WHAT HAPPENED BEFORE THE BIG BANG?

The theoretical physicist PAUL DAVIES is this year's winner of the Templeton Prize for Progress in Religion. In a new essay, he tackles one of the trickiest questions of existence. Illustration by EMMA PARKER

Well, what did happen before the Big Bang? Few schoolchildren have failed to frustrate their parents with questions of this sort. It often starts with puzzlement over whether space "goes on forever", or where humans came from, or how the planet Earth formed. In the end, the line of questioning always seems to get back to the ultimate origin of things: the Big Bang. "But what caused that?"

Children grow up with an intuitive sense of cause and effect. Events in the physical world aren't supposed to "just happen". Something makes them happen. Even when the rabbit appears convincingly from the hat, trickery is suspected. So could the entire universe simply pop into existence, magically, for no actual reason at all?

This simple, schoolchild query has exercised the intellects of generations of philosophers, scientists, and theologians. Many have avoided it as an impenetrable mystery. Others have tried to define it away. Most have got themselves into an awful tangle just thinking about it.

The problem, at rock bottom, is this: If no-thing happens without a cause, then something must have caused the universe to appear. But then we are faced with the inevitable question of what caused that something. And so on in an infinite regress. Some people simply proclaim that God created the universe. But children always want to know who created God, and that line of questioning gets uncomfortably difficult.

One evasive tactic is to claim that the universe didn't have a beginning, that it has existed for all eternity. Unfortunately, there are many scientific reasons why this obvious idea is unsound. For starters, given an infinite amount of time, anything that can happen will already have happened, for if a physical process is likely to occur with a certain nonzero probability - however small - then given an infinite amount of time the process must occur, with probability one. By now, the universe should have reached some sort of final state in which all possible physical processes have run their course. Furthermore, you don't explain the existence of the universe by asserting that it has always existed. That is rather like saying that nobody wrote the Bible: it was just copied from earlier versions. Quite apart from all this, there is very good evidence that the universe did come into existence in a Big Bang, about fifteen billion years ago. The effects of that primeval explosion are clearly detectable today - in the fact that the universe is still expanding, and is filled with an afterglow of radiant heat.

So we are faced with the problem of what happened beforehand to trigger the Big Bang. Journalists love to taunt scientists with this question when they complain about the money being spent on science. Actually, the answer (in my opinion) was spotted a long time ago, by one Augustine of Hippo, a Christian saint who lived in the fifth century. In those days before science, cosmology was a branch of theology, and the taunt came not from journalists, but from pagans: "What was God doing before he made the universe?" they asked. "Busy creating Hell for the likes of you!" was the standard reply.

But Augustine was more subtle. The world, he claimed, was made "not in time, but simultaneously with time".

In other words, the origin of the universe - what we now call the big bang - was not simply the sudden appearance of matter in an eternally pre-existing void, but the coming into being of time itself. Time began with the cosmic origin. There was no "before", no endless ocean of time for a god, or a physical process, to wear itself out in infinite preparation.

Remarkably, modern science has arrived at more or less the same conclusion as Augustine, based on what we now know about the nature of space, time, and gravitation. It was Albert Einstein who taught us that time and space are not merely an immutable arena in which the great cosmic drama is acted out, but are part of the cast - part of the physical universe. As physical entities, time and space can change - suffer distortions - as a result of gravitational processes. Gravitational theory predicts that under the extreme conditions that prevailed in the early universe, space and time may have been so distorted that there existed a boundary, or "singularity", at which the distortion of space-time was infinite, and therefore, through which space and time cannot have continued. Thus, physics predicts that time was indeed bounded in the past as Augustine claimed. It did not stretch back for all eternity.

If the Big Bang was the beginning of time itself, then any discussion about what happened before the Big Bang, or what caused it - in the usual sense of physical causation - is simply meaningless. Unfortunately, many children, and adults, too, regard this answer as disingenuous. There must be more to it than that, they object.

Indeed there is. After all, why should time suddenly "switch on"? What explanation can be given for such a singular event? Until recently, it seemed that any explanation of the initial "singularity" that marked the origin of time would have to lie beyond the scope of science. However, it all depends on what is meant by "explanation". As I remarked, all children have a good idea of the notion of cause and effect, and usually, an explanation of an event entails finding something that caused it. It turns out, however, that there are physical events which do not have well-defined causes in the manner of the everyday world. These events belong to a weird branch of scientific inquiry called quantum physics.

Mostly, quantum events occur at the atomic level; we don't experience them in daily life. On the scale of atoms and molecules, the usual common- sense rules of cause and effect are suspended. The rule of law is replaced by a sort of anarchy or chaos, and things happen spontaneously - for no particular reason. Particles of matter may simply pop into existence without warning, and then equally abruptly disappear again. Or a particle in one place may suddenly materialize in another place, or reverse its direction of motion. Again, these are real effects occurring on an atomic scale, and they can be demonstrated experimentally.

A typical quantum process is the decay of a radioactive nucleus. If you ask why a given nucleus decayed at one particular moment rather than at some other, there is no answer. The event "just happened" at that moment, that's all. You cannot predict these occurrences. All you can do is give the probability - there is a 50-50 chance that a given nucleus will decay in, say, one hour. This uncertainty is not simply a result of our ignorance of all the little forces and influences that try to make the nucleus decay; it is inherent in nature itself, a basic part of quantum reality.

The lesson of quantum physics is this: Something that "just happens" need not actually violate the laws of physics. The abrupt and uncaused appearance of something can occur within the scope of scientific law, once quantum laws have been taken into account. Nature apparently has the capacity for genuine spontaneity.

It is, of course, a big step from the spontaneous and uncaused appearance of a subatomic particle - something that is routinely observed in particle accelerators - to the spontaneous and uncaused appearance of the universe. But the loophole is there. If, as astronomers believe, the primeval universe was compressed to a very small size, then quantum effects must have once been important on a cosmic scale. Even if we don't have a precise idea of exactly what took place at the beginning, we can at least see that the origin of the universe from nothing need not be unlawful or unnatural or unscientific. In short, it need not have been a supernatural event.

Inevitably, scientists will not be content to leave it at that. We would like to flesh out the details of this profound concept. There is even a subject devoted to it, called quantum cosmology. Two famous quantum cosmologists, James Hartle and Stephen Hawking, came up with a clever idea that goes back to Einstein. Einstein not only found that space and time are part of the physical universe; he also found that they are linked in a very intimate way. In fact, space on its own and time on its own are no longer properly valid concepts. Instead, we must deal with a unified "space-time" continuum. Space has three dimensions, and time has one, so space-time is a four-dimensional continuum.

In spite of the space-time linkage, however, space is space and time is time under almost all circumstances. Whatever space-time distortions gravitation may produce, they never turn space into time or time into space. An exception arises, though, when quantum effects are taken into account. That all-important intrinsic uncertainty that afflicts quantum systems can be applied to space-time, too. In this case, the uncertainty can, under special circumstances, affect the identities of space and time. For a very, very brief duration, it is possible for time and space to merge in identity, for time to become, so to speak, spacelike - just another dimension of space.

The spatialization of time is not something abrupt; it is a continuous process. Viewed in reverse as the temporalization of (one dimension of) space, it implies that time can emerge out of space in a continuous process. (By continuous, I mean that the time-like quality of a dimension, as opposed to its spacelike quality, is not an all-or-nothing affair; there are shades in between. This vague statement can be made quite precise mathematically.)

The essence of the Hartle-Hawking idea is that the Big Bang was not the abrupt switch- ing on of time at some singular first moment, but the emergence of time from space in an ultra-rapid but nevertheless continuous manner. On a human time scale, the Big Bang was very much a sudden, explosive origin of space, time, and matter. But look very, very closely at that first tiny fraction of a second and you find that there was no precise and sudden beginning at all. So here we have a theory of the origin of the universe that seems to say two contradictory things: First, time did not always exist; and second, there was no first moment of time. Such are the oddities of quantum physics.

Even with these further details thrown in, many people feel cheated. They want to ask why these weird things happened, why there is a universe, and why this universe. Perhaps science cannot answer such questions. Science is good at telling us how, but not so good on the why. Maybe there isn't a why. To wonder why is very human, but perhaps there is no answer in human terms to such deep questions of existence. Or perhaps there is, but we are looking at the problem in the wrong way.

Well, I didn't promise to provide the answers to life, the universe, and everything, but I have at least given a plausible answer to the question I started out with: What happened before the Big Bang?

The answer is: Nothing.

From 'How Things Are: A Science Tool-Kit for the Mind', edited by John Brockman and Katinka Matson, to be published on 27 July by Weidenfeld & Nicolson at pounds 18.99