Black holes can’t trash info about what they swallow—and that’s a problem

There are three numbers

It takes just three numbers to describe a black hole. We are done if I tell you the mass, electric charge, and spin. All we need to know about it is that.

We can use those three numbers to figure out how a black hole will interact with its environment, how objects around it will respond to it, and how the black hole will evolve in the future.

Black holes are very easy to understand. You wouldn't be able to tell them apart if I gave them the same mass, charge, and spin. You wouldn't know that I swapped places if I didn't look.

When you see a black hole, you don't know what it is. It could have been done with a combination of mass squeezed into a small amount. The core of a dying star could have been that dense. There could have been a lot of kittens.

The history is meaningless if the mass, charge, and spin are the same. There is no information about the original material. Do you think it does?

Founding charters

Depending on who you ask and what mood they're in, "information" can be a loaded term. Our understanding of how physical systems evolve and how we construct our theories of physics are closely linked.

We think that physics is a good way to understand the universe. The power of prediction is one way that physics is useful. If I give you a list of all the information about a system, I should be able to show you how that system will change. It's also true. To figure out how the system got to its current state, you can run all the math backwards.

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The core of physics is determinism and reversibility, two concepts that are known as determinism and reversibility, respectively. We wouldn't be able to do much work if our theories of physics didn't have these properties

Both of these concepts apply to quantum mechanics. The limits on what we can measure about the Universe are put in place by quantum mechanics. We can replace a classical state with a fuzzier quantum state and move on with our lives, since the quantum state changes according to the Schrdinger equation, which supports both determinism and reversibility.

Information is not lost when you burn a book; it’s merely scrambled.

This one-two punch of determinism and reversibility means that, in terms of physics, information must be preserved during any process. It can’t be either created or destroyed—if we were to add or remove information willy-nilly, we wouldn’t be able to predict the future or read the past. Any loss or gain means there would either be missing information or extra information, so all of physics would crumble to dust.

There are a lot of processes that seem to destroy information, but we don't keep track of them. There is a burning of a book. If I gave you a pile of ashes, you wouldn't be able to reassemble the book. If you have a powerful microscope and can watch me burn the book, you could track the motion of every molecule in the process. The book could be reconstructed by reversing all those motions. When you burn a book, you don't lose information.

All this business about information is not a problem in the classical view. The one-way boundary at the black hole's surface is what makes it unique. The information won't be seen again in this universe. It doesn't matter whether the black hole was formed from dying stars or cats. The information is hidden from view.