Beyond Copenhagen: Following the Trail of Decoherence in Feynman's Light Microscope

Abstract

Feynman's light microscope invites us to reconsider what we thought we knew about quantum reality. Rather than invoking wavefunction collapse to predict the loss of fringes in a monitored interferometer, Feynman analyzes the problem in terms of a disturbance. This approach raises the question of whether the classical world, including its localized particles and definite measurement outcomes, might emerge as the universe evolves smoothly according to Schr\"odinger's equation. Treating the particle and its environment as an entangled system, unmodified quantum mechanics shows remarkable success toward this end. This is the purview of decoherence theory. How we then think about macroscopic reality becomes dependent on how we think about microscopic reality. Is quantum mechanics successful because it describes what microscopic particles are really doing, such as traveling both interferometer paths at the same time? Or is the wavefunction only a mathematical tool which predicts measurement outcomes but does not describe microscopic reality? Both options are uncomfortable. The first implies that each moment in time branches into a vast number of divergent macroscopic realities. The second represents, for many practitioners, a weakened view of science. This article is written to be accessible to anyone with an undergraduate course in quantum mechanics.