Earlier this year, Marcelo Gleiser published over at Nautilus a nice reflection on the quest for unity in physics. As in his other popular writings on physics and astronomy (including an essay for our sister publication The New Atlantis), Gleiser paints a winsome portrait of physics as an “ever-changing, self-correcting description of natural phenomena,” characterized first and foremost by a kind of intellectual humility.
Physics is not just an empirically grounded science, Gleiser reminds us, it is also a precise — exact — science. But that precision comes at a price: answers can only be given once we specify the particular (and often highly unusual) conditions under which its findings are valid. Once they fix these conditions, however, physicists can formulate incomparably exact and experimentally confirmable answers to myriad questions about the nature of the universe — about space, time, matter, and motion, as well as the origins of our cosmos.
Certain physical theories are capable of providing an enormous range of such answers. Consider quantum theory and Einstein’s general theory of relativity. These theories are able to explain a broad range of empirical phenomena as well as unify physical theories that were once distinct or even appeared irreconcilable. Thus Einstein’s theory subsumed classical mechanics and electrodynamics under the principle of relativity (and thereby transformed both), and quantum theory unified wave and particle descriptions of matter (and thereby transformed both).* Because of this impressive power to unify, such theories are often described by physicists in aesthetic terms — and may even be preferred by them partly on that basis — as “elegant” or even “beautiful.”
Quantum mechanics and relativistic physics are the two dominant frameworks for physics today — sometimes characterized (if imprecisely) as the physics of the very large and the physics of the very small. And yet, however powerful, these two frameworks are not themselves unified, and, in some crucial ways remain irreconcilable. This is where string theory comes in, which ostensibly offers hope for a grand unified vision for physics, marrying the macroscopic universe to the subatomic. According to Gleiser, however, far from unifying physics, string theory has produced a kind of “crisis.” The theory, with its countless mathematical solutions, winds up postulating ever-more physical universes with ever-less contact with our own, empirical one. Though string theory is often lauded for its formal beauty, many physicists worry that it has lost touch with physical reality.
Something has gone wrong with our quest for unity. But what?
According to Gleiser, the overvaluation of elegance and beauty has seduced researchers down unproductive paths of inquiry. The problem is that we ask too much of physics. Gleiser thinks we should replace the proud ambition for a grand unified theory — and its implicit and “dogmatic” equation of truth and beauty — with a more modest image of physics as the careful and piecemeal study of nature in which only “incremental” progress is possible. The “angst of not knowing it all” — of lacking a unified theory — would thereby be “exorcised,” allowing us to embrace “our imperfect and incomplete grasp on physical reality.”
The Quest for Reality
There’s much to be said for this modest vision, and Gleiser is surely right that something has gone astray in the search for a unified theory as he describes it. But his alternative proposal could be helpfully fleshed out with a few concepts borrowed from philosophy.
Philosophers of science debate about whether scientific theories should be understood as providing true accounts of reality or just empirical descriptions. The question is metaphysical. Does physics tell us what’s real? Or is it merely useful? If the former, a position usually labeled “realism,” then we can reasonably assume that physical reality is composed of whatever entities are postulated by our leading theories — elementary particles or quantum fields or a four-dimensional space-time manifold. If the latter, a position labeled anti-realism (or sometimes instrumentalism), these things are simply façons de parler, useful fictions that help physicists to catalogue observations and make predictions, rather than real features of reality. If you want to know what reality is like, don’t ask physicists, say the anti-realists. Their theories are, at best, only empirically adequate.
Gleiser’s view is a little ambiguous in this context. He proposes thinking of physics “as a descriptive mode of explanation, free of the unifying quest.” The word “explanation” suggests that physics helps us get at reality by telling us why things are the way they are — realism. But Gleiser also says that physics “sets aside metaphysical expectations about the nature of reality, which have more to do with how we search for meaning as humans than with how nature actually works.” And that sounds a lot like anti-realism.
But the realism/anti-realism dichotomy may be misleading. There seems to be little doubt that practicing physicists (think of Einstein or Max Planck) do — and ought to — take themselves to be seeking after the nature of things. In this sense, they are “realists.” What’s more, we non-scientists certainly do — and ought to — incorporate their findings into our picture of reality. In this sense, we, too, are realists. However, physicists are also a pretty modest bunch. When pressed, they will typically tell us that we shouldn’t demand too much of their theories, that more research is needed or that new evidence might lead them to abandon their current approach. In this sense, physicists don’t seem much like realists at all.
This is where Gleiser’s picture fits in nicely, by recognizing the unique epistemological power of physics while stressing the modesty that it demands. Of course, sophisticated realists will admit that our current scientific theories do not adequately describe reality. Their point is that we should aim for such a description as a goal. Gleiser seems to suggest that goal is simply unattainable.
I think he’s right. But must we therefore abandon “metaphysical expectations about the nature of reality” as well? Why not accept that a metaphysical impulse underlies and drives the scientific enterprise while also recognizing that scientific theories will never be definitive? The result would be neither realism nor anti-realism, exactly. Or, put differently, the result would be a kind of tempered realism, one that accepts natural science as our most reliable way to understand — to explain — nature but that does not expect its theories to provide a metaphysically ultimate account of reality.
What about the quest for unity? The more modest vision I’m proposing — similar to but distinct from Gleiser’s — is perfectly compatible with that quest, while granting that unity may never be perfectly instantiated in any particular theory. Unity, in other words, may be a goal of scientific explanation, even if it is not attainable. Instead, we might think of unity as a kind of regulative ideal — something that guides and orients research. String theory’s vice, on this account, would not be seeking unity — it would be pursuing unity as a goal to be reached rather than an ideal to be approximated. In so doing, other theoretical virtues — parsimony, predictive power, empirical robustness — fall by the wayside.
Perhaps relativity and quantum theory will never be unified. But the quest to unify them — if properly tempered by the type of humility Gleiser highlights — might nevertheless produce unexpected and fruitful results. Consider that one of Einstein’s motivations in developing his relativistic theories was to unify the two leading, though disparate paradigms of nineteenth-century physics. By following that path, Einstein helped effectuate a scientific revolution the implications of which we still do not adequately understand — evidence, perhaps, that the metaphysical quest to explain nature should engender rather than preclude intellectual humility.
* CORRECTION: A previous version of this article claimed that Einstein’s theory “unified”classical mechanics and electrodynamics. That characterization suggests, misleadingly, that the theory of relativity is itself a unified field theory, whereas finding such a unified theory was one of Einstein’s life-long and unsuccessful ambitions. (H/t Don Howard.)