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Three numbers.
In general relativity, just three numbers are enough to describe a black hole 100 percent completely, unambiguously. If I tell you the mass, electric charge, and spin (ie, angular momentum) of a black hole, we’re done. That’s all we ever know about it and all we need to describe its features.
These three numbers allow us to calculate everything about how the black hole will interact with its environment, how the objects around it will react to it, and how the black hole will evolve in the future.
For all their violent gravitational powers and unholy exotic nature, black holes are surprisingly simple. If I gave you two black holes with the same mass, charge, and spin, you wouldn’t be able to tell them apart. If I changed places without you looking, you wouldn’t know I did it.
It also means that when you see a fully formed black hole, you have no idea what created it. Any combination of mass compressed into a small enough volume could do the job. It could be the very dense core of a dying star. It could have been an extremely dense litter of forgotten adorable kittens.
As long as the mass, charge, and spin are the same, history doesn’t matter. No information survives about the original material that formed the black hole. Or does it?
Articles of Incorporation
“Information” is a somewhat loaded term; can take on different definitions depending on who you ask and what mood they’re in. In physics, the concept of information is closely related to our understanding of how physical systems evolve and how we construct our theories of physics.
We like to think that physics is a relatively useful paradigm for understanding the Universe in which we live. One of the ways physics is useful is in its predictive power. If I give you a list of all the information about a system, I can apply my laws of physics and theories to tell you how that system will evolve. The opposite is also true. Now, if I tell you the state of a system, you can work all the math backwards to figure out how the system got to its current state.
These two concepts are known as determinism (I can predict the future) and reversibility (I can read the past) and is almost the fundamental core of physics. Without these features in our physics theories, we wouldn’t be able to do much.
These two concepts also apply to quantum mechanics. Yes, quantum mechanics puts severe limits on what we can measure about the Universe, but that doesn’t mean all bets are off. Instead, we can simply replace the sharply defined classical state with a more fuzzy quantum state and go on with our lives; the quantum state evolves according to the Schrödinger equation, which advocates both determinism and reversibility, so we’re all good.
When you burn the book, the data is not lost; it’s just messed up.
This one-two punch of determinism and reversibility means that information must be preserved during any process in physics. It can neither be created nor destroyed—if we inevitably add or remove information, we cannot predict the future or read the past. Any loss or gain means that there will be either missing information or additional information, so all physics will crumble to dust.
There are many such processes it seems to destroy information, but that’s simply because we’re not watching closely enough. Take, for example, the burning of a book. If I give you a pile of ashes, it will seem irreversible: There is no way to put the book back together. But if you have a powerful enough microscope at your disposal (and a lot of patience) and can watch me burn the book, in principle at least, it’s good enough – you can watch and watch it move. each molecule in the process. You can then reverse all these actions and all these interactions to rebuild the book. When you burn the book, the data is not lost; it’s just messed up.
In the traditional, classical view of black holes, all this data work is not a problem at all. The information that goes into creating a black hole is simply hidden behind the event horizon—the one-sided boundary on the black hole’s surface makes it unique. Once there, the information will never appear again in this Universe. Whether a black hole is formed from dying stars or crushed kittens, practically issue Information may not be destroyed, but it is forever hidden from our prying eyes.
