Departure(27)
Mike squints at me, but I glance over at Bob, interested.
“The rate at which time passes changes throughout the universe, depending on gravity and velocity. Let me give you an example. Let’s say twins were born today. One is placed on a spaceship and launched into space. The ship simply orbits our solar system, but it does it at an incredible speed—say ninety-nine-point-nine percent of the speed of light. That’s what Einstein correctly identified as the speed limit for mass in our universe, though we’re pretty sure some particles are capable of faster than light travel—which, by the way, opens all sorts of possibilities: quantum entanglement that enables data to travel faster than light, for one. But Einstein’s limit, at least for particles with mass, may still hold.” Bob stops and scans our blank expressions. “Anyway, back to the twins: one on Earth, one in a ship in space, going really, really fast. In fifty years the ship returns. The twin who stayed on Earth is fifty—a middle-aged man. The one on the spaceship? Still a baby, though he’s aged a little, since the ship couldn’t reach the speed of light without transforming into energy, and it takes some time to get up to speed. Bottom line: moving fast slows down time. So does gravity.
“Here’s a real-life example: GPS. GPS was developed by the Department of Defense in the seventies to help get military assets exactly where they needed to be. It currently consists of twenty-four satellites in high orbit, around twenty thousand kilometers from Earth’s surface. That’s so far up there that Earth’s gravity doesn’t exert the same influence on the curvature of space-time. As I said, gravity slows time down. The stronger the gravity, the slower time passes. So the closer to Earth you are, the slower time goes. If you get close enough to very, very strong gravity, say a black hole, time almost stands still. If you crossed the event horizon of a black hole in a spaceship, you would watch the entire fate of the universe unfold in the seconds before you were sucked into the center.
“But away from gravity, time goes faster—you experience more time, like a video on fast-forward. That’s what happens to GPS satellites. General relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by forty-five microseconds each day. So for every day that passes here on Earth, up there, twenty thousand kilometers away from the gravity we experience, the GPS satellites experience one day and forty-five microseconds. Doesn’t sound like much, but it’s time travel. The satellites are moving into our future. But that’s only half of what’s going on up there.”
Mike rubs his eyelids. “You’re making my brain hurt, Bob.”
“Stay with me here, Mike. There’s another part of the GPS time travel puzzle: velocity. Remember our example with the twins?”
Bob waits, but neither Mike nor I volunteer an answer.
“Right. So like our spaceship, these GPS satellites are flying really fast. They’re not in geosynchronous orbit, like many people think. They circle the globe roughly every twelve hours, and they have to move at about fourteen thousand kilometers per hour to do that. That’s fast. The speed of light is around a million kilometers per hour, so it’s only a fraction of that, but still fast enough to dilate time. But in this instance, instead of speeding up time, the velocity actually slows it down. Remember our twin on the spaceship? Time flowed slower for him. Gravity and velocity both slow time down. Special relativity predicts that, based on their velocity of fourteen thousand kilometers per hour, we should see these GPS clocks ticking more slowly by about seven microseconds per day—and they do. So the satellites’ velocity slows time down for them by seven microseconds, while the lower gravity up there speeds it up by forty-five. When you put the effects predicted by special and general relativity together, each satellite should travel forward in time by about thirty-eight microseconds per day. And that’s exactly what they do: clocks on the GPS satellites record thirty-eight microseconds each day that we don’t observe here on Earth.”
“What does this have to do with our flight, Bob?” Mike asks.
“Everything. In fact, if we had landed at Heathrow, we would have traveled slightly back in time. JFK to Heathrow is a seven-hour flight, most of it at about thirty to forty thousand feet, flying at around six hundred miles per hour. We would have landed slightly younger than everyone who stayed on the ground. The time difference would have been insubstantial: a fraction of a second, maybe a hundred nanoseconds, but nevertheless, less time would have passed for us than them. And it gets even stranger: if we’d flown westward, against Earth’s rotation—say, from JFK to Honolulu—we would have had a lower velocity than clocks on the ground, and landed slightly older.
“Bottom line: the closer you are to strong gravity, and the faster you move, the slower time goes. If you go fast enough, you can almost stop the flow of time, though you experience it as normal—from your point of view, the world outside you progresses at a faster rate.”
“Interesting,” I murmur, still taking it in. “But you’re talking about fractions of a second.” I motion to the structure around us. “It seems like a lot more time has passed than that.”
“True. My working theory, Nick, is that our plane passed through a patch of space-time where gravity was distorted. It’s the only reasonable explanation, given current scientific understanding. A gravimetric distortion would dilate space-time, making time flow slower or, in our case, faster. Say this distortion created a bubble in space-time, and our plane was in this bubble, where time passed at an incredible rate. If the bubble popped, it would dump us out at whatever time the clock stopped. There are only two possibilities: the gravimetric distortion was a natural occurrence—”