In 1976, Stephen Hawking proposed that Black Holes, do something impossible according to the laws of quantum mechanics. They destroy information. This is a paradox because information should be conserved in the universe. If it is not, then it would mean that causality could be violated. Random inexplicable events could happen not linked to prior events. This has implications for the way we think our universe works. The order of events, time, and the deterministic laws depend on information conservation.

What does information have to do with black holes? The way physicists think of information is not like on a hard drive or a book. They describe it as the number of yes/no questions that must be answered to fully specify the properties of a system. This is also the way entropy is defined in physics. Entropy and information are linked. The more information that is necessary to specify a system, the higher the entropy of that system.

One of the basic laws of physics is that information is never lost. It always stays in the universe. In terms of entropy, the second law thermodynamics states that entropy can never decrease. So if information could be erased, that would mean that entropy could be decreased, which is not possible.

If you burn a book, the book’s information doesn’t really get destroyed because the information is still retained in the universe. All the elementary particles of the book are preserved in the soot and smoke resulting from the burning. In principle, if we had the technical capability to look at the quantum state of all those elementary particles, even after the burning process, we could recreate their quantum state immediately prior to that, and then immediately prior to that, and so on until we recreated the book.

How is this information conserved? This is closely related to determinism in physics. The fundamental laws of physics are deterministic, i.e. given the full information about an initial state of a system, you can in principle predict the evolution of that system. That is, you can predict its state in any later moment in time. And you can also reconstruct the earlier states of the system.

There is a cause and effect for all interactions. What follows is that you can’t have two different initial states evolve into exactly the same final state, because then you would not be able, even in principle, to reconstruct the previous state of the system. So, “information loss” for a physicist means: having many different initial states evolve into exactly the same final state. If that were the case, the idea of determinism would be lost. Conservation of information, therefore, is what makes the universe deterministic. And we can infer from this, a predictability and causality in the universe.

If a book falls into a black hole, it’s not accessible to anyone outside the black hole and is lost forever. But prior to Stephen Hawking, this was not considered a big deal, because it was thought the information in the book was at least contained inside the black hole, and therefore still in the universe. We can’t get to it, but it was there.

But this idea was shattered when Hawking showed that Black holes evaporate slowly over time, and that given enough time, the entire black hole would evaporate into photons or black body radiation. But this radiation does not contain enough information to recreate the book. So where does the information go?

There are several theories. One is that the information that goes into making a black hole is somehow encoded in the evaporated Hawking radiation. A recent paper suggests that when the effects of quantum gravity are taken into account, Hawking radiation could contain information. Others suggest that there is a correlation between every radiated particle of a black hole and the information that fell into it similar to entanglement. Another theory is that Black holes are gateways to other universes and that the seemingly lost information is actually stored in another universe. Bottom line is that this paradox is not resolved.

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