How gravity slows down time

Time does not tick at the same rate everywhere. The closer a clock is to a heavy object, the slower it runs. This is called gravitational time dilation. Einstein predicted it, and experiments have confirmed it again and again.

On Earth the effect is tiny. Atomic clocks at different heights drift apart by mere nanoseconds. But it adds up: over Earth's whole 4.6-billion-year history, a clock at the height of Mount Everest would end up about 39 hours ahead of one at sea level. Earth's core, deep in the gravity well, is in effect about two and a half years younger than its surface.

You rely on this every day. GPS satellites orbit high above us, where gravity is weaker, so their clocks run faster by about 45.8 microseconds a day. Their speed slows them a little, but overall they gain about 38.6 microseconds daily. Left uncorrected, your map position would drift by roughly 11 kilometres every day.

Now go somewhere extreme. A black hole is so compact that not even light escapes it. To someone watching from far away, a clock near a black hole ticks slowly, and anything falling in seems to slow down, turn ever redder, and never quite reach the edge. Yet the person falling in notices nothing special at that moment, and by their own watch they cross the edge after a finite time.

Time, it turns out, is not a universal backdrop. It stretches with gravity.