However, that radiation carries no information with it, which brings up a nasty paradox: information goes in, but doesn’t come out, and then the black hole goes away. So what happened to all the information?
✅ In this context, information is the list of all the properties of all the particles that fell into the black hole—in other words, everything you need to reconstruct the original objects that fell in. Instead, what comes out of a black hole, due to Hawking radiation, is just a bunch of random particles. You can’t tell what fell in based on the radiation coming out.
A major clue came in the decades that followed Hawking’s extraordinary discovery. One way to measure the amount of information is through entropy, a thermodynamic concept that is loosely related to the amount of disorder in a system. Black holes have a surprising property: their entropy is proportional to their surface area, not their volume. In other words, the amount of information in a black hole is related to its two-dimensional surface, not its three-dimensional volume.
✅ The concept of entropy describes a system’s tendency to move from order toward disorder, because there are so many more ways for a disordered state to exist than an ordered state. For example, you could clean your room, and there’s only one way for that room to be clean. Yet, there are countless ways for it not to be clean, or become chaotic, like adding a smudge of dirt or a stray sock in a corner. So over time, entropy must increase. That goes for any system in the universe, not just your room.
This is pretty much unlike every other object in the entire universe, and so naturally a lot of physicists all of sudden became very interested in black holes, with top-tier physicists like Leonard Susskind leading the charge into this new land of the holographic principle.
The name comes from holography itself. Ever see a hologram in real life, and it looks like the image leaps out at you? That’s because the hologram encodes all the three-dimensional information in a two-dimensional surface.