Gravity Has No Mechanism
Newton gave us the law of gravity — the exact formula, the inverse-square dependence, the constant G — and immediately refused to speculate about what gravity is or how it works. "Hypotheses non fingo" (I frame no hypotheses), he wrote. He could calculate what gravity does with extraordinary precision; he had no idea how it does it.
Feynman treats this as still true today, and worth saying plainly: we do not know what gravity is. We have excellent mathematical descriptions — Newton's F = Gmm'/r² for most purposes, Einstein's general relativity for strong fields and precise timing — but neither of these is a mechanism. They tell you how to compute the force; they do not explain what is doing the forcing.
What "No Mechanism" Means
The distinction Feynman draws is between a law (a reliable mathematical pattern) and a mechanism (a physical process that produces the pattern). The electromagnetic force, for example, can be understood as photons being exchanged between charged particles — a mechanism, however abstract. Gravity has no analogous quantum carrier. The hypothetical graviton has never been detected, and there is no consistent quantum field theory of gravity.
General relativity describes gravity as the curvature of spacetime caused by mass-energy: massive objects warp the geometry of space and time, and other objects follow the straightest possible paths (geodesics) through that curved geometry. This is a beautiful and accurate description. But it raises rather than settles the mechanical question: what is it about mass that curves spacetime? We don't know.
The Scale Problem
At atomic scales, the gravitational force is so weak (10⁻⁴⁰ times the electromagnetic force between two electrons) that quantum effects have never been observed for gravity. There is no experimental evidence to guide a quantum theory of gravity, and no theoretical consensus on how to build one. String theory and loop quantum gravity are candidates; neither is confirmed. The inability to quantize gravity consistently is the outstanding problem of fundamental physics.
Why the Ignorance Is Interesting
Most of physics has been able to advance by finding mechanisms — by asking "what is actually happening at a finer level?" and answering experimentally. Gravity resists this. The reason may be that gravity is not a force in the usual quantum-field-theory sense but something else entirely — a geometrical property of the fabric of the universe. If so, the concept of a "mechanism" may simply not apply.
Feynman's point is that honest physics includes naming what it doesn't know. The success of Newton's law in predicting planetary orbits, tides, and spacecraft trajectories is not the same as understanding gravity.
Connections
- universal-gravitation — the law we do have, without mechanism
- general-relativity-gravity — the best description, which raises more questions than it answers
- four-fundamental-forces — gravity is the one force not yet unified with the others
- experiment-as-sole-judge — the mechanism question must wait for experiments that probe the gravitational quantum regime