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Foundations of Physics

Special States Demand a Force for the Observer

Lawrence S. Schulman

Published Jul 28, 2016• 7 citations
Publisher

Abstract

The “special state” understanding of the measurement process is presented, namely there is no “measurement process,” only unitary time evolution. However, in contrast to the many worlds interpretation, there is only one world. How this can be accomplished and how statistical mechanics is changed as a result are also discussed. The focus though is on experimental tests of this theory and the in-principle realization that this can give rise to feasible experimental tests. Those tests rely on the particular feature of having only one world, so that any change in the wave function must have a proximate cause, and it is the detection of that cause that constitutes the test. In a companion article there is further exploration concerning the details of the test. In addition, in the present article, the special state theory is extended theoretically through evidence of the uniqueness of the Cauchy distribution as well as explicit recognition of the role of entanglement.

DOI
10.1007/s10701-016-0025-8

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The “special state” understanding of the measurement process is presented, namely there is no “measurement process,” only unitary time evolution. However, in contrast to the many worlds interpretation, there is only one world. How this can be accomplished and how statistical mechanics is changed as a result are also discussed. The focus though is on experimental tests of this theory and the in-principle realization that this can give rise to feasible experimental tests. Those tests rely on the particular feature of having only one world, so that any change in the wave function must have a proximate cause, and it is the detection of that cause that constitutes the test. In a companion article there is further exploration concerning the details of the test. In addition, in the present article, the special state theory is extended theoretically through evidence of the uniqueness of the Cauchy distribution as well as explicit recognition of the role of entanglement.
By a radical modification of statistical mechanics the measurement process of quantum mechanics can be described in terms of pure, unitary time evolution, with no wave function collapse or many‐world ideas. The key notion is “special states,” rare microscopic states of a complex system. Recovering the standard probabilities requires of this theory the appearance of Cauchy‐distributed noise in some measurement processes. This article treats experimental situations where such noise might be detected and correlated with the need or absence of need for special states. Included in this possibility are “delayed choice” experiments, in which the correlation contravenes conventional ideas on causality. Background material on all topics is provided.
Establishing (or falsifying) the special state theory of quantum measurement is a program with both theoretical and experimental directions. The special state theory has only pure unitary time evolution, like the many worlds interpretation, but only has one world. How this can be accomplished requires both “special states” and significant modification of the usual assumptions about the arrow of time. All this is reviewed below. Experimentally, proposals for tests already exist and the problems are first the practical one of doing the experiment and second the suggesting of other experiments. On the theoretical level, many problems remain and among them are the impact of particle statistics on the availability of special states, finding a way to estimate their abundance and the possibility of using a computer for this purpose. Regarding the arrow of time, there is an early proposal of J. A. Wheeler that may be implementable with implications for cosmology.
An experimental test of the “special state” theory of quantum measurement is proposed. It should be feasible with present-day laboratory equipment and involves a slightly elaborated Stern–Gerlach setup. The “special state” theory is conservative with respect to quantum mechanics, but radical with respect to statistical mechanics, in particular regarding the arrow of time. In this article background material is given on both quantum measurement and statistical mechanics aspects. For example, it is shown that future boundary conditions would not contradict experience, indicating that the fundamental equal-a-priori-probability assumption at the foundations of statistical mechanics is far too strong (since future conditioning reduces the class of allowed states). The test is based on a feature of this theory that was found necessary in order to recover standard (Born) probabilities in quantum measurements. Specifically, certain systems should have “noise” whose amplitude follows the long-tailed Cauchy distribution. This distribution is marked by the occasional occurrence of extremely large signals as well as a non-self-averaging property. The proposed test is a variant of the Stern–Gerlach experiment in which protocols are devised, some of which will require the presence of this noise, some of which will not. The likely observational schemes would involve the distinction between detection and non-detection of that “noise”. The signal to be detected (or not) would be either single photons or electric fields (and related excitations) in the neighborhood of the ends of the magnets.
Quantum mechanics famously posits two distinct evolution rules for the state of a system: a deterministic, unitary time evolution under the Schr¨odinger equation, and a probabilistic “collapse” of the wavefunction upon measurement as dictated by the Born rule. The apparent inconsistency of these two dynamical laws – one continuous and one discontinuous – constitutes the quantum measurement problem. This foundational problem, which underlies the emergence of definite outcomes from quantum superpositions, has persisted since the theory’s inception. In this review, we present a comprehensive and rigorous examination of the measurement problem and survey the leading approaches that seek to resolve or circumvent it. We begin by formulating the measurement problem and its conceptual challenges. We then discuss in detail the major interpretations of quantum mechanics and theoretical frameworks addressing the problem: the Copenhagen interpretation, Everett’s many-worlds interpretation, de Broglie–Bohm pilot-wave theory, objective collapse models, the role of decoherence and environment-induced superselection, and epistemic approaches such as QBism. For each interpretation, we describe the core principles and mathematical formalism, assessing how (and whether) it attempts to solve the measurement problem. We also review modern developments and experiments relevant to quantum measurements, including tests of Bell inequalities, observations of decoherence and macroscopic superpositions, weak measurements and quantum eraser experiments, and thought experiments ‘a la Wigner’s friend. We highlight how these results inform the ongoing debate. Finally, we discuss open problems and challenges that remain in fully resolving the measurement problem.
There are several theories or processes which may underlie quantum mechanics and make it deterministic. Some references are given in the main text. Any such theory, plus a number of reasonable assumptions, implies the existence of what I have called ``special" states. The assumptions are conservation laws, obedience (up to a point) of Schrodinger's equation, and a single world, in the sense of the many worlds interpretation (the last one a consequence of any deterministic theory). This article also, for clarity, gives an example of a ``special" state. There is an experimental test of the ``special" state theory.
In most theories of the quantum measurement process changes in an observer’s perception of a state can take place without forces, as for example if a state is prepared in an eigenstate of JxJ_xJx​ (x component of angular momentum) but JzJ_zJz​ is measured. In the “special state” theory (explained in the previous article) any change in wave function requires forces. This allows experimental tests to distinguish these ideas and in the present article two examples of such tests are considered. The first is a kind of double Stern–Gerlach experiment, the second a check for angular momentum changes in a polarizer.