1. Basics of The Quantum Measurement Problem

It is presumably well-known that the process by which the probability densities computed from quantum theory are converted into definite observational facts remains profoundly mysterious. The conceptual model of this process depends entirely on the interpretation of quantum mechanics favored by the analysts. The role and relevance of consciousness in this process varies enormously among interpretations. Some would regard the process as essentially irrelevant to consciousness except to the extent that it is part of the physical world in which humans and their minds exist. One example of such a view is seen in the "many worlds" or "many views of one world" interpretation (Everett, 1957; Squires, 1987). In this model, both an experiment and its conscious observer continue a deterministic evolution according to the equations of quantum mechanics and there is no probabilistic transition to definite outcomes; the appearance of such a transition to observers is essentially an illusion arising from the fact that the observing instrument correlates each state of the quantum system with a state of the observer that corresponds to having observed the particular quantum state in question, rather than the entire superposition. Other theories argue that so-called wave function collapse is an actual physical process proceeding independently of observation or measurement as such; classical objects composed of huge numbers of quantum particles spontaneously maintain themselves in classical states regardless of anyone’s observations (Ghirardi et al. 1986; Joos et al. 2003). In contrast, the view championed by Wigner and various others holds that conscious observation is the fundamental cause of the transition from probabilities to definite outcomes (Von Neumann, 1955; Wigner, 1963, 1964; Stapp, 2001). In general, those holding this view contend that no observation has actually been made until some conscious being has become aware of the results. It is interesting to note that a fundamental relationship between consciousness and quantum measurement is also required by a view almost diametrically opposed to the Wignerian approach. In the speculative model of Penrose, quantum measurement or "wave function collapse" is viewed as a strictly physical process, but this process is fundamental to the existence of consciousness, and only systems that can maintain certain kinds of quantum superposition and collapse them in particular ways are capable of being conscious (Penrose, 1989, 1994).

One of the most commonly used experimental tools for examining quantum superposition and its breakdown is some form of the Mach-Zehnder interferometer, in which a light beam is split into separate paths and then reunited at another beam splitter. By suitably chosen geometry it is possible to arrange that the reunited beam interferes with itself. Such interference can take place only if the individual quanta of the split beam are free to propagate in a superposed state along both paths between the initial and final beam splitters. If a "which-way" measurement intervenes to establish which branch of the apparatus contains a particular photon, the interference at the final beam splitter is lost, a fact that can be established by the detection of a change in the normal interference pattern. An important point is that interference is destroyed by the mere existence of apparatus capable of performing a which-way measurement, whether or not it actually measures anything. As dramatized by the "Elitzur-Vaidman bomb test" thought experiment, the presence of a photon detector in one branch of the interferometer destroys interference even if it fails to detect anything because the photon traveled through the other branch (Elitzur and Vaidman, 1993). This can, of course, be viewed as a simple instance of the general principle that experiments on quantum objects examine only the properties they are designed to examine. An interferometer with a photon detector embedded in one branch of the light path is, by construction, an instrument for establishing which way the initial beam splitter directed the incident photon, and naturally cannot generate interference effects downstream of the detector. The general properties of the Mach-Zehnder interferometer and its sensitivity to which-way measurements are central to the following discussion.

2. Empirical Data Involving Consciousness

When considering the role of consciousness in the physical universe, physicists are somewhat handicapped by the fact that their traditional experimental techniques do not involve human beings as part of the apparatus, or even as subjects of inquiry. Such tests are generally considered to be part of other fields of science. Moreover, specific inquiries regarding consciousness are seldom useful or informative from a physical-science perspective because of the techniques necessarily used in the so-called "soft" sciences such as psychology, although in recent years there has been considerable input to such studies from allied fields such as neurophysiology.

It is therefore unsurprising that experiments in relatively remote and unfamiliar fields gather little notice in the physics literature even on those occasions when they do have significant implications for physics. Nevertheless, these investigations sometimes produce data that are relevant to fundamental physical theories. Since theoretical predictions must be checked against observational data, it seems inappropriate to ignore such data even when they come from unexpected sources.

The particular data in question come from experiments using a Michelson interferometer, which can be considered as a Mach-Zehnder interferometer in which the light paths are overlaid by reflection so that the same beam splitter can be used both to separate and to reunite the beams. It was found that human subjects were apparently able to perform a limited which-way measurement despite the absence of conventional detection apparatus in the light path (Radin 2008). This experiment was a refinement and extension of earlier experimental work by other researchers that had shown mixed and inconclusive results (Jeffers and Sloan, 1992; Ibison and Jeffers, 1998). It is a noteworthy feature of all these investigations that no attempt was made to evaluate the human subjects' success in actually acquiring which-way information; the existence of a which-way measurement was inferred from its physical effects on an interference pattern. While it might seem peculiar to design an interferometry experiment around a human subject's cognitive state, the primary hypothesis was derived from speculations about human capabilities which, while controversial, are supported by an extensive technical literature (Pratt et al. 1940; Rhine, 1971; Targ and Puthoff, 1974; Parker, 1975; Honorton, 1977; Tart et al. 1979; Bem and Honorton 1994; May, 1996; Brown, 2005). For consideration of the physical nature of consciousness the most interesting thing about this experiment is not the history leading to the adoption of its design, but the fact that it seems to have shown detectable physical consequences of a human subject's mental state in a physically separated, isolated, and shielded experimental system.

A brief overview of the Radin (2008) experimental report is necessary before discussion of its theoretical implications can proceed. The primary empirical finding is that a particular state of consciousness induced by human subjects had the effect of performing a which-way measurement, albeit one with fairly low quantum efficiency, on a nearby Michelson interferometer, which was shielded against electromagnetic interference and vibration. (The report's methodology section refers to a "double-steel-walled, electrically and acoustically shielded room" resting on a vibration isolation mat and with further layers of vibration isolation between the room's interior floor and the experimental apparatus. The implication of a separate floor suggests that the experimental space might more appropriately have been called a steel-walled box rather than a steel-walled room.) The refinements to earlier experiments (Jeffers and Sloan, 1992; Ibison and Jeffers, 1998) consisted largely in this superior isolation, and the change from a simple double-slit interference apparatus to the Michelson interferometer, allowing significant spatial separation of the beam paths. The method for measuring the extent of interference in the apparatus was to measure total light intensity in portions of the interference pattern that showed strong differences between the two-path pattern with interference and a one-path pattern without mutual interference.

Unfortunately, the report does not give actual light intensities for these measurements, instead presenting them either in arbitrary units or in statistical figures of merit. In these statistical terms, then, one can say that subjects skilled in maintaining a state of focused attention produced a change of the measured intensity amounting to 4.28 times the measurement uncertainty. Unskilled subjects produced an apparent change of only 0.29 times the measurement uncertainty, while control runs with no subjects, performed immediately after the sessions with skilled subjects, produced an apparent change of 0.46 times the measurement uncertainty. The changes referred to are the changes between the measured light intensity when the subject was, or was not, maintaining a state of focused attentiveness directed at the location of one branch of the interferometer. The classification of "skilled" or "unskilled" subjects is relevant due to the difficulty most untrained people experience in maintaining such focused mental states over significant intervals of time, as discussed in the report.

What makes these empirical findings of particular theoretical interest is the absence of any conventional measuring device in the interferometer beam path. It was noted above that the presence of apparatus capable of conducting a which-way measurement is sufficient to destroy the ability of the two light paths to interfere with each other. In this case, however, there are two conditions, one with full interference and one with reduced interference, in which from a conventional viewpoint no additional measuring apparatus has been introduced. This seems to force the conclusion that the which-way apparatus in this case consists of the specific cognitive state of the human subjects during the periods when they were instructed to maintain focused attention on the apparatus. This conclusion is less peculiar than it might seem at first inspection. On the presumption that a cognitive state must equate to a particular physical configuration of the subject's brain activity, there is a clearly identifiable physical difference between the two experimental conditions. Runs showing evidence of which-way measurement activity correspond to one condition, and the runs lacking such evidence correspond to the other.

It is, of course, somewhat unexpected that whatever physical changes the subjects are inducing in their brains by entering a state of focused attention should have an effect on instrumentation several meters away and heavily shielded. While a detailed physical mechanism is obscure, there is no shortage of roughly analogous situations in known experiments. Two examples seem particularly apropos. In the Aharonov-Bohm solenoid effect, electron beams propagating through a space free of electric and magnetic fields show changes in relative phase from the presence of a magnetic field completely confined within a solenoid in a region of space isolated from the beam paths (Aharonov and Bohm, 1959, 1961). So-called quantum eraser experiments actively rely upon the fact that an interference pattern can be altered or eliminated by remote manipulations of entangled partners of the photons actually participating in the interference (Kim et al. 2000). There is, of course, no reason to expect that Radin's subjects are actually acting as sources of a field-free potential, or have access to particles entangled with the interferometer's photon stream. These specific examples are presented not as plausible mechanisms for the observed effect, but simply as illustrations of the fact that known physical effects are entirely capable of circumventing methods of isolation, separation, or shielding which naοve physical intuition might expect to be adequate to protect an experimental system from outside influences.

3. Conclusions

It is premature to draw overly strong conclusions from a single experiment, not yet replicated by other researchers. (Although the experiment is itself a conceptual replication, the earlier experiments showed conflicting results and must be considered inconclusive.) Nevertheless, if these preliminary findings are confirmed, it would follow that at the very least we now know how to build an instrument to detect remotely the presence of one particular cognitive state in a human being, and that the physical mechanism mediating this detection is not yet understood. The observed phenomenon involves the resolution of a quantum superposition into distinct spatial states with definite locations, apparently in response to a purely cognitive and internal effort by a human subject. This fact suggests that further and more thorough investigations using this experimental paradigm might be able to throw considerable light on the relationship between human consciousness and the process that remains one of the fundamental mysteries of quantum mechanics.

Acknowledgments The author is indebted to Dean Radin for a number of informal presentations and communications without which this work would have remained unknown to him.