David Bohm and Basil Hiley worked together for twenty years and between them developed a very unusual approach to quantum theory. Bohm died in 1992, but by then the book was almost complete. It is a magnificent monument to one of this century’s finest and most attractive minds.
Painfully shy, and finding few fellow physicists willing to give a hearing to his new ideas, Bohm struggled for four decades to get beyond the orthodox views that he had himself defended in his Quantum Theory of 1951, long the subject’s standard textbook, but which later put him in mind of Escher’s Waterfall, whose careful construction cannot hide the fact that the water must at some stage be flowing uphill. He found himself proposing that far separated events were correlated by influences which acted instantaneously. True, we couldn’t use these influences for faster-than-light signalling. Nevertheless, they would show that there was an absolute ‘now’, a uniquely correct way of splitting space from time, of the sort denied by Einstein’s special relativity.
Unpopular stuff, this. Yet Bohm made matters worse when he sprang to Einstein’s defence in an area where the great man was generally considered to have blundered – the blunder being summed up in the remark that God didn’t play dice. Bohm worked his way round to something very reminiscent of Louis de Broglie’s early approach to quantum theory. Particle positions, apparently just matters of chance, were in fact under the control of fully deterministic ‘pilot waves’. Over the years, Bohm and Hiley developed this theory in detail, guarding it against the criticisms which had led de Broglie to abandon it. Not even Bohm’s status as Fellow of the Royal Society and professor at Birkbeck could persuade many physicists to listen to a theory of this kind. And their tendency to block their ears was if anything made worse when he spoke of the ‘undividedness’ of the universe. While his reputation kept growing with the general public, swelled by such fine books as Wholeness and the Implicate Order (1980), physicists found something else to occupy their minds.
The Undivided Universe is Bohm’s final plea for their understanding. He and Hiley introduce their results in considerable mathematical detail, with difficult excursions into philosophy. Anyone likely to be put off by technical arguments – or the mere sight of mathematical equations – should read one of Bohm’s other books instead. On the assumption that some readers will be willing to give this one a try, I will concentrate here on one or two things to look out for.
To begin with, there is the ‘pilot-wave’ model of the famous double-slit experiment. Electrons fly towards two openings in a screen. Even if only one election is in flight at any one moment, ‘interference patterns’ are formed on the photographic plate on which they eventually land. It is as if the electrons were each of them a wave which traversed both openings. The two resulting waves would then ‘interfere’, reinforcing each other in some places and cancelling each other out at others. Always, however, such waves would finish by collapsing down to single landing places. The wave-patterns would control the likelihood that particular landing places would be chosen, but according to the orthodox ‘Copenhagen’ interpretation of quantum theory, there would be absolutely nothing that decided exactly which landing place would be chosen by any given electron. The waves might thus be considered ‘mere waves of probability’.
Bohm and Hiley instead hold that they are actual waves: waves which guide electrons to one place rather than another. The guidance, however, has results unpredictable by anyone. Think of the guidance given to a falling marble by a wall bristling with pegs: the slightest change in how you drop the marble makes it bounce down a different sequence of pegs, ending in a different spot.
In developing this theme, Bohm and Hiley introduced a highly unpopular element: the effect of the pilot waves depended on their shape, not their intensity. This is one basis for the notion of an undivided universe, since the waves could in this case spread to any distance without losing their power to guide. But how could a very spread-out, very weak wave have powerful effects on elementary particles such as electrons? Bohm and Hiley reply by pointing to the way huge ships can be controlled by tiny radio impulses. This suggests to them that elementary particles aren’t elementary in the sense of being simple entities, but are rather like radios, with complex innards. This helps narrow the gap between the ‘bruteness’ of matter and the subtlety of consciousness.
How about instantaneously communicated influences? Bohm and Hiley point to recent experiments inspired by the late John Bell. Particles generated in pairs can move apart while maintaining a mysterious connectedness. Observing the state of one, an experimenter can be sure that its distant partner is in a correlated state, in much the same way as examining one half of a fish and finding that it’s the head will tell you that me fish’s other half is the tail, even when it is in New York and you are in London. In one respect, however, the case of the particle partners is very unlike that of the fish and more like tossing a coin in New York and knowing at once, when you see it landing heads, that a second coin in London is landing tails. Paradoxical though this may be, it is accepted by almost every physicist. What distinguishes Bohm and Hiley is their eagerness to use it as a reason for speaking of ‘non-local’ influences: influences that help to pull the universe into a single whole.
Themes like these are woven into a book of tremendous richness, its main ideas far too many to be listed. One of them is particularly important. An ink blob in glycerine, trapped between a fixed cylinder and a rotating one, can be utterly smeared out, ‘enfolded’, and then almost magically reassembled when the rotation is reversed. It was seeing this done on television which first led Bohm to speak of an ‘implicate’ or ‘enfolded’ order in the universe, an order allowing seemingly scattered and chopped-up patterns to be startlingly recovered at distant points in space or time. His knowledge of holograms, which again hide patterns in spread-out forms that permit of almost magical reassembly, convinced him that he was on the right track. He then applied the idea of ‘enfoldment’, and of its subsequent reversal (‘unfolding’), to very many areas.
The book discusses quite a few of these. Particles, for instance, could be considered as waves continually diverging and reconverting. Or memories could be spread throughout the brain, yet be ready for recovery without long searching. Again, the unity of consciousness at any single moment (or else over several moments, as when we experience successive notes as a single tune) could be explained: each element in the whole would be ‘enfolded’ in the others. There could be an indefinite number of levels of enfoldment, each interacting with the level above and the level below.
Bohm and Hiley may distress many of their admirers by giving no fundamental role to observers in quantum theory, and by the determinism which they initially build into their pilot waves. Still, they insist that indeterminism (which they often present as something glorious, essential to true creativity) could always underlie any apparent determinism. Also that distinctions of mind and matter, observer and observed, can become very blurred when everything is enfolded in everything else.
The book’s last chapter is thoroughly uninhibited on this topic. It goes so far as to suggest that enfoldments have given rise to a ‘collective mind’ which might extend indefinitely beyond the human species. A soul of the biosphere, the solar system, the galaxy, the cosmos? No wonder so many physicists are reluctant to touch this kind of thing. But even supposing that it were very largely wrong, The Undivided Universe would remain a remarkable piece of work.
After the enormous press coverage of A Brief History of Time, all the world knows that Stephen Hawking has motor neurone disease, can speak only with a computer-synthesised voice controlled by the few fingers that he can move, and fills the same Cambridge chair as Newton did. The 14 essays of the new book, together with a Christmas Day radio interview of 1992, form a very mixed bunch. They cover Hawking’s early years, his experiences with slowly progressing paralysis, his views about the human race and its probable future, and some of his physical and cosmological ideas. These are introduced at levels varying from the elementary to the fairly advanced: one of the essays is his Inaugural Lecture of 1980. Mathematical formulas are avoided.
Hawking’s essay number five, ‘A Brief History of A Brief History’, tells us that despite dust-jacket photographs – not, he emphasises, a matter under his control – of the severely crippled author, the huge sales of the earlier book came as a big surprise both to him and to his publishers. I find this hard to understand. Hawking’s personal history was scarcely mentioned in the book itself, but journalists would scarcely have been likely to overlook it. His unremarkable performance at school (he was nearly demoted to a ‘B’ stream); his managing to get a first at Oxford despite working at physics for only about an hour a day; his later sudden energy, inspired by his wife and by the idea that he was unlikely to live much longer; his rapid rise to the Lucasian Professorship as his health continued to crumble; his unwillingness to allow his condition to end scientific work of first-rate importance, although at one stage his only means of communication was by waggling his eyebrows; his probing of the cosmos from the depths of a wheelchair; what journalist worth his salt would disregard these things? The sales figures were not surprising.
It so happens that the book was very well worth reading. People in universities, and philosophers in particular, tended to complain about its obscurities, but popular writing almost never escapes that kind of reaction. And the book’s central ideas made it of greater philosophical interest than almost all the volumes ever written by philosophers.
Black Holes and Baby Universes will help those eager to learn more about Hawking’s stubborn war against his paralysed condition. It will also be certain to upset the same philosophers as before, not entirely unwarrantably. Hawking’s ideas are often very exciting. One would love to understand them. But often this is no easy task – and not always just because physics and cosmology are difficult subjects. At many crucial points his explanations are highly oracular. Quite often their obscurity is recognised good-humouredly, apologetically; sometimes, though, it is defended in words which can easily sound too bellicose. Hawking is a warrior through and through – a great contrast with David Bohm.
Take one of the book’s chief ideas: that all the details of the universe, including the workings of our minds, could be explained by a Theory of Everything – some fairly simple set of laws dictating events right back to the beginning of time. These laws might actually be discovered within our lifetimes, Hawking thinks. What form would a Theory of Everything take? The Inaugural Lecture bets fairly heavily on a theory called ‘N=8 Extended Supergravity’. In the book’s later essays attention centres on something else. In the Sixties, Hawking and Roger Penrose seemed to have shown that any universe beginning in a state of infinite density, a ‘singularity’, which is how our universe was thought to have begun, would necessarily begin unpredictably. All Theories of Everything would fail at the very earliest times. But Hawking later came to believe that quantum physics allowed the beginning – at least as modelled in ‘imaginary time’, which is in some ways more like space than like ordinary time – to be entirely free of singularities. All of the universe’s events, from start to finish, might then be dictated by a few fairly simple mathematical equations. And although the universe wouldn’t have existed eternally, there would have been no time before it began, just as there is no place on Earth to the north of the North Pole. There would thus have been no need for a divine being to make things spring into existence at one moment rather than another.
Hawking admits that he cannot explain why the universe began: what it is that, as he puts it, ‘breathes fire into the equations’. Again, he points to evidence which could well show that the universe is fine-tuned to life’s requirements. Very small changes in such things as the strength of electromagnetism, or the mass difference between the neutron and the proton, could well have made it a universe in which life would never have evolved. As he remarks, this might suggest that there exist hugely many universes, their characters varying randomly. Necessarily, we living beings would then find ourselves in one of the universes which just happened to be life-permitting. But when you talk of this or that ‘just happening’ to be so, a Theory of Everything is of course in difficulty.
There are two other important problems for any such theory. The first comes from Hawking’s most famous discovery: that black holes aren’t completely black. A black hole was traditionally described as a system whose gravitational pull was so strong that light itself couldn’t escape from it. To everybody’s surprise, Hawking showed that light – together with particles of all kinds, including (on very, very rare occasions) particles which simply chanced to form television sets or the works of Proust in ten leatherbound volumes – could tunnel out of black holes by exploiting quantum theory’s Uncertainty Principle. Black holes would therefore ‘evaporate’, at first very slowly but eventually with extreme violence. Their final explosions might release energies equivalent to those of ten-million-megaton H-bombs. One strong possibility is that black holes would give rise to separate ‘baby universes’ which would later rejoin our universe – as black holes evaporating into – it at unpredictable times and places. Here would be a reason why a Theory of Everything could fail.
The second problem is still more interesting – and highlights the difference between the warriorship of Stephen Hawking and the quieter advocacy of David Bohm. The fact is that Hawking’s model of the universe incorporates many-worlds quantum theory. The key ideas of this theory were formulated by Hugh Everett nearly forty years ago. Quantum physics appears to tell us that the universe is intrinsically unpredictable. Events keep developing in fuzzy ways. The fuzziness then ‘collapses’ into something definite, but one never can say just what a collapse will lead to. Look at an electron at one moment. Nobody can tell precisely when and where this particular electron will next be seen. Everett suggested, however, that collapses never really occur. Instead, the universe continually streams into branches which develop differently. Observers branch like everything else. Thus you can never say exactly what you – ‘you’ being just one branch of the you of a moment ago – will see until you actually look. The big question is, whether Hawking really believes any of this. He answers oracularly but with no apologies – even, indeed, a little aggressively.
I teach philosophy for a living. I can see why other philosophers, including those who admire Hawking’s work as greatly as I do, can often read him only with sighs of frustration. Look at his essay number twelve. This discusses human freedom against the background of his belief that the world is fully deterministic – so that if it could somehow be rewound and started off again precisely as before, then everyone would do precisely what they did on the first occasion. Hawking ties himself in knots. He could be expected to opt for the ‘compatibilist’ view, defended by philosophers in their thousands, that even such decision-making machines as advanced computers, while remaining fully deterministic, could still really make decisions, in which case they should be described as free. However, it seems never to have entered his head that freedom and deterministic decision-making might be compatible. He declares that, because we are fully deterministic, we cannot be free. Yet he then goes on to say – in maddening disregard of the fact that one ought to do something only if free to do it or not do it – that we all ought to pretend to ourselves that we are free, although we aren’t. What a tangle! Yet why, after all, should a philosopher expect Hawking to discuss freedom expertly? Does it matter if he gets tangled up? Surely not. It is, however, immensely frustrating that one can’t make out where he stands on such philosophically fascinating questions as whether many-worlds quantum theory is right. Does Hawking’s universe actually branch, or doesn’t it?
He appears to reply in two flatly contradictory ways. He tells us firmly that there isn’t just a single history of the universe. Instead there is a collection of all physically possible histories. One and the same cat can die of old age in one history, while dying much earlier by violence in another. These branches are ‘equally real’. People who refuse to believe this have missed ‘the whole point of quantum mechanics’. And yet, virtually in the same breath, Hawking says that all this concerns ‘just a mathematical model’ and that it makes no sense to ask whether any theory of physics ‘corresponds to reality’. He also says that philosophers (and, while he avoids spelling it out, many top-flight physicists too) are tediously out of date when they keep puzzling over the foundations of quantum mechanics.
It would be nice to think that, in other equally real universe-branches, Hawking has written just as interestingly but a little less oracularly and a little more mildly.