Beyond a Theory of Everything

On the very large and very small versus the very, very complex.

The LHC hasn’t yet provided its first results, the much-anticipated answers to questions we’ve been asking for so long. But they should surely come in 2009, bringing us closer to understanding the bedrock nature of particles, space, and time — toward a unified theory of the basic forces. This would push forward a program that started with Newton (who showed that the force that made the apple fall was same one holding the planets in orbit), and continued through Faraday, Maxwell, Einstein, Weinberg/Salam, and others in a distinguished roll call.

Most exciting of all would be clues to the ultimate unification between the force of gravity, which governs cosmic scales, and the forces of the microworld. Indeed, the quest for a unified theory engages huge numbers of the most talented young theorists (too many, in my opinion — most would derive more satisfaction, and contribute more to science, if they focused on other scientific frontiers that are less intensively studied). But while unified theories are sometimes called “theories of everything,” this phrase is misleading and hubristic. Such theories offer absolutely zero help to 99 percent of scientists. Chemists and biologists don’t fret about their ignorance of subnuclear physics, still less about the mysterious “deep structure” of space and time.

String theory, or some alternative to it, might indeed unify two great scientific frontiers, the very big and the very small — and that would be an immense intellectual triumph. But a third frontier, the very complex, is perhaps the most challenging of all.
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In terms of scale, the most complex entities we know of — ourselves — are midway between atoms and stars. It would take about as many human bodies to make up a star as there are atoms in each of us. Living things are very large compared to atoms: They must be big enough to have layer upon layer of intricate structure. But they cannot be too large, otherwise they would be crushed by gravity.

It may seem topsy-turvy, then, that astronomers can speak confidently about things billions of light-years away, whereas things on the seemingly more graspable human scale, such as theories of diet and child care, are notorious for their lack of consensual progress. But stars are simple. They’re so big and hot that their content is broken down into simple atoms; none matches the intricate structure of even an insect, let alone the human brain.

The sciences are sometimes likened to different floors of a tall building: particle physics on the ground floor, then the rest of physics, then chemistry, then biology, and so forth, all the way up to psychology, with the economists in the penthouse. There is a corresponding hierarchy of complexity — atoms, molecules, cells, organisms, ecosystems, and so forth.

But the analogy of a building is poor. A building, especially a high one, needs secure foundations. But the “higher level” sciences that deal with complex systems each have their own autonomous concepts. An insecure base doesn’t imperil them, as it would a building.

Scientists who try to understand why flows go turbulent, how taps drip, or why waves break, treat the fluid as a continuum: Subatomic details are irrelevant. Even if we could solve Schrödinger’s equation for all the atoms in the flow, the solution would offer no insight into turbulence.

We can predict with confidence that an albatross will return to its nest having wandered 10,000 kilometers or more over the southern ocean. Such a prediction would be impossible — not just in practice, but even in principle — if we considered the albatross as an assemblage of electrons, protons, and neutrons. Animal behavior is best understood in terms of goals and survival rather than any concepts used by physicists or chemists.

Finding the “read out” of the human genome, discovering the string of molecules that encode our genetic inheritance, is an amazing achievement. But it is just the prelude to the far greater challenge of post-genomic science: understanding how the genetic code triggers the assembly of proteins and expresses itself in a developing embryo. Other aspects of biology, especially the nature of the brain, pose challenges at a higher level of the hierarchy that can barely yet be formulated.

Problems in biology and in environmental and human sciences remain unsolved because scientists have yet to elucidate the complex patterns, structures, and interconnections — not because we don’t understand subatomic physics well enough.

If Newton and Einstein are the icons of “unification,” then Charles Darwin is the icon of complexity. In the famous concluding words of On the Origin of Species, he showed how “whilst this planet has been cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved.”

2009 is a double anniversary for Darwin: the 200th of his birth, and the 150th of the publication of his great book. The focus on his intellectual legacy will be a fitting reminder that our beautiful and wonderful everyday world presents intellectual challenges just as daunting as those of the cosmos and the quantum.  — Sir Martin Rees is president of The Royal Society and Master of Trinity College, Cambridge.