Two people can walk straight and still collide, and that is how gravity works
Falling objects are following the straightest paths through curved spacetime, not being pulled by a force.
Veritasium · What Everyone Gets Wrong About GravityImagine you and a friend stand 1,000 kilometers apart on the equator. You both start walking due north, each holding a compass and never turning left or right.
At first you stay far apart. Then mile by mile you drift closer, until near the top of the world your paths meet and you walk straight into one another. It feels as if some invisible force pulled you together. No force touched either of you. You both walked straight.
That is gravity.
To understand it, think about airplanes. A flight from New York to Paris does not follow a straight line on a flat map. It arcs far north, over Newfoundland and Ireland. The pilots are not detouring. Because Earth is curved, that arc is the shortest distance between the two cities. Mathematicians call such shortest paths on a curved surface geodesics, meaning the straightest possible lines in a world that is not flat.
Einstein argued that space and time together form a kind of four-dimensional surface that can also curve, and that massive things like Earth dent it. A falling apple, an orbiting astronaut, and a planet circling the sun are all doing what you and your friend did on the equator. They are moving straight along geodesics through curved spacetime.
From the outside, their paths look curved. The apple drops, the astronaut circles. From the inside, nothing is pulling them. They are simply going as straight as the shape of the universe allows.
That flips the familiar picture. We tend to think gravity reaches up and yanks things down. In this view, coming together is just what straight lines do when the ground beneath them, spacetime itself, is curved.
Questions people ask
If gravity is not pulling me down, why do I feel heavy?
You feel your weight because the ground stops you from following your natural straight path. As spacetime curves you downward, the floor pushes upward on your feet, and that push is what your nerves register as heaviness. In free fall, with nothing in the way, you would feel weightless.
Why do heavy and light objects fall at the same rate?
Because they are all doing the same thing: moving straight through the same curved spacetime. A bowling ball and a feather would follow identical paths if air were not in the way. Air resistance, not gravity, is what normally makes the feather drift.
What does this have to do with astronauts floating on the space station?
They are weightless for the same reason a skydiver feels weightless before the parachute opens. Nothing is pushing on them, so they sail along a geodesic, a straight path through curved spacetime. Their path looks like a circle around Earth only because we are ignoring the time part of that four-dimensional trip.