Gravity

Drop a pencil and it falls. That is gravity, the most familiar force there is and also the most mysterious. It is far weaker than the other forces of nature, so weak that a small magnet can lift a paperclip against the pull of the entire planet. Yet because it reaches across any distance and never cancels out, it is gravity that shapes everything larger than a mountain: planets, stars, galaxies and the universe as a whole.

Newton's universal law

Isaac Newton's great insight, in the 1680s, was that the force pulling an apple to the ground is the same force holding the Moon in its orbit. Every object attracts every other, with a pull that grows with their masses and weakens with the square of the distance between them: twice as far, a quarter the pull. The Moon is falling toward the Earth all the time, but it is also moving sideways so fast that it keeps missing, circling for ever. Every orbit, from a satellite to a galaxy, is a fall that never lands.

Newton's law explained the tides, the shape of the Earth, the paths of comets, and, two centuries later, the existence of Neptune before anyone saw it. It is still accurate enough to send spacecraft to Pluto. But Newton himself was troubled by it. How could the Sun pull on the Earth across 150 million kilometres of empty space? He admitted he had no idea.

Why everything falls at the same rate

Galileo had already noticed something odd: ignoring air resistance, a heavy ball and a light one dropped together land together. In 1971 an astronaut on the Moon dropped a hammer and a feather and they hit the ground at the same moment. Gravity pulls harder on the heavier object, but the heavier object is also harder to move, and the two effects cancel exactly. No other force behaves this way. Einstein thought this was the most important clue in physics.

Einstein's answer

In 1915 Einstein proposed that gravity is not a force at all. Mass bends the fabric of space and time, and objects simply follow the straightest paths available through that curved geometry. The Earth is not pulled by the Sun; it is coasting along a curve that the Sun's mass has made in spacetime. That is why everything falls at the same rate: they are all following the same curve. The usual picture is a bowling ball on a trampoline, with marbles rolling around the dent it makes.

General relativity made predictions Newton's law could not. Light, having no mass, should still bend around the Sun, and in 1919 astronomers measured exactly that during an eclipse, making Einstein famous overnight. Clocks should run slower deeper in a gravity well, and they do: the atomic clocks on GPS satellites, higher up where gravity is weaker, gain about 45 microseconds a day on clocks at the surface, and without correcting for it the system would be out by kilometres within a day. Mercury's orbit should drift in a particular way, and it does. And collapsing stars should be able to curve spacetime so severely that nothing escapes, which is a black hole.

Ripples

Einstein's equations also predicted that accelerating masses should send ripples through spacetime itself, gravitational waves, so faint that he doubted they could ever be detected. A century later, in September 2015, two detectors in the United States, with arms four kilometres long, measured a stretch and squeeze of less than a thousandth the width of a proton: the echo of two black holes, thirty times the mass of the Sun each, merging 1.3 billion years ago. Since then hundreds of such signals have been recorded, opening a new way of observing the universe, by feeling it shake.

Gravity at every scale

Gravity made the universe we see. In the almost smooth gas left by the Big Bang, slightly denser regions pulled in their surroundings, and over hundreds of millions of years those clumps became galaxies and stars. Gravity squeezes the cores of stars until hydrogen fuses, so every star shines because of it. It holds the atmosphere to the Earth, keeps the oceans in their basins, and sets the limit on how tall a mountain or how big an animal can be. Your weight is simply the Earth's spacetime curvature pressing you against the floor; in orbit, in free fall, astronauts feel none, though gravity there is nearly as strong as on the ground.

What we still don't know

General relativity works perfectly on every scale tested, from laboratories to colliding black holes. Quantum mechanics works perfectly for everything small. They cannot both be right as they stand, because they describe space and time in incompatible ways, and in the centre of a black hole or at the first instant of the Big Bang, where both matter, physics has no working theory at all. Why gravity is so much weaker than the other forces, and what dark matter and dark energy, which show themselves only through gravity, really are, remain among the largest open questions in science. The most familiar force is still the least understood.