What Is Gravity?

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.

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.

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.

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

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.

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.

Tides are a direct consequence of the inverse square law. The Moon pulls on every part of the Earth, but not equally: the side facing the Moon is closer and is pulled slightly more strongly than the centre of the Earth, while the far side is pulled slightly less. Relative to the Earth as a whole, the ocean on the near side is drawn towards the Moon and the ocean on the far side is effectively left behind. This stretching produces two bulges of water on opposite sides of the planet.

As the Earth rotates through both bulges, many coastlines see two high tides and two low tides roughly every day, with the timing shifting by about fifty minutes daily because the Moon moves along its orbit. The Sun also raises tides, a little under half as strong as the Moon's because it is so much farther away. When Sun and Moon line up at new and full moon, their effects combine into larger spring tides; when they pull at right angles, smaller neap tides follow. Coastline shape then greatly modifies these simple patterns.

Newton's law contains a number, the gravitational constant G, that sets how strong the attraction is, but Newton had no way to measure it, because the pull between everyday objects is tiny. The first laboratory measurement came from Henry Cavendish in 1797 and 1798. He hung a rod with a small lead ball at each end from a thin wire, then placed two large lead spheres near the small ones. The gravitational pull between them twisted the wire by a minute angle, which he watched through a telescope from outside the room so that his body heat and movements would not disturb the delicate apparatus.

Cavendish described his aim as finding the density of the Earth, and the experiment is often called weighing the Earth. Once G is known, the familiar strength of gravity at the surface reveals Earth's mass, about six million billion billion kilograms. Cavendish found that Earth is around five and a half times as dense as water, very close to the modern value. Surface rocks are only about half as dense as that, so the interior must contain something much heavier, an early clue to the iron-rich core. Combining G with the orbits of planets and moons also gives the masses of the Sun and Jupiter.

The familiar figure of 9.8 metres per second squared is an average. At the equator, falling objects accelerate at about 9.78, while at the poles the figure is about 9.83. Two effects combine. Earth's spin carries the equator round fastest, so part of gravity's pull is taken up keeping you moving in a circle, and the planet bulges at the equator, putting you slightly farther from its centre. A spring scale would therefore show a 60 kilogram person reading about 0.3 kilograms more at a pole. An old-fashioned balance would show no change, because it compares two masses that are affected equally.

Smaller differences come from what lies underground. Dense rock such as an ore body pulls slightly harder than lighter sediment, and sensitive instruments called gravimeters can detect the difference, which is why geologists use gravity surveys to search for minerals and to map buried structures. Since 2002, pairs of satellites have done the same from orbit. The GRACE missions fly two spacecraft one behind the other and measure tiny changes in the distance between them as they pass over regions of greater or lesser mass. Their maps have tracked ice loss from Greenland and Antarctica and the draining of underground water supplies in places such as northern India.

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.