2014 : WHAT SCIENTIFIC IDEA IS READY FOR RETIREMENT?

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Theoretical Physicist, Dartmouth College; author, The Island of Knowledge
Unification

There! I said it! The venerable notion of Unification needs to go. I don't mean the smaller unifications that we scientists search for all the time, connecting as few principles with as many natural phenomena as possible. This sort of scientific economy is a major foundational stone for what we do: we search and we simplify. Over the centuries, scientists have done wonders following this motto. Newton's law of universal gravity, the laws of thermodynamics, electromagnetism, universal behavior in phase transitions…

The trouble starts when we take this idea too far and search for the über-unification, the theory of everything, the arch-reductionist notion that all forces of Nature are merely manifestations of a single force. This is the idea that needs to go. And I say this with a heavy heart, given that my early career aspirations and formative years were very much fueled by the impulse to unify it all.

The idea of unification is quite old, as old as Western philosophy. Thales, the first pre-Socratic philosopher, already posited that "all is water," thus dreaming up a single material principle to describe all of Nature. Plato proposed elusive geometrical forms as the archetypal structures behind all there is. Math became equated with beauty and beauty with truth. From there, the highest of post-Plato aspirations was to erect a purely mathematical explanation for all there is, the all-encompassing cosmic blueprint, the masterwork of a supreme intelligence. Needless to say, the whole thing was always about our intelligence, even if often blamed on some foggy "Mind of God" metaphor.

We explain the world the way we think about it. There is no way out of our minds.

The impulse to unify it all runs deep in the souls of mathematicians and theoretical physicists, from the Langlands program to superstring theory. But here is the rub: pure mathematics is not physics. The power of mathematics comes precisely from its detachment from physical reality. A mathematician can create any universe she wants, and play all sorts of games with it. A physicist can't, for his job is to describe Nature as we perceive it. Nevertheless, the unification game has been an integral part of physics since Galileo, and has produced what it should: approximate unifications. Yes, even the most sacred of our unifications are only approximations. Take, for example, electromagnetism. The equations describing electricity and magnetism are only perfectly symmetric in the absence of any sources of charge or magnetism, that is, in empty space. Or take the famous (and beautiful) Standard Model of particle physics, based on the "unification" of electromagnetism and the weak nuclear force. Here again, we don’t have a real unification since the theory retains two forces all along. (In more technical jargon, there are two coupling constants and two gauge groups.) A real unification, such as the conjectured Grand Unification between the strong, the weak, and the electromagnetic forces, proposed 40 years ago, remains unfulfilled.

So, what's going on? Why do so many insist in finding the One in Nature while Nature keeps telling us that it's really about the many?

For one thing, the scientific impulse to unify is cryptoreligious. The West has bathed in monotheism for thousands of years, and even in polytheistic cultures there is always an alpha-God in charge (Zeus, Ra, Para-Brahman…) For another, there is something deeply appealing in equating all of Nature to a single creative principle: to decipher the "mind of God" is to be special, is to answer to a higher calling. Pure mathematicians who believe in the reality of mathematical truths are monks of a secret order, open only to the initiated. In the case of high-energy physics, all unification theories rely on sophisticated mathematics related to pure geometric structures: the belief is that Nature's ultimate code exists in the ethereal world of mathematical truths and that we can decipher it.

Recent experimental data has been devastating to such belief. No trace of supersymmetric particles, of extra dimensions, or of dark matter of any sort, all long-awaited signatures of unification physics. Maybe something will come up: to find we must search. The trouble with unification in high-energy physics is that you can always push it beyond the experimental range. "The Large Hadron Collider got to 7 TeV and found nothing? No problem! Who said Nature should opt for the simplest versions of unification? Maybe it’s all happening at much higher energies, well beyond its reach."

There is nothing wrong with this kind of position. You can believe it until you die and die happy. Or you can conclude that what we do best is to construct approximate models of how Nature works and that the symmetries we find are only descriptions of what really goes on.

Perfection is too hard a burden to impose on Nature.

People often see this kind of argument as defeatist, or as coming from someone who got frustrated and gave up. (As in "he lost his faith.") Big mistake. To search for simplicity is essential to what scientists do. It's what I do. There are essential organizing principles in Nature, and the laws we find are excellent ways to describe them. But the laws are many, not one. We are successful pattern-seeking rational mammals. That, alone, is cause for celebration. However, let us not confuse our descriptions and models with reality. We may hold perfection in our mind's eye as a sort of ethereal muse. Meanwhile, Nature is out there, doing its thing. That we manage to catch a glimpse of its inner workings is nothing short of wonderful. And that should be good enough.