The Washington Post has a review, and introduction therewith, of a curious new book called Ka, authored by John Crowley (acclaimed author of Great Work of Time). It is narrated from the POV of a crow named Dar Oakley, who journeys repeatedly into the realm of the dead with a human companion. A para from the WaPo piece caught my attention for its allusion to an unsolved problem in physics:
In many cultures, crows have long been regarded as “death-birds.” Eaters of carrion and corpses, they are sometimes even said to convey the soul into the afterlife. Crowley’s title itself alludes to this notion: Dar Oakley croaks out “ka,” which isn’t just a variant spelling of “caw,” but also the ancient Egyptian word for the spiritual self that survives the decay of the body. Yet what actually remains of us after our bones have been picked clean? Might our spirits then dwell in some Happy Valley or will we suffer in eternal torment? Could death itself be simply an adventure-rich dream from which we never awake? Who knows? The narrator, who might be a writer, says of his dead and much-missed wife Debra that “the ultimate continuation of her is me.” What, however, becomes of Debra when he too is dead?
What indeed. The question is left unanswered so the reader can confront the unanswerability supposedly implicit in this riddle. But while this scheme may be acceptable in a book-length “exploration of the bond between the living and the dead”, physicists don’t have much of a choice. They really want to know, would love to know, how a very similar situation plays out in the quantum realm.
It’s called the black hole information paradox. A black hole is a single point in space around which spacetime is folded into a sphere. This means that if you get trapped in this region of spacetime, you’re locked in. You can’t leave the sphere. The surface of this sphere is called the event horizon: it’s the shortest distance from the black hole from which you can pull away.
Now, there’s no way to tell two black holes apart if their mass, angular momentum and electric charge are the same. This is called the no-hair conjecture. This means that whatever a black hole swallows – whether it be physical matter or information as a sequence of 0s and 1s encoded as an electromagnetic signal – doesn’t retain its original shape or patterns. They become lost, observable only in changes to the black hole’s mass, angular momentum and/or electric charge.
In 1974, Stephen Hawking, Alexei Starobinsky and Yakov Zel’dovich found that, thanks to quantum mechanical effects near an event horizon, the black hole within could be emitting radiation out into space. So assuming a black hole contains a finite amount of energy and has stopped eating material/info from the outside, it will evaporate slowly over time and vanish. This is where the information paradox kicks in.
You’re obviously thinking the info the black hole once swallowed was all converted into energy and emitted as Hawking radiation. This is actually where the problem begins. Quantum mechanics may be whimsically counterintuitive about what it allows nature to do at its smallest scale. But it does have some rules of its own that it always follows. One of them is that information is always conserved, that when information passes into a black hole, it can’t be converted into the same energy mulch that everything else is converted to.
We don’t know what happens to the ‘spirit’ of Debra when Dar Oakley passes away. And we don’t know what happens to the information inside a black hole when the latter evaporates.
Black holes are unique objects of study for classical and non-classical physicists alike because they combine the consequences of both general relativity and quantum mechanics. Those pursuing a unified theory, broadly called quantum gravity, hope that data about black holes will help them find a way to reconcile the laws of nature at the biggest and smallest scales. Resolving the black hole information paradox is one such path.
For example, string theory, which is a technical framework that gives physicists and mathematicians the tools to solve problems in quantum gravity, proposes a way out in the name of the holographic principle. It states (in highly simplified terms) that the information trapped by a black hole is actually trapped along the event horizon and doesn’t fall inside it. Over time, fluctuations on the horizon release the information out. However, neither the complete shape and consequences of this theory nor some contradictory predictions are fully understood.
Even whether humans will be able to resolve this paradox in their lifetime at all remains to be seen – but it’s important to hope that such a thing is possible and that the story of a black hole’s life can be told from start to finish someday. Crowley also tries to answer Dar Oakley’s question about Debra’s fate thus (according to the WaPo review):
“Maybe not, said the Skeleton. But look at it this way. When you return home, you’ll tell the story of how you sought it and failed, and that story will be told and told again. And when you’re dead yourself, the story will go on being told, and in that telling you’ll speak and act and be alive again.”
Featured image credit: Free-Photos/pixabay.