The probability operator is derived from first principles for an equilibrium quantum system. It is also shown that the superposition states collapse into a mixture of states giving the conventional von Neumann trace form for the quantum average. The mechanism for the collapse is found to be quite general: it results from the conservation law for a conserved, exchangeable variable (such as energy) and the entanglement of the total system wave function that necessarily follows. The relevance of the present results to the einselection mechanism for decoherence, to the quantum measurement problem, and to the classical nature of the macroscopic world are discussed.
The probability operator is derived from first principles for an equilibrium quantum system. It is also shown that the superposition states collapse into a mixture of states giving the conventional von Neumann trace form for the quantum average. The mechanism for the collapse is found to be quite general: it results from the conservation law for a conserved, exchangeable variable (such as energy) and the entanglement of the total system wave function that necessarily follows. The relevance of the present results to the einselection mechanism for decoherence, to the quantum measurement problem, and to the classical nature of the macroscopic world are discussed.
Entanglement with the reservoir causes the wave function of an open quantum system to collapse into a mixture of entropy states. This gives the von Neumann trace form for the statistical average. The mechanism is related to environmental selection. The wave space of a closed quantum system has uniform weight, which means that microstates have equal weight. The Maxwell–Boltzmann form for the equilibrium probability operator arises from the weight of reservoir microstates. The collapse of an open quantum system gives rise to the classical universe and Hamilton’s equations of motion from the underlying Schrödinger equation.
The exchange of conserved variables between a subsystem and a reservoir or environment entangles their wave functions. Their subsequent collapse, together with the uniform weight of wave space, gives rise to quantum statistical mechanics formulated as the trace of the probability and observable operators. A dual representation in position and momentum space allows a permutation loop expansion for particle symmetrization, and a second expansion accounting for the non-commutativity of the position and momentum operators. The leading term gives classical statistical mechanics, and higher-order terms give successive quantum corrections that become increasingly important for low temperatures, high densities and small masses. The probability density in phase space is stable under the stochastic dissipative equations of motion. Trapped Bose-Einstein condensates may be modeled as a system of non-interacting harmonic oscillators. Most bosons in the condensate are in the ground state, in contrast to the -transition in 4He where the fraction in the ground state is negligible.
From a classical perspective, a quantum state is a surprising mixture of information and ignorance. If a system is sitting in an eigenstate of an observable, we can predict with confidence that a measurement result will yield the appropriate eigenvalue. With a superposition, certainty is not granted to us. Unlike the closest possible classical case, where a superposition represents a genuine blending of classical states, the quantum superposition is a strange overlapping of states simultaneously occupied by the system. On measurement, this ontologically curious animal collapses into one of the overlapping states, a process that is not well understood to this day. The collapse is random, a-causal, but governed by quantum amplitudes. Heading into a measurement, we do not know what we are going to get out. Given the evident link between entropy and information, the probabilities and uncertainties baked into a quantum state may well be a fertile ground for another form of entropy. The density matrix gives us a flexible tool for studying the properties of quantum ensembles of any kind. In this chapter, we will see how von Neumann helped develop and use the density matrix to study the problem of measurement and the information/entropy content of quantum states .
The seventh chapter is concerned with the formulation of quantum statistical mechanics from first principles. It is shown that interactions with the reservoir or environment cause the subsystem wave function to collapse into a mixture of pure entropy states, which obey the axioms of classical probability theory. A dual representation in position and momentum states allows a permutation loop expansion for particle symmetrisation, and a second expansion accounting for the non-commutativity of the position and momentum operators. The leading term gives classical statistical mechanics, and higher order terms gives successive quantum corrections that become increasingly important for low temperatures, high densities, and small masses. From the permutation loop expansion, explicit results for the quantum ideal gas and the quantum harmonic oscillator are derived. The role of entropy in the classical manifestation of the universe given the underlying quantum mechanical wave equation is discussed.
The quantum measurement problem, i.e., the apparent conflict between unitary quantum evolution and non-unitary, stochastic wave-function collapse, remains unresolved a century after the formulation of quantum mechanics. We first review the standard picture, from the Copenhagen prescription with its Heisenberg cut, through von Neumann's movable cut, to environment-induced decoherence, which explains the emergence of stable classical records but still presupposes the Born rule and therefore cannot by itself replace the measurement postulates. We then connect this problem to the Maximum Entropy Principle in two complementary ways. First, decoherence drives the measurement apparatus toward the least biased state compatible with the dynamically protected pointer distribution, so that the emergent collapse may be viewed as a thermodynamic relaxation toward constrained maximum entropy. Second, we propose a Timeless Histories formulation of quantum mechanics in which the primitive objects are ordered sequences of events rather than evolving wave functions. Their conditional probabilities are assigned, relative to specified refinements, by a single Born-Boltzmann rule that combines the quadratic structure of Born's rule with Boltzmann's counting of equally probable microstates. The textbook form of Born's rule, the projection postulate, density operators, and wave functions are then recovered as derived informational constructs, while collapse becomes Bayesian conditioning on recorded events. Time and space enter only subsequently through unitary translation symmetry. Since the probability rule is postulated without reference to any physical collapse or classical observer, the usual circularity of the decoherence program is avoided. Decoherence can then be invoked to explain the stability and approximate additivity of macroscopic records, i.e., the emergence of classicality.
The assumption that wave function collapse is induced by the interactions that generate decoherence leads to a stochastic collapse equation that does not require the introduction of any new physical constants and that is consistent with conservation laws. The collapse operator is based on the interaction energy, with a variable timing parameter related to the rate at which individual interactions generate the branching process. The approximate localization of physical systems follows from the distance-dependent nature of the interactions. The equation is consistent with strict conservation of momentum and orbital angular momentum, and it is also consistent with energy conservation within the accuracy allowed by the limited forms of energy that can be described within nonrelativistic theory. A relativistic extension of the proposal is outlined.