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Does It Matter If Black Holes Are Popping into Existence around Us All the Time?

The views expressed are those of the author and are not necessarily those of Scientific American.

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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.”

Photo credit: iStockPhoto

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. promytius 11:01 am 07/16/2012

    How can any sentient say things don’t come out of a black hole when we have photographic evidence? X-ray, infra-red, ultraviolet and every other type of image of jets spewing out from black holes – so what is this crap about never getting out? What are they arguing, and what is in those images? How is this interesting; it’s enraging, not engaging.

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  2. 2. gmusser 11:59 am 07/16/2012

    @ promytius The jets you refer to do not come from inside the event horizon

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  3. 3. terribletony 1:04 pm 07/16/2012


    There’s no doubt that black holes do eject material, the real debate is over whether or not this material contains any information on the material that originally fell in.

    If that information is irreversibly lost, then black holes are time-irreversible; i.e. the laws of physics don’t hold when the event is run in reverse.

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  4. 4. suitti 3:18 pm 07/16/2012

    And yet, Hawking Radiation comes out. The caution is that Hawking Radiation is unconfirmed.

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  5. 5. Postman1 10:55 pm 07/16/2012

    A well written and very interesting article and it certainly makes one think. Thank you GM.

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  6. 6. 11:45 pm 07/16/2012

    I had a thought. If Space Time pulls matter into a black hole what comes out the other side? Is it possible that matter as we know gets converted into dark matter as it passes through a black hole? If space is 70% of the mass of the universe, and is expanding at an ever accelerating rate, is it possible that matter as we know it is being converted to dark matter at a very high rate of speed? Just a thought to ponder.

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  7. 7. vinodkumarsehgal 1:21 am 07/17/2012

    Both Relativists and Quantum Physicists are unable to resolve the conundrum of fate of matter falling into a BH with their respective theories. On the contrary, they are willing to sacrifice Laws of conservation of energy and momentum to support their point of view. If within the existing paradigm, problem is not being resolved, why don’t they explore other avenues viz 5th and higher dimensions of universe and anti-gravity. Within a BH under extreme conditions of gravity, existing space-time may break down, have a connect with 5th dimension leading to link of in-falling matter, either in existing form or in some transformed energy form, with higher dimension. Alternatively, anti-gravity may develop and cause repulsion to the in-falling matter, after a certain critical density of in-falling mass/energy. The repulsed mass/energy might not have been detected. A few months ago, SA had published an article by G. Musser on dark matter describing the concept of “melting of space-time” within a BH by Eric Verlinde. I would request G. Musser that whenever he publishes such article on any matter, having large diversity of views, he should preferably project a holistic view by indicating all the contemporary and competing concepts.

    Any BH, however massive it might be in universe, has a limited life span since its birth. There is also the possibility that in none of the BHs which Physicists have so far observed, critical density for repulsion might have had developed.

    If existing physical theories are unable to solve the issue, it will be unscientific to resolve the issue thru forced interpretation or by invoking the violation of First principles of Natute like violation of principle of conservation of energy/momentum

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  8. 8. EmilMottola 10:51 am 07/17/2012

    Einstein’s General Relativity is a classical theory–a very successful one in its domain of applicability just as Newton’s was. But we know that matter is quantum, so classical theories of any kind cannot be fully consistent with nature. And despite its impressive successes in relatively weak fields, Einstein’s theory has not been well tested in cases of strong gravitational fields, so of course as scientists we should have an open mind to the evidence as it comes in. Black holes are particularly interesting because they bring the clash between the Quantum Mechanics and Statistical Mechanics, both extremely well-tested by innumerable experiments, and General Relativity to a head. It is well to remember that General Relativity can only make predictions if the right side of Einstein’s eqs. for “matter” are supplied from the outside. If this matter is fully quantum then “black holes” as they are described in textbooks as mathematical vacuum solutions of Einstein’s equations may not apply to real physical objects at all. Finally, it may not be Quantum Mechanics or General Relativity that is at fault, but the unchecked assumptions about how quantum matter behaves in strong gravitational fields as sources to Einstein’s eqs. that should be reexamined.

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  9. 9. alan borky 10:06 am 07/18/2012

    @gmusser ‘The jets you refer to do not come from inside the event horizon’.

    Or so they say which ultimately’s both promytius and indeed Unruh’s point.

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  10. 10. gmusser 1:17 pm 07/18/2012

    @alan borky The observed astrophysical jets coming from (presumed) black holes are quite well explained by current theory. Bill Unruh is talking about something completely different, namely the Hawking effect.

    @craig@HighMileageExperts There probably isn’t “another side” — that’s the whole point. The issue with dark matter is that the big bang just couldn’t have created enough ordinary matter to account for the missing mass in galaxies and larger structures, even if that ordinary matter collapsed to black holes.

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