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Quantum and the Limits of Empiricism: Recent Challenges to the Orthodox Theory

Christopher Norris, Cardiff University

I

The measurement-problem in first cropped up in the 1930s during a famous series of debates between Einstein and Bohr concerning the physical adequacy – or ‘completeness’ – of the orthodox quantum theory. 1 Einstein’s strongly-held view was that the theory as it stood could not be ‘com- plete’ since (1) it involved such classically unthinkable phenomena as / particle dualism and superposition; (2) it conceived the uncertainty-relations as somehow intrinsic to the quantum domain rather than as products of our limited knowledge or powers of precise observation/measurement; and (3) it thus imputed a strictly (irreducibly) probabilistic character to quantum-phys- ical objects, processes, or events. Moreover (4) the orthodox theory required that any empirical results with respect to (e.g.) particle position or were dependent on the kind of measurement conducted or the kind of experi- mental set-up deployed to detect or observe the relevant properties. In which case (5) there was no question – according to the orthodox theory – of ventur- ing ‘beyond’ those paradoxical appearances in order to postulate a deeper level of objective (observer-independent) where the ground-rules of classical would regain their hold and where any such uncertainties could be placed on the side of our limited knowledge or restricted powers of epistemic access. Rather we should take it (so Bohr argued) that quantum physics marked an absolute break with that whole way of thinking that char- acterised classical physics and whose chief tenets were precisely the doctrines that Einstein so strenuously sought to uphold.

1 See especially A. Einstein, B. Podolsky and N. Rosen, ‘Can Quantum-Mechanical Description of Physical Reality be Considered Complete?’, Physical Review, series 2, Vol. 47 (1935), pp. 777–80; , article in response under the same title, Physical Review, Vol. 48 (1935), pp. 696– 702; Bohr, ‘Conversation with Einstein on Epistemological Problems in ’, in P.A. Schilpp (ed.), : philosopher-scientist (La Salle: Open Court, 1969), pp. 199–241; Einstein, ‘Autobiographical Notes’ and ‘Reply to Criticisms’, in Schilpp (op. cit.), pp. 3–94 and 665–88; also Arthur Fine, The Shaky Game: Einstein, realism, and quantum theory (Chicago: University of Chicago Press, 1936); Don Howard, ‘Was Einstein Really a Realist?’, Perspectives on Science, Vol. 1 (1993), pp. 204–51; M. Klein, ‘The First Phase of the Bohr-Einstein Dialogue’, Historical Studies in the Physical Sciences, Vol. 2 (1970), pp. 1–39. 220 Christopher Norris

Whence Einstein’s further objection: that the orthodox theory had no means of explaining the so-called ‘collapse of the wavepacket’ or transition from the realm of quantum uncertainty to that of definite values with regard to space- time location or other such straightforward facts of everyday experience. 2 That is to say, ‘phenomena’ like superposition or wave/particle dualism were plainly not observed to take place in the macrophysical domain and must therefore be subject to reduction – or resolution into one or the other determinate state – at some definite yet so-far unspecified point on the scale of increasing quantum numbers. (Such was the problem most graphically illustrated by the dead-and- alive/neither-dead-nor-alive predicament of Schrödinger’s ‘superposed’ cat. 3) These various criticisms of orthodox QM were all summed up in Einstein’s verdict that the theory must be ‘incomplete’ – or lacking some crucial depth- explanatory resource – in so far as it manifestly failed to provide an intelligible picture of quantum-physical reality that would satisfy the joint constraints of empirical adequacy and objective truth. That is, it stopped short at just the point where any reputable theory would pass beyond the empirical data to a depth-explanatory hypothesis whose truth-value was dependent on the way things stood in physical reality rather than the way they happened to appear according to our present-best means of observation-measurement. Thus the issue fell out between those (principally Bohr and Heisenberg) who took the orthodox quantum theory to be ‘complete’ in all essential respects and those (chief among them Einstein, Schrödinger, de Broglie, and Bohm) whotookittoberadicallyincomplete in so far as it failed – or dogmatically refused – to conceive of an objective quantum reality beyond the phenomenal appearances. From this latter point of view any adequate physical theory must satisfy not only the orthodox requirement, i.e., that it match the full range of empirical data but also the further realist constraint that it provide a depth-onto- logical account of quantum processes and events. 4 The Bohm interpretation did just that by developing de Broglie’s pilot-wave theory and showing in detail how any given particle could be taken to possess precise simultaneous values of position and momentum that perfectly matched the established QM empirical- predictive results. On his account, moreover, these were objective values which pertained to the particle quite aside from any issue of the limits imposed by our capacities of measurement or observation. In which case one could think of it – in realist terms – as having a continuous trajectory and remaining ‘the same’

2 See J.A. Wheeler and W.H. Zurek (eds.), Quantum Theory and Measurement (Princeton, N.J.: Princeton University Press, 1983). 3 For a good popularising account, see John Gribbin, In Search of Schrödinger’s Cat: quantum physics and reality (New York: , 1984). 4 See entries for Einstein, Note 1 above; also Louis de Broglie, Physics and Microphysics (New York: Harper & Row, 1960); Schrödinger, Letters on Wave Mechanics (op. cit.); , Causality and Chance in (London: Routledge & Kegan Paul, 1957); David Bohm and B.J. Hiley, The Undivided Universe: an ontological interpretation of quantum theory (London: Routledge, 1993).