Adrian Kent

University of Cambridge
Emily Adlam, DAMTP, University of Cambridge

Quasiclassical events in Relativistic Quantum Field Theory

Our perceptions seem to tell us that there are real and definite events taking place in the world around us. We see supernovae explode, comets collide with planets, and indeed the results of past cosmic events from the Big Bang onwards. At smaller scales, we see Geiger counters clicking when atoms decay, molecules combining in chemical processes, and so on. Yet it has been hard to understand how such events can arise according to our most fundamental and most successful theory, relativistic quantum theory. This has led many physicists either to postulate some sort of "many worlds" picture, according to which the events we see represent only those in one of many effectively independent "worlds", or to suggest that quantum theory may ultimately need to be replaced. This project pursues and tests a new idea, in which there is just one "world" of events, but they can be described by rules that are consistent with existing relativistic quantum theory. Excitingly, these rules also allow new ways of extending quantum theory to different theories which predict different cosmic effects. We will use this for new tests of standard quantum theory on large scales using cosmological data.

The project will develop recent work of the PI showing that a quasiclassical description of physics, including measurement events, emerges
from simple natural rules in relativistic quantum field theory. Specifically, when initial data and asymptotic final data are combined appropriately, a generalised expectation value for the stress-energy tensor gives a description of quasiclassical reality, with effective "collapses" taking place during measurement events. Unlike other approaches to defining measurement events, this prescription is explicitly Lorentz covariant. It also respects standard quantum dynamics. The description of quantum reality augments the unitarily evolving quantum state but does not change its evolution. Effectively, in Everettian language, it gives a precise rule for picking out one quasiclassical world from the universal wave function. This has been shown in toy models, and we aim to give a persuasive description in more detailed models using good semi-rigorous approximations to relativistic quantum field theory. While the standard rule respects quantum dynamics, it also offers new ways of generalizing quantum dynamics to alternatives that are experimentally and cosmologically testable. We will characterize the simplest and most interesting alternatives and their signatures in cosmological data, in order to characterize how well quantum theory has truly been tested at large scale.

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