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Quantum Theory

Quantum theory grew out of a series of anomalies in the picture of matter and light offered by Newtonian physics - in particular associated with black-body radiation, the photo-electric effect, and the need to devise a model of the atom consistent with the newly discovered subatomic particles.

Important principles of quantum theory include its statistical nature, and the uncertainty principle which sets a limit on our knowledge of physical systems. The implications of the theory for the nature of reality are much discussed (see Implications of the new physics). Most quantum theorists accept an intrinsic element of probability in fundamental physics, and also the need to see systems as wholes rather than merely dissecting them into their simplest components.

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Physics

The empirical basis for quantum physics lies in such phenomena as blackbody radiation, the photoelectric effect, the specific heats of solids, the stability of the structure and the emission spectrum of atoms, all of which remained unexplainable in terms of classical physics. In 1901, Max Planck solved the blackbody problem by proposing that energy is quantized: it is available in discrete, not continuous, amounts. The quantization of light as ‘photons’ by Einstein in 1905 explained the photoelectric effect as well as the specific heat two years later. In 1913 Niels Bohr predicted the emission spectrum for hydrogen with a simple ‘planetary’ model of the atom in which the angular momentum of the orbiting electron, and thus the size of its orbits, are quantized. In 1924, Louis de Broglie attributed wave-like behavior to particles as the converse of energy quantization. Based on this idea, Erwin Schrödinger developed the wave equation which has proved to be foundational for quantum mechanics, Werner Heisenberg announced the uncertainty principle (and an alternative, but mathematically, equivalent formulation to that of Schrödinger), Wolfgang Pauli discovered the exclusion principle; by the end of the decade (nonrelativistic) quantum mechanics was basically complete.

Conceptual Problems

Still, almost a century later, major conceptual problems persist in interpreting quantum mechanics:

  • the Schrödinger equation propagates continuously in time but ‘collapses’ discontinuously in a process not described by the Schrödinger equation when a particle interacts with a classical system (often called ‘the measurement problem’);

  • the Schrödinger equation describes the propagation of the wave function ψ but this is a complex variable whose squared value ψ2 represents information about the quantum system;

  • a composite quantum system displays a holistic character entirely unlike classical composite systems (what can be called ‘whole-part causality’ as distinct from ‘whole-part constraints’): once interacting, now vastly separated, particles continue to act in some ways as though they remained part of a single system, as underscored by the “EPR” paradox in the 1930s and Bell’s theorem in the 1960s and now referred to as ‘non-locality’ and ‘non-separability’;

  • ’chance’ in quantum mechanics (i.e., quantum statistics) is not only strikingly different from classical chance (as in the familiar ‘bell curve’), it actually gives rise, in a ‘bottom-up’ way, to the basic features of the classical world, including the impenetrability of matter.

Philosophical Issues

Quantum mechanics can be interpreted philosophically in a variety of conflicting ways, and so far we know of no experimental basis for choosing definitively between them. These include ontological indeterminism (Heisenberg), ontological determinism (Einstein, David Bohm --- as stressed recently by Jim Cushing), or many worlds ( Everett ); as involving consciousness (Von Neumann, Eugene Wigner, Roger Penrose), non-standard logic (Gribb), or consistent histories (Bob Griffiths, Chris Clarke). It is particularly important to note that Bohm’s approach assumes an underlying, deterministic ontology; the implications of his approach for a philosohy of nature and for theology have received some attention. It is also important to recognize that all of these interpretations challenge classical ontology, with its core concepts of waves, particles and locality, as well as a critical realist philosophy of nature. In any case, quantum mechanics, at least compared to the other sciences surveyed here, can plausibly be said to offer the strongest reasons for expecting that the ontology of nature at the lowest levels at least is indeterministic.

Contributed by: Dr. Robert Russell - Dr. Christopher Southgate

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