quantum mechanics
Quantum theory, introduced by Max Planck (1858–1947) in 1900, was the first serious scientific departure from Newtonian mechanics. It involved supposing that certain physical quantities can only assume discrete values. In the following two decades it was applied successfully to different physical problems by Einstein and the Danish physicist Neils Bohr (1885–1962). It was superseded by quantum mechanics in the years following 1924, when the French physicist Louis de Broglie (1892–1987) introduced the idea that a particle may also be regarded as a wave. The Schrödinger wave equation relates the energy of a system to a wave function: the square of the amplitude of the wave is proportional to the probability of a particle being found in a specified position. The wave function expresses the lack of possibility of defining both the position and momentum of a particle (see Heisenberg uncertainty principle ). The allowed wave functions, or ‘eigenfunctions’, have ‘eigen-values’ that describe stationary states of the system.
Part of the difficulty with the notions involved is that a system may be in an indeterminate state at a time, characterized only by the probability of some result for an observation, but then ‘become’ determinate (‘the collapse of the wave packet’) when an observation is made (see also Einstein–Podolsky–Rosen thought experiment, Schrödinger's cat ). It is as if there is nothing but a potential for observation or a probability wave before observation is made, but when an observation is made the wave becomes a particle. The wave–particle duality seems to block any way of conceiving of physical reality in quantum terms. In the famous two-slit experiment, an electron is fired at a screen with two slits (like a tennis ball thrown at a wall with two doors in it). If one puts detectors at each slit, every electron passing the screen is observed to go through exactly one slit. But when the detectors are taken away, the electron acts like a wave process going through both slits, and interfering with itself. A particle such as an electron is usually thought of as always having an exact position, but its wave nature ensures that the amplitude of its waves is not absolutely zero anywhere (there is therefore a finite probability of it ‘tunnelling’ from one position to emerge at another).
The unquestionable success of quantum mechanics has generated a large philosophical debate about its ultimate intelligibility and its metaphysical implications. The wave–particle duality is already a departure from ordinary ways of conceiving of things in space, and its difficulty is compounded by the probabilistic nature of the fundamental states of a system as they are conceived in quantum mechanics. Philosophical options for interpreting quantum mechanics have included variations of the belief that it is at best an incomplete description of a better-behaved classical underlying reality (Einstein), the Copenhagen interpretation according to which there are no objective unobserved events in the micro-world (Bohr and W. K. Heisenberg, 1901–76), an ‘acausal’ view of the collapse of the wave packet (J. von Neumann, 1903–57), and a ‘many worlds’ interpretation in which time forks perpetually towards innumerable futures, so that different states of the same system exist in different parallel universes (H. Everett).
In recent years the proliferation of subatomic particles (there are 36 kinds of quark alone, in six flavours and three colours) has prompted physicists to look in various directions for unification. One avenue of approach is superstring theory, in which the four-dimensional world is thought of as the upshot of the collapse of a ten-dimensional world, with the four primary physical forces (gravity, electromagnetism, and the strong and weak nuclear forces) becoming seen as the result of the fracture of one primary force. While the scientific acceptability of such theories is a matter for physics, their ultimate intelligibility plainly requires some philosophical reflection. See also Bell's theorem.

Philosophy dictionary. . 2011.

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