Quantum TheoryQuantum
theory grew out of a series of anomalies in the picture of matter and light
offered by Newtonian physics  in
particular associated with blackbody radiation, the photoelectric 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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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
wavelike 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 ψ^{2} represents information about
the quantum system;

a composite quantum system
displays a holistic character entirely unlike classical composite systems
(what can be called ‘wholepart causality’ as distinct from ‘wholepart
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 ‘nonlocality’ and ‘nonseparability’;

’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 ‘bottomup’ 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),
nonstandard 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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