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FQXI ARTICLE

January 21, 2021

Behind the Shadows

Imagining Schrödinger’s cat peering out from behind a black hole could lead to a theory of quantum gravity.

FQXi Awardees: Joe Polchinski

December 15, 2011

JOE POLCHINSKI

Bumps on the road to quantum gravity will not deter avid biker Polchinski.

Armed with a $54,329 grant from the Foundational Questions Institute, Polchinski is investigating the fate of a shadowy version of an infamous quantum cat hiding behind the horizon of a black hole. If successful, the picture that emerges from his musings could provide physicists with a glimpse at the ultimate structure of spacetime. In turn, that may point them toward the long-sought theory of quantum gravity, which would unify the two pillars of modern physics: quantum mechanics, which deals with the physics of the very small, and general relativity, which describes how very massive objects warp spacetime.

A Hole Lot of Problems

Those hoping to construct a theory of quantum gravity often look to black holes as the perfect theoretical laboratory for pondering what happens when quantum mechanics and relativity meet because they contain huge masses confined within a very small region. In particular, in the 1970s, Stephen Hawking at Cambridge University and Hebrew University physicist Jacob Bekenstein showed that a black hole’s entropy—classically, the number of possible configurations of particles in a system—is proportional to its surface area. This was surprising because every other object in the universe has an entropy that is proportional to its volume. Physicists describe this property as "holographic" because it encodes three-dimensional information into a two-dimensional equation. "The ultimate unification of quantum mechanics and gravity will almost certainly be based on the holographic principle," says Polchinski.

Another famous black-hole paradox posed by Hawking highlights the clash between classical theories (such as general relativity) and quantum mechanics: On the one hand, physicists expect that a quantum particle falling into a black hole should have any information about its original state destroyed—but, on the other hand, this stands in clear violation of the quantum tenet that information is always preserved. The conflict has led Hawking and others to conclude that, to marry the two extremes, there must be profound changes either to quantum theory or to our understanding of spacetime. Polchinski is among the camp that believes we must dispense with the latter, repainting spacetime as a consequence of a deeper underlying structure. "The paradoxes point to a radical picture in which spacetime is not fundamental, but rather emerges from a projection in a lower dimensional space," Polchinski explains.

In an effort to find out how spacetime might emerge—just as our mental images of three-dimensional animals are generated by the movement of two-dimensional shadow puppets—Polchinski and others are now working on a model that views the entire universe as a giant hologram of information. "Polchinski’s work has led to deep insights into the quantum structure of space-time," says physicist Alex Maloney, at McGill University in Montreal, who works on a similar FQXi-funded project.

The question is whether such a counterintuitive beginning can yield a viable theory. "This is so different from the way that physics has always worked that we have no intuition for how it happens," admits Polchinski.

The Hows and Whys of Spacetime

It’s an intellectual challenge he seems to have spent his whole life preparing for. As a child growing up in Tucson, Arizona, Polchinski was fascinated by his illustrated "How and Why Wonder Books," which introduced him to different areas of science, such as atomic energy and electricity. Later, in high school, he recalls wondering a lot about gravity: how strong it was and how fast it traveled. But things really clicked for Polchinski when he went to Caltech as a undergraduate and stumbled across Feynman’s famous lectures. After that, there was no holding him back.

SPOT THE CAT

A supermassive black hole gobbles a star in this artist’s conception

of a long-predicted astronomical event confirmed by NASA’s Chandra

and ESA’s XMM-Newton X-ray Observatories.

Credit: NASA/CXC/M.Weiss

"Mandelstam was way ahead of his time and many of the problems he gave me are only now being solved," Polchinski says.

The ultimate unification of quantum

mechanics and gravity will almost

certainly be based on the holographic

principle.

mechanics and gravity will almost

certainly be based on the holographic

principle.

- Joe Polchinski

Maldacena, now at the Institute for Advanced Study at Princeton, believes that Polchinski has the right credentials to tackle these issues. "Joe has been a leader in this field and has had many important conceptual insights during his career," he says.

Hidden Cats

Holography also implies that the information contained within everything that ever fell into a black hole is entirely contained in quantum fluctuations of the surface of the hole’s event horizon. Polchinski is considering a thought experiment to test this: If Schrödinger’s cat—teetering in a quantum state in which it is both alive and dead —were hidden behind the horizon of a black hole, could we ever measure its state in a dual theory?

Polchinski argues that the answer is "yes," implying that a single set of building blocks is recycled to construct both the inside and the outside of the black hole. "This helps to sharpen the meaning of holography and the way that space and time emerge," says Polchinski.

So how far away is he from finding a unifying theory? "We keep learning new and surprising things, with duality and holography being the latest," says Polchinski. "But all we can say is that we are not finished yet."

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SRIDATTADEV wrote on December 19, 2011

Dear All,

Conscience or Universal I or singularity is the cosmological constant.

Space-time emerges from the absolute or universal I.

Love,

Sridattadev.

Dear All,

Conscience or Universal I or singularity is the cosmological constant.

Space-time emerges from the absolute or universal I.

Love,

Sridattadev.

LAWRENCE B. CROWELL wrote on December 16, 2011

The entanglement of the two states changes. From the perspective of the exterior observer the particle which falls into the black hole is redshifted and time dilated according to the delay coordinate

τ = r – 2m ln(1 – 2m/r)

so that it is never observed to reach the horizon, and only does so as τ -- > -∞. Let Alice be the exterior observer and Bob an observer who falls in with the other EPR pair. Assume Alice performs a measurement and sends the result by a...

The entanglement of the two states changes. From the perspective of the exterior observer the particle which falls into the black hole is redshifted and time dilated according to the delay coordinate

τ = r – 2m ln(1 – 2m/r)

so that it is never observed to reach the horizon, and only does so as τ -- > -∞. Let Alice be the exterior observer and Bob an observer who falls in with the other EPR pair. Assume Alice performs a measurement and sends the result by a...

WILHELMUS DE WILDE DE WILDE wrote on December 16, 2011

The origin of the question is if space/time is constituted of grains or not. Holographic information is information on a larger scale as for instance the Planck length. A black hole is also an "event" not on a quantum scale, (that also counts for a cat). So when we are placing in our "experioence" a cat in a black hole then indeed we pass a border : the event horizon of the black hole, this event horizon only exists in the scale of our perceptional causal reality. If we make the same journey...

The origin of the question is if space/time is constituted of grains or not. Holographic information is information on a larger scale as for instance the Planck length. A black hole is also an "event" not on a quantum scale, (that also counts for a cat). So when we are placing in our "experioence" a cat in a black hole then indeed we pass a border : the event horizon of the black hole, this event horizon only exists in the scale of our perceptional causal reality. If we make the same journey...

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