Everything with mass pulls on everything else. That one rule holds the Moon up, bends light, and rings the universe like a bell.
move your pointerdrag a finger · then scroll
Gravity pulls harder on the hammer. But the hammer is harder to move, by exactly the same factor. The mass cancels, and in a vacuum, everything falls together.
m·a = m·g ⇒ a = g
Galileo argued it. Apollo 15 filmed it on the Moon.
The pull spreads out over a sphere. Twice as far, the same pull is spread over four times the area, so you feel a quarter of it. Ten times as far, a hundredth. It never reaches zero.
F = G·m₁m₂ / r²
Drag the planet.
Newton imagined a cannon on a very tall mountain. Fire the ball fast enough and the ground curves away beneath it as quickly as it drops. It falls forever and never lands.
The Moon is doing exactly this, right now.
Einstein's picture has no pull at all. Mass dents the geometry of spacetime, and everything simply follows the straightest path available. Near a mass, the straightest path bends.
Gμν = 8πG·Tμν
Move the pointerDrag to move the mass. Watch the orbits react.
Two black holes spiraling together shake the geometry around them. The ripples cross the universe at the speed of light, stretching and squeezing everything they pass through.
On 14 September 2015 one passed through Earth. It changed the length of a 4 km detector arm by less than the width of a proton.
The merger leaves one black hole behind. Light has no mass, so Newton's formula says gravity should ignore it. But light travels through space, and the space itself is curved. Pass near and your path bends. Pass too near and you never leave.
Move the black holeDrag the black hole through the beam.