| Themes > Science > Physics > Quantum Physics > Quantum Physics Concepts | |||
The fundamental laws of quantum physics were discovered independently in 1925 by Werner Heisenberg and in 1926 by Erwin Schrödinger in response to puzzling experimental evidence that contradicted the fundamental concepts of classical physics. For example, electrons (which were previously thought to be particles) were found to exhibit properties of waves. Conversely, light (which was previously thought to be waves) was found to exhibit properties of particles. This confusion of classical distinctions between particles and waves was resolved by Niels Bohr's principle of complementarity, according to which the wave and particle concepts are understood to be mutually exclusive but both necessary for a complete description of quantum phenomena. A consequence of this wave-particle duality is that all matter has a wave aspect, and cannot be said to have a definite localized position at all times. Moreover, by virtue of their nonlocal wave properties, pairs of spatially separated particles sometimes exhibit nonlocal correlations in their attributes. Another consequence of the wave-particle duality is a corresponding duality between the unobserved and the observed. This duality raises puzzling questions regarding the nature of measurement in quantum mechanics: how is it that the wave suddenly changes into a particle, and how is this sudden transformation related to observation? A deeper understanding of these subtle
issues requires some basic understanding of the way quantum physics
describes phenomena. According to quantum physics, the state of an
unobserved quantum of matter or light (such as an electron or photon) is
represented by a solution to Schrödinger's wave equation. This solution
is a quantum wave function y(x) whose intensity |y(x)|2
at any particular position x represents the probability of
observing the quantum at that position. When the quantum is observed,
however, it is seen to have a definite actual position, and the wave
function no longer properly describes the quantum. Thus, when the quantum
is unobserved, it is a nonlocal wave of probable positions; and when the
quantum is observed, it is a particle having a definite localized
position. As a result, both the particle and wave concepts are required to
completely characterize a quantum: the particle concept is required to
describe its particle-like behavior when observed, while the wave concept
is require to describe its wave-like behavior when unobserved. The
particle and wave concepts are called "complementary"
descriptions because they are both needed to characterize the observed and
unobserved aspects of any quantum, as illustrated in the following table.
Although observation is evidently necessary to bring about the transition from possible to actual, the fundamental nature of observation in quantum theory remains somewhat mysterious. This problem of measurement derives from the fact that, prior to observation the quantum is described as being a nonlocal wave of probability spread throughout space, while after observation only one of the possible values is actualized. Thus, observation involves a discontinuous "collapse" (also called a "projection") of the quantum wave function from a continuum of possibilities to a single actualized value. This projection, however, is an ad hoc element of the formalism, and is not a lawful transformation that is governed by Schrödinger's wave equation. There is no explanation for how, when, or where this mysterious projection happens. Moreover, when the projection takes place, the laws of quantum physics do not predict which of the possible values will be actualized in any given observation, thus violating classical determinism and introducing an element of acausality and spontaneity into the theory at a fundamental level. In a fundamental analysis of the quantum measurement process, John von Neumann argued that consciousness is required to explain the projection of the wave function from possibility to actuality. In particular, he reasoned that because all physical interactions are governed by Schrödinger's wave equation, the projection that is associated with observation must be attributed to a non-physical consciousness that is not governed by physical law. According to von Neumann, this activity of consciousness only serves to cause the projection, and does not select or influence the particular value actualized. There is thus a spontaneity inherent in the projection that takes place in the transition from the unobserved to the observed. |
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