The information paradox at the heart of every black hole

When something falls into a black hole, where does the material ultimately go, and will it, or the quantum information that it contained, ever come back out again? According to Einstein’s general relativity, those answers are simple: as soon as anything physical — matter, antimatter, radiation, etc. — crosses over the event horizon, it’s irretrievably gone. It can add to certain properties, like mass, electric charge, and angular momentum, that the black hole possesses, but little else.

Instead, that infalling material goes swiftly toward and eventually into the central singularity, and will never escape again. But our Universe isn’t governed by general relativity alone, but also by quantum physics. According to our best understanding of quantum physics, there’s much more that needs to be considered.

Not only are there other quantum properties inherent to the raw ingredients that go into making a black hole — baryon number, lepton number, color charge, spin, lepton family number, weak isospin and hypercharge, etc. — but the fabric of spacetime itself, which contains the black hole, ought to be quantum in nature as well. Because of the quantum properties our Universe possesses, black holes do not remain static, but rather evaporate over time: emitting Hawking radiation as a key part of the process. When black holes do evaporate, then, what happens to the information that went into creating them?

Is the information preserved? Is it encoded in the outgoing radiation? These questions are at the heart of perhaps the greatest paradox of all: the black hole information paradox.

Here’s both what we know and what we still need to find out. When two particles are entangled in the quantum mechanical sense, it’s as though some sort of hidden, invisible connection exists between them, and that a seemingly “random” property of one member of the pair can be known to better than “random guessing” simply by measuring the properties of the other member. Entanglement, as far as we can tell, is instantaneous, although no actual information appears to ever be transmitted from one member of the pair to the other.

When a physicist talks about information, they don’t necessarily mean what we conventionally think of as information: a string of letters, numbers, symbols, or anything else that can be encoded with bits like 0s or 1s. Conventionally, this is often described as “the number of yes/no questions that must be answered to fully specify the properties of your physical system,” although even that description has limitations. These are all certainly examples of information, but those examples don’t encompass all the various types of information that exist.