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An Impossible Experiment Shows How Black Holes Might Burp Back Information

A way around the black hole information paradox?
December 28, 2015, 12:30pm
Image: Wikipedia

Physicists at Caltech have devised a way to coax information back from the so-far unknowable depths of a black hole. While it's an impossible scheme to implement experimentally and involves the recapture of information from just a single particle, it's an interesting illustration of both the difficulties in truly understanding black holes and of quantum entanglement, the quantum mechanical property that seems to unify separate, individual particles across unlimited spans of time and space.


The Caltech group's work is described in an open-access paper posted to the arXiv pre-print server and slated for publication in the Physical Review Letters.

One of the most vexing features of black holes is known as the "black hole information paradox." Simply put, it would seem that black holes have no memory. If you or I were to leap across some event horizon, a black hole's gravitational point of no return, any and all information about us would be lost. No more probabilities; no more determination; no more causes and effects. History itself would seem to vanish.

Physicists don't like this very much because it violates a seemingly obvious rule called unitarity. What this says, basically, is that the probabilities of some event occurring should always add up to one. There should never be more than a 100 percent chance of something occurring because what would that even mean?

And yet this is what life in a black hole implies. No more unitarity. Huh.

The experiment proposed in the Caltech paper doesn't get rid of the unitarity problem, but it offers something of a way in. It relies on quantum entanglement. This is where two quantum particles prepared in the same quantum state get to share that state even as they're separated across vast distances. Entanglement is open to interpretation, but it's sort of as if a single particle is allowed to exist in two places at once.

Maybe you can already see where this is going. One particle goes in, vanishing in the most profound way a thing can vanish, but it's mirror image is left out here. That's sort of it, but we'll need to exploit a property of black holes called Hawking radiation.

Hawking radiation is how black holes slowly evaporate and eventually die. Thanks to the quantum mechanical ban on complete simultaneous knowledge of both a particle's position and momentum, we wind up with a curious omnipresent fizz of "virtual" particles. Simply: Because we can't ever know a quantum particle completely, we can't ever say that one is definitely not there. Consequently, there are always these pairs of virtual particles, one normal particle and one antiparticle, blinking in and out of existence.


It seems like the sudden emergence of particles from empty space would break all kinds of rules, but because one is an antimatter particle and the other is "normal," they usually annihilate each other immediately. At the edge of a black hole, however, sometimes one falls in and the other survives to radiate away as Hawking radiation. One of the particles thus adds one particle to the universe, while the other subtracts one from the black hole.

This is how a black hole ultimately evaporates away.

Now, the Caltech group has figured out how to snag some information from an electron dropped into a black hole using these virtual photon pairs and a totally unrealistic measurement of a black hole's combined angular momentum, or spin.

By taking this measurement along with a measurement of the surviving photon's own spin, the black hole itself becomes entangled with the photon. Next, the electron is dropped into the black hole and the black hole's angular momentum is measured again, this time with the electron. Since measurement has an irreversible effect on a quantum state, this action is reflected in the surviving photon from the virtual pair. In this way information from the "lost" electron is transmitted back out of the black hole.

So, yes, this is very unrealistic and, even if it weren't, the information gleaned is extremely, extremely minimal. But still.

"This falls far short of a resolution to the information-loss problem," the paper concedes, "but it does provide a concrete illustration of how information can escape from a black hole in certain special circumstances, and is similar in spirit to earlier discussions about using conserved quantities to recover black hole information."