A Falling Man Feels No Weight Because Falling Is Like Floating in Deep Space
Dropped objects hover beside him, and no onboard test can tell the difference.
Veritasium · What Everyone Gets Wrong About GravityAlbert Einstein, the physicist behind relativity, said his most joyful insight came from picturing a person slipping off a rooftop. On the way down, Einstein realized, the man would feel no weight. Anything he let go of would not fall away from him. It would hover beside him, or drift in a straight line at a steady speed if he gave it a nudge.
That is exactly what life feels like far from any planet, in a spaceship coasting at a steady speed. Out in deep space, you float, objects stay where you leave them, and a gentle push sends them gliding straight. Physicists have a name for that condition, an inertial observer, meaning someone who feels no push or pull and for whom the ordinary laws of physics hold.
Einstein's leap, called the equivalence principle, was to trust what those observers feel over how they look from afar. Since both float, both count as inertial observers, for whom the ordinary laws of physics hold.
It sounds backward, because from the ground we see the falling man gain speed by about 9.8 meters per second, every second, until he hits. But Einstein argued that the man's own floating feeling is the test that matters, so his fall counts as unaccelerated, inertial motion.
Consider that spaceship again, now drifting toward a distant planet. Like the falling man, the pilot keeps floating even as an outside watcher sees the ship curve toward the planet and move faster as it nears. Inside, he notices nothing. He still floats, and an onboard device that measures acceleration never flickers. His ride stays inertial all the way down, right until the moment it crashes.
Questions people ask
If he is not accelerating, why do we see him fall faster and faster?
You see him cover more ground with each passing second, so in your frame he is speeding up. In his frame, he feels no push, and it is the Earth that rushes up to meet him. Relativity trusts what the falling observer feels, not how the motion looks from far away.
What would an accelerometer carried by the falling man show?
It would read zero the whole way down, just as it would in deep space. An accelerometer measures the pushes acting on it, not how fast the ground thinks you are moving. It would spike only at impact, when the ground stops you.
Does air resistance spoil the weightlessness?
This goes beyond the video, but yes, strictly speaking. Air pushes up on a falling person and on dropped objects, so a skydiver feels wind and falls at a limited top speed. The perfect weightlessness Einstein imagined would require a vacuum, where nothing touches or drags on the faller.