A reason why we do not observe Schr\"odinger's cats

TL;DR

The paper proposes a dynamical explanation for the absence of macroscopic superpositions, showing that quantum superpositions of macrostates rapidly collapse due to internal stochastic effects, deriving the Born rule without additional postulates.

Contribution

It introduces a model where macroscopic superpositions are quickly reduced through unitary dynamics, providing a natural solution to the measurement problem and deriving the Born rule.

Findings

Superpositions of macrostates are rapidly reduced by internal stochastic effects.

The model predicts collapse timescales consistent with observations.

Probabilities follow the Born rule without extra assumptions.

Abstract

A reason is discussed (may be not the only one) for why we do not see any superposition of macroscopic states in the real world. Under the general assumption that quantum macrostates are statistical ensembles of microstates, it is shown that any superposition of macrostates is reduced in a very short time by the unitary dynamics of the ordinary Schr\"odinger equation, deducing the Born rule without having to postulate it. In more detail, the macroscopic and microscopic degrees of freedom are decoupled in the Schr\"odinger equation, yielding an effective stochastic equation for the macroscopic variables, with the ensemble average of the microscopic amplitudes that acts as a self-generated internal white noise. The stochastic equation is shown to be a reducing It\^o equation if some general causality conditions are met, predicting a very quick collapse of any macroscopic superposition upon formation, with probabilities which satisfy the Born rule. In the context of the von Neumann measurement scheme, the relevance of the result is discussed as a simple dynamical solution of the measurement problem.