The Effects of Time Dilation

Scientists have also determined that when black holes, including spinning ones, interact with normal space, they can create strange time distortions. That is, the passage of time experienced by an observer located outside a hole looking in will be markedly different from that of an observer located inside a hole looking out.

Consider the example of two astronauts in a spaceship orbiting the black hole from a safe distance. One exits the ship and propels himself toward the hole, which draws him in. From the point of view of his friend aboard the ship, he will spiral inward increasingly slowly and eventually become frozen on the hole’s spinning event horizon. Round and round he will go, getting closer and closer to the horizon for years, and indeed forever; but he will never seem to pass through it into the hole. However, the point of view of the astronaut who approaches the horizon will be quite different. Time will seem to pass normally for him; he will feel himself journey from the ship to the horizon and then cross over the horizon into the black hole, all in only a few minutes.

If humanity ever develops sufficiently advanced technology, this time differential, called time dilation, might be exploited to allow people to propel themselves forward in time. (It must be emphasized that such ventures would take place outside the black hole and be unrelated to the even weirder distortions of space and time that might exist inside.) Gribbin explains how such journeys might be accomplished. A group of astronauts would bid farewell to observers on a space station orbiting far from a black hole and fly a ship toward the hole’s event horizon. “The longer the astronauts spend near the event horizon, and the closer they get to it,” Gribbin says,

the stronger the effect will be. You don’t even need enormously powerful rockets to take advantage of the effect, because the astronauts could use a judicious, short-lived blast on their rockets to set their spacecraft falling on an open orbit down into the region of highly distorted spacetime, leaving the observers behind. . . . The falling spacecraft would coast in . . . being accelerated by the gravity of the black hole up to the point of closest approach. Then, it would whip around the hole very sharply . . . and climb out again, now being slowed all the time by gravity. At the farthest distance from the hole, the astronauts could fire their rockets briefly again, to put the spacecraft back alongside the space station of the observers, who [would be] ready to compare clocks.

When the astronauts and observers do compare clocks, they will find a noticeable difference. Whereas the astronauts may have experienced the passage of only a few hours, the observers’ clocks will record that several weeks or months have gone by. If the astronauts are careful to choose just the right orbit and speed around the event horizon, they might be able to leap ahead dozens, hundreds, or even thousands of years. They could not travel backward to their starting point, however, as the effects of time dilation work in only one direction forward (at least in ordinary space). Also, in longer journeys through time, the original observers will grow old and die while the astronauts are away; so each time the travelers visit the space station, they will be greeted by a new group of observers.