What Is Gravity?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Newton described gravity as an attraction between masses, and that description still works well enough to fly spacecraft. Einstein reframed it as the bending of spacetime by mass and energy. Both are useful, and the second explains things the first cannot.
Newton's version
Newton's law of universal gravitation says every mass attracts every other mass with a force proportional to the two masses and inversely proportional to the square of the distance between them.
Doubling the distance therefore reduces the force to a quarter. The insight that made it universal was recognising that the force pulling an apple down is the same force keeping the Moon in orbit. The Moon is falling continuously, but moving sideways fast enough that it keeps missing the Earth.
Why everything falls at the same rate
Drop a hammer and a feather in a vacuum and they land together, a demonstration performed on the Moon by an Apollo astronaut. In air, the feather loses because of resistance, not gravity.
The reason is that a heavier object experiences a stronger gravitational force but also has more inertia resisting acceleration, and the two effects cancel exactly. Why they cancel so precisely was a puzzle Newton could not answer, and it became the starting point for Einstein.
Einstein's reframing
General relativity, published in 1915, describes gravity not as a force but as geometry. Mass and energy curve spacetime, and objects move along the straightest available paths through that curved geometry.
The Earth does not pull the Moon with an invisible rope. The Earth distorts the spacetime around it, and the Moon simply follows the shape. On this view a person standing still on the ground is being accelerated upwards by the floor, and someone in free fall is the one moving naturally.
What relativity predicted that Newton did not
General relativity has passed a series of demanding tests:
- •The precise shift in Mercury's orbit, unexplained by Newton
- •Light bending around the Sun, confirmed during a 1919 eclipse
- •Time running slower in stronger gravity, which GPS satellites must correct for
- •Gravitational waves from merging black holes, first detected in 2015
- •Black hole shadows matching predicted size and shape
What is still unresolved
Gravity is by far the weakest of the four fundamental forces, which is why a small magnet can lift a paperclip against the pull of the entire planet. Why it is so weak is an open question.
The bigger problem is that general relativity and quantum mechanics have never been successfully combined. Each works superbly in its own domain, but they give incompatible answers where both should apply, such as inside black holes or at the very beginning of the universe. Producing a theory of quantum gravity remains one of the central unfinished tasks in physics.
Why you feel weightless in orbit
Astronauts on a space station are not beyond Earth's gravity. At that altitude gravity is still roughly ninety percent of its strength at the surface, which is why the station stays in orbit at all.
What they experience is free fall. The station and everything in it are falling towards Earth continuously while moving sideways fast enough to keep missing, so there is no floor pushing up against them. Weightlessness is the absence of that supporting force rather than the absence of gravity, which is why the correct technical term is microgravity.
The takeaway
Gravity is described by Newton as an attractive force between masses and by Einstein as the curvature of spacetime, the second explaining more and passing harder tests, while reconciling it with quantum mechanics remains unsolved.