At quite an early age, I became closely associated with the so-called arrow-of-time problem. This has to do with the mystery of why most physical processes in the universe seem to go one way in time, whereas the underlying laws of physics that govern them are reversible — they have no preferred time orientation. I got into this because of a couple of papers by John Wheeler and Richard Feynman, in which they tried to explain how, for example, radio signals always arrive at the receiver after they leave the transmitter and never before. They had a clever way of starting out with time-symmetric electromagnetic waves (forwards and backwards in time, symmetrically), and recovering the purely time-forwards waves by appealing to cosmology — that is, taking into account a whole universe full of emitters and absorbers of electromagnetic waves. This led me to investigate a wide range of other topics in which time symmetry gets broken. When I was twenty four, I wrote a book on the subject, called The Physics of Time Asymmetry. It was really just a preliminary skirmish with an enormously complicated topic, but lots of influential people, like Wheeler, Roger Penrose, and Martin Gardner, said nice things about it. Even Feynman recommended it to a colleague!
In terms of actual discoveries, my name is most often coupled to a weird effect I found in the theory of quantum fields. Imagine a total vacuum, devoid of all particles including photons. Now suppose you accelerate through that emptiness, what do you see? Nothing? In fact, you see a bath of heat radiation, even though your nonaccelerating friend still sees absolutely nothing. The effect, which is closely related to Stephen Hawking's discovery that black holes radiate heat, was discovered independently by Bill Unruh, at the University of British Columbia. I wrote up this result in the mid-seventies, almost casually. The effect is very small, and not hard to prove, and I didn't think many people would be interested, but papers still appear at the rate of several a year, elaborating on this or that aspect of "acceleration radiation."
After this success, I worked on the theory of quantum fields in curved spacetime — that is, in the presence of gravitational fields. The book I wrote with my student Nick Birrell, called Quantum Fields in Curved Space, remains the principal text on the subject, I am pleased to say. A lot of my investigations concerned the behavior of quantum fields in certain model universes that were simple to study. We were interested in many questions. Could the expansion of the universe create particles? How did the quantum field get disturbed by the gravitational field of the universe, and how did this disturbance in turn react gravitationally? One of these model universes is named after the Dutch cosmologist Willem de Sitter, and together with another student, Tim Bunch, I spent a lot of time looking at it. Among many results emerged the concept of a particularly interesting quantum-vacuum state, still known as the Bunch-Davies vacuum. It never occurred to me at the time — the late seventies — that this stuff would find any real applications. How delighted I was when suddenly de Sitter space became of central importance in Alan Guth's inflationary-universe scenario, and people began using the Bunch-Davies vacuum in their calculations!
I also did a lot of work on black holes and their thermodynamic properties, discovering, for example, that if a black hole carries a large enough electric charge it can remain in equilibrium with a surrounding heat bath instead of evaporating away in the manner Hawking first described. I've always been fascinated by Penrose's belief that gravitation represents a sort of entropy in its own right, and many papers I wrote in the eighties were attempts to flesh out this idea, but without complete success.
It's very curious how Alan Guth got into the inflationary scenario. He was trying to solve a rather specific problem connected with magnetic monopoles. The standard hot-big-bang theory, combined with our best knowledge of particle physics, indicated that the universe ought to be stuffed full of magnetic monopoles, and yet we don't see any. The question was, How had they been eliminated? One obvious way to get rid of them is to have the universe inflate by some large factor that simply dilutes the density of these monopoles.
Guth wasn't a cosmologist; he was a particle physicist trying to get rid of monopoles, and so he proposed the idea that the universe, during its first split second, suddenly jumped in size by a huge amount. Plausible answers to key cosmological questions — such as whether the universe is expanding at precisely the rate to escape its own gravitational pull, and whether the quantum fluctuations around that precise rate would give the sort of spectrum just observed by the COBE satellite — came as a bonus. The fascinating thing is that what Guth did was to come in the back door and discover this immensely rich seam of ideas, which he then successfully quarried. His inflationary theory, inevitably refined, is now pretty much the standard cosmological scenario for the origin of the universe.
Only twenty-five years ago it was not considered appropriate to consider the physical mechanism of the birth of the universe. I remember a lecture I attended as a graduate student at University College London. This was a couple of years after the discovery in 1965 of the cosmic microwave background radiation, and the implications of that discovery had not yet generally sunk in. A professor was talking about how theorists had computed, based on the existence of this radiation, that there would be about 25 percent helium and 75 percent hydrogen in the universe, and that this had come from an analysis of the nuclear processes that took place in the first few minutes after the big bang. Everyone in the lecture hall fell about laughing, because they thought it was so absurd and audacious to talk about the first three minutes after the big bang, just on the basis of the discovery of this radiation. Now, of course, it is absolutely standard cosmological theory. We feel we understand the first few minutes of the universe very well.
What we now find is that the big bang has gone from being merely a description of the origin of the universe to being an explanation. That's a key difference. Simply saying that things just happen that way — in other words, to say that things are the way they are because they were the way they were — constitutes a description. What we now have is something much closer to a scientific explanation, in which not only can we account for the fact that there was a bang but also a lot of the specific features of the big bang now emerge from well-formulated physical theory, instead of being put in as ad-hoc initial conditions. That's the big difference. The latest COBE discovery adds enormously to the strength of the big-bang theory as a proper theory and not just a description.
The evolution of the big-bang theory leads to a discussion of the anthropic principle, which says that the world we see must reflect, to some extent, the fact that we're here to see it — not only here but here at this particular location in space and time. There are different variants of the anthropic cosmological principle, and how much credence you can give it depends on which one you're talking about. What's quite clear is that there must be an anthropic companion to our science. To take a trivial and extreme example: most of the universe is empty space, and yet we find ourselves on the surface of a planet. We're therefore in a very atypical location, but of course it's no surprise that we're in this atypical location, because we couldn't live out there in space.
Obviously, there's an anthropic factor to what we observe and the position in the universe from which we observe it, or maybe the time, the epoch, that we observe it. Having said that, the question is whether it's just a comment about the universe or in some sense an explanation for some features of the universe. If there's only one universe, it's just a comment on it. But if we imagine that there is a whole ensemble of universes — a huge variety, with different conditions, different laws — then it starts to become an explanation, or a selection principle. Part of the reason for the order we observe in the universe is that this is one of the few universes out of the whole ensemble that is cognizable. Some people have tried to carry this principle to a ludicrous extreme by making out that ultimately there are no laws of nature at all, that there is only chaos, that the lawfulness of the universe is merely explained by the fact that we've selected it from this infinite variety of essentially chaotic worlds. That is demonstrably false, and an unreasonable extrapolation of the whole anthropic idea.
It's remarkable that the universe is lawful, that there exist underlying rational principles which govern the way the universe behaves. We can't account for that just on the basis of the fact that we're here to see it, as some people have tried to do. There's a dual principle at work. There's a principle of rationality that says that the world is fashioned in a way that provides it with a rational order, a mathematical order. There's a selective principle — which is an anthropic principle — that says that maybe out of a large variety of different possible worlds this type of world is the one we observe.
We can't avoid some anthropic component in our science, which is interesting, because after three hundred years we finally realize that we do matter. Our vantage point in the universe is relevant to our science. But it's very easy to misconstrue the anthropic principle, and draw ridiculous conclusions from it. You have to be very careful how you state it. What it is not saying is that our existence somehow exercises a theological or causative compulsion for the universe to have certain laws or certain initial conditions. It doesn't work like that. We're not, by our own existence, creating such a universe.
We are now very close to identifying the nature of the fundamental building blocks out of which the world is put together. This reductionist path is tremendously important and has exercised an enormous influence in the thinking of physicists, but it's only a part of the story. To say that the world is built up of a collection of certain particles playing certain roles of interaction is one thing. But to give an explanation of problems like the origin of life, the origin of consciousness — problems that refer to highly complex systems — that's quite another. To talk about complexity, we have to realize that there are systems the behavior of which can be understood only by looking at the collective and organizational aspects, instead of the individual constituents. It's impossible to explain the behavior of these so-called adaptive complex systems in a purely reductionistic manner and expect to build up from that. These are systems like biological organisms, which appear to respond to and adapt in accordance with their environment.
I look forward to a time when the biologists stop berating the physicists for abandoning reductionism. At the moment, the biologists are strongly and evangelically reductionistic, and any suggestion by physicists that one can deviate from the path of strict reductionism tends to evoke a rap over the knuckles from the biologists. My personal belief is that biologists tend to be uncompromising and reductionistic because they're still feeling somewhat insecure with their basic dogma, whereas physicists have three hundred years of secure foundation for their subject, so they can afford to be a bit more freewheeling in their speculation about these complex systems. I hope to see this cultural division between these two communities dissolve away within the next ten to twenty years, so that they'll be able to talk to each other in the same language.
There are two paths in investigating the world: the reductionist path and the synthetic path. In the science of complexity, it's essential to recognize that there is this second path. Complexity amounts to more than mere complication. It's more than just a large number of simple systems coming together in conjunction. Complex systems really do have their own laws and principles, and their own internal logic.
In the next few decades, physics will be going in the direction of complexity. One of the key questions for physics is, Can the reductionist program be completed? Stephen Hawking said, in his famous 1979 address on his inauguration to the Lucasian Chair, that the end might be in sight for theoretical physics, by which he meant that the end of this reductionist program might be in sight. Indeed, we may complete it and be able to write down a formula you could wear on your T shirt — some mathematical statement, or a set of principles encapsulated in a single piece of mathematics, describing all the fundamental particles and forces out of which the world is built.
That would still leave this path of complexity, this synthetic or holistic way of looking at the world. There, what I see as the real excitement is the dissolving away of the division between physics and biology. We see a very curious phenomenon at the moment: while physicists are increasingly recognizing the importance of looking at the collective, organizational, and qualitative features of complex systems, and recognizing that they have their own laws and principles and qualities, in a way that makes them every bit as fundamental as the elementary particles out of which the world is built, at that same time the biologists are going the other way, becoming overly reductionistic and regarding life as nothing but a collection of individual particles interacting in an unwitting manner by means of blind and purposeless forces.
It's often said that if we had a theory of everything, everything would be explained. But when physicists talk about a theory of everything, they don't mean literally everything. They don't mean a theory that would explain how the stock market rises and falls, still less something that would explain the origin of life. They mean a theory that accounts for all these fundamental units out of which the world is built.
Excerpted from The Third Culture: Beyond the Scientific Revolution by John Brockman (Simon & Schuster, 1995) . Copyright © 1995 by John Brockman. All rights reserved.
