How does gravity fit with quantum mechanics?

Physics owns two crown jewels. General relativity describes gravity as the smooth curving of spacetime, and has passed every test from GPS clocks to colliding black holes. Quantum mechanics describes everything else, matter and the other forces, as fields of discrete, probabilistic quanta, and has passed every test down to parts per trillion. Each is nearly flawless in its territory; they disagree about the nature of reality itself, one deterministic geometry, the other probabilistic fields on a fixed stage. Einstein noted as early as 1916 that quantum theory would have to modify gravitation. A century later, the modification has not been found, or rather, too many candidates have and no experiment can yet referee.

Why it matters

The seam between the theories runs through the most interesting places in the universe: the center of every black hole and the first instant of the Big Bang, where both theories apply and their combination yields nonsense. Quantum gravity is the tollbooth every deep cosmological question must eventually pass, including this catalog’s entries on time’s arrow and black hole information. It is also a matter of principle: physics cannot rest on two mutually contradictory foundations. One of them, at minimum, is an approximation to something deeper, and finding out which is the discipline’s oldest unfinished assignment.

What has been tried

The obvious move, quantizing gravity like the other forces, fails: the calculation generates infinities that cannot be tamed, signaling that new physics enters at the Planck scale, a hundred billion billion times beyond the reach of any collider. In that experimental vacuum, research programs multiplied. String theory dissolves particles into vibrating strings, demanding extra dimensions and yielding, through holography, real insight into gravity’s quantum bookkeeping. Loop quantum gravity quantizes spacetime itself into discrete atoms of geometry. Asymptotic safety bets the infinities cure themselves; causal set theory builds spacetime from bare ordered events; emergent gravity proposes gravity is not fundamental at all but thermodynamics in disguise. Each program has internal successes; none has made a confirmed prediction.

Where the edge is

The frontier is turning experimental at last, from below: tabletop proposals aim to test whether gravity can entangle two tiny masses, which would prove gravity is quantum in nature without reaching the Planck scale, and cosmological surveys hunt imprints of quantum spacetime in primordial gravitational waves. A null result either way would be historic.

What would count as an answer

One experiment where the theories' union is exercised and a candidate theory’s prediction is confirmed. Physics has waited a century, which is, by this catalog’s standards, not even old.