02.06.2007

Physics Will Not Achieve a Theory of Everything

I'm optimistic that physics will not achieve a Theory of Everything.

That might seem an odd thing to be optimistic about. Many of my colleagues in physics are inspired by the prospect of achieving a Theory of Everything. Some even claim that they've already got it. (Acknowledging, to be sure, that perhaps a few i's remain to be dotted or a few t's to be crossed.) My advice, dear colleagues: Be careful what you wish for. If you reflect for a moment on what the words actually mean, a Theory of Everything may not appear so attractive. It would imply that the world could no longer surprise us, and had no more to teach us.

I don't buy it. I'm optimistic that the world will continue to surprise us in fascinating and fundamental ways.  

Simply writing down the laws or equations is a long way from being able to anticipate their consequences. Few physicists—and no sober ones—seriously expect future work in fundamental physics to exhaust, for example, neuroscience. 

A less literal reading of "Theory of Everything" is closer to what physicists who use it mean by it. It's supposed to be a theory, not really of everything, but of "everything fundamental". And here "fundamental" is also being used in an unusual, technical sense. A more precise word here might be "basic" or "irreducible". That is, the physicists' Theory of Everything is supposed to provide all the laws that can't be derived logically, even in principle, from other laws. The structure of DNA surely emerges—in principle—from the equations of the standard model, and I strongly suspect that the possibility of Mind does too. So those phenomena, while they are vastly important and clearly fundamental in the usual sense, aren't fundamental in the technical sense, and elucidating them is not part of a Theory of Everything. 

I think we're about to enter a new Golden Age in fundamental physics. The Large Hadron Collider (LHC), which should begin to operate at CERN, near Geneva, starting in summer 2007, will probe the behavior of matter at energies higher than ever accessed before. There is no consensus about what we'll find there. I'm still fond of a calculation that Savas Dimopoulos, Stuart Raby and I did in 1981. We found—speaking roughly—that we could unify the description of fundamental interactions (gauge unification) only within an expanded version of relativity, which includes transformations of spin (supersymmetry). To make that dual unification we had to bring in new particles, which were too heavy to be observed at the time, but ought to be coming into range at the LHC. If they do exist we'll have a new world of phenomena to discover and explore. The astronomical riddle of dark matter could well be found there. Several competing ideas are in play, as well. The point is that whatever happens, experimenters will be making fundamental discoveries that take us by surprise. That would be impossible, if we had a Theory of Everything in the sense just described—that is, of everything fundamental.

In recent months a different, much weaker notion of what a "Theory of Everything" might accomplish has gained ground, largely inspired by developments in string theory. In this concept, the Theory provides a unique set of equations, but those equations that have many solutions, which are realized in different parts of the Universe. One speaks instead of a multiverse, composed of many domains, each forming a universe in itself, each with its own distinctive laws. Now even the fundamental—i.e., basic, irreducible—laws are beyond the power of the Theory to supply, since they vary from universe to universe. At this point the contrast between the grandeur of the words "Theory of Everything" and the meager information delivered becomes grotesque.

The glamour of the quest for a Theory of Everything, or a Final Theory, harks back Einstein's long quest for his version, a Unified Field Theory. Lest we forget, that quest was fruitless. During his great creative period, Einstein produced marvelous theories of particular things: Brownian motion, the photoelectric effect, the electrodynamics of moving bodies, the equality of inertial and gravitational mass. I take inspiration from the early Einstein, the creative opportunist who consulted Nature, rather than the later "all-or-nothing" romantic who tried (and failed) to dictate to Her. I'm optimistic that She'll continue to surprise me, and my successors, for a long time.