Black hole, cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black hole can be formed by the death of a massive star. When such a star has exhausted the internal thermonuclear fuels in its core at the end of its life, the core becomes unstable and gravitationally collapses inward upon itself, and the star's outer layers are blown away. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity. At the simplest level, there are three kinds of black holes: stellar-mass black holes, supermassive black holes and intermediate-mass black holes.
Stellar-mass black holes form when very large stars finish burning their fuel and collapse into themselves. Supermassive black holes live in the centers of most galaxies, and likely grow to their extreme sizes — up to tens of billions of times more massive than our Sun — by consuming stars and merging with other black holes. Intermediate-mass black holes are still mysterious, and only a few suspected examples have been discovered, but astronomers think they may form through a similar process of accretion, just on a smaller scale.
Einstein taught us that gravity warps space itself, causing it to curve. So given a dense enough object, space-time can become so warped that it twists in on itself, burrowing a hole through the very fabric of reality.
A massive star that has run out of fuel can produce the kind of extreme density needed to create such a mangled bit of world. As it buckles under its own weight and collapses inward, space-time caves in with it. The gravitational field becomes so strong that not even light can escape, rendering the region where the star used to be profoundly dark: a black hole. The instant you enter the black hole, reality would split in two. In one, you would be instantly incinerated, and in the other you would plunge into the black hole utterly unharmed.
As you go deeper into the black hole, space becomes ever more curvy until, at the centre, it becomes infinitely curved. This is the singularity. Space and time cease to be meaningful ideas, and the laws of physics as we know them — all of which require space and time — no longer apply. What happens here, no one knows. Another universe? Oblivion?
The event horizon is ablaze with energy. Quantum effects at the edge create streams of hot particles that radiate back out into the universe. This is called Hawking radiation. As you accelerate toward the event horizon, we see you stretch and contort, as if we were viewing you through a giant magnifying glass. What's more, the closer you get to the horizon the more you appear to move in slow motion. When you reach the horizon, we see you freeze, like someone has hit the pause button. You remain plastered there, motionless, stretched across the surface of the horizon as a growing heat begins to engulf you. You are slowly obliterated by the stretching of space, the stopping of time and the fires of Hawking radiation. Before you ever cross over into the black hole's darkness, you're reduced to ash. From your point of view. Now, something even stranger happens: nothing.
You sail straight into nature's most ominous destination without so much as a bump or a jiggle – and certainly no stretching, slowing or scalding radiation. That's because you're in freefall, and therefore you feel no gravity. After all, the event horizon is not like a brick wall floating in space. It's an artefact of perspective. An observer who remains outside the black hole can't see through it, but that's not your problem. As far as you're concerned there is no horizon. If the black hole were smaller you'd have a problem. The force of gravity would be much stronger at your feet than at your head, stretching you out like a piece of spaghetti. In fact, in a big enough black hole, you could live out the rest of your life pretty normally before dying at the singularity.
How normal could it really be, you might wonder, given that you're being sucked toward a rupture in the space-time continuum, pulled along against your will, unable to head back the other way? But when you think about it, we all know that feeling, not from our experience with space but with time. Time only goes forwards, never backwards, and it pulls us along against our will, preventing us from turning around.
Black holes warp space and time to such an extreme that inside the black hole's horizon, space and time actually swap roles. In a sense, it really is time that pulls you in toward the singularity. You can't turn around and escape the black hole, any more than you can turn around and travel back to the past. you're inside the black hole, surrounded by nothing weirder than empty space.
From an outside point of view, you really have been burned to a crisp at the horizon. It's not an illusion. We could even collect your ashes and send them back to your loved ones. In fact, the laws of nature require that you remain outside the black hole as seen from our perspective. That's because quantum physics demands that information can never be lost. Every bit of information that accounts for your existence has to stay on the outside of the horizon, lest our laws of physics be broken.
So the laws of physics require that you be both outside the black hole in a pile of ashes and inside the black hole alive and well. Last but not least, there's a third law of physics that says information can't be cloned. You have to be in two places, but there can only be one copy of you. Somehow, the laws of physics point us towards a conclusion that seems rather nonsensical:
There is no paradox, because no one person ever sees your clone. We only see one copy of you. You only see one copy of you. You and we can never compare notes. And there's no third observer who can see both inside and outside a black hole simultaneously. So, no laws of physics are broken. Unless, that is, you demand to know which story is really true. Are you really dead or are you really alive?
The great secret that black holes have revealed to us is that there really is no. Reality depends on whom you ask. There is our reality and there is your reality. End of story.