Exploring a Black Hole
If an astronaut were going in a black hole, it would be a one-way trip. Long before the astronaut even arrived at the event horizon, he would encounter tremendous tidal forces exerted by the black hole. Imagine, for example, that the astronaut is falling feet first. The gravitational force pulling on his legs would be considerably greater than the gravity pulling on his head. The difference between the two forces would stretch the astronaut like taffy.
As if that wasn't bad enough, every single atom in the astronaut's body would be pulled toward the singularity, located at the black hole's center. For the astronaut it might feel like being squeezed by a giant fist.
After being stretched and squeezed, the astronaut would look like a strand of spaghetti.
Instead, let's imagine that we could send a robot probe, one that could somehow stay intact despite the gravity. Let us also say that this robot has a light source and a clock on it.
If we were watching the robot back on Earth, we would notice a strange phenomenon. The light source would start to change color. If it started out blue, it would turn green and then yellow and then get red as it approached the black hole.
This is because light is made of particles called photons. A photon's color is directly related to its energy. For example, photons from the blue end of the spectrum have a higher energy than photons from the red end. When a photon uses up energy, it shifts toward the red end of the spectrum. As the photons move away from the black hole, they use some of their energy trying to escape from the black hole's gravity. The closer the photons are to the event horizon, the more energy they need to pull away.
Eventually, as the robot gets closer to the event horizon, the light source will disappear from view. The energy of the photons of light has been reduced so much that it can only be detected by infrared and radio telescopes. Photons which start from just above the event horizon end up losing almost all of their energy. Theoretically, these photons from the light source would still reach us back on Earth, but by then their energy would be so low that no known scientific device would be able to detect them.
Meanwhile, the clock on the side of our robot would start to act strangely as well. According to Einstein's theory of relativity, time slows down in a strong gravitational field--at least as viewed by an outside observer. As the probe got nearer and nearer to the black hole, astronomers back on Earth would notice that the clock was ticking more and more slowly.
The clock would continue to slow down, until it arrived at the event horizon, where it would stop completely.
According to Einstein's theory of relativity, however, from the perspective of the robot, time would not seem to be affected in any way. The probe would arrive at the event horizon and enter the black hole without the clock slowing for even an instant. Yet our robot will only have a fraction of a second to notice this peculiar law of nature, at which point it would be pulled toward the center of the black hole, where it would hit the singularity and be crushed to infinite density.