It may well have been the liveliest hour and a half I’ve ever spent in the company of theoretical physicists. In April, during a workshop I was attending on black holes, Bill Unruh gave a talk that challenged his colleagues on a point almost all of them thought had been settled in the mid-1980s. His colleagues challenged him back. The room throbbed with debate. At most conferences I’ve been to, one speaker presents his or her ideas, the next speaker presents his or her ideas, which might be exactly the opposite, nobody responds to what any else says, and nothing gets resolved. Everyone shuffles off to lunch, leaving onlookers not knowing what to think. Well, I still don’t know what to think of Unruh’s arguments, but it was invigorating to see great minds engage with one another.
Unruh bit off a piece of the central question in the search for a unified theory: What happens to stuff that falls into a black hole? No place else in the universe brings modern theories into such direct conflict. Einstein’s general theory of relativity says black holes are one-way streets: their gravity is so intense that nothing going down the drain can ever get back out again. Quantum theory says black holes are two-way streets: all processes are reversible in time, so whatever falls in has to be able to get back out in some form or other.
People who specialize in relativity, such as Unruh, not surprisingly tend to blame quantum theory for the trouble. They suggest that the time-reversibility of quantum theory, or, more strictly, its unitarity, fails. For their part, people who specialize in quantum theory tend to find fault with relativity. They think Einstein’s brainchild must break down, loosening the hole’s gravitational clutches.
In 1984 three people in the latter camp came up with a knockout blow. Tom Banks, Michael Peskin, and Leonard Susskind claimed that if unitarity failed, so would the laws of conservation of energy and momentum. Their reasoning was a straightforward application of the second law of thermodynamics. Any irreversible process creates entropy and heat. Under ordinary circumstances, the heat comes from burning fuel or some other source of energy; here, it has no source. It comes out of nothingness.
A failure of basic conservation laws is bad enough. What makes it really bad is that its effects would not be confined to black holes out in deep space, but should afflict our planet, too. Black holes pop into and wink out of existence all around us all the time. Quantum physicists are ordinarily untroubled by this ethereal effervescence—it is a measure of the weirdness of their theories that space could sizzle with short-lived black holes, like so many Pop Rocks, and theorists scarcely bat an eye. It gets their attention only if energy conservation fails.
Susskind has compared the failure of energy conservation to being pregnant: there’s no such thing as just a little energy non-conservation. Once you have any, you’re cooked. Literally. All those black holes would turn space into a giant heating coil and roast us to a temperature of 1032 degrees. The fact we are not being roasted alive means unitarity holds, ergo relativity must break down.
Not so fast, says Unruh. He is an avuncular, frizzy-haired Canadian who has a way of making you question things you thought you were sure about. One morning at the workshop, which was being held at the Kavli Institute for Theoretical Physics in Santa Barbara, I went to fill my water bottle and bumped into Unruh and another physicist, Ted Jacobson, in the hallway. We got to talking about our bike rides to campus, and I commented on some bluffs to the east. Unruh asked how tall they were. Jacobson and I both estimated 100 feet. Bill had never even seen the bluffs, but, based on some general remarks about local topography, doubted our estimate so vehemently that Jacobson and I lost all our initial conviction.
His doubts about toasty black holes go back to the mid-’90s, when he and fellow relativity expert Bob Wald cited new work in the physics of materials which suggested that an irreversible process does not necessarily generate heat. Nikolai Prokof’ev and Philip Stamp had recently described how a process can instead cause spinning particles to begin precessing like quantum versions of a wobbling top. Causing a particle to precess does not require any expenditure of energy, so it is not subject to the same thermodynamic restrictions that apply to other processes.
Experiments have since confirmed Prokof’ev and Stamp’s idea, as I learned last year when Stamp spoke at a meeting organized by the Foundational Questions Institute. Stamp breathes the same contrarian fire as Unruh. Quantum theory may be the best-tested theory in the history of science, but that didn’t stop him from cautioning that it may not be the final word: “I don’t think it’s a good idea to be too sure of it.”
In Santa Barbara, Unruh suggested that if the inner mechanism of a black hole behaves like a bunch of wobbly particles, it could swallow material irreversibly without roasting the universe in return. After an hour and a half of vigorous exchange, few if anyone seemed convinced, and Unruh became steadily more self-deprecating. “I accept my and Bob Wald’s position is a minority view,” he said at last. “This is the last gasp. As Popper [sic] said, you just have to wait for us to die out. But sometimes the troglodytes are right.”
Minority view though it is, Unruh’s position still gives his colleagues pause. Three weeks ago, I attended a conference on quantum gravity at the Nordic Institute of Theoretical Physics in Stockholm, organized by physicist and blogger extraordinaire Sabine Hossenfelder. Hossenfelder had the good fortune to bring together German, Italian, and Spanish physicists while their national soccer teams were clashing at the Euro 2012 championship. There’s nothing like epic sporting rivalry and Swedish microbrews to liven up physics discussions.
Chatting over lunch, two of the attendees discovered they had both been thinking about Unruh’s arguments and whether irreversibility necessarily toasted energy and momentum conservation. Jonathan Oppenheim, who has been toying with alternatives to quantum mechanics as a way to unpick the mysteries of black holes, said that holes popping up here and there wouldn’t alter the overall symmetries of space and time, which underpin the conservation laws. Luis Garay discussed how no clock is perfect. Unavoidable timing errors can cause quantum waves to fall out of sync irreversibly without flouting any conservation law. Like Unruh, Oppenheim and Garay were circumspect. It wasn’t that they thought the Banks, Peskin, and Susskind paper had to be wrong. They just thought no one could securely pronounce on it.
A few weeks after our encounter in the hallway at Santa Barbara, Jacobson told me he had checked Google Earth and found that the bluffs were indeed about 100 feet tall. Unruh was wrong about their height after all. Still, he was right to force us to think twice. Unusually among contrarians, he applies his same skeptical instincts to himself. “If you think you know the answer to something, then you stop looking for the answer,” he said during his talk. “For the young people: don’t assume the answers are known, no matter how confident the speaker may be—and that includes me.”