It has become fashionable lately to speculate that science-specifically, physics, the king of sciences-may be coming to an end. Some physicists believe that they are on the brink of finding a "theory of everything," or TOE, that will describe all of the forces and particles of nature in one equation that could be written on a T-shirt. As ever, the popularisations are trailing some way behind what the scientists themselves are saying. As long ago as 1980, Stephen Hawking gave a lecture entitled "Is the End of Theoretical Physics in Sight?", in which he suggested that the TOE might be no more than 10 or 20 years away. Since then, he has made simi-lar speculations, but the TOE always remains 10 or 20 years into the future, whenever the talk is given.
But what would such a TOE look like, if it were ever attained? Today, we know of four fundamental forces which rule the universe. Gravity and electromagnetism are familiar in the everyday world, and there are two forces, the strong and weak interactions, that operate only on a sub-atomic scale. The particles that interact with the aid of these forces-the entire material world-come in just two families, the quarks (which make up things such as protons and neutrons in atoms, and therefore the bulk of everyday matter) and the leptons (which include the electrons that orbit in the outer regions of atoms). And that is it. Already scientists have found a way to combine their description of the electromagnetic force and the weak force in one package, called the electroweak interaction. And there is indeed a realistic hope of finding a TOE to describe all four forces in one equation.
But would that be the end of physics? There are lessons to be drawn from history. A hundred years ago, physicists knew about electromagnetism and gravity, but had no inkling that the other two forces existed. They were just coming to terms with the idea of atoms, but it was only in 1897 that JJ Thomson first found evidence that the electron was a piece that could be chipped off an atom (the evidence was not compelling until 1899), so that atoms were not indivisible. After that discovery at the end of the 19th century, there was a widespread belief that physics was at an end. Then came the theory of relativity, quantum physics and the discovery of subatomic particles.
So the first question to ask advocates of the idea of the end of science is how they know that there are no deeper layers of the particle world yet to be probed. Of course, they do not. To put things in perspective, there is some reason to think that the smallest scale on which any particle could have a meaningful existence would be the so-called Planck scale, where quantum effects make even the concepts of space and time fuzzy. I will not trouble you with all the zeros that have to be written out after a decimal point before you get down to the Planck scale, but in terms of ratios, the distance we still have to go from the scale of quarks to the Planck scale is the same as the ratio of the diameter of an atom to the orbit of the Moon. The orbit of the Moon was first satisfactorily described by Isaac Newton a little over 300 years ago; quarks were explained a little over 20 years ago. Even allowing for the greater pace of science today, it seems to smack of overconfidence to expect as much progress in the next 10 years as in the past 300.
You may object that there is no evidence for further layers of matter within the quark. But there is! The package I have described, four forces and two families of particles, is called the standard model of particle physics. And the one thing all particle physicists know for certain is that the standard model is imperfect. It almost explains everything we have yet observed, but not quite. There are a few loose strands at the edges of the standard model, and physicists are continually picking at these strands, trying to find the one to pull which will make the model unravel. For, of course, to a physicist the excitement lies not in proving what is already known to another decimal place, but in discovering something new. They would all be delighted if the standard model proved to be no more the last word than the 19th century model of the atom as an indestructible billiard ball. And one of those loose strands is the new evidence coming from high energy particle collisions at an accelerator in Germany (experiments carried out within the past year, and soon to be repeated) that there may be a previously unknown force operating on a distance scale smaller than the size of a quark.
this is still less than half the story. Even if all the particles and forces are already known, and even if a TOE is found in the next 10 or 20 years, would that leave physicists out of work? Far from it. The great physicist Richard Feynman used to make an analogy with a game of chess. A child of five can learn the rules-how a knight moves, the role of the pawn and so on. Indeed, a child of 13 has just become an international master. But the greatest chess player who ever lived can spend a lifetime applying those rules, and still find new ways for them to interact, producing new games of chess. The equation that could be written on a T-shirt would not be the last word in physics, but the basic rule book, from which you would still have to explain the complexity of the universe.
That, indeed, is the key to the next develop- ment in physics-complexity. It is all very well speculating about probing deeper into the structure within the atom, but what about the structure in the universe at large? If the universe began in the hot fireball of a big bang some 15 billion years ago (as an overwhelming weight of evidence still suggests), how did it evolve to produce galaxies and stars, planets and people?
The keyword here is "evolve." Forget the end of science polemics. The most important science book published in 1997 is one which carries the opposite message, and which also carries the rather startling title, The Life of the Cosmos. It comes from Lee Smolin, a physicist based in New York, and it elaborates a theme that has been developed over the past few years by Smolin himself, Andrei Linde in California, and a handful of other researchers. Their thesis is that the way the universe works can best be understood not simply by applying the rules of physics worked out by Newton and Einstein, but by taking account as well of the rules of evolution worked out by Darwin-the theory of natural selection. The universe itself, and its main components (notably galaxies such our own Milky Way) may literally be alive, and, more to the point, may have evolved by natural selection from a simpler state to produce the complexity we see around us.
To take literally the equations of the general theory of relativity, the big bang itself emerged from a point of infinite density, known as a singularity. There is another place where singularities are known to occur-at the heart of a black hole. As Roger Penrose and Stephen Hawking proved in the 1960s, the expanding universe is described by exactly the same equations as a collapsing black hole, but with the opposite direction of time.
As far as I am aware, I was the first person to describe the universe as the inside of a black hole, in an unsigned editorial commentary in the journal Nature, where I used to work (volume 232 page 440, 1971). If all the complexity of galaxies, stars, planets and organic life has emerged from the singularity in which our universe was born, within a black hole, could not something similar be happening to the singularities at the heart of other black holes?
The most basic view of what might happen to a collapsing singularity to turn it into the kind of expanding singularity that we see in our universe is that there is simply a "bounce," turning collapse into expansion. Unfortunately, that will not do as an explanation. A singularity forming from a collapse within our three dimensions of space and one of time cannot turn itself around and explode back outwards in the same three dimensions of space and one of time. But in the 1980s relativists realised that there is nothing to stop the material that falls into a singularity from being shunted through a kind of spacetime warp and emerging as an expanding singularity in another set of dimensions-another spacetime.
Mathematically, this "new" spacetime is represented by a set of four dimensions (three of space and one of time), just like our own but with all of the new dimensions at right angles to all of the familiar dimensions of our own spacetime. Every singularity, on this picture, has its own set of spacetime dimensions, forming a bubble universe within the framework of some "super" spacetime, which we can refer to simply as "superspace."
One way to picture what this involves is to use the old analogy between the three dimensions of expanding space around us and the two-dimensional expanding surface of a balloon that is being steadily filled with air. The analogy is not with the volume of air inside the balloon, but with the expanding skin of the balloon, stretching uniformly in two dimensions, but curved around upon itself in a closed surface. Imagine a black hole as forming from a tiny pimple on the surface of the balloon, a small piece of the stretching rubber that gets pinched off, and starts to expand in its own right. There is a new bubble, attached to the original balloon by a tiny, narrow throat-the black hole. And this new bubble can expand away happily in its own right, to become as big as the original balloon, or even bigger, without the skin of the original balloon (the original universe) being affected at all. There can be many bubbles growing out of the skin (the spacetime) of the original universe in this way at the same time. And, of course, new bubbles can grow out of the skin of each new universe, ad infinitum.
The dramatic implication is that many-perhaps all-of the black holes that form in our universe may be the seeds of new universes. And, of course, our own universe may have been born in this way out of a black hole in another universe. This means that the universe may not be unique. Instead, it may be one of a population of universes, interconnected by what physicists call wormholes.
The key element that Smolin has introduced into the argument is the idea that every time a black hole collapses into a singularity and a new baby universe is formed, the basic laws of physics are altered slightly as spacetime it-self is crushed out of existence and reshaped. The process is analogous (perhaps more than analogous) to the way mutations provide the variability among organic life forms on which natural selection can operate. Each baby universe is, says Smolin, not a replica of its parent, but a slightly mutated form.
The original, natural state of such baby universes is to expand out to only about the Planck length, before collapsing once again. But if the random changes in the workings of the laws of physics-the mutations-happen to allow a little bit more expansion, a baby universe will grow a little larger. If it becomes big enough, it may separate into two, or several, different regions, that each collapse to make a new singularity, and thereby trigger the birth of a new universe. Those new universes will also be slightly different from their parents. Some may lose the ability to grow much larger than the Planck length and will fade back into the quantum foam. But some may have a little more inflation still than their parents, growing even larger, producing more black holes and giving birth to more baby universes in their turn. The number of new universes that are produced in each generation will be roughly proportional to the volume of the parent universe. There is even an element of competition involved, as the many baby universes are in some sense vying with one another, jostling for spacetime elbow room within superspace.
Heredity is an essential feature of life, and this description of the evolution of universes works in a similar manner to living systems. On this picture, universes pass on their characteristics to their offspring with only minor changes, just as people pass on their characteristics to their children with only minor changes.
Universes that are "successful" are the ones that leave most offspring. Provided that the random mutations are indeed small, there will be a genuinely evolutionary process favouring larger and larger universes. Once universes start to be big enough to allow stars to form, in succeeding generations of universes there will be a natural evolution, a drift in the laws of physics, to favour the production of the kinds of stars that will eventually form black holes.
The end product of this process should be not one but many universes which are all about as big as it is possible to get while still being inside a black hole, and in which the parameters of physics are such that the formation of stars and black holes is favoured. Our universe exactly matches that description.
This explains the otherwise baffling mystery of why the universe we live in should be "set up" in what seems, at first sight, such an unusual way. Just as you would not expect a random collection of chemicals to suddenly organise themselves into a human being, so you would not expect a random collection of physical laws emerging from a singularity to give rise to a universe such as the one we live in. Before Charles Darwin and Alfred Wallace came up with the idea of natural selection, many people believed that the only way to explain the existence of so unlikely an organism as a human being was by supernatural intervention; recently, the apparent unlikelihood of the universe has led some people to suggest that the big bang itself may have resulted from supernatural intervention. But there may no longer be any basis for invoking the supernatural. We live in a universe which is exactly the most likely kind of universe to exist, if there are many living universes that have evolved in the same way that living things on Earth have evolved. The fact that our universe is "just right" for organic life forms such as ourselves turns out to be no more than a side effect of the fact that it is "just right" for the production of black holes and baby universes. Cosmologists are now having to learn to think like biologists and ecologists, and to develop their ideas not within the context of a single, unique universe, but in the context of an evolving population of universes. Each universe starts from its own big bang, but all the universes are interconnected in complex ways by black hole "umbilical cords," and closely related universes share the "genetic" influence of a similar set of physical laws.
But the realisation that our universe is just one among many, that it is alive and that no supernatural influences need be invoked to explain its existence, is still not the most dramatic conclusion we can draw from the new cosmology. Although it is now clear that the universe has not been set up for our benefit, and that the existence of organic life forms on Earth is a minor side effect of an evolutionary process involving universes, galaxies and stars, nevertheless it is clear that the existence of life forms such as ourselves is an inevitable side effect of those greater evolutionary processes.
The same laws of physics apply throughout our universe and throughout many other universes besides. Organic (carbon based) material occurs in profusion between the stars of a spiral galaxy such as our Milky Way. This carbon-rich material seems to be crucially involved in the processes which allow gas clouds to cool and new stars to form, so a universe that is good at making black holes will also be good at making carbon based compounds. Those compounds will undoubtedly seed any Earth-like planet that forms with each new generation of stars.
Astronomers calculate that there may be as many as 1020 planets suitable for life forms such as ourselves in our universe. We see the components of organic life everywhere in the universe, and the chances are that most of those 100,000,000,000,000,000,000 planets actually are carriers of our kind of life, in the same way that Earth is a carrier of life. The birth of the living universe inevitably gave rise to the birth of living planets. Which still leaves physicists the task of explaining just how complexity arose in a hot universe expanding out of a big bang. The end of science has been exaggerated. Indeed, you ain't seen nothin' yet.
