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
QSpace Latest
PressRelease: Shining a light on the roots of plant “intelligence”
All living organisms emit a low level of light radiation, but the origin and function of these ‘biophotons’ are not yet fully understood. An international team of physicists, funded by the Foundational Questions Institute, FQxI, has proposed a new approach for investigating this phenomenon based on statistical analyses of this emission. Their aim is to test whether biophotons can play a role in the transport of information within and between living organisms, and whether monitoring biophotons could contribute to the development of medical techniques for the early diagnosis of various diseases. Their analyses of the measurements of the faint glow emitted by lentil seeds support models for the emergence of a kind of plant ‘intelligence,’ in which the biophotonic emission carries information and may thus be used by plants as a means to communicate. The team reported this and reviewed the history of biophotons in an article in the journal Applied Sciences in June 2024.