A Heuristic Approach to the Quantum Measurement Problem: How to Distinguish Particle Detectors from Ordinary Objects

Elementary particle detectors fall broadly into only two classes: phase-transformation devices, such as the bubble chamber, and charge-transfer devices like the Geiger-Mueller tube. Quantum measurements are seen to involve transitions from a long-lived metastable state (e. g., superheated liquid or a gas of atoms between charged capacitor plates) to a thermodinamically stable condition. A detector is then a specially prepared object undergoing a metastable-to-stable transformation that is significantly enhanced by the presence of the measured particle, which behaves, in some sense, as the seed of a process of heterogeneous nucleation. Based on this understanding of the operation of a conventional detector, and using results of orthogonality-catastrophe theory, we argue that, in the thermodynamic limit, the pre-measurement Hamiltonian is not the same as that describing the detector during or after the interaction with a particle and, thus, that superpositions of pointer states (Schroedinger cats) are unphysical because their time evolution is ill defined. Examples of particle-induced changes in the Hamiltonian are also given for ordinary systems whose macroscopic parameters are susceptible to radiation damage, but are not modified by the interaction with a single particle.