By Valerie Ross
You’re squeezed into a middle seat, two rows from the back of the plane. It’s barely two hours into your cross-country flight, though you’d swear it’s been longer. Does it just seem like the minutes of your trip are crawling by — or does time actually pass more slowly for people who are mid-flight than for people on the ground?
Many of us have heard the idea that time doesn’t pass at the same rate for everyone. It’s a common narrative in science fiction, one that has its roots in Einstein’s theory of relativity. The story starts, let’s say, with two twins, one of whom stays on Earth while the other clambers aboard a rocket that’s making a round-trip journey, at a substantial fraction of the speed of light, to a planet in a not-too-distant solar system. When the traveling twin returns to earth, he’s aged more slowly, and now he’s younger than the twin who stayed behind.
This familiar — and paradoxical — plotline comes from a particular tenet of relativity theory known as time dilation. It predicts that a fast-moving clock will tick at a slower rate than a stationary one — or, a man on an interstellar voyage will age more slowly than his twin back on Earth. But time dilation also says that velocity isn’t the only thing that affects the rate at which clocks tick, or people age; gravity does, too. A clock in a stronger gravitational field (the Earth’s surface, let’s say) will have a slower tick rate than a clock subject to weaker gravity (such as a few miles up into the atmosphere).