Originally published August 16, 1999 at Edge. Published on KurzweilAI.net August 3, 2001.

JULIAN BARBOUR: The question I’m always asking myself is, what is the universe and how does it work? I come at it from the point of view of fundamental physics, basic questions of quantum mechanics and its relationship to classical mechanics. Quantum mechanics was discovered in 1925-1926, and it gave a completely new picture of physics which was extraordinarily surprising, and it’s still very difficult to understand. It suggests the world is not at all like we see it. That has remained a really big problem, and it’s getting more and more discussion, more and more interest from people. This is what I’m really thinking about; how to reconcile the fact that the world seems to be classical, we seem to have unique past, things seem to be in definite positions, and have a definite future–that’s what it seems to be like, but quantum mechanics tells us that it is different–not like that at all. The aim is to try and find a description of the entire universe that is quantum mechanical and understand how it nevertheless it can look like the classical world that we actually see and experience.

I came into it quite by chance by reading a newspaper article about the attempts that the great Paul Dirac, one of the discoverers of quantum mechanics, was making about 40 years ago to bring it together with Einstein’s general theory of relativity. He’d come across a rather surprising fact and this led him to question whether the picture of space–time that was the whole basis of Einstein’s theory really was as fundamental as people had thought. This prompted me to think about time itself. For nearly 36 years now, I’ve been thinking about time and trying to understand it at the most fundamental level. If you look at the history of physics, surprisingly few people have really thought about time and what it truly is. Even Einstein only thought about certain aspects about time; he never asked what it means to say that a second today is the same as a second tomorrow. This is a very fundamental question. Einstein somehow assumed that it is meaningful, but he never actually asked how does that come about and how can that be? He never defined the notion of duration. So there are aspects of time that haven’t been fully studied, in my opinion.

JB: Can you give me another example besides duration?

BARBOUR: Certainly. One of the great questions in physics is whether there’s some sort of invisible framework in which everything unfolds. Newton introduced the notions of absolute space and absolute time. Absolute space is like a translucent glass block that stretches from infinity to infinity; it’s a fixed frame of reference in which everything happens. Newtonian time is like some invisible river that flows uniformly for ever. The trouble with this is that we can’t see this invisible framework, all we see are things moving relative to each other. This is the relational viewpoint, as opposed to the absolute viewpoint of Newton. The challenge has been to create a theory containing genuine relationships between genuine things, and not relationships between real things and unobservable things. That’s what I’ve spent a lot of my time working on. It’s given me the ideas which I’m trying now to develop into a complete cosmology, a complete explanation of what the universe is.

JB: Did you ever get to talk to Dirac?

BARBOUR: I tried. I was studying in Munich when I read the article about him. I got so hooked on the issue of time that I went back to England try and see Dirac in Cambridge. I actually spoke to him on the phone, but he wasn’t a very talkative person and he wasn’t all that interested in meeting somebody who’d got half–thought–out ideas about time. I can certainly understand that.

JB: Are the ideas full–baked now?

BARBOUR: They’re certainly not as half–baked as they were. They’ve definitely taken quite a shape. I hope some at least will have a place in the new picture of the universe for which so many physicists are groping, one that is completely quantum–mechanical and not half quantum and half classical. What my Italian collaborator Bruno Bertotti and I managed to show is that the world is strongly relational according to the physics as we know it now, but this hasn’t been properly recognized. The people like Leibniz and Ernst Mach who criticized Newton really were right. Einstein somehow or other put this into his theory without anyone, including Einstein himself, properly appreciating it. The world is relational. It is about how real things relate to real things. This is potentially important for how we try to picture the quantum universe.

JB: How do you fit into the leading edge of today’s research–string theorists, quantum gravity people?

BARBOUR: My work has little direct connection to what the string theorists do. There are two main approaches to quantum gravity, and one of them is definitely much more popular than the other, that’s the string theory line. I’m following a line that is at least twice as old but followed by far fewer people. It is closely related to basic questions–what is time, what is space, what is motion? Science has its fashions. String theorists are a bit like a pack of hounds following an extremely promising scent. But it is a particular scent. If they lose the trail, nothing will come of the great chase. In contrast, those basic questions will never go away. In fact, if string theory is successful, it will be very interesting to see how it does answer them.

JB: What is distinctive about your approach?

BARBOUR: My basic idea is that time as such does not exist. There is no invisible river of time. But there are things that you could call instants of time, or ‘Nows’. As we live, we seem to move through a succession of Nows, and the question is, what are they? They are arrangements of everything in the universe relative to each other in any moment, for example, now.

We have the strong impression that you and I are sitting opposite each other, that there’s a bunch of flowers on the table, that there’s a chair there and things like that–they are there in definite positions relative to each other. I aim to abstract away everything we cannot see (directly or indirectly) and simply keep this idea of many different things coexisting at once in a definite mutual relationship. The interconnected totality becomes my basic thing, a Now. There are many such Nows, all different from each other. That’s my ontology of the universe–there are Nows, nothing more, nothing less.

JB: But where does our experience of the flow of time come from?

BARBOUR: That has always proved to be difficult to attack, because if you try to get your hands on time, it’s always slipping through your fingers. People are sure that it’s there but they can’t get hold of it. Now my feeling is that they can’t get hold of it is because it isn’t there at all. That what we think is the flow of time–and even seeing motion–is actually an illusion. But I come to that after seeing what the quantum mechanics of the complete universe might be like.

JB: Sounds tough. Have you got a simple picture?

BARBOUR: Let’s take a simple model; suppose there were just three particles in the universe and nothing else. In some instant they would be in certain positions relative to each other and would form some triangle. Newton claimed that this triangle has in addition some position in absolute space and that it’s changing in time. What I’m saying is that there isn’t any of that external framework of space and time, there’s just the possible triangles that the particles form. The triangles do not occur somewhere in absolute space at some instant of time, some Now. The triangles are the Nows. You are forced to some view like this if the invisible framework is denied. If we had a universe with a million particles in it there would be some relative configuration of those million particles and nothing else. That would form one Now, and all the different ways you could arrange all the million particles would make all the different possible Nows. I think the actual Nows of this universe are more sophisticated constructs involving fields, but Nows formed by arrangements of particles can get the idea across.

JB: Didn’t Einstein abolish Nows?

BARBOUR: In fact no. He only showed that they do not follow one another in a unique sequence. There is no absolute simultaneity in the universe, or at least not in the classical universe. But relative simultaneity remains, and Nows as I define them form an integral part of Einstein’s theory. Actually the discovery of Dirac that started all my interest in time was that Nows appeared to be far more significant in the quantum world than one might have expected coming from the normal interpretation of Einstein’s relativity.

Continued at: http://www.edge.org/3rd_culture/barbour/barbour_p3.html

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