Black holes are the collapsed remnants of massive stars and are location of intense gravity in the universe. Let's imagine that two people, Anita and Carlos, are floating in space near a black hole. Normally, the vacuum of space would cause all the liquids inside Anita and Carlos to boil away and expand. This would cause them to swell and pop like overfilled balloons (ugh!). Luckily our two friends have nice warm (and pressurized) space suits.

Unlucky Carlos, he drew the short straw and is now drifting, spacesuit and all, toward the event horizon of the black hole. Anita, meanwhile, is waiting patiently for Carlos to cross the horizon so she can record the effects on a handy notepad.

As Carlos falls closer and closer to the event horizon, he begins to notice something strange. Anita seems to be moving very quickly. Her heart is beating amazingly fast and her hair seems to be growing right before Carlos's eyes. Then she seems to be quite still; Carlos realizes that she has grown old and died, in what seemed to him to be only a few seconds. Even more amazing, Carlos begins to see the entire universe evolve, grow old, and finally go dark all in the blink of an eye. For Carlos, time outside the black hole seemed to speed up.

Let us now return to Anita and her note pad. After Carlos drew the short straw, she watched him fall toward the event horizon of the black hole. As he got closer and closer, he seemed to move slower and slower. His heartbeat slowed down, he quit blinking his eyes, and as he reached the event horizon he seemed to almost freeze in place. From Anita's point of view, Carlos stopped moving, in space and in time. His heart stopped beating, he stopped breathing, and everything about Carlos seemed to come to a stand still (like a video on pause).

To understand what is happening to our two explorers, we need to understand the effect of strong gravitational fields like the one around the black hole.

Light, and in fact time itself, is affected by gravity. You can think about a gravitational field much like a large flat plane. When an object with mass (like a planet, a star, or in our case, a black hole) is present, it causes forms a dent or depression in the plane. The curvature around this depression is a representation of the force of gravity. As another object comes close to this depression it begins to roll downhill toward the central object. If we think about the opposite situation, something trying to escape the central object by going uphill, we can begin to understand what is happening to Anita and Carlos.

You can appreciate that you will travel more slowly when running uphill than when running on level ground. The steeper the hill is, the slower you will go. A similar effect causes objects to have trouble escaping the depression once they are inside it. As Carlos falls in toward the event horizon, the curvature of space-time around him gets steeper and steeper. This means it is harder and harder for light to escape, as it is slowed down by the intense gravity of the black hole.

Time is also slowed by this effect. It slows down for Carlos as he falls closer and closer to the event horizon of the black hole. At the exact moment that he reaches that limit, Anita sees Carlos freeze; time stops for Carlos, from Anita's point of view.

This effect is not limited to the regions around black holes. Anywhere that you compare a stronger gravitational field with a weaker one, you will find a difference in the rate at which time passes. If you shift from a planet with a stronger surface gravity to one with a weaker force, this effect will come into play, or if you simply move closer to or further away from a massive (satellite-, planet-, or star-sized) object. The stronger the contrast in the gravitational field, the larger the difference in the time dilation factor will be.