plato.stanford.edu/entries/qt-measurement -> plato.stanford.edu/entries/qt-measurement/
The Encyclopedia Now Needs Your Support Please Read How You Can Help Keep the Encyclopedia Free Measurement in Quantum Theory From the inception of Quantum Mechanics (QM) the concept of measurement h as proved a source of difficulty. The Einstein-Bohr debates, out of whic h both the Einstein Podolski Rosen paradox and Schrdinger's cat paradox developed, centered upon this difficulty. The problem of measurement in quantum mechanics arises out of the fact that several principles of the theory appear to be in conflict. In particular, the dynamic principles of quantum mechanics seem to be in conflict with the postulate of collap se. David Albert puts the problem nicely when he says: The dynamics and the postulate of collapse are flatly in contradiction w ith one another ... the postulate of collapse seems to be right about w hat happens when we make measurements, and the dynamics seems to be biz arrely wrong about what happens when we make measurements, and yet the dynamics seems to be right about what happens whenever we aren't making measurements. The measurement problem is not just an interpretational problem internal to QM. It raises broader issues as well, such more general philosophical debates between, on the one hand, Cartesian and Lockean accounts of obs ervation as the creation of "inner reflections" and, on the other, neo-K antean conceptions of observation as a quasi-externalized physiological process. In this article I trace the history of these debates, and indic ate some of the interpretative strategies that they stimulated.
Related Entries The Birth of the Measurement Problem The measurement problem in QM (Quantum Mechanics) grew out of early debat es over Niels Bohr's "Copenhagen interpretation". Bohr maintained that t he physical properties of quantum systems depend in a fundamental way up on experimental conditions, including conditions of measurement. This do ctrine appeared explicitly in Bohr's 1935 reply to Einstein, Podolski, a nd Rosen: "The procedure of measurement has an essential influence on th e conditions on which the very definition of the physical quantities in question rests" (Bohr 1935, 1025;
character of all physical phenomena, a cha racter which depends essentially upon the state of motion of the observe r" (Bohr 1929, 73). In general terms, then, Bohr proposed that, like tem poral relations in special relativity, properties in QM exhibit a hidden relationalism - "hidden", that is, from a classical, Newtonian point of view. Paul Feyerabend gave a clear exposition of this Bohrian position in his "Problems of Microphysics" essay (Feyerabend, 1962). It can also be found in earlier commentaries upon Bohr by Vladimir Fock and Philip F rank (Jammer 1974, section 65). Many of Bohr's colleagues, including his young protege Werner Heisenberg, misunderstood or rejected the relationalist metaphysics underpinning Bo hr's endorsement of .
It is meaningless to assign Q a value q for S at t unles s Q is measured to have value q for S at t Heisenberg's approach, as presented in The Physical Principles of the Qua ntum Theory, quickly became a popular way of reading (or misreading, as Bohr would claim) the philosophically more forbidding complexities of th e Copenhagen interpretation.
which denied that the numerical value of a physical quantity has no meaning whatsoever until an observation has been performed" (Jammer 1974, 246). Bohr disagreed with Heisenberg's extreme positivistic gloss of the Copenh agen interpretation that reduced questions of "definability to measurabi lity" (Jammer 1974, 69). Heisenbe rg reports a discussion that arose while preparing his 1927 Zeitschrift fr Physik paper in the following terms: "I remember that it ended with my breaking out in tears because I just couldn't stand this pressure fro m Bohr" (Jammer 1974, 65). Nevertheless, the two men agreed in broad ter ms that ways of describing quantum systems depended upon experimental co nditions.
Einstein's disagreement with the Copenhagen school came to a head in the famous exchange with Bohr at the fifth Solvay conference (1927) and in the no less famous Einstein, Podolski, Rosen paper of 1935. Arguing f rom what might be called a "realist" position, Einstein contended that u nder ideal conditions observations (and measurements more generally) fun ction like "mirrors" (or, as Crary argues, camera obscura) reflecting an independently existing reality (Crary 1995, 48). the value of a physical quantity, the n there exists an element of physical reality corresponding to this phys ical quantity" (Einstein et al 1935, 778). This criterion characterizes physical reality in terms of objectivity, meaning its independence from any direct measurement. By implication, then, when a direct measurement of physical reality occurs it merely passively reflects rather than acti vely constituting that which is observed. Einstein's position has roots in Cartesian as well as empiricist, and spe cifically Lockean, notions of perception. This realist position opposes the Kantian metaphor of the "veil of perception" that pictures the appar atus of observation as like a pair of spectacles through which a highly mediated sight of the world can be glimpsed. To be specific, according t o Kant, rather than simply reflecting an independently existing reality, "appearances" are constituted through the act of perception in a way th at conforms them to the fundamental categories of sensible intuition. the trans cendental object remains unknown to us" (Kant 1973, 85). By contrast, the realism that I am associating with Einstein takes the po int of view that, insofar as they are real, when we observe rain drops u nder ideal conditions we are seeing objects "in themselves", that is, as they exist independently of being perceived. In other words, not only d o the rain drops exist independently of our observations but also, in ob serving them, what we see reflects how they really are.
influence on the co nditions on which the very definition of the physical quantities in ques tion rests" (Bohr 1935, 1025). As Henry Folse points out, however, it is misleading to take the parallel between Bohr and Kant too far (Folse 1985, 49 and 217-221). Bohr strong ly opposed the Kantian position that "space and time as well as cause an d effect had to be taken as a priori categories for the comprehension of all knowledge" (Folse 1985, 218). This opposition between Bohr and Kant reflected a deeper division. Whereas for Kant "concepts played their ro le prior to experience and give form to what is experienced" (Folse, 220 ), for Bohr it was the other way around, that is, objective reality, in particular conditions of observation, determine the applicability of con cepts. Thus, although for Bohr no less than for Kant, observation took o n a role in determining the forms that structure the world of visible ob jects, the two men conceived the way in which that role is discharged qu ite differently. For Kant subjective experience was structured in terms of certain prior forms, whereas Bohr argued for a hidden relationalism i n the domain of appearances, contending that the properties in terms of which a system is described are relative to the conditions of measuremen t This difference between Bohr and Kant may be seen as an aspect, indeed ra dicalization, of a more general shift in nineteenth century conceptions of vision, exemplified in Johannes Mller's compendious summary of curre nt physiology, Handbuch der Physiologie des Menschen (1833). Mller (a m entor of the influential physicist Hermann von Helmholtz) may be seen as physiologizing the Kantian conception of observation.
work, in spite of his praise of Kant, implies something q uite different. In particular, it was as sumed that observable phenomena were conditioned, not by universal forms of sensible intuition, but rather by the sorts of external physical fac tors that affected bodily and specifically physiological processes in ge neral. Bohr extended the nineteenth century concept by proposing that the "exter nal procedures" that influence vision affect not only how we see but als o the scientific concepts in terms of which what we see should...
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