SYDNEY

Imagine you can play God and fiddle with the settings of the great cosmic machine. Turn this knob and make electrons a bit heavier; twiddle that one and make gravitation a trifle weaker. What would be the effect? The universe would look very different - so different, in fact, that there wouldn't be anyone around to see the result, because the existence of life depends rather critically on the actual settings that Mother Nature selected.

Scientists have long puzzled over this rather contrived state of affairs. Why is nature so ingeniously, one might even say suspiciously, friendly to life? What do the laws of physics care about life and consciousness that they should conspire to make a hospitable universe? It's almost as if a Grand Designer had it all figured out.

The fashionable scientific response to this cosmic conundrum is to invoke the so-called multiverse theory. The idea here is that what we have hitherto been calling "the universe" is nothing of the sort. It is but a small component within a vast assemblage of other universes that together make up a "multiverse."

It is but a small extra step to conjecture that each universe comes with its own knob settings. They could be random, as if the endless succession of universes is the product of the proverbial monkey at a typewriter. Almost all universes are incompatible with life, and so go unseen and unlamented. Only in that handful where, by chance, the settings are just right will life emerge; then beings such as ourselves will marvel at how propitiously fine-tuned their universe is.

But we would be wrong to attribute this suitability to design. It is entirely the result of self-selection: we simply could not exist in biologically hostile universes, no matter how many there were.

This idea of multiple universes, or multiple realities, has been around in philosophical circles for centuries. The scientific justification for it, however, is new.

One argument stems from the "big bang" theory: according to the standard model, shortly after the universe exploded into existence about 14 billion years ago, it suddenly jumped in size by an enormous factor. This "inflation" can best be understood by imagining that the observable universe is, relatively speaking, a tiny blob of space buried deep within a vast labyrinth of interconnected cosmic regions. Under this theory, if you took a God's-eye view of the multiverse, you would see big bangs aplenty generating a tangled melee of universes enveloped in a superstructure of frenetically inflating space. Though individual universes may live and die, the multiverse is forever.

Some scientists now suspect that many traditional laws of physics might in fact be merely local bylaws, restricted to limited regions of space. Many physicists now think that there are more than three spatial dimensions, for example, since certain theories of subatomic matter are neater in 9 or 10 dimensions. So maybe three is a lucky number that just happened by accident in our cosmic neighborhood - other universes may have five or seven dimensions.

Life would probably be impossible with more (or less) than three dimensions to work with, so our seeing three is then no surprise. Similar arguments apply to other supposedly fixed properties of the cosmos, such as the strengths of the fundamental forces or the masses of the various subatomic particles. Perhaps these parameters were all fluke products of cosmic luck, and our exquisitely friendly "universe" is but a minute oasis of fecundity amid a sterile space-time desert.

How seriously can we take this explanation for the friendliness of nature? Not very, I think. For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there are an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification.

Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence it requires the same leap of faith.

At the same time, the multiverse theory also explains too much. Appealing to everything in general to explain something in particular is really no explanation at all. To a scientist, it is just as unsatisfying as simply declaring, "God made it that way!"

Problems also crop up in the small print. Among the myriad universes similar to ours will be some in which technological civilizations advance to the point of being able to simulate consciousness. Eventually, entire virtual worlds will be created inside computers, their conscious inhabitants unaware that they are the simulated products of somebody else's technology. For every original world, there will be a stupendous number of available virtual worlds - some of which would even include machines simulating virtual worlds of their own, and so on ad infinitum.

Taking the multiverse theory at face value, therefore, means accepting that virtual worlds are more numerous than "real" ones. There is no reason to expect our world - the one in which you are reading this right now - to be real as opposed to a simulation. And the simulated inhabitants of a virtual world stand in the same relationship to the simulating system as human beings stand in relation to the traditional Creator.

Far from doing away with a transcendent Creator, the multiverse theory actually injects that very concept at almost every level of its logical structure. Gods and
worlds, creators and creatures, lie embedded in each other, forming an infinite regress in unbounded space.

This reductio ad absurdum of the multiverse theory reveals what a very slippery slope it is indeed. Since Copernicus, our view of the universe has enlarged by a factor of a billion billion. The cosmic vista stretches one hundred billion trillion miles in all directions - that's a 1 with 23 zeros. Now we are being urged to accept that even this vast region is just a minuscule fragment of the whole.

But caution is strongly advised. The history of science rarely repeats itself. Maybe there is some restricted form of multiverse, but if the concept is pushed too far, then the rationally ordered (and apparently real) world we perceive gets gobbled up in an infinitely complex charade, with the truth lying forever beyond our ken.

Paul Davies, professor of natural philosophy at the Australian Center for astrobiology, is author of "How to Build a Time Machine."