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FQXI ARTICLE

June 5, 2020

Dropping Schrödinger’s Cat Into a Black Hole

Combining gravity with the process that transforms the fuzzy uncertainty of the quantum realm into the definite classical world we see around us could lead to a theory of quantum gravity.

FQXi Awardees: Raphael Bousso

May 5, 2016

Raphael Bousso

University of California, Berkeley

That goal is tougher than it sounds because when physicists usually attempt this feat, their quantum calculations tell them that an infinite amount of stuff happens within every tiny portion of spacetime—even though common sense tells them this is nonsense. So now Bousso, at the University of California, Berkeley, is trying to use gravity to tame those unwieldy quantum equations, potentially leading to a theory of quantum gravity, which could in turn help explain the physics of the early cosmos and black holes.

"We want to know how to describe our universe, we want to know how did that start," says Bousso. His work is even more pressing since the LIGO collaboration announced the first detection of gravitational waves—ripples through spacetime that were set off when two black holes collided—in February 2016. "What happens inside the black holes that we now know happily merge somewhere far away and make gravitational waves?" Bousso asks.

As the world evolves

it branches into a

monstrous network

of entanglement

it branches into a

monstrous network

of entanglement

- Leonard Susskind

It’s often said that this superposition only collapses once the box is opened and someone peers inside, sealing the precarious feline’s fate. But it’s not quite that simple. In the 1980s, physicist and FQXi member Wojciech Zurek, now at Los Alamos National Laboratory in New Mexico, and others suggested that superpositions can be snapped if the quantum object—in this case, the cat—interacts with the environment, without the need for a conscious observer. It turns out that just a few photons of heat jostling the cat are enough to do the job. This process is called "quantum decoherence." But what exactly happens in the box is still something of a mystery, and one with which Bousso is now grappling.

To this end, and with the aid of an FQXi grant of over $140,000, Bousso is now examining a quantity that changes during decoherence called "entanglement entropy." This provides a mathematical measure of how strongly regions within a quantum system—say Schrödinger’s boxed cat—are connected to, and interact with, regions in the system’s environment.

Massaging Infinity

The calculations are tricky because when you try to conscientiously add up all the connections between the regions inside and outside the system in question, the answer hits infinity—because you can divide up the spacetime on either side of the boundary a countless number of times. But physicists have worked out ways to massage away these infinities in situations where gravity is extremely weak, allowing them to get sensible answers for the level of entanglement entropy and, hence, decoherence.

Heart of Darkness

What would happen to a cat, or anything else, that fell into a black hole?

Credit: NASA/JPL-Caltech

Bousso is now investigating whether taken together entanglement entropy and gravitational entropy can be used to quantify how much happens in cosmological settings, without giving ridiculous, infinite answers. For Bousso, this isn’t just about imagining ever more gruesome deaths of pets, but about understanding black hole physics, in general, and potentially even developing a theory of quantum gravity.

Leonard Susskind, an FQXi member and physicist at Stanford University, in Palo Alto, California, is impressed by Bousso’s approach. "As the world evolves it branches into a monstrous network of entanglement," he says. "I strongly applaud Bousso’s bold attempt to quantify this network." Though Susskind adds that entanglement may not turn out to be the only ingredient necessary to quantify what happens, it is certainly an important one. "It will surely lead to new insights into gravity and quantum mechanics," says Susskind.

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GEORGE JACKSON wrote on March 8, 2018

There is still a long way to the good understanding of the surrounding world. I'm not quite sure if human beings will ever be able to grasp the essence of the universe, but we can at least trying to with such theories as Schrodinger's Cat being tossed into a black hole. Thanks for this interesting article.

Best wishes,

George

There is still a long way to the good understanding of the surrounding world. I'm not quite sure if human beings will ever be able to grasp the essence of the universe, but we can at least trying to with such theories as Schrodinger's Cat being tossed into a black hole. Thanks for this interesting article.

Best wishes,

George

DURGA DAS DATTA. wrote on July 1, 2016

We are assuming all electrons and protons etc etc are exactly equal. We assume radioactive decay is just statistical average. We cannot specify which but we can give a statistical average. We think of cat may be alive or dead at the same time until we observe. Because we do not know if any decay of radio activity taken place. These are all our probabilistic quantum physics and we say there is no cause and effect . The biggest mistake in quantum physics that we treat all elementary...

We are assuming all electrons and protons etc etc are exactly equal. We assume radioactive decay is just statistical average. We cannot specify which but we can give a statistical average. We think of cat may be alive or dead at the same time until we observe. Because we do not know if any decay of radio activity taken place. These are all our probabilistic quantum physics and we say there is no cause and effect . The biggest mistake in quantum physics that we treat all elementary...

JOE FISHER wrote on May 22, 2016

Leicester City won the Premier League Championship title for the first time in 132 years. Hibernians won the Scottish Football Association Scottish Cup for the first time in 114 years. Perhaps my infinite surface assertion will replace the Einstein utterly unrealistic Theory of Relativity: General and Special that is now 107 years old. I hope so.

Leicester City won the Premier League Championship title for the first time in 132 years. Hibernians won the Scottish Football Association Scottish Cup for the first time in 114 years. Perhaps my infinite surface assertion will replace the Einstein utterly unrealistic Theory of Relativity: General and Special that is now 107 years old. I hope so.

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