I begin with Wikipedia:
A theory of everything (ToE), final theory, ultimate theory, or master theory is a hypothetical single, all-encompassing, coherent theoretical framework of physics that fully explains and links together all physical aspects of the universe. Finding a ToE is one of the major unsolved problems in physics. Over the past few centuries, two theoretical frameworks have been developed that, as a whole, most closely resemble a ToE. These two theories upon which all modern physics rests are general relativity (GR) and quantum field theory (QFT).
Michael Brooks, in “Has This Physicist Found the Key to Reality?” (The New Statesman, October 21, 2016), puts it this way:
In relativity, time is a mischievous sprite: there is no such thing as a universe-wide “now”. . .
. . . and movement through space makes once-reliable measures such as length and time intervals stretch and squeeze like putty in Einstein’s hands. Space and time are no longer the plain stage on which our lives play out: they are curved, with a geometry that depends on the mass and energy in any particular region. Worse, this curvature determines our movements. Falling because of gravity is in fact falling because of curves in space and time. Gravity is not so much a force as a geometric state of the universe.
The other troublesome theory is quantum mechanics [the core of QFT], which describes the subatomic world. It, too, is a century old, and it has proved just as disorienting as relativity. As [Carlo] Rovelli puts it, quantum mechanics “reveals to us that, the more we look at the detail of the world, the less constant it is. The world is not made up of tiny pebbles, it is a world of vibrations, a continuous fluctuation, a microscopic swarming of fleeting micro-events.”
But . . .
. . . here is the most disturbing point. Both of these theories are right, in the sense that their predictions have been borne out in countless experiments. And both must be wrong, too. We know that because they contradict one another, and because each fails to take the other into account when trying to explain how the universe works.
All of this is well-known and has been for a long time. I repeat it only to set the stage for my amateur view of the problem.
As is my wont, I turn to baseball for a metaphor. A pitcher who throws a fastball relies in part on gravity to make the pitch hard to hit. Whatever else the ball does because of the release velocity, angle of release, and spin imparted to the ball at the point of release, it also drops a bit from its apparent trajectory because of gravity.
What’s going on inside the ball as it makes it way to home plate? Nothing obvious. The rubber-and-cork core (the “pill”) and the various yarns that aare wound around it remain stationary relative to each other, thanks to the tightness of the cover, the tightness of the winding, and the adhesives that are used on the pill and the top layer of wound yarn. (See this video for a complete explanation of how a baseball is manufactured.)
But that’s only part of the story. The cover and the things inside it are composed of molecules, atoms, and various subatomic particles. The subatomic particles, if not the atoms and molecules, are in constant motion throughout the flight of the ball. Yet that motion is so weak that it has no effect on the motion of the ball as it moves toward the plate. (If there’s a physicist in the house, he will correct me if I’m wrong.)
In sum: The trajectory of the baseball (due in part to gravity) is independent of the quantum mechanical effects simultaneously at work inside the baseball. Perhaps the the universe is like that. Perhaps there’s no need for a theory of everything. In fact, such a theory may be a will-o-the-wisp — the unicorn of physics.