down the street, or of a piece of paper being consumed by fire, or a time-lapse film of somebody growing old or something like that, we can damn well tell very easily whether we are being shown the film forward or in reverse, okay? Now, that is very directly at odds with what we take ourselves to have very good reasons for believing about the structure of the fundamental physical laws, which are supposed to govern all of these process; namely, that those laws are perfectly symmetric under time reversal, okay? To put it slightly differently, on the level of fundamental physical law there doesn't seem to be any distinction whatsoever between past and future, okay? Moving from the present into the future ought to look, statistically speaking, just like moving from the present into the past if those laws are true.

On the other hand, it's probably not an exaggeration to say that the most basic, primordial, unavoidable feature of our everyday experience of being in the world is that there is all the difference in the world between the past and the future. In our everyday experience of the world there is an extremely vivid and pronounced temporal direction, okay? On the level of the fundamental laws, that temporal direction apparently completely vanishes, okay? Once again, there's a question, and like I say, this question has a similar structure to the question in the measurement problem in that it's a clash between our fundamental equations of motion and our everyday experience, macroscopic experience, of being in the world. There's a question with time about how to put these two things together.

Once again, it appears as if although the theory does an extremely good job of predicting the motions of elementary particles and so on and so forth, there's got to be something wrong with it, okay, because we have -- although we have very good, clear quantitative experience in the laboratory which bears out these fully time-reversal symmetric laws, at some point there's got to be something wrong with them, because the world that we live in manifestly not even close to being time-reversal symmetric. And once again there are proposals on the table for how to fiddle around with the theory, adding a new law governing initial conditions, for example. There are all kinds of proposals about how to deal with this, but this has been -- this is a very fundamental challenge. Although we've got laws that are doing a fantastic job on the micro level, there's some way in which these laws manifestly get things wrong on the macro level, and we need to figure out what to do about it.

Recorded on December 16, 2009

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