UNIVERSE OR MULTIVERSE?
Edited by Bernard Carr
Cambridge University Press 2007 £45
There's a story told about a university that fell on hard times. The Chancellor reasoned that given the collective scientific knowledge of his academic staff if they were to invest in a racehorse they could be sure of a winner. So he e-mailed the professors. The engineers replied that for the best mechanical advantage a horse with long legs was the best bet. The biologists pointed out that there was a genetic link between efficient muscles and brown coat colour. After a long delay the cosmologists finally came back with a request for further funding to develop their very promising model of a spherical horse!
Like all good jokes, there is a tiny grain of truth in that story. Though to be fair to cosmologists, it is not normally their modelling which is flaky just that they dare to venture further into the unknown than most other disciplines. The big change that has come about in the last decade or two is that observational physics and astronomy and mathematical modelling are now able to take cosmology into areas that used to be restricted to theologians and philosophers. It is no longer considered out of bounds to ask 'why' questions as well as 'what and how' questions. In this case, asking a why questions about our Universe produces a descriptive answer that goes way, way beyond what is observable. For some, that we live in a Multiverse is in fact one of the more logical answers to the why question of the Universe. For others, including the philosopher Neil Manson, "the multiverse is the last resort for the desperate atheist".
It all began in 1973 when Brandon Carter, at the Paris Observatory, coined the term "the anthropic principle". He was speaking at a 500th anniversary conference about Copernicus and wanted something to counteract the Copernican arguments that there is nothing special about our place in the universe -- a typical solar system among billions. He pointed out that our very presence as observers of the Universe restricts the sort of Universe we can observe to one that is able to produce complex life such as ourselves and which has had enough time to do so. Carter rather regrets using the word Anthropic now because it implies something special about humans in particular whereas he intended it to refer to any complex observer. But the name stuck.
It turns out that our place in the Universe does seem to be very special indeed, and not simply our position in space and time. If any one of about 19 different seemingly fundamental constants of nature were any different, the Universe would not be suitable for the production or sustenance of carbon-based life and we would not be there to observe it. At present, we know of no underlying reasons in physics why those constants should have the values they do. They include the ratios of the strengths of the different fundamental forces in physics, such as gravity and electromagnetism and the resonances and coupling constants which mean that atoms such as carbon and oxygen get formed in stars and blown out to form new solar systems. They also include the stability of matter, the number of spatial dimensions and the longevity of stars.
The classic work on this is The Anthropic Cosmological Principle by John Barrow and Frank Tipler, published in 1986 before the term Multiverse was even in widespread use, though the book does discuss the 'many worlds hypothesis' as it was then known. Much of that excellent work was devoted to analysing the problem posed by design arguments in many fields of science and it is interesting to see how far studies have come since then.
The time is now right for Universe or Multiverse? It developed out of a series of workshops in Stanford USA and Cambridge UK supported by the Templeton Foundation. The chapters are mostly adaptations of papers delivered then by a spectacular array of 27 cosmologists, mathematicians and philosophers. The volume has been edited by Bernard Carr who provides an excellent overview in his introduction and first chapter. The authors of following chapters include the likes of Stephen Hawking, Martin Rees, Andrei Linde, George Ellis, Leonard Susskind, Max Tegmark, Steven Weinberg, and Paul Davies. I have to confess that the mathematics in some of the chapters was way beyond me but others are lucid narratives and all reward a little perseverance.
The "?" in the title is significant and much of the discussion centres around whether multiple universes are the best explanation for anthropic selection. The term multiverse can cover a number of ideas. There is the quantum multiverse of parallel universes expressing each possible outcome of every quantum event. There is the multiverse which is essentially an extension of our own Universe into regions beyond the light horizon and thus causally unconnected with our world. Then there is the idea of multiple distinct universes either completely separate from or in dimensions in some sense orthogonal to our own; a sort of foam of universes. That's a concept given theoretical underpinning by the latest developments of string theory, M. theory, which suggests that there could be universes with 10 (to the power of 500) different vacuum states. In any of these scenarios, it is reasonable to imagine universes with different physical laws from our own and it therefore follows naturally that we can observe only ones which are suitable for our existence. As Martin Rees puts it, our laws of physics are just local bylaws in the Multiverse. The alternative is much harder for an atheist or agnostic scientist to handle. That suggests that the universe was in some sense designed with the purpose of producing complexity and life or at least that purpose is somehow written in to the laws of physics at a deeper level.
It is sometimes said (particularly by theologians) that God defies proof. But there are ways in which we could investigate how precisely "designed" the Universe actually is. In most of the parameters that make our Universe bio friendly, there is room for variation, albeit very slight. The question is, is it perfect or is it 'near enough'. Take the orbit of the Earth for example. If it were highly elliptical, the oceans would freeze for part of the year and boil six months later. Life would be impossible. But it would seem to be a remarkable coincidence if our orbit were precisely circular. In this case, we know that it is not exactly circular, just within tolerable limits. For one parameter, there might be quite wide safe margins but for 19, it would be like a 19-dimensional object: it's much more likely, were it left to chance, that we would be near the edge of viability in at least some parameters rather than in the perfect, safe place in the middle. So we just need to measure the actual values of the constants, work out what range would be viable and we would have some idea of how well "designed" our Universe is. Anthropic selection from the Multiverse would be much more likely to place us on the edge of viability.
As philosopher Nick Bostrom points out, we have to be very careful not to be swayed too far by selection effects. Of course the laws of nature seem bio-friendly because we can only observe in a bio friendly universe. But it is possible to take that argument much further! Sooner or later, he suggests, a technological civilisation will emerge which can command infinite computer power. It will want to run simulations of its own genesis that are accurate in every detail and it is likely to run these simulations many times. It follows therefore that we are much more likely to be living in a simulated Universe than a real one! It seems absurd, if not to the makers of the film The Matrix. But is it any more absurd than a Multiverse?
So can we find any satisfying and consistent line of explanation? I find the idea of a tower of turtles to be a useful metaphor here. Paul Davies mentions it briefly in the final chapter and has developed it elsewhere. You remember the idea: the world is supported on the back of a giant turtle. So what supports the turtle? The answer is, it's turtles all the way down! In a sense, the Multiverse is like an infinite tower of turtles; it solves the immediate problem but substitutes one that is even worse. As Davies points out, invoking either something more fundamental in the laws of physics or a creator God does not help much. Both are like levitating super turtles and you end up with a simplistic argument over whose turtle is best! Davies' own tentative explanation is more like a loop of turtles in which complex life, mind perhaps, somehow influences the selection of the physical laws of the universe in a process of backwards causation, rather as an observer influences the outcome of a quantum process. It's not a very popular idea so far, but it does seem to be about the only way out of turtle trouble and into a self consistent explanation. And it puts mind or consciousness centre stage.
Some may find it unsatisfying that this is the book that reaches no firm conclusion. Like a whodunnit with the last chapter missing. I find it exciting to think that there are some things that we don't yet know and yet about which people now have the language and skills to argue. This book really does lie at the frontier of cosmology, philosophy and possibly even theology. It is essential reading for anyone who wants to consider these ideas in depth. So it is a pity that it has been priced at a level that only specialists can afford. But it does go to show just how far cosmology has come on the road from pure speculation to exact science. This is no spherical horse. It contains ideas that can gallop.
Martin Redfern is a senior BBC radio producer with the World Service. He enjoys wild speculation in a way that only a journalist can, and is currently en-route to Antarctica in pursuit of stories from the edge of our Earth. He is an editorial advisor to, Science, People & Politics (ISSN:1751-598X).
I updated html mark up code today 12.10.11. I have made no alterations to text since publication. HG.