Spin Foam

Michael Redhead

  • Three Roads to Quantum Gravity: A New Understanding of Space, Time and the Universe by Lee Smolin
    Phoenix, 231 pp, £6.99, August 2001, ISBN 0 7538 1261 4

The old notions of space and time are currently being turned upside down by theoretical physicists in their attempt to reconcile the two great pillars of 20th-century physics: quantum theory and Einstein’s general theory of relativity. Lee Smolin, a major contributor to the subject, brings us right up to date with it in this book. It’s written for general readers, but is more than just another work of popular science: it is a serious attempt at clarifying the author’s own thoughts about the significance and interpretation of the highly mathematical theories he is discussing. The book belongs to a new genre of science writing, in which the author also tells the story of his own involvement in the research, so giving it a striking freshness. It’s reminiscent in this way of Brian Greene’s very successful The Elegant Universe, a book which covers some, but by no means all, of the same ground as Smolin. Indeed, the two complement each other nicely, for anyone who wants to understand, in more than a superficial way, what is currently happening to the deep conceptual foundations of physics.

At this level of enquiry, physics rapidly merges with philosophy. Neither Smolin nor Greene is a professional philosopher, but that isn’t to say that philosophers shouldn’t now be spending more of their time poring over the Physical Review and Reviews of Modern Physics. There have always been two traditions in metaphysics, characterised by Peter Strawson as ‘descriptive’ and ‘revisionary’. Descriptive metaphysics seeks to uncover how we do, or more ambitiously must, conceptualise the world we find ourselves in. Revisionary metaphysics seeks, by contrast, to show that our ordinary thinking about the world, or more generally reality, is quite wrong and needs radical revision. The great revisionists have been philosophers like Plato, Leibniz and Berkeley, weavers of fantastical metaphysical speculations. Descriptive metaphysics began in effect with Kant, and has been much in vogue ever since. Indeed, many modern analytic philosophers have rejected revisionary metaphysics altogether as wild and baseless.

Now, however, with modern theoretical physics as its conduit, revisionary metaphysics is back with a vengeance. No longer unbridled speculation, it emerges from attempts to remove glaring defects in theories that are empirically very well attested. Proof for the latest theories, as we shall see, is probably beyond experimental reach at present, but the speculations of theoretical physics might still, one day, face the ‘tribunal of sense experience’, as Quine put it. Quine argued, admittedly, that the tribunal’s verdict would never be unequivocal, but some sort of empirical check on the physicists’ speculations should be a possibility for the future. This has led some philosophers to take the view that modern physics is a new sort of ‘experimental metaphysics’, in Abner Shimony’s phrase. If that’s right, then philosophers should sit up and take notice of what physicists like Greene and Smolin are writing, the trouble being that the theories involve horrendously complicated mathematics. However, philosophers are now emerging who are equally qualified in physics and philosophy, and who aim to be taken seriously in both disciplines.

Smolin’s book begins by asking how it is that the quantum theory (QT) and the general theory of relativity (GR) in their present state are both so amazingly successful, when each is also profoundly unsatisfactory. The basic problem is that QT as currently formulated denies the most fundamental insight of GR, so that to make GR conform to QT, we must either change it to accommodate something that denies its own fundamental principles – not a very attractive prospect – or try to change radically the way we do QT.

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