The concept of the physical state of a system is ubiquitous in physics but is usually presented in terms of specific cases. For example, the state of a point particle of mass m is completely characterized by its position and momentum. There is a tendency to consider such states as "real", i.e., as physical properties of a system. This rarely causes problems in classical physics but the notion of real quantum states has contributed mightily to the philosophical conundrums associated with quantum mechanics. The Einstein-Rosen-Podolsky paradox is a prime example. In fact, quantum states are not physical properties of a system but rather subjective descriptions that depend on the information available to a particular observer. This realization goes a long way toward resolving such dilemmas as Schrödinger's cat, wave function collapse, quantum non-locality, and parallel universes.
The concept of the physical state of a system is ubiquitous in physics but is usually presented in terms of specific cases. For example, the state of a point particle of mass m is completely characterized by its position and momentum. There is a tendency to consider such states as "real", i.e., as physical properties of a system. This rarely causes problems in classical physics but the notion of real quantum states has contributed mightily to the philosophical conundrums associated with quantum mechanics. The Einstein-Rosen-Podolsky paradox is a prime example. In fact, quantum states are not physical properties of a system but rather subjective descriptions that depend on the information available to a particular observer. This realization goes a long way toward resolving such dilemmas as Schrödinger's cat, wave function collapse, quantum non-locality, and parallel universes.
The concept of "reality" is often raised in the context of philosophical foundations of physics or interpretations of quantum mechanics. When this term is so raised, it is a warning to me that I am about to be led down a rabbit hole. Such diversions usually lead nowhere unless you consider endless discussions of Schrodinger's cat, wave function collapse, quantum non-locality, and parallel universes to be useful. A prime example is the famous Einstein, Podolsky, and Rosen paper wherein they concluded that the quantum wave function cannot provide a complete description of physical reality. In this essay I suggest that, in physics discourse, the term "reality" should be avoided at all costs.
Classical and quantum properties We shall use the term physical property to refer to something which can be said to be either true or false for a particular physical system. Thus “the energy is between 10 and 12 μJ” or “the particle is between x 1 and x 2 ” are examples of physical properties. One must distinguish between a physical property and a physical variable , such as the position or energy or momentum of a particle. A physical variable can take on different numerical values, depending upon the state of the system, whereas a physical property is either a true or a false description of a particular physical system at a particular time. A physical variable taking on a particular value, or lying in some range of values, is an example of a physical property. In the case of a classical mechanical system, a physical property is always associated with some subset of points in its phase space. Consider, for example, a harmonic oscillator whose phase space is the x, p plane shown in Fig. 2.1 on page 12. The property that its energy is equal to some value E 0 > 0 is associated with a set of points on an ellipse centered at the origin. The property that the energy is less than E 0 is associated with the set of points inside this ellipse. The property that the position x lies between x 1 and x 2 corresponds to the set of points inside a vertical band shown cross-hatched in this figure, and so forth.
The paper attempts to convince that the orthodox interpretation of quantum mechanics does not contradict philosophical realism by throwing light onto certain properties of quantum systems that seem to have escaped attention as yet. The exposition starts with the philosophical notions of realism. Then, the quantum mechanics as it is usually taught is demoted to a mere part of the theory called phenomenology of observations, and the common impression about its contradiction to realism is explained. The main idea of the paper, the physical notion of intrinsic properties, is introduced and many examples thereof are given. It replaces the irritating dichotomy of quantum and classical worlds by a much softer difference between intrinsic and extrinsic properties, which concern equally microscopic and macroscopic systems. Finally, the classicality and the quantum measurement are analyzed and found to present some still unsolved problems. A possible way of dealing with the Schrödinger cat is suggested that is based on the intrinsic properties. A simple quantum model of one classical property illustrates how our philosophy may work.
Abstract: The standard formulation of Quantum Mechanics has raised from its beginning animated discussions about the interpretation of the counterintuitive properties of mental objects (wave functions or Schrödinger waves) introduced to represent the properties of the physical objects.Two questions have since then been formulated to which a universally accepted answer is still lacking. The first one (Bohr, von Neumann) concerns the ontological nature of physical reality (the existence of classical objects) and the role of the observer (wave packet collapse) in assessing it. The second one is the non local character of quantum physical quantities (Einstein Podolski Rosen [EPR] long distance correlation of particles). An alternative formulation of Quantum Mechanics, originally proposed in 1932 by Eugene Wigner, taken up by Richard Feynman in 1987, and reelaborated by myself in the years from 1998 to 2003, is possible. The mental objects of standard Quantum Mechanics (Schrödinger waves) no longer appear in this new formulation and are replaced by new ones (Wigner functions) which do not show any more the puzzling properties which worried Einstein. My conclusion from the preceding discussion is that different explanations of a given set of experimental data may be derived according to the different nature of the mental objects introduced to represent the properties of the physical objects involved. The confusion between these two kind of objects may be, however, very misleading. I will finally discuss two examples of this conclusion from Biology
Mature theories of physical science associate states with systems, for instance with particles or waves in classical physics. The quantities whose values comprise states in classical mechanics, namely position and momentum, are chosen, first of all, for predictive purposes. Everything that one would want to say about classical systems can, it seems, be said in terms of properties: specification of the properties of a system gives a complete specification of its state, and conversely. Quantum mechanics associates states with physical systems; whatever else these might be, systems are the particulars which bear physical properties. The former is non-reduced state, the quantum state proper, while the latter is the reduced state, the class of quantitative properties possessed by the system. The state therefore determines which quantities and which properties are instantiated. The non-reductive account of states was initially appealing because it seemed to promise some additional explanatory scope.
Quantum non-integrable systems have pseudochaos in the phase of the eigenfunctions. In particular, correlation function of wave functions at two different points disappears, when observation process, which requires space and/or time average over a small range, is taken into account. This gives rise to the realization of decoherence in measuring processes. By virtue of this property of quantum chaos, various problems and paradoxes are explained in the framework of conventional quantum mechanics. The subjects treated here are the duality of wave and particle, the wave function collapse in measurement, Stern–Gerlach experiments, Schrödinger's cat paradox, the Einstein–Podolsky–Rosen paradox and quantum Zeno effect.