Supersymmetry by Gordon Kane, Perseus Books, 拢17.95, ISBN 0738202037
Supersymmetry: The quantum theory of fields (volume III) by Steven Weinberg, Cambridge University Press (1999), 拢32.50, ISBN 0521660009
SOMETHING really is rotten at the heart of the standard model. Tough luck for the one-size-fits-all mock-up of the Universe鈥檚 origins and workings. Since the 1970s particle experimenters have dotted the model鈥檚 Is and crossed the Ts, their lives enlivened by the odd rumour, soon squashed, of a flaw in the model. You can鈥檛 blame particle physicists for being impatient to see supersymmetry confirmed. Their field has had precious few headlines in recent years.
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As Gordon Kane makes admirably clear in Supersymmetry, the standard model is full of holes 鈥� it contains too many free parameters and there are unnaturally peculiar relationships between its families of particles. For one thing, no one can make sense of the values of the particles鈥� masses. Worse, the model fails if you try to apply it to super-high-energy conditions of the Universe during the first instants of the big bang. So, though current experiments say the model is hunky-dory, theorists are certain it cannot possibly be right. They say the model needs to incorporate a special type of symmetry, known as supersymmetry.
Particle physicists are willing to go even further. Supersymmetry theory is too beautiful to be wrong, they say. All of them are rooting for it: the theorists love its elegance, the experimenters are beguiled by its ready testability. The problem is, no one has yet found a single piece of evidence to confirm it. Nature can be so perverse.
The theory of supersymmetry was proposed in the early 1970s by several groups of European theoreticians. They saw that there was an imbalance in the way the standard model treats bosons, the particles that have spins measured in whole numbers (0, 1, 2 and so on), and the way it treats fermions, which have spins that are odd multiples of a half (1/2, 3/2, 5/2 and so on). In the supersymmetric version of the model, the equations remain the same if the terms describing bosons are swapped with those describing fermions.
The result is a theory that is backwards-compatible with the standard model 鈥� it reproduces all the old model鈥檚 successes 鈥� but naturally solves its key problems, notably how to account for the masses of the particles in its families.
Happily, there are not one but two curtain raisers for the symmetry鈥檚 supposedly impending discovery. Steven Weinberg, one of the greatest theoreticians in the past 50 years, has written a magisterial, no-holds-barred account of the theory in all its glory, while fellow theorist Kane provides a gentle explanation for the mathematically challenged of what all the fuss will be about when the experimenters have come up with the goods.
After reading these two books, you feel as though you鈥檝e proof-read a play before its premiere: all that鈥檚 needed is for the actors 鈥� in this case, the particles 鈥� to hurry up, learn their words, and get on stage.
Take your partners
Kane spares us the terrifying mathematics and concentrates on describing their implications. It turns out, remarkably, that every known particle must have a hitherto undetected partner, known as a sparticle. For example, the photon (with spin 1) has a corresponding spin 1/2 sparticle, the photino. Likewise, the electron and the quarks (all with spin 1/2) each have spinless sparticles, the selectron and the squarks. No wonder that the patois of supersymmetry enthusiasts has been dubbed 鈥渟language鈥�.
The symmetry of the theory goes well beyond its predicted particles. If nature really is supersymmetric then we shall have to revise our view of space and time. Whereas time and space are conventionally measured using numbers (8 pm, 400 metres above sea level), time and space in a supersymmetric universe is measured in quantum mechanical terms. Einstein, always a quantum sceptic, would have been appalled.
The main strength of Kane鈥檚 book is the clarity with which it sets out the theory鈥檚 predictions. He tells us where sparticles are most likely to show up: they should be found at CERN鈥檚 Large Hadron Collider near Geneva after it鈥檚 switched on in 2005, but the Fermilab accelerator in the US may beat the Europeans to it, possibly as early as next year. Kane describes clearly what could be the most fascinating coup of all: the possibility that sparticles make up the 90 per cent of the Universe鈥檚 mass not accounted for by astronomers. If true then it would follow that the Universe predominantly consists not of the matter that we are made of (electrons, quarks and so on), but of supersymmetric exotica. Now that would be a headline . . .
For the most part, Kane addresses himself to readers who know a little physics but want to know a lot more. But occasionally he forgets his audience. For example, only a page after he introduces Planck鈥檚 constant without explanation, he gives us a swift, kindergarten lesson in powers-of-ten notation.
Weinberg is in no doubt at all about his audience 鈥� they are the cr猫me de la cr猫me of theoretical physics graduate students. Supersymmetry, the third volume of his treatise on quantum field theory, is a tour de force of clear explication, catering for those gifted with the mathematical firepower and physical insight to follow science at this stratospheric level. The rest of us can only stand back in awe and be grateful that a leading theoretician has invested so much of his time in preparing a definitive text for succeeding generations. They will be grateful to Weinberg for setting out the bones of the subject so clearly, giving intricate details of proofs and calculations that other authors vexatiously keep to themselves. It will be fascinating to see how much of the theory has to be modified in the light of the planned experiments.
The greatest discovery of all would, of course, be that supersymmetry is wrong. For the first theorist to find a loophole or for the experimenter who refutes one of its key predictions, Stockholm awaits. Most particle physicists would be humiliated, but they would be compensated by a revolution in their subject. An intriguing scenario, but I wouldn鈥檛 bet on it.
