Should You Believe What Stephen Hawking Says About Black Holes?

What are astronomy's most intriguing puzzles?
Feb. 13 2014 6:36 AM

Did Stephen Hawking Just Eliminate Black Holes?

Celebrity science can be weirder than quantum mechanics.

Massive Attack
The black hole at the heart of the M87 galaxy is surrounded by a maelstrom of hot gas.

Composite image courtesy of NASA. X-ray: NASA/CXC/KIPAC/N. Werner et al. Radio: NSF/NRAO/AUI/W. Cotton

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.

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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.

Despite years of effort, physicists have yet to unify gravity and quantum mechanics. The theory astronomers use to understand black holes is Einstein’s general theory of relativity. While nobody has directly observed event horizons for a variety of reasons (not least because they are very small, which makes it challenging even for big telescopes), astronomers have confirmed many predictions about their nature. The quantum character of black holes, however, is beyond reach for the foreseeable future: Few expect we’ll soon be able to observe the faint glow of Hawking radiation against stronger light sources.

Hawking’s own thinking about black holes has changed over time. That’s no criticism: Evidence in science often requires us to reassess our thinking. In this case, Hawking originally argued that black holes violated quantum mechanics by destroying information, then backed off from that assertion based on ideas derived from string theory (namely, the holographic principle). Not everyone agrees with his change of heart, though: The more recent model he used doesn’t correspond directly to our reality, and it may not have an analog for the universe we inhabit. The new talk suggests he has now moved on from both earlier ideas. That’s partly what raises doubts in my mind about the “no event horizons” proposal in the online summary. Is this based on our cosmos or yet another imaginary one of the sort physicists are fond of inventing to guide their thinking? In my reading, it’s hard to tell, and in the absence of a full explanation we are free to project our own feelings about both Hawking and his science onto the few details available.

Stephen Hawking
Stephen Hawking.

Photo by Mike Hutchings/Reuters

Within physics, Hawking’s fame can seem kind of odd. Einstein made foundational contributions to both branches of modern physics—relativity and quantum mechanics—so his fame is assured. Hawking, on the other hand, works in a rarified subfield of gravitational physics, far removed from the realm of immediate testability. Most physicists don’t learn about Hawking radiation, singularity theorems, or black hole information paradoxes, yet outside academic departments Hawking is far better known than the architects of quantum physics, whose work underpins all of modern technology.

Hawking’s popular reputation was created through his best-selling book, A Brief History of Time, and the accompanying video program. Much of his fame is tied to a remarkable circumstance of his life: He has lived with amyotrophic lateral sclerosis for 50 years when few with the disease survive one-tenth of that time. Hawking’s electric wheelchair and attached computer that enables him to speak are as iconic as Einstein’s crazy hair.

In her excellent book Hawking Incorporated, Hélène Mialet examines the disconnect between the legend of Hawking the solitary genius of science and the reality of his life. That includes the entourage of people required to do everything from handling his bodily needs to assisting him with working out the mathematical equations he can no longer write down. These students and other assistants are not always credited by the public, even though their work is essential in translating Hawking’s thoughts into the symbolic form that is the language of theoretical physics. “The closer we get to the scientist’s [Hawking’s] body,” she writes, “the greater access we have to the extension of his distributed body: his assistants, computer, and students. Finally … we believe we have found the man because we are in the presence of his body, but that is when a multiplicity of Hawkings suddenly appears.” To put it another way, because of Hawking’s inability to communicate directly, we project our own feelings onto him.

That’s why it’s doubly necessary to separate Hawking the scientist from his science. Hawking isn’t a god among mortals; He has admitted to being wrong in his thinking in the past, so we shouldn’t assume everything he says is absolute truth now. He is an authority on black holes, but he’s not the only one—and there’s reason to believe we don’t know the whole story of his thinking on them now.

Such is the danger of celebrity in science, and why I think it’s a bad thing as a general rule. This paper becomes important not because of what it does or doesn’t say, but because A Famous Scientist wrote it. While the value of some scientific research is obvious immediately, the true worth of many theories is often unclear until later, after all the preliminary mess has been cleaned up.

Matthew R. Francis is a physicist, science writer, public speaker, educator, and frequent wearer of jaunty hats. He blogs at Galileo’s Pendulum.

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