Semi-classical approximations based on Bohmian mechanics

TL;DR

This paper explores a Bohmian mechanics-based semi-classical approximation that treats some degrees of freedom classically, providing potentially more accurate results than traditional methods, with applications in quantum gravity and gauge theories.

Contribution

It introduces a novel semi-classical approach based on Bohmian mechanics, extending its application to quantum electrodynamics and quantum gravity, and discusses gauge invariance considerations.

Findings

Bohmian semi-classical approximation aligns more closely with quantum results.

Application to quantum gravity offers new semi-classical insights.

Framework accommodates gauge symmetries with careful degree of freedom separation.

Abstract

Semi-classical theories are approximations to quantum theory that treat some degrees of freedom classically and others quantum mechanically. In the usual approach, the quantum degrees of freedom are described by a wave function which evolves according to some Schr\"odinger equation with a Hamiltonian that depends on the classical degrees of freedom. The classical degrees of freedom satisfy classical equations that depend on the expectation values of quantum operators. In this paper, we study an alternative approach based on Bohmian mechanics. In Bohmian mechanics the quantum system is not only described by the wave function, but also with additional variables such as particle positions or fields. By letting the classical equations of motion depend on these variables, rather than the quantum expectation values, a semi-classical approximation is obtained that is closer to the exact quantum results than the usual approach. We discuss the Bohmian semi-classical approximation in various contexts, such as non-relativistic quantum mechanics, quantum electrodynamics and quantum gravity. The main motivation comes from quantum gravity. The quest for a quantum theory for gravity is still going on. Therefore a semi-classical approach where gravity is treated classically may be an approximation that already captures some quantum gravitational aspects. The Bohmian semi-classical theories will be derived from the full Bohmian theories. In the case there are gauge symmetries, like in quantum electrodynamics or quantum gravity, special care is required. In order to derive a consistent semi-classical theory it will be necessary to isolate gauge-independent dependent degrees of freedom from gauge degrees of freedom and consider the approximation where some of the former are considered classical.