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Zenith Grant Awardee

Raphael Bousso

University of California, Berkeley

Project Title

Quantum Information, Quantum Gravity, and Quantifying How Much Happens

Project Summary

Theories predict what should happen; experiments measure what does happen. But what does it mean for something to “happen”? Physics is a quantitative science: its most stunning insights are invariably rooted in associating numbers with the concepts it attempts to describe. So if we wish to understand what events are, we should first figure out how many there are. In a given region or matter system, what should we compute to determine how much “happens”? In tackling this question, I will explore a profound connection between the notion of “happening,” and recent advances in the study of quantum information and of black holes. In quantum mechanics, how much “happens” can be measured in terms of entanglement, the “spooky action” that troubled Einstein. But in quantum field theory—the guise in which quantum mechanics actually appears to us—the entanglement is always infinite. More refined notions from quantum information theory will have to be employed to capture how much “happens.” On the largest scales, in cosmology, even these sophisticated tools are undermined by the dominant role of the gravitational force. The only way forward appears to be to include quantum gravity—the entanglement of spacetime itself—in our accounting of what happens.

Technical Abstract

The goal of this project is to quantify how much “happens” in a given spacetime region. In Quantum Mechanics, decoherence is necessary for the outcome of a measurement to “happen.” A pointer becomes entangled with an environment that cannot be monitored. I propose to quantify how much “happened” in a region by quantifying the entanglement of the region with its exterior. This is nontrivial in Quantum Field Theory, since bounded regions have divergent entanglement entropy. Recently, the notion of vacuum-subtracted entropy has yielded a finite, cutoff- independent measure of entanglement. I will pursue an appropriate construction involving this quantity to capture how much “happens” in weakly gravitating or isolated regions. However, vacuum-subtraction cannot be performed in cosmology. Intriguingly, we may be forced to include gravitational degrees of freedom in accounting for what “happens” in an expanding universe. I will explore the role of the generalized entropy, which remains finite. Thus, the study of a seemingly mundane question—how much happens?—leads to a fascinating exploration of the interplay between quantum mechanics, quantum information theory, and quantum gravity

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