Alan Saunders: Hello, and welcome to The Philosopher's Zone. I'm Alan Saunders.

And this week: Time.

Yes, time and relative dimensions in space. Time, as we all know, is money, but what else is it? Would it exist if nothing were changing or moving? Does it really have a direction? Are the future and the past real? Will the future be infinite? Was, or is, the past infinite?

These are some of the questions that philosophers have asked about time, and that's before we get to relativity, black holes, time travel or the idea that time might go in circles.

SONG: I got you, babe/RADIO SMASHING

Alan Saunders: Bill Murray in the movie Groundhog Day registering his objection to circularity by smashing the radio that wakes him every morning to a day that is exactly like the day before. The radio will be back tomorrow though, playing the same song.

Well there's much talk of time going on now - (we might find out later what we mean by 'now') in the Philosophy Department at the University of Sydney. And to talk about that, I'm joined now by Associate Professor David Braddon-Mitchell who's also an Associate of the University's Centre for Time. David, welcome back to The Philosopher's Zone.

David Braddon-Mitchell: Thanks Alan, glad to be back.

Alan Saunders: Let's begin with a question of just three words, but a very difficult one: What is time?

David Braddon-Mitchell: I think time is a direction or a dimension; it's the other one besides the three spatial directions or dimensions that we're familiar with. So if you describe the way the whole universe is, and where things are, you shouldn't say they're in someplace given by the three dimensions. We normally used to specify them by four dimensions, the fourth one is time.

Alan Saunders: It's not quite clear that as the way that you've put it, it's not clear that you've quite captured the notion that time involves movement, it involves change. I mean Aristotle said that 'time is the measurer of change'.

David Braddon-Mitchell: That's right. I think the contemporary philosophical orthodoxy, (maybe it's not orthodoxy, maybe it's only the contemporary philosophical majority) has a picture of time that's deeply counter-intuitive. In one sense there is no change. In one sense there is no motion. So on this story about what motion is, for example, it's for there to be things connected to each other, where at different points on a time scale they're in different places on a space scale. So there's just something which is eternally in various locations, and if you can trace its route in a certain way, it counts as being a moving thing.

Alan Saunders: So it's not a question of befores and afters. I mean I might say that time is what allows us to arrange events in some sort of sequence, so that I can say that Marilyn Monroe lived before Paris Hilton, and that Paris Hilton lives at the same time as Britney Spears for example.

David Braddon-Mitchell: This picture does allow you to have befores and afters, so all of Marilyn Monroe's bits, she has those little tiny bits that were only a foot tall, and those wizened - no, there weren't any wizened bits because she tied too young. Sadly, yes. The rather less-wizened bit later, are arranged in a sequence which are indeed related by the before-after relation. There's now however, what there isn't is an objective now and an objective future and an objective past on this picture. All there is, is before and after.

Alan Saunders: Well I want to come on to that in a minute. But another possibility, another definition that's been offered, is that time is an ordering of events and we can determine this order by causality. I turn the gas on (because I've got a very old-fashioned kitchen), I turn the gas on, and the kettle boils. So that gives us a way of ordering events and therefore of saying what time is.

David Braddon-Mitchell: That's a really intriguing thing you should have said that, because one of the most interesting frontiers of research into time, is casting doubt on that very way of drawing the audience. It looks like there's a causal ordering when you look at the level of big things, but as for example, Hew Price, in our Centre for Time, has been arguing over the last ten years I think, when you look at the microscopic level you just don't see any of that order. The laws of nature work forwards, the laws of nature work backwards, and this appearance of order, and this appearance of causation at the level of ordinary person-sized objects, seems to depend more on just local facts about how things are decaying; it makes it look like there's a temporary local order. It has nothing to do with underlying physical causation as far as we can tell.

Alan Saunders: You were telling me that you had an egg muffin for lunch. Did your having that egg muffin cause a chicken to have laid an egg?

David Braddon-Mitchell: No, because on this picture macroscopic things like muffins don't cause big things like eggs. What happens is there's a whole constellation of particles in the past and you can apply the laws of nature forward and predict my eating the muffin, but similarly the whole cluster of particles in the world at the muffin eating time, and you can apply those laws backwards and find the chicken. So it's very counter-intuitive.

Alan Saunders: Does the second law of thermodynamics help us here? 'Heat can't of itself pass from a colder body to a hotter'. If I put an ice cube into that kettle after I've taken it off the gas, the water will become cooler, the ice cube won't become hotter, it won't heat the water, it won't give any of its own heat to the water. So anything you're steadily cooling down, or as physicists say, 'entropy in the universe is increasing', doesn't that give us a clear, one direction for time? It gives us an arrow of time?

David Braddon-Mitchell: This is the scariest thing in the whole area I think. It looks that way. It always looks like there's decay and it always looks like there's an increase in disorder. But if you look at what the second law really says, or at least what the more sophisticated recent versions of expression of it say, what they say is that order will always go to a certain average, so if there's extreme disorder, you should expect order. If there's extreme order you should expect disorder. And the reason we see what looks like this tendency towards disorder in the universe, is just apparently the extraordinarily flukey fact that the universe began with massive amounts of water, and so the natural statistical fluctuation is meaning that it's going to the mean, which means we've got disorder.

Alan Saunders: Let's look at another issue, something to which you've already alluded: I would say this is the commonsense view. Naturally we all thing that our own sense of things is the commonsense - I would certainly say that the future doesn't exist, and the present does, but then I'm left the question, what about the past? But before we get to that question though, how does the present exist? If I say the present is now, well it's not now, because now that I've uttered that sentence, it's in the past, in fact it was going into the past even while I was uttering it. So what do we mean by 'the present'?

David Braddon-Mitchell: That's a very interesting question. One problem with trying to come up with an account of the present is that you might be tempted to think that whenever you're saying something or thinking something, the present is all the time sort of simultaneous with you. But it turns out simultaneity is something which is very observer-dependent. So there is no observer independent fact of the matter about what's simultaneous with you. So from my perspective there might be someone who is simultaneous with me, but from their perspective they might be simultaneous with someone who in turn is not simultaneous with me. So if this idea of simultaneity plays any role in the present, means that the present is going to be a strange thing, a thing that varies from perspective to perspective and person to person.

Alan Saunders: Let's look at the existence or the status of past, present and future. Now there seem to be three competing theories: the presentists, the growing universe theory, and what's called eternalism, or the block universe theory. Can you describe these different views?

David Braddon-Mitchell: Well the three alternatives are firstly, everything exists; that's the block universe view, the block universe says that all of space and time is one ginormous, eternal thing, and it has parts which are temporal parts, if you like. So the moment that we're in now, is one of the parts, there are lots of future parts, all of which exist, and lots of past parts, all of which exist. So this is the Yes, it's all there view, that's the eternalist view.

Then there's I think what is the intuitive view for many people, it's the present is really special because the present is real. The present is all that there is, there's other stuff of the used to be, there's other stuff that there will be, but in no sense does it actually exist in any timeless sense. Like the presentist view in which the universe is incredibly thin, it's just now.

The third view is the growing universe view, and I think this is motivated by the idea that the past has happened, it's kind of real, it's fixed, and the future because it hasn't happened, is open and could be any one of many ways, so it can't really be there. So in this view yes, the past really exists, it's perfectly solid, and it comes up to the present and the present is a kind of a border between existence and non-existence, between being and non-being.

Alan Saunders: Let's just have a closer look at the first one you mentioned, the block universe theory which is that - which are called equal status to past, present and future. That seems to imply that we don't need grammatical tenses, we don't need past, present and future. So instead of saying 'Collingwood will win the game', we say 'Collingwood do win at time-T', and time-T is after the time of this utterance. Can we make any sense of that at all?

David Braddon-Mitchell: I understood what you just said! So I've made sense of it. It doesn't tell you you can't use locations like 'will' and so forth of course, because 'will' just means in a temporal direction forward from here. So these expressions become what are causing jargon indexicals. 'It will happen' means it happens forwarded here temporarily, 'it did happen' means it happened backwards from here temporarily. Just as long as we know where we are, we've got no problems with using those expressions, or when we are I should say.

Alan Saunders: There are other difficulties with the idea that there's no difference between past, present and future; it's not clear how this allows for the possibility of movement and change.

David Braddon-Mitchell: What it allows for is a kind of slightly counter-intuitive substitute for movement in change. So change becomes some of your parts having certain property at one time, and then a later part of you having the property at another time. So I regrettably am changing, my knees are much dodgier than they were a couple of years ago. On this story what that means is there are me parts, which exist in 2008, which have dodgy knees, and me parts which exist in 2005 that had less dodgy knees.

Alan Saunders: Let's consider if the future is as real as the present and the past, doesn't that mean it's fixed? Isn't this a deterministic theory? It means there's nothing we can to affect what will happen?

David Braddon-Mitchell: It's not necessarily deterministic because to be deterministic would mean that the way the laws of nature are at a certain time would guarantee things look a certain way in the future. But it does mean the future is fixed. It means there's a fact of the matter about how it is.

Alan Saunders: Which of these three views do you favour?

David Braddon-Mitchell: I think I favour the block universe view, simply because it has the best fit with science and because the counter-intuitive things about it can be explained away, and one thing I regrettably find the more I do philosophy is that the world might be very counter-intuitive, and commonsense might be an awfully bad guide to how things are.

Alan Saunders: So you do believe that the future exists?

David Braddon-Mitchell: Probably, he says weasley.

Alan Saunders: What about the past? What is the status of the past when, to take an example from something you've written, when more than 2,000 years ago Julius Caesar crossed the Rubicon, he thought he was doing so in the present, and you say what I find a very odd thing, you say 'Yes, that's still real, that past is real, but when he thought he was crossing the Rubicon in the present more than 2,000 years ago, he was right, and now he's wrong. It seems very odd that he can be made wrong about something that was right, merely by (if you'll pardon the expression) the passage of time.

David Braddon-Mitchell: This is a very interesting point and I think this is where there's a dispute I suppose between the block people, block universe people, and the growing salami people. The pedantists haven't got problem of course because there is no Julius Caesar for them.

Alan Saunders: Now hang on, let's talk about the growing salami principle, which is appropriate given that we're talking about Julius Caesar.

David Braddon-Mitchell: Did I say 'growing salami' without being forewarned. Okay we'll talk about the growing salami then. The Italian sausage of the legions.

This view that says that the past exists but not the future is sometimes called the 'growing salami' philosophers like Broad and Michael Tooley, are well-known for that view. So the idea is that the universe adds slices as now gets later. At the beginning of the universe you had only a couple of little slices in that piece of metal at the bottom of a salami that holds it together, and then as time goes on, slice after slice, after slice is added, and at any given time the latest slice which is then added to the salami, is the kind of border between existence and non-existence. And that's the view which has this I agree, absolutely puzzling sort of claim about say Caesar crossing the Rubicon, because on this view, Caesar is back there, having crossed the Rubicon, but there's an objective fact about when now is. Now is the slice, and so suppose we are living in the objective present, then if the objective present is this time that we're in now, then Caesar is back there saying something false, when he says, 'I am now crossing the Rubicon'. And that's nuts. And this is one of the reasons why I favour the block universe view because the block universe view says that there's no objective fact about now. Now just means this time, in which case of course Caesar is right when he says 'I'm now crossing the Rubicon', because he's referring to this time, and of course we are right when we say 'We are now in an ABC studio', because we're referring to this time.

Alan Saunders: Yes I mean I think we're quite happy to allow that a statement of belief may be made false by the passage of time. I mean the statement 'Julius Caesar is a Roman General' would have been true had I uttered it more than 2,000 years ago, but it's false if I utter it now. However it seems odd to say, that Caesar's belief about these things is rendered false by the passage of time.

David Braddon-Mitchell: That's right, the peculiar thing about this view is that Caesar himself is back there in the past falsely saying I am now crossing the Rubicon. Whereas on the other views, what he says at the time he says it, is right.

Alan Saunders: Do salami people and block people lead very different lives? Does it change your world view? Or your approach to Italian food?

David Braddon-Mitchell: I don't think it does change your world view, but I suspect that world views may play an illicit role in shaping people's intuitions as to which ones they believe. So the salami people like the fact that on their view there's no facts about the future, and they look forward to ever-tastier slices being added to it. Whereas someone like me wonders what those slices that are already there are going to be like.

Alan Saunders: There is of course a view actually favoured by the English philosopher, R.G. Collingwood which is that the past doesn't exist and that when you talk about the past, all you're talking about is the evidence that we have now - I might be misquoting Collingwood here, I'm quoting from memory - is the evidence that we have now, the historical evidence.

David Braddon-Mitchell: No that's right. Michael Dummett famously defended the same view. It's called anti-realism about the past. So these are a species of the presentists, that we talked about earlier, people who think that all there is, is what's in the present. And they think of what makes claims about the past true are the little traces, and bits of evidence and as the evidence is destroyed, what's true about the past gets thinner and thinner, so you know, in 100AD when there were tiny traces of garlic left in some camp near the Rubicon, it was true that Caesar ate garlic sausage, but now that all evidence is destroyed, there's no truth about what he ate.

Alan Saunders: Now let's see what we've got time left to discuss. If there were no minds, would there be time? I mean we can make plausible hypotheses about what happened in the universe before there was intelligent life, and we talk about it in terms of millions and millions of years, we go back to the Big Bang, and we'll come in a minute to whether there was time before the Big Bang, but was there in fact time back then over those millions of years?

David Braddon-Mitchell: Yes. Simple answer. It might require a bigger answer. Certainly if there was no intelligent life there'd be no-one to wonder what happened in the past, there'd be no-one to wonder what the future is like. But one can imagine the universe having evolved in many ways, most of which of course have no life in them, because we now know that it was incredibly flukey things about nature and the Big Bang which made it possible for life to exist at all. And it's hard to believe that nature of space and time itself would be any different in all these vast majority of ways the universe could have been in which we sadly would have been lacking.

Alan Saunders: Well I'm moving gradually towards the Big Bang, but we can fairly easily understand the notion of time stretching infinitely into the future, though I believe there are people who doubt that. But does it stretch infinitely back into the past?

David Braddon-Mitchell: Probably not. One of the things about the block picture of the universe, and indeed others as well, is that you see space and time as a kind of volume. It might be a finite volume, or it might be infinite volume, but as far as we know in the time-like directions there's a point of singularity and there's nothing before it. So there's no time for there to be more timing. There's no space time for there to be more timing.

Alan Saunders: So there's no time before the Big Bang?

David Braddon-Mitchell: No time before the Big Bang.

Alan Saunders: Which makes it rather odd to talk about the Big Bang as an event, I mean what's it an event in?

David Braddon-Mitchell: Yes. Interestingly, physics breaks down at ten to the minus fourteenth of a second or some tiny period after the Big Bang. All people can really do is speculate. I like to think that philosophy is not just speculation so I will decline to speculate what happened at those extraordinarily early times. The Big Bang itself doesn't seem to be an event. It's the way things were in the very, very early climates of the universe.

Alan Saunders: OK, well that's one end of the salami. Does the salami have another end? Is it an infinite salami going forward?

David Braddon-Mitchell: Well this is beyond mere philosophy and we need to talk to our colleagues in cosmology. It could have an end, and I'm happy to say that philosophically, so there are scientific models where there's a Big Crunch as well as a Big Bang, all the space and time is contained, that in some sense it has a beginning and an end. There are other models from which it just goes on forever.

Similarly the fact that there's no time before the Big Bang is only a product of how things contingently are. There's nothing philosophically wrong with a difference-based time which is extended infinitely in all directions, it's just that it seems it doesn't.

Alan Saunders: Various philosophers and not a few religions have perhaps got round, perhaps not, some of these problems by assuming that time is circular, and of course Nietzsche rather famously talked about 'eternal recurrence' and various people have talked about how it's all going to happen again.

David Braddon-Mitchell: Well that's one of those things which is logically possible, but I see no reason to believe it's actually true. There are fascinating models though on which you have an end of the universe and there's nothing wrong with a description of the way the universe is, in which the end of the universe and the beginning of the universe are the very same point. And if that's the case, yes, it is one perfectly circular connected thing.

Alan Saunders: Does this mean that we're doomed to an eternity of this interview?

David Braddon-Mitchell: I think so. It will turn out this interview will come back again and again and again. But then, would all those interviews in fact be the same interviews?

Alan Saunders: Well if you believe the great German philosopher, Gottfried Leibnitz, they presumably would be the same interview.

David Braddon-Mitchell: So they're indiscernible and hence identical, yes, I'm inclined to go with that.

Alan Saunders: Yes. What about the notion, we know that time flies, but does it flow? Isaac Newton was a bit flow-man, he was very keen on the idea of time flowing.

David Braddon-Mitchell: Flow is a fascinating idea. It's certainly how time seems to be. You get this sense that you're sort of washed down a river watching events unfold around you, always in the kind of now, thinking back to how things used to be. My hunch is that this is an illusion. It's an illusion caused by the fact that at any given moment, we have memories of the past, and the next bit of us, just a few seconds later, has memories of memories, and the bit of us that's minutes later after that, has memories of memories and memories, and this creates a sense in which all our parts spread out through time are constantly thinking about the past and about the future and imagining that they're the very same thing. So there's a part of us at one time and a part of us at the next time, and a part of us at a time after that, and each of them has memories of the past, and they don't just remember the past, they can remember a memory of the past, and remember remembering remembering the past and so on and so forth, and that's what creates this sort of illusion that we are one thing moving through time instead of the succession of things glued together in time, which is a picture that I think is probably right.

Alan Saunders: Well I've run out of time puns to use on this program, so I will just say David Braddon-Mitchell Associate Professor of Philosophy at the University of Sydney, thank you very much for joining us.

David Braddon-Mitchell: Thank you, Alan.

Alan Saunders: Time present and time past

Are both perhaps present in time future

And time future contained in time past.

If all time is eternally present,

All time is unredeemable.

What might have been is an abstraction

Remaining a perpetual possibility

Only in a world of speculation.

What might have been and what has been

Point to one end, which is always present.

Footfalls echo in the memory

Down the passage which we did not take

Towards the door we never opened

Into the rose garden.

Some thoughts on time from T.S. Eliot's poem Burnt Norton. Eliot, by the way wrote a thesis on the English philosopher, F.H. Bradley who was one of those thinkers who argued that time is just an appearance, reality is static.

For more on time, check out the links on our website. Production on the show is by Kyla Slaven, with technical production this week by Menny Wessershtrom and John Diamond. I'm Alan Saunders and I shall, if you'll allow me to use the future tense, be back next week with another Philosopher's Zone.