  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.
