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FQXI ARTICLE

November 21, 2014

Black Holes: Paradox Regained

In 2004, Stephen Hawking famously conceded that black holes do not devour all information when they swallow matter—seemingly resolving the *black hole information paradox* that had perplexed physicists for decades. But some argue that the paradox remains open, and we must abandon our simple picture of spacetime to unravel it.

FQXi Awardees: Steve Giddings

March 18, 2013

STEVE GIDDINGS

University of California, Santa Barbara

Growing up in Salt Lake City, Utah, Giddings was not only bewitched by black holes, he was also one of those rare teenagers who questioned the logic of the standard explanations. "That doesn’t make sense that nothing makes sense," he recalls thinking to himself. "There’s got to be some answer."

More than two decades later, Giddings, is still intrigued by black holes. In the intervening years, there have been a number of plot twists in their tale, mostly surrounding the debate over whether black holes also destroy information as they munch on matter, violating the laws of quantum mechanics that say that information cannot be lost.

The question seemed to be settled in 2004, when Stephen Hawking—the main proponent of the notion that information is lost—famously changed his mind over the issue. While Giddings thinks that Hawking was right to say that information isn’t lost in black holes, he notes that we still lack a full explanation of what happens to information. To really understand how information survives, Giddings argues, we have to throw out our familiar conception of spacetime.

The black hole information paradox goes back to the mid-1970s, when Hawking and fellow physicist Jacob Bekenstein of the Hebrew University in Jerusalem, Israel, showed that black holes should gradually radiate thermal energy. But this led to a conundrum: Consider two high energy particles that are smashed head on (say, within a particle accelerator, such as the Large Hadron Collider (LHC)) creating a black hole. According to Hawking’s original argument, the black hole would give off so-called

The trouble does not end there. In 1984, physicists Tom Banks, of the University of California, Santa Cruz, and Leonard Susskind and Michael Peskin, of Stanford University, argued that information loss in the context of black holes would lead to a unreasonably hot universe. This happens because the vacuum of spacetime is not empty, but rather it is a foaming sea in which virtual particles are forming and disappearing all the time. Quantum mechanics does not prevent the same from happening with virtual, microscopic black holes. If these black holes lose information, the net consequence of this process, Banks, Susskind and Peskin showed, is that the temperature of the vacuum would be a whopping 1.4 x 10

Paradox Lost?

The

Prior to Hawking’s turnaround, other physicists had also claimed to have solved the information paradox, by considering string theory, which posits that elementary particles are composed of tiny, vibrating strings. The mathematics of string theory allowed them to relate the physics on the surface of an object with its contents (thanks to the so-called

Locality is at the heart

of why information can’t

escape from black holes.

of why information can’t

escape from black holes.

- Steve Giddings

Despite learning from one of the discipline’s masters, Giddings feels string theory hasn’t yet fulfilled its promises. For one, he thinks it hasn’t shown conclusively that the black hole information paradox can be solved using the AdS/CFT correspondence. The correspondence essentially links a lower-dimensional theory with a higher dimensional theory. The solution for the black hole information paradox works only if there is a very precise one-to-one correspondence between each and every aspect of the two theories. "But do we really have a precise correspondence, or is there detail lost in going from the higher dimensional theory to the lower dimensional theory?" says Giddings. As far as he is concerned, the answers to these questions haven’t been established yet.

As a result, he views the claim that string theory provides a way out of the black hole information paradox as premature and incomplete. That means that either Hawking’s original view was correct and that information

DO BLACK HOLES OBLITERATE INFORMATION AS THEY DEVOUR STARS?

Artist’s depiction of a supermassive black hole ripping apart and consuming

part of a star—an event confirmed by NASA’s Chandra and ESA’s XMM-Newton

X-ray Observations.

Credit: NASA/CXC/M.Weiss

Quantum field theory explains how all known particles interact with the fundamental forces (except gravity, that is). The basic concept in question here is the assumed

So, thought Giddings, what if quantum field theory is actually non-local? Physicists have considered this possibility in the past and it turns out the answer cannot be that easy. Simply violating non-locality in quantum theory creates its own paradoxes: If faster-than-light signaling was allowed, you could, for instance, send a message back in time to persuade someone to kill your own grandfather before he met your grandmother—and thus you wouldn’t exist to send the signal. "Non-locality in quantum field theory is bad. It can get you in all kinds of trouble," says Giddings. "So, in fact, locality appears robust and well-defined in quantum field theory."

But Giddings has identified a key, and debatable, assumption in those locality arguments: "All of that assumes that there is some pre-existing spacetime that defines what it means for things to travel faster than the speed of light or not." Giddings and his colleague Donald Marolf, also at UCSB, are now using a $60,862 grant from FQXi to investigate physics without invoking such a pre-existing spacetime.

Information Exchange

One proposal being studied describes black holes and their environments as a network of

Giddings’ preoccupation with the black hole information paradox has not gone unnoticed by his peers. "Steve is one of the few people who understand the information problem in its full form, and his early work has been very useful in helping people understand the details of Hawking radiation," says Samir Mathur of Ohio State University in Columbus.

There are, however, serious implications of such an approach. "The broad lesson is that spacetime is doomed," says Giddings. There is no underlying spacetime. It only emerges in the absence of the kind of extreme boundary conditions experienced in black holes, or in the early universe.

So, how does one test such a theory? It could potentially be tested at future particle accelerators, more powerful than the LHC. First, microscopic black holes will need to be created in highly energetic particle collisions (something that has not yet been seen at the LHC). But more important is measuring the outcome of such collisions. It’s not enough to measure the macro properties such as position and momentum of particles emanating from the micro-black-hole, that look like Hawking radiation. Physicists will have to measure their quantum correlations, to see if information is conserved. "That’s pretty futuristic," says Giddings.

Spacetime is doomed.

There is no underlying

spacetime.

There is no underlying

spacetime.

- Steve Giddings

Such large spacetime fluctuations would occur at distances that can’t be observed, however, either in the variations in the temperature of the cosmic microwave background, which contains an imprint of primordial fluctuations of spacetime, or by the detection of gravitational waves themselves. But Giddings thinks that a modified quantum gravity theory (which combines general relativity with quantum mechanics) with some non-locality might be used to resolve some of the puzzles of inflation. This might result in small but measurable effects in our observable universe. "The idea is that somehow you look for signals in sufficiently precise cosmological observations," he says. "But we don’t have something concrete we can point to yet and say, ‘go look at this and you’ll find it’. It’s still too early."

Rewriting quantum gravity as a non-local theory could help reconcile the seemingly intractable differences between general relativity and quantum theory. Washington Taylor, an expert on quantum gravity at the Massachusetts Institute of Technology, thinks that there’s merit to Giddings’ approach. "It is very plausible that quantum gravity modifies the conventional notion of locality in quantum field theory. In fact, I think this is very likely," he says. "Steve is a talented and creative physicist, so I think it’s likely he will find some interesting results from his current line of inquiry."

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STEVE AGNEW wrote on May 18, 2014

It gets a little confusing commenting on an article by one person, Anil Ananthaswamy, about what another person, Steve Giddings, says about gravity force and black holes and what Giddings says about what even other people say.

Giddings wrote even more on Edge.org,

"Naive modifications of locality—as often proposed by physicists "on the fringe," generically lead to disastrous collapse of the entire framework of quantum field theory, which not only has been experimentally tested...

It gets a little confusing commenting on an article by one person, Anil Ananthaswamy, about what another person, Steve Giddings, says about gravity force and black holes and what Giddings says about what even other people say.

Giddings wrote even more on Edge.org,

"Naive modifications of locality—as often proposed by physicists "on the fringe," generically lead to disastrous collapse of the entire framework of quantum field theory, which not only has been experimentally tested...

JOHN C HODGE wrote on April 9, 2014

They used Quantum field theory explains how all known particles interact with the fundamental forces (except gravity, that is). The information could go to the exception, into a gravity field. Perhaps, we need to reconsider what gravity is or, rather, how it functions. This is more than combining QM and GR. This is forming a completely new model that can correspond to both QM and GR with appropriate, but different, approximations. STOE correspondence to general relativity and quantum mechanics...

They used Quantum field theory explains how all known particles interact with the fundamental forces (except gravity, that is). The information could go to the exception, into a gravity field. Perhaps, we need to reconsider what gravity is or, rather, how it functions. This is more than combining QM and GR. This is forming a completely new model that can correspond to both QM and GR with appropriate, but different, approximations. STOE correspondence to general relativity and quantum mechanics...

ANONYMOUS wrote on April 1, 2014

Here is the interesting article that may shed a light on the information paradox.

Suppose you store information of a book in the z-directional spin states of stream of electrons. Then later, in order to extract the information, but by mistake, you measure their spin states in x-direction. So you mess up the information stored in the stream of electrons. But conventional interpretation of quantum mechanics does not insist this measurement destroys the information, because the wave function...

Here is the interesting article that may shed a light on the information paradox.

Suppose you store information of a book in the z-directional spin states of stream of electrons. Then later, in order to extract the information, but by mistake, you measure their spin states in x-direction. So you mess up the information stored in the stream of electrons. But conventional interpretation of quantum mechanics does not insist this measurement destroys the information, because the wave function...

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