9.81

Drop anything and it gains almost ten metres per second of speed, every second. Gravity keeps the air on the planet, the Moon on its track and the coffee in your cup. It is by far the weakest of the four forces, and still it runs the largest scales there are.

Metres per second squared, standard gravity

Einstein swapped the invisible force for geometry. Mass and energy curve spacetime, and anything in motion simply follows the straightest available path through that curved landscape. A planet is not tugged by the Sun; it rolls along a slope the Sun has pressed into space and time.

General relativity, Einstein 1915

Newton pictured a cannon on a mountain, firing faster and faster. At one speed the ball drops exactly as quickly as the round Earth curves away beneath it, so it never comes down. Every satellite, every space station and the Moon itself are falling around us, all the time.

Newton, A Treatise of the System of the World, 1728

Squeeze enough mass into a small enough space and not even light is fast enough to leave. The boundary where that happens is the event horizon. Karl Schwarzschild found the solution that fixes its size only weeks after Einstein published his equations; the name black hole caught on half a century later.

Schwarzschild 1916, Wheeler 1967

In 1687 Newton writes down one law for apples and planets. In 1915 Einstein turns it into geometry, and in 1919 an eclipse shows starlight bending around the Sun. Gravitational waves are heard in 2015, and the first image of a black hole's shadow is released in 2019. Some predictions waited a century for their proof.

Milestones, a selection

1,560 ly

Closer than most people guess. Gaia BH1, found in 2022, lies about 1,560 light-years away and betrays itself only through the star circling it. The giant at the heart of our galaxy is some 27,000 light-years off. Around a hundred million more are thought to drift through the Milky Way unseen.

Distances in light-years, rounded

6.5 bn

The black hole in the galaxy M87 weighs about six and a half billion suns, and it was the first ever imaged. A few giant galaxies hold heavier ones still, and distant quasars may hide the biggest of all, but those masses are inferred indirectly and the estimates swing widely. Next to these, our own centre is a lightweight.

M87*, in solar masses, EHT 2019

It raises the tides twice a day and keeps an atmosphere wrapped around the planet. Satellite navigation only works because its clocks are corrected for time passing faster in orbit, by roughly 38 microseconds a day. Space probes borrow speed from planets to reach the edge of the solar system.

Applications, not speculation

Near a small black hole the difference in pull between head and feet would stretch you thin long before the horizon. Near a giant one you could cross it without noticing anything at all. To anyone watching from afar you would seem to slow down, redden and fade at the edge.

A thought experiment

Galaxies spin faster than their visible matter allows, so physicists propose dark matter. The expansion of the universe is speeding up, so they add dark energy. And general relativity still refuses to merge with quantum mechanics. The force we have known longest is the one we understand least.

Rubin 1970, Planck 2018

Everything in the universe is falling toward something. Sources for the figures are listed below.

Sources follow