Just three numbers – that’s all it takes to describe a black hole completely and 100% unambiguously in general relativity. If I tell you the mass, electric charge, and spin (i.e. angular momentum) of a black hole, we’re done. That’s all we will know about it and all we need to describe its features.
These three numbers allow us to calculate everything about how the black hole interacts with its environment, how the objects around it will respond to it, and how the black hole will develop in the future.
For all their fierce gravitational capabilities and their unholy alien nature, black holes are surprisingly simple. If I gave you two black holes with exactly the same mass, charge, and spin, you wouldn’t be able to tell them apart. If you switch places without looking, you won’t know I did.
It also means that when you see a fully formed black hole, you have no idea why it formed. Any combination of compact mass in a small enough volume can do the job. It could be the super-dense core of a dying star. It would have been very thick litter of adorable cats that had been crushed into oblivion.
As long as mass, charge, and rotation are the same, history is irrelevant. There is no information about the original material that created the black hole. Or is it?
“Information” is a slightly loaded term; They can take different definitions depending on who you ask and what mood they are in. In physics, the concept of information is closely related to our understanding of how physical systems develop and how we construct our theories in physics.
We like to think that physics is a relatively useful model for understanding the universe in which we live. One of the ways that physics benefits is in its ability to predict. If I give you a list of all the information about a system, I should be able to apply the laws and theories of physics to tell you how that system will develop. And the opposite is also true. If I tell you the state of the system now, you can run all the math back to see how the system got to its current state.
These two concepts are known as inevitability (I can predict the future) and reflection (I can read the past) And it’s pretty much the foundational core of physics. If our theories of physics didn’t have these properties, we wouldn’t be able to get much work done.
These two concepts also apply to quantum mechanics. Yes, quantum mechanics places strict limits on what we can measure about the universe, but that doesn’t mean all bets are off. Instead, we can simply replace a well-defined classical state with a more fuzzy quantum state and move on with our lives; The quantum state evolves according to the Schrödinger equation, which supports both determinism and reversibility, so we are all good.
Information is not lost when burning a book; It’s just a mixture.
This double whammy of determinism and reversibility means that, in terms of physics, information must be preserved during any process. It cannot be created or destroyed – If we add or remove information individually, we will not be able to predict the future or read the past. Any loss or gain means there will be either missing information or extra information, so all physics will fall to dust.
There are many processes that He appears To destroy information, but that’s only because we don’t follow carefully enough. Take, for example, burning a book. If I give you a pile of ashes, this will seem irreversible: there is no way you can put the book back together. But if you have a powerful enough microscope at your disposal (and a lot of patience) and you have to keep an eye on me as the book burns, you can – in principle at least, which is good enough – watch and track the movement of every molecule in the process. You can then reverse all of those movements and all of those interactions to rebuild the book. Information is not lost when burning a book; It’s just a mixture.
In the classic, traditional view of black holes, all this information-related work is not a problem at all. The information that was used to build the black hole is simply obscured far behind the event horizon – the one-way boundary on the surface of the black hole that makes it so unique. Once there, the information in this universe will never be seen again. Whether a black hole formed from dying stars or squashed cats, it doesn’t happen practically issue. The information may not be destroyed, but it is permanently hidden from prying eyes.