Stephen Hawking is a celebrity as well as a scientist, more famous for being famous than he is for his accomplishments in theoretical physics. To many, he’s nearly as well known as Albert Einstein (another rare physics celebrity), his face as familiar as those of movie stars and athletes. As a result, his public statements often attract a disproportionate amount of media attention.
A case in point: Hawking recently posted online a summary of a talk he gave in August 2013 on black holes. The text is brief, contains none of the equations essential in theoretical physics, and bears few specifics. However, unwrapping what it says involves us in the mysteries of black holes, quantum physics, and the nature of scientific celebrity.
Despite the paucity of information in the paper, the science side of the Internet exploded with excitement (and occasional criticism). News sites proclaimed, “Hawking said there are no black holes!” and other variations on the theme. The headlines were not terribly far off. However, Hawking’s actual statements were more subtle and frankly more vague than that.
The truth is, we don’t know the details of what Hawking actually has in mind. It’s unclear from the summary, published on the public research website arXiv.org, whether he’s describing our universe or a model version, or even whether this is a sketch of preliminary ideas. It hasn’t been peer reviewed. If anyone else had posted the document, you wouldn’t be reading this piece.
So what was Hawking’s talk actually about?
His radical proposal is that the event horizon of a black hole—the boundary beyond which everything is trapped by the black hole’s gravity and nothing can return to the outside universe—is not a permanent feature. Instead, Hawking calls it an “apparent horizon,” one that persists for a long time and reproduces the features of what we observe in telescopes as black holes.
This idea is both more and less radical than it sounds. The term black hole can have any of three meanings. Astronomers mean an extremely compact object with too much mass to be a neutron star or anything else; these are very common in the cosmos. They are found at the centers of nearly every large galaxy, including our own Milky Way. Theoretical physicists like me tend to use the event horizon itself as the definition of a black hole; that’s why it’s legitimate to say that Hawking’s idea would do away with black holes as they are conventionally understood. The third meaning involves the hypothetical quantum nature of black holes, an area of research pioneered by Hawking and a handful of other physicists in the 1970s.
Hawking was motivated to ponder the “apparent horizon” concept by the ongoing debate over the black hole information paradox. That’s a problem he described in 1976 at the intersection of quantum mechanics and gravitation. In quantum physics, certain pieces of information—the identity of particles, for example—must be preserved in some way. You can create and destroy particles via specific interactions, but the information is sacrosanct. Black holes, however, seem to violate that rule: When a particle falls in, much of the information is lost to the outside universe. A black hole cares not whether it’s made of electrons or photons or dark matter: The only thing an outside observer can measure is the black hole’s mass, spin, and (in the unlikely case there’s enough build-up) electric charge.
The problem gets worse when you bring in a discovery Hawking made in the 1970s: Black holes slowly lose mass, radiating away by emitting particles. This Hawking radiation has nothing to do with the matter falling into the black hole initially, so by the end of the evaporation process, any information about what made the black hole is long gone. That’s a serious violation of the laws of quantum physics, but Hawking initially thought that incompatibility is just how things must be.