Emergent gravity may be a dead idea, but it’s not a bad one

It claims that the force of gravity is a mere illusion, more akin to friction or heat—a property that emerges from some deeper physical interaction. This emergent gravity idea might hold the key to rewriting one of the fundamental forces of nature—and it could explain the mysterious nature of dark matter.

But in the years since its original proposal, it has not held up well to either experiment or further theoretical inquiry. Emergent gravity may not be a right answer. But it is a clever one, and it's still worth considering, as it may hold the seeds of a greater understanding.

To understand what emergent gravity means, we first have to clarify what gravity is supposed to be emerging from and what the word “emergent” even means.

Emergence is an old concept that appears and reappears in many contexts, from physics to philosophy to art. In physics especially, emergence refers to a clear but slightly uncomfortable fact: Despite our deep understanding of the innermost workings of nature at the subatomic scale, we often can’t use that knowledge to describe most of the systems we actually care about.

One way to view nature is as a vast hierarchy. At the “bottom” of the hierarchy are the quantum fields, which we use quantum field theory to understand. On top of that are all the myriad subatomic and atomic interactions, also governed by quantum mechanics. Above that is chemistry, where the quantum starts mattering less. And on top of all that, far, far removed from quantum fields, are all the wonderful branches of science and their various tools that describe all manner of phenomena: astrophysics, oceanography, geology, sociology, and so on.

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