Philosophers
Mortimer Adler Rogers Albritton Alexander of Aphrodisias Samuel Alexander William Alston Anaximander G.E.M.Anscombe Anselm Louise Antony Thomas Aquinas Aristotle David Armstrong Harald Atmanspacher Robert Audi Augustine J.L.Austin A.J.Ayer Alexander Bain Mark Balaguer Jeffrey Barrett William Belsham Henri Bergson George Berkeley Isaiah Berlin Richard J. Bernstein Bernard Berofsky Robert Bishop Max Black Susanne Bobzien Emil du BoisReymond Hilary Bok Laurence BonJour George Boole Émile Boutroux F.H.Bradley C.D.Broad Michael Burke Lawrence Cahoone C.A.Campbell Joseph Keim Campbell Rudolf Carnap Carneades Ernst Cassirer David Chalmers Roderick Chisholm Chrysippus Cicero Randolph Clarke Samuel Clarke Anthony Collins Antonella Corradini Diodorus Cronus Jonathan Dancy Donald Davidson Mario De Caro Democritus Daniel Dennett Jacques Derrida René Descartes Richard Double Fred Dretske John Dupré John Earman Laura Waddell Ekstrom Epictetus Epicurus Herbert Feigl Arthur Fine John Martin Fischer Owen Flanagan Luciano Floridi Philippa Foot Alfred Fouilleé Harry Frankfurt Richard L. Franklin Michael Frede Gottlob Frege Peter Geach Edmund Gettier Carl Ginet Alvin Goldman Gorgias Nicholas St. John Green H.Paul Grice Ian Hacking Ishtiyaque Haji Stuart Hampshire W.F.R.Hardie Sam Harris William Hasker R.M.Hare Georg W.F. Hegel Martin Heidegger Heraclitus R.E.Hobart Thomas Hobbes David Hodgson Shadsworth Hodgson Baron d'Holbach Ted Honderich Pamela Huby David Hume Ferenc Huoranszki William James Lord Kames Robert Kane Immanuel Kant Tomis Kapitan Jaegwon Kim William King Hilary Kornblith Christine Korsgaard Saul Kripke Andrea Lavazza Christoph Lehner Keith Lehrer Gottfried Leibniz Jules Lequyer Leucippus Michael Levin George Henry Lewes C.I.Lewis David Lewis Peter Lipton C. Lloyd Morgan John Locke Michael Lockwood E. Jonathan Lowe John R. Lucas Lucretius Alasdair MacIntyre Ruth Barcan Marcus James Martineau Storrs McCall Hugh McCann Colin McGinn Michael McKenna Brian McLaughlin John McTaggart Paul E. Meehl Uwe Meixner Alfred Mele Trenton Merricks John Stuart Mill Dickinson Miller G.E.Moore Thomas Nagel Otto Neurath Friedrich Nietzsche John Norton P.H.NowellSmith Robert Nozick William of Ockham Timothy O'Connor Parmenides David F. Pears Charles Sanders Peirce Derk Pereboom Steven Pinker Plato Karl Popper Porphyry Huw Price H.A.Prichard Protagoras Hilary Putnam Willard van Orman Quine Frank Ramsey Ayn Rand Michael Rea Thomas Reid Charles Renouvier Nicholas Rescher C.W.Rietdijk Richard Rorty Josiah Royce Bertrand Russell Paul Russell Gilbert Ryle JeanPaul Sartre Kenneth Sayre T.M.Scanlon Moritz Schlick Arthur Schopenhauer John Searle Wilfrid Sellars Alan Sidelle Ted Sider Henry Sidgwick Walter SinnottArmstrong J.J.C.Smart Saul Smilansky Michael Smith Baruch Spinoza L. Susan Stebbing Isabelle Stengers George F. Stout Galen Strawson Peter Strawson Eleonore Stump Francisco Suárez Richard Taylor Kevin Timpe Mark Twain Peter Unger Peter van Inwagen Manuel Vargas John Venn Kadri Vihvelin Voltaire G.H. von Wright David Foster Wallace R. Jay Wallace W.G.Ward Ted Warfield Roy Weatherford C.F. von Weizsäcker William Whewell Alfred North Whitehead David Widerker David Wiggins Bernard Williams Timothy Williamson Ludwig Wittgenstein Susan Wolf Scientists Michael Arbib Walter Baade Bernard Baars Leslie Ballentine Gregory Bateson John S. Bell Mara Beller Charles Bennett Ludwig von Bertalanffy Susan Blackmore Margaret Boden David Bohm Niels Bohr Ludwig Boltzmann Emile Borel Max Born Satyendra Nath Bose Walther Bothe Hans Briegel Leon Brillouin Stephen Brush Henry Thomas Buckle S. H. Burbury Donald Campbell Anthony Cashmore Eric Chaisson JeanPierre Changeux Arthur Holly Compton John Conway John Cramer E. P. Culverwell Olivier Darrigol Charles Darwin Richard Dawkins Terrence Deacon Lüder Deecke Richard Dedekind Louis de Broglie Max Delbrück Abraham de Moivre Paul Dirac Hans Driesch John Eccles Arthur Stanley Eddington Gerald Edelman Paul Ehrenfest Albert Einstein Hugh Everett, III Franz Exner Richard Feynman R. A. Fisher Joseph Fourier Philipp Frank Lila Gatlin Michael Gazzaniga GianCarlo Ghirardi J. Willard Gibbs Nicolas Gisin Paul Glimcher Thomas Gold A.O.Gomes Brian Goodwin Joshua Greene Jacques Hadamard Mark Hadley Patrick Haggard Stuart Hameroff Augustin Hamon Sam Harris Hyman Hartman JohnDylan Haynes Donald Hebb Martin Heisenberg Werner Heisenberg John Herschel Art Hobson Jesper Hoffmeyer E. T. Jaynes William Stanley Jevons Roman Jakobson Pascual Jordan Ruth E. Kastner Stuart Kauffman Martin J. Klein Simon Kochen Hans Kornhuber Stephen Kosslyn Ladislav Kovàč Leopold Kronecker Rolf Landauer Alfred Landé PierreSimon Laplace David Layzer Benjamin Libet Seth Lloyd Hendrik Lorentz Josef Loschmidt Ernst Mach Donald MacKay Henry Margenau James Clerk Maxwell Ernst Mayr John McCarthy Ulrich Mohrhoff Jacques Monod Emmy Noether Abraham Pais Howard Pattee Wolfgang Pauli Massimo Pauri Roger Penrose Steven Pinker Colin Pittendrigh Max Planck Susan Pockett Henri Poincaré Daniel Pollen Ilya Prigogine Hans Primas Adolphe Quételet Jürgen Renn/a> Juan Roederer Jerome Rothstein David Ruelle Tilman Sauer Erwin Schrödinger Aaron Schurger Claude Shannon David Shiang Herbert Simon Dean Keith Simonton B. F. Skinner Lee Smolin Roger Sperry John Stachel Henry Stapp Tom Stonier Antoine Suarez Leo Szilard Max Tegmark William Thomson (Kelvin) Giulio Tononi Peter Tse Vlatko Vedral Heinz von Foerster John von Neumann John B. Watson Daniel Wegner Steven Weinberg Paul A. Weiss John Wheeler Wilhelm Wien Norbert Wiener Eugene Wigner E. O. Wilson H. Dieter Zeh Ernst Zermelo Wojciech Zurek Konrad Zuse Fritz Zwicky Presentations Biosemiotics Free Will Mental Causation James Symposium 
Decoherence
Decoherence is a broad explanation for the lack of uniquely quantum effects in macroscopic objects. Decoherence theorists say in particular that it explains the absence of superpositions of live and dead Schrödinger Cats.
The "decoherence program" of H. Dieter Zeh, Wojciech Zurek, and their colleagues is an attempt to describe the "appearance" or "emergence" of a classical world from the microscopic quantum world. Decoherence theorists trace the emergence of classical properties to the interaction of quantum systems with the environment, generally the exchange of photons between quantum systems and the environment, but also the collisions of particles (which are mediated by virtual photons). No physical system is ever completely isolated from the environment. A perfectly isolated system is by definition unobservable and thus of no interest to an experimental physicist, though theoreticians often work with such ideas. Although isolation is one of the fundamental principles of all physics experiments, it is the case that it is practically impossible to prevent highenergy particles and photons from passing through any experiment.
Denials of Standard Quantum Physics
Most decoherence theorists subscribe to what they call a "universally valid quantum theory." Despite the name, this theory denies one of the basic hypotheses of standard quantum physics, namely the collapse of the quantummechanical wave function (Dirac's projection postulate), which is the ultimate source of chance, indeterminism, free will, and creativity.
"Universally valid" refers to the WheelerEverettDeWittWigner view that replaces the wavefunction collapse with a splitting of a universal wave function Ψ into separate branches or components, each of which contains all the material of the universe just before the quantum event, typically a quantum measurement. Where standard quantum theory predicts the probabilities of the measurement yielding two or more eigenvalues of the physical observable being measured, the "Many Worlds" theory assumes that new worlds are created with each world realizing one of those eigenvalues, all other things about the new worlds being the same as before the measurement. John Bell described the manyworlds theory as "extravagant." We find it extremely so, since it blatantly violates the most fundamental conservation laws of physics. In order to create another parallel universe, it must double the amount of energy, mass, charge, etc. And the new universe must be as large as our observable universe. All this because they find the idea of the collapse of the wave function nonintuitive, which in some respects it is. But in other respects it is simply the actualization of a single outcome from among many alternative possibilities. Some of the decoherence theorists appear to share a dislike of indeterminism, exemplified by Albert Einstein's famous dictum "God does not play dice." Einstein objected to determinism, even more strongly, he objected in his famous EPR paper to the nonlocal character of quantum reality, which suggested to him that "influences" are traveling faster than the speed of light, violating his theory of special relativity. And if Einstein disliked the measurement of one quantum particle instantly altering the properties of another particle a significant distance away (in this universe), what would he have thought about duplicating the entire observable universe contents in an unobservable parallel branch of the universal wave function Ψ?
Decoherence in Standard Quantum Physics
Decoherence can be separated from the "many worlds" and "nocollapse" theories. The core idea is that classical macroscopic properties depend on decoherence of quantum properties, especially the interference of different components of a coherent quantum system. We can endorse that view and show that it is not classical properties that "emerge" under conditions of decoherence, but quantum properties that show up when we look at sufficiently isolated systems small enough to exhibit coherence.
Paul Dirac described the breakdown of classical mechanics as "an inadequacy of its concepts to supply us with a description of atomic events." (Dirac, p.3) The early Greek philosophers Democritus, Leucippus, and Epicurus argued that large objects were made from smaller objects, but there comes a size when something is absolutely small. Quantum mechanics defines the absolutely small as objects that cannot be seen without disturbing them. Decoherence says that the very act of looking at a quantum system destroys the coherence that reflects its fundamental quantum nature. Dirac defined an object to be "big" when the disturbance accompanying our observation of it may be neglected, and "small" when the disturbance cannot be neglected. There comes a size when every attempt to minimize the disturbance fails. Dirac says "there is a limit to the fineness of our powers of observation and the smallness of the accompanying disturbance  a limit which is inherent in the nature of things and can never be surpassed by improved techniques or skill on the part of the observer.." An important consequence of absolute smallness is that we must revise our idea of causality. If a system is small, we cannot observe it without producing a serious disturbance and hence we cannot expect to find causal and deterministic connections connections between our observations. There is an unavoidable indeterminacy in measurement of quantum systems, so that we can only calculate the probability of various possible measurements. Under pressure from Niels Bohr, Werner Heisenberg modified his original idea that indeterminacy is always a result of observations (see Heisenberg's Microscope). Indeterminacy is an intrinsic property of quantum objects in space and time and does not depend on observations by conscious observers (with Ph.D.'s, as John Bell quipped). Note that quantum systems can make informationgenerating measurements on themselves, which the decoherence theorists accept.
A TwoState Example of Coherence and Decoherence
Consider the famous TwoSlit Experiment.
We can label the probabilityamplitude wave function passing through the left hand slit in the figure ψ_{left} and the waves passing through the righthand slit ψ_{right}. These are coherent and show the characteristic quantum interference fringes on the detector screen (a photographic plate or CCD array). This is the case even if the intensity of particles is so low that only one particle at a time arrives at the screen. In a dramatic experimental proof of decoherence, Gerhard Rempe sent matter waves of heavy Rubidium atoms through two slits. He then irradiated the left slit with microwaves that could excite the hyperfine structure in Rb atoms passing through that slit. As he turned up the intensity, the interference fringes diminished in proportion to the number of photons falling on the left slit. The photons decohere the otherwise coherent wave functions.
The QuantumClassical "Boundary"
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