Mathematically speaking, anything could become a black hole if you were to compress it into a small enough space. That’s right, you, me, everything in the universe has what is known as a “Schwarzchild radius”: a tiny, tiny amount of space that, were you to collapse the entire mass of the object into, it’s density would be so great that its gravitational pull would be so great that not even light could escape from it. You would have a black hole. If you were to compress Mount Everest into something smaller than a nanometer, you would have a black hole. And if you were to compress the entire Earth down to the size of a peanut, you would have a black hole. But, fortunately for us, there is no known way to compress Everest or Earth in that fashion. But a star, many, many, many times larger than our own sun, has a much larger Schwartzchild radius, and when it runs out of fuel and can no longer keep itself hot enough, it collapses to a single, infinitesimally-small point known as a “singularity.” It’s density will be infinite, and, so, it’s gravitational pull will be so strong that nothing can escape not even light.
Jumping Into a Black Hole
What would it look like from the outside?
Well, we know that gravitational fields bend space and time. Stars behind our sun will actually appear to be in slightly different locations from Earth because the sun’s gravitational field bends the light coming from those stars. When it comes to the gravitational fields of larger objects, like entire galaxies or, for that matter, a black hole, the effect is even nuttier. Light coming from object’s behind them is significantly distorted, producing smears and smudges. This is known as “gravitational lensing.”
For the sake of simplicity, let’s jump into a simple black hole, one that doesn’t have a charge and isn’t moving. And, also, isn’t already sucking up a bunch of matter so it’s just there on its own.
As we approach, the distortion of the sky grows greater and greater. A larger and larger portion of our field of view looking forward into the black hole will be filled with darkness. At this point, where half of our field of view has been swallowed up in darkness, we have reached the “Photon Sphere.” At this point, light is not going to necessarily get sucked into the black hole, but it doesn’t necessarily leave it either. Instead, at this magical point in space, light, photons, can actually orbit the black hole. If you were to stop here for a moment and look to the side, you could theoretically see the back of your own head, because light reflecting off the back of your head would travel all the way around the sphere of the black hole, right back to your face.