To Unify or Not to Unify?

Since the earliest days of Western philosophy, we can identify an irresistible push toward unification—a description of Nature based on a single principle. In the East, we can also find this urge in the Taoist unity of the Tao or the Hindu concept of Brahman, the ultimate reality underlying all phenomena. The notion that all is one seems to be an attribute of human thinking itself, as we imagine the world in which we evolved and exist through reason and intuition.

The scientific urge to find a single, simple substrate of physical reality owes much to its Greek precedents and, of course, to more than three thousand years of monotheistic thinking. The Judeo-Christian-Islamic God is a single absolute power, omnipresent, omniscient, the nexus of all Creation. The patriarchs of what we call modern science—Copernicus, Galileo, Kepler, Descartes, Newton—all found inspiration from their faiths, even if, for each of them, there was also disagreement with ecclesiastical authorities.

Many scientists may feel uncomfortable with this kind of association, but the cultural roots of unification ideas in ancient philosophy and religion are undeniable. There is a deep need to unify, to find the simplest possible formulation of the natural laws.

Isaiah Berlin called this sort of monism and its derivatives, material or otherwise, the “Ionian Fallacy,” declaring the notion meaningless: “A sentence of the form ‘Everything consists of . . .’ or ‘Everything is . . .’ or ‘Nothing is . . .’ unless it is empirical . . . states nothing, since a proposition which cannot be significantly denied or doubted can offer us no information” (Concepts and Categories, 1979). In other words, authoritative all-encompassing statements, which cannot be comparatively contrasted or measured, carry no information: they are articles of faith, not reason. Despite Berlin’s wise admonition, the rush for unifying is as strong as ever.

In high-energy physics, the branch of physics occupied with finding the fundamental material constituents of physical reality and the interactions between them, this trend follows from the spectacular success of reductionism as a strategy for uncovering Nature’s mechanistic underpinnings. From planetary orbits to the properties of atoms, it is undeniable that reductionism works, especially as it makes use of the concept of symmetry. Reductionism and symmetry unite in the dominant monist effort of our time, the construction of the theory of everything (TOE), encompassing all particles and their interactions under a single overarching principle.

The goal is to go beyond current understanding as encompassed in the so-called Standard Model of particle physics, which maps matter into twelve fundamental particles (the electron being the most popular) and three forces (electromagnetism and two forces that act only within subnuclear distances, the strong and weak nuclear forces). To these, we add the Higgs boson particle and the force of gravity, which is a universal attractive interaction that acts on anything that has matter (or energy, given that matter can be understood as a manifestation of energy through the E=mc² relation).

According to modern high-energy physics, the forces between matter particles are themselves described through the exchange of particles during interactions. A suggestive image is that of two ice skaters drifting next to one another on a frozen lake, throwing tennis balls at one another; the closer they are, the more balls they throw and the higher the intensity of the exchange. The world becomes the playground for two kinds of particles, those comprising matter and those comprising the interactions between them.

Beyond the Standard Model

The Standard Model doesn’t satisfy the monist goal of unity. Even if it has been experimentally shown to manifest an approximate unification of the electromagnetic and weak nuclear forces above a certain energy (equivalent to a few hundred times the mass of a proton divided by c²), it’s still not what one would consider a true unified theory, where all the forces behave as one.

The current favorite for the unification of the four fundamental forces is known as superstring theory, a formulation that calls for a radical ontological change in the way physics portrays matter. As opposed to the atomistic “basic building blocks” picture of elementary particles, superstring theory proposes that the fundamental material entities are wiggling tubes of energy that exist in nine spatial dimensions. Just as guitar strings vibrating at different frequencies produce different sounds, the different vibrations of fundamental strings represent different particles of matter, ideally those mapped by the Standard Model and a few new ones. Vibrating superstrings can also emulate the particles responsible for transmitting the forces between material particles. In this picture, the whole of physical reality is reducible to vibrating superstrings, the ultimate monistic theory.

Superstrings call for two major extensions of physical reality, the existence of extra spatial dimensions and the existence of supersymmetry (hence the “super” in superstrings), a hypothetical symmetry of Nature where particles of matter and particles of force are interconvertible. Both extensions present serious challenges for empirical validation. Extra spatial dimensions, if they exist, may be effectively impossible to detect due to their smallness. Most models ask for dimensions curled into balls of size of order 10¯³³ cm, the so-called Planck length, well beyond any current or future detectability. In fact, the Planck length delimits the region where space-time itself needs to be described quantum mechanically, compromising the very notion of detecting a specific length scale. It is possible that the length scale for the extra dimensions is larger, but so far, no evidence has been found.

Supersymmetry had been proposed in the mid-1970s to cure certain pathologies related to quantizing gravity and with the disparity of energies between the weak and gravitational forces (the Hierarchy Problem). It predicts the existence of at least twice as many particles as in the Standard Model, since it gives each a supersymmetric partner. Of those, most models predict that only the lightest supersymmetric particle will be stable and thus more easily detectable at the LHC or in another experiment. Despite intense searches, no supersymmetric particle has been detected.

The next few years will be decisive to the future of high-energy physics. Either theories proposed almost five decades ago will finally be vindicated, or they will have to be abandoned. If some physicists choose not to abandon them, even without empirical support, they will be forced to defend a radically new way of constructing a description of reality, based more on metaphysical assumptions than on verifiable hypotheses. Defenders of such a position have compared it to atoms, proposed first in Ancient Greece. Atoms were an idea for millennia, they claim, and were only vindicated in the early twentieth century. Maybe supersymmetry and superstrings are like that?

The key difference is that atoms are about the structure of matter—its perceived granularity—while supersymmetry proposes a radical shift in our understanding of Nature whereby matter and the forces acting on it are essentially the same thing. If this equivalence is realized only at extremely high energies, beyond the grasp of foreseeable experimentation, we will not be able to validate it. The same if this symmetry requires the existence of extremely small extra spatial dimensions.

Of course, it may be possible that our current ideas of unification are childish and will be supplanted by new concepts in the future. It’s hard to predict what we don’t know. (Only that it’s a lot.)

However, it’s also possible that we are reaching a stage whereby Nature is sending us a message that goes against our urge to unify. It is entirely possible that Nature is not simple at its most fundamental layers and that the forces are what they are, each its own thing, despite some similarities.

To accept this may be hard to those who aren’t ready to jettison a monistic view of the world. On the other hand, there is no reason for Nature to comply to our aesthetic expectations. Its beauty may be in it being almost perfect.

Templeton Prize winner Marcelo Gleiser is a professor of natural philosophy, physics and astronomy at Dartmouth College.