Good Morning Starshine

The science press today has been hailing a solution to a conundrum between general relativity and quantum mechanics identified by Stephen Hawking nearly fifty years ago. The quandary concerns the nature of black holes, those locations predicted by general relativity throughout the universe where mass has become so highly concentrated the resulting gravitational field prevents anything, even light, from escaping. Quantum mechanics, however, provides a workaround to this one-way trip, since in the its formulation the locations of microscopic particles are governed by probability and not absolutes. In this sense, particles can occasionally spontaneously “tunnel” from where they’re supposed to be, and by extension these locations must include black holes. (Quantum tunneling actually happens, and indeed the computer you are reading this e-mail on uses transistors relying on this very property.) So, one foundational theory of physics says nothing comes out of a black hole and another says that some stuff can come out of a black hole. That latter stuff can be the packets of light called photons, producing what has come to be known as “Hawking radiation.” Interestingly, the extrapolation of this idea is that black holes actually evaporate over enormous amounts of time. Another way to look at this conflict is about information, i.e., according to general relativity when stuff falls into a black hole some amount of information is removed from the larger universe, but the quantum mechanical formation says that information is not lost after all, indeed some of it is encoded in Hawking radiation.

So what gives? In the new theory published today, a team of theoretical physicists claim to have worked out a scheme that resolves the paradox. Namely, the material that falls into a black hole actually causes extremely minute imprints on its gravitational field depending on its composition, meaning that information about the black hole’s contents actually is able to escape a black hole after all. Put another way, two black holes of exactly the same mass will have slightly different gravitational fields based on what each ate in the past. In classical general relativity, only the mass (well, also the spin and charge) describes a black hole and there are no other defining characteristics, a model which American physicist John Wheeler called the “no hair theorem.” In the new model published recently, the team posit that black holes are actually quantum mechanically “hairy,” with more information about their contents theoretically retrievable. It seems, however, there is little chance of detecting these kind of extremely minute imprints in the gravitational field of a black hole, but who knows, even Einstein thought it would be impossible for gravitational waves to be detected. Anyway, for those who’d like a better description of this new model, there’s a nice article about it in the UK newspaper, The Guardian. Also, here’s a link to the paper itself in the Physical Review Letters for those who’d like to go straight to the source. By the way, “Good Morning Starshine” is a song from the musical “Hair.” :-)