Clocks that run slow

Einstein's theory makes a strange claim. Gravity slows time. A clock closer to a heavy object ticks more slowly than one farther away. This isn't a flaw in the clocks. Everything slows together, atoms, heartbeats, the aging of cells. Whoever is near the heavy object feels perfectly normal. Only when two people compare watches afterward does the difference show.

It happens here too

The effect is real near Earth, just tiny. GPS satellites orbit about 20,000 kilometers up, where gravity is weaker, so their clocks run fast. Their speed slows them a little, but the net result is a gain of roughly 38 millionths of a second a day. Engineers adjust for it. If they didn't, the position on your phone's map would drift by kilometers within a day.

In 2010, researchers in the United States lifted one extremely precise clock about a third of a meter higher than another. The higher clock ran measurably faster. By the same logic, over a lifetime your head ages a few hundred billionths of a second more than your feet.

Near a black hole, it's enormous

Close to a black hole, the slowing stops being a rounding error. Here's how fast a clock would tick compared with one far away, if it could hover in place near a black hole that isn't spinning. Distances are measured from the center, in multiples of the horizon's radius.

Hovering that close would take an impossibly powerful rocket. But the numbers show the shape of it. The last stretch before the horizon is where time really drags.

Choose a radius and press Play to compare elapsed time. Each clock feels normal beside itself; the difference appears when their readings are compared.

Two views of one fall

Say you fall in while a friend watches from a safe distance. You'd notice nothing odd about your own watch, and you'd cross the horizon without any pause. Your friend sees something else. You seem to move more and more slowly as you approach the edge. Your light gets redder and dimmer and finally fades out. They never quite see you cross.

Both accounts are correct for the person telling them. That's the part that takes a while to sit comfortably.

The film Interstellar has a planet where one hour equals seven years back home. Its science adviser, the physicist Kip Thorne, worked out that this needs a giant black hole spinning almost as fast as physics allows. Possible on paper. Nobody has found one like it.

Caught in the act

In 2018 astronomers watched a star called S2 swing close past Sagittarius A*, at roughly 120 times the distance from Earth to the Sun. As it passed, its light came out slightly stretched toward red, by the amount Einstein's theory predicts. That stretch is slowed time, seen through light. It's the same effect, measured near a real black hole.