What is gravity? The question physics hasn't finished answering

What the phenomenon is

Gravity is the pull that anything with mass exerts on anything else with mass, and the reason light itself bends when it passes something as massive as the Sun. Physics has two separate, thoroughly tested descriptions of how it behaves: Isaac Newton's, from 1687, and Albert Einstein's, from 1915. Both make predictions that have been checked and confirmed, from falling apples to colliding black holes.

What physics does not have is a single, complete explanation of what gravity is at every scale. Newton's and Einstein's theories describe gravity's effects with remarkable accuracy, but they describe it in incompatible terms, and nobody has combined either one with quantum mechanics, the physics of the very small, into one working theory.

The popular claim

A common assumption is that Einstein simply proved Newton wrong, the way a later discovery retires an old one overnight. Newton's 1687 law of gravitation then gets treated as a piece of history, something superseded rather than something still in daily use.

That is not how physicists treat it. NASA's own teaching material on the law notes it “governs the motion of the planets in their orbits, guides spacecraft to their destinations, and even keeps our feet firmly on the ground” — it is still the tool of first resort, because Einstein's equations reduce to Newton's under everyday, weak-gravity conditions.

Newton's law: gravity as a pull between masses

Newton set out the law in the Principia, the printing of which was finished in the summer of 1687: every particle of matter attracts every other particle, with a force that grows with their masses and shrinks with the square of the distance between them. NASA's material on the law credits Newton with seeing that “an apple falling from a tree, the Moon orbiting the Earth, and the planets orbiting the Sun” all follow the same rule.

The law is precise enough that it still guides spacecraft today. What it cannot explain is why mass attracts mass in the first place, or what happens to gravity at very high speeds or in very strong fields — the gap Einstein's theory was built to close, more than two centuries later.

Einstein's general relativity, and how it has been tested

In 1915, Einstein proposed that mass and energy curve space and time, and that what we feel as gravity is simply objects following the straightest possible path through that curved geometry. As the planet closest to the Sun, Mercury feels the Sun's gravity most strongly, and NASA notes this gives its orbit “a wobble” that Newton's law alone could not fully account for; general relativity did.

The most famous test came on May 29, 1919. Two observing teams, one led by Dr. Crommelin of the Royal Greenwich Observatory and the other by Prof. Eddington of Cambridge University, photographed stars in the Hyades cluster as their light passed close to the Sun during a total solar eclipse. Einstein's theory predicted the Sun's gravity would bend that starlight twice as much as Newton's theory predicted, and NASA's account of the experiment confirms it “verified that the Sun's gravity bends light,” shifting the stars' apparent positions just as general relativity called for.

General relativity is not only historical, either: it runs continuously in everyday technology. GPS satellite clocks sit in weaker gravity than clocks on the ground, so relativity makes them run faster; left uncorrected, NASA notes, the drift is a matter of “millionths of a second” per day, but without correcting for it, GPS “would never work” and would “guide you miles out of your way.”

Gravitational waves and the 2017 Nobel Prize

Einstein's equations also predicted that violent events, such as two black holes spiraling into each other, send ripples through spacetime itself. For a century nobody detected one directly. That changed on September 14, 2015, at 5:51 a.m. Eastern time, when the twin detectors of the Laser Interferometer Gravitational-Wave Observatory, in Livingston, Louisiana, and Hanford, Washington, picked up a signal from two black holes, each about 30 times the mass of the Sun, merging roughly 1.3 billion light-years away, MIT News reported.

The discovery was not announced to the public until February 11, 2016, nearly five months later. The collision, NASA's Jet Propulsion Laboratory noted, released more energy in a fraction of a second than the light of every star in the visible universe combined, some 50 times over. MIT News described it as a test of Einstein “in a regime where his theory has never been tested before” — fields strong enough that, as the report put it, “Newton's gravity doesn't work at all.”

The 2017 Nobel Prize in Physics went to three of the discovery's leaders: Rainer Weiss of MIT, and Barry Barish and Kip Thorne of Caltech, “for decisive contributions to the LIGO detector and the observation of gravitational waves,” as the American Physical Society put it. A fourth key figure, Ronald Drever of Caltech, had died in March 2017 and, under Nobel rules against posthumous awards, could not be added to the prize.

What is still debated

Every test so far — Mercury's orbit, the 1919 eclipse, GPS, gravitational waves — confirms general relativity at the scale of planets, stars, and black holes. The open question sits at the opposite end of the size scale: physics has no working theory of gravity for individual particles, the domain of quantum mechanics.

The two frameworks disagree at a basic level. Quantum mechanics treats time as universal and absolute; general relativity treats it as relative, woven into a four-dimensional spacetime that bends and stretches. Quanta Magazine describes the resulting puzzle as the “problem of time”: unifying the two theories means reconciling those contradictory notions of what time is, and physicists pursuing one leading approach, holography, are candid about where they stand. “We don't have a good idea for how to understand the emergence of time,” the physicist Brian Swingle told the magazine.

No experiment has yet probed gravity at the quantum scale, and no proposed theory of quantum gravity has been tested. That is what keeps the question open: not a disagreement over data, since there is essentially none yet at that scale, but the absence of any experiment able to referee between competing ideas.

Questions people ask

Verdict

Gravity is the best-tested large-scale physics there is, and an unsolved problem at the smallest scale. Newton's 1687 law and Einstein's 1915 general relativity both work exactly as predicted, from falling apples to GPS satellites to the black hole collision LIGO detected in 2015. Real or Legend has filed this one as unsolved, because the question in the title — what gravity is, in a way that also fits with quantum mechanics — has no accepted answer yet, measured or otherwise.