BROTHER, CAN YOU SPARE A PARADIGM:

At the close of the 20th century, the logical end-point of physics seemed clear: unify all physical phenomena under the umbrella of a single, unique 'Theory of Everything' (ToE). Indeed, many were convinced that this goal was well within reach: in his 1980 inaugural lecture Is the end in sight for theoretical physics?, Stephen Hawking, the physicist perhaps most closely associated with the quest for the ToE in the public eye, speculated that this journey might be completed before the turn of the millennium.

More than twenty years after, a ToE has not manifested--and moreover, seems in some ways more distant than ever. Confidence in the erstwhile 'only game in town', string/M-theory, has been waning in the face of floundering attempts to make contact with the real world. Without much hope of guidance from experiment, some have even been questioning whether the theory is 'proper science' at all--or, conversely, whether it requires a reworking of scientific methodology towards a 'post-empirical' framework from the ground up. But in the wake of string theory's troubles, no other contender has risen up to take center stage. [...]

But Whence X?

The X-theory is a theory of a particular kind: it takes initially disparate phenomena--general relativity and the standard model of particle physics--and unifies them in a more fundamental setting, namely, the behavior of the X-entities. This style of theorizing has a long pedigree in Western science and philosophy. The pre-Socratic philosopher Thales of Miletus, who lived around 600 BCE, proposed that ultimately, everything is water. Water's mutability is, after all, readily observable: it transforms to ice and steam, back and forth. Why not into other forms?

Later, in the 5th century BCE, Leucippus and his pupil Democritus developed the theory of atomism--that every phenomenon ultimately can be reduced to the motion of certain most fundamental entities--atoms--in the void. In honor of the centrality of this idea to much of what followed, in science and philosophy alike, I will thus call this style of theorizing 'atomistic': a theory is atomistic when it seeks to unify phenomena by reducing them to the behavior of entities at a more fundamental level. Note, however, that I take 'atomistic' in a broad sense, here: the unification of electric and magnetic phenomena in the behavior of the electromagnetic field, for instance, is atomistic in this sense.

Thus, complex, disparate phenomena are unified by reduction to the (often simpler or 'more elegant') behavior of more fundamental entities. The periodic table finds its explanation in the structure of atoms. The 'particle zoo' containing more than one hundred hadrons is reduced to different bound states of six quarks. Thus goes the standard 'success story' of physics. With the X-entities, we are at the final rung: the dynamics of space and time, leptons and quarks, gluons, photons, W- and Z-bosons, and the Higgs field--all find a natural explanation in X-theory.

But of course, a glaring question remains open: just as we could've asked Thales 'whence water?', as we could've asked the string theorists 'whence strings?', for any atomistic X-theory, we will always finally ask: but whence X? Or, as the question was formulated by Hawking in A Brief History of Time: "What is it that breathes fire into the equations and makes a universe for them to describe?"

There seem to be two options. Either, there is some further justification, some theory that accounts for X, for the fire bringing it to life. Then, X-theory wasn't the ultimate theory after all--but whatever replaces it, if it does so in atomistic style, will be subject to the same question. Or, there is no such justification--Xs and their behavior are just a brute fact about the universe.

But in the latter case, what have we really learned? What does X-theory provide us with beyond a single, perhaps more convenient, framework to describe the phenomena of physics? 

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