Wigner's friend

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Wigner's friend is an extension of the thought experiment " Schrödinger's cat " by Eugene Paul Wigner and relates to the measurement problem of quantum mechanics. The example illustrates an idealistic or subjectivistic interpretation of quantum mechanics or a subsequent speculation to such. In the “Wigners-Freund” thought experiment, no decoherence effects are taken into account, which is why the argumentation based on it was rejected by Wigner himself in the 1970s and is considered obsolete in today's scientific and philosophical discourse.

Basics and thought experiment

The axioms of quantum mechanics postulate in current versions that quantum mechanical systems are in a so-called superposition state up to the point in time of a measurement (in the vast majority of cases) . As soon as we measure, however, we do not “see” a fuzzy “superimposed” value, but an exact discrete state value. Explaining this is one of the most important challenges for interpretations of quantum mechanics, the so-called quantum mechanical measurement problem . Various common treatment methods now identify the point in time of the measurement with the point in time at which the system changes from an " entangled " superposition state to a "reduced" unambiguous state .

Wigner's thought experiment is intended to illustrate a somewhat different view . Assume that not only a cat, but also an observer (Wigner's friend) is in a system that either triggers the death of the cat by means of a quantum experiment or does not. At the latest as soon as an external observer (Wigner) learns the measurement result from him, there is a clear description of the measured system status - that much seems beyond dispute. It is unclear, however, whether the reduction occurred earlier - namely when Wigner's friend (consciously) saw it. If there were a material device instead of Wigner's friend, its condition would be entangled with that of the cat and the (quantum mechanical) trigger before it died. Such a device would be in a superposition state itself. From the perspective of the second observer, however, the reduction does not take place until later. Since both observers are of the same kind, this is paradoxical.

Philosophical discussion

Wigner himself concluded from this that it is the non-material consciousness of the observer that distinguishes the observer from the material world (thus at least represented an ontological dualism : there is at least one other type of being besides material, if not idealism : everything is immaterial ).

According to Wigner, this boundary between the material and the immaterial would be the boundary between quantum mechanics and classical mechanics , which is called the Heisenberg cut . Such a limit is usually not formulated, but material systems and “conscious observers” are treated the same in principle. However, since the measurement problem is simply kept open and the conscious observer is at least the last station between superposition and reduction and some theorists already have dualistic or idealistic or constructivistic ideas, since von Neumann some interpreters have given consciousness a constitutive role for the quantum collapse or . assigned the generation of reality in general . A well-known representative of such theories is, for example, Henry Stapp .

More common views tend to explain reduction or decoherence through interaction with macroscopic objects . As long as the term “macroscopic” can be physically specified (this is controversial), it seems - as critics of theories of “collapse qua consciousness” object - to avoid importing extra-physical components into quantum mechanics. In addition, so many critics, the physically imprecise term “consciousness” is notoriously unclear, for example with regard to the criteria for when consciousness is present.

The paradox outlined could apparently also simply be avoided if the thesis is abandoned that the reduction in the system state only takes place at the time of notification. In certain idealistic interpretations, however, this should not happen. Instead, alternative ontologies and epistemologies are proposed to analyze the example and to use it to illustrate an idealistic theory.

An extreme example is the constructivist systems theorist John L. Casti . For the second observer, the first observer, like the cat, belongs to the system of the wave function . For Wigner, the whole world is part of it. As a result, consciousness is assigned the decisive role in reducing the wave function. Conversely, "the things in the world" out there "are little more than useful constructions ". A real world independent of consciousness does not exist at all. Speculations like that of Casti have so far found little support among experts in the philosophy of physics . But they have similarities to other statements by constructivist theorists on the philosophy of quantum mechanics.

Perhaps closest to Wigner's interpretation and constructivist conceptions is the class of relativistic interpretations. According to this, the description of a system state is relative to the descriptive system, analogous to the fact that space and time concepts are relative to the motion (and gravitation ) of the descriptive system (an analogy to the theory of relativity ). It is unclear how exactly this analogy should be worked out. According to one variant of these theories (as represented by Rovelli, for example), objectivity only occurs when descriptive systems interact.

Among the various realistic interpretations of quantum mechanics, Everett's Many Worlds Interpretation deserves a mention. The different system states are therefore not realized superimposed in this world, but distributed over different worlds. As soon as we measure, we learn which of them is our world. One of the problems of this interpretation is the explanation of the probability assignment for the expected measurement results. Wigner's interpretation also seems to have this problem.

The many minds theories are a complicated hybrid . According to these, the different system states are distributed among different consciousnesses (one and the same person). These theories have more realistic features, but have some similarities with Wigner's theoretical sketch, "Collapse qua consciousness" and some relativistic theories.

Wigner scenario seems more likely to be non-realistic invite responses. From the class of interpretations of a generally non-realistic type, van Fraassen's well-elaborated conception is worth mentioning, which is based less on the special status of consciousness than on more general, non-realistic scientific-theoretical foundations.

Fiction

  • In Stephen Baxter's science fiction book Time Like Infinity (dt. The weave of infinity ) there is a sect , the "Friends of Wigner" called and justifies destructive with their interpretation of the paradox described acts.

Individual evidence

  1. ↑ “In fact, Wigner later abandoned his views on the special role of consciousness in quantum measurement once he became aware of Zeh's paper of 1970.” Quotation from M. Schlosshauer: Decoherence and the Quantum-to-Classical Transition. Springer, 2007, ISBN 978-3-540-35773-5 , p. 365.
  2. ↑ Today, Wigner's conjecture about the role of the mind in the quantum measurement process is no longer part of physics, but rather part of the history of physics. from: O. Freire: Orthodoxy and Heterodoxy in the Research on the Foundations of Quantum Physics: EP Wigner's Case. In: B. de Sousa Santos (Ed.): Cognitive justice in a global world: prudent knowledges for a decent life. Lexington Books, 2007, p. 221. arxiv : physics / 0602028 .
  3. ↑ As Wigner himself describes the experiment, it is about a photon that hits the eye of Wigner's friend. Because it is a complication of Schrödinger's cat and it becomes more illustrative with it, the latter is often put in the laboratory of Wigner's friend.
  4. ↑ Also Walter Heitler , Fritz London , Fred Alan Wolf , William A. Tiller , John Hagelin , Stuart Hameroff , Bernard Baars , Amit Goswami , Russell Targ, Nick Herbert, Jeffrey M. Schwartz, Menas Kafatos, Keith Ward and others have such perceptions represented. See also the link selection in the English Wikipedia.
  5. ↑ John L. Casti : Loss of Truth, -Streitfragen der Naturwissenschaften- , Munich 1990, p. 549.
  6. ↑ See, for example, Merriam; Rovelli and Laudisa give an overview; see. also the literature there.
  7. ↑ See, inter alia, H. Putnam: A Philosopher looks at quantum mechanics.

literature

  • David Z Albert, Hilary Putnam : Further adventures of Wigner's friend. In: Topoi. 14, 1995, pp. 17-22.
  • J. Barrett: The Quantum Mechanics of Minds and Worlds. Oxford UP, 2001, ISBN 0-19-924743-9 , pp. 227-232.
  • D. Mayr: Comment on Putnam's 'Quantum mechanics and the observer'. In: Knowledge. 16, 1981.
  • Paul Merriam ,: On the Relativity of Quantum Superpositions. ( Memento of August 7, 2008 in the Internet Archive ) 1997.
  • Paul Merriam: Quantum Relativity: Physical Laws Must be Invariant Over Quantum Systems. In: Physics Essays. 2005. arxiv : quant-ph / 0506228
  • Hilary Putnam: Quantum Mechanics and the Observer. In: Knowledge. 16, 1981, pp. 193-219. (Reprinted in: Realism and Reason. (= Philosophical Papers. Volume 3). Cambridge University Press, Cambridge 1983, pp. 248–270)
  • Hilary Putnam: A Philosopher Looks at Quantum Mechanics. In: Robert G. Colodny (Ed.): Beyond the Edge of Certainty: Essays in Contemporary Science and Philosophy. Prentice-Hall, Englewood Cliffs, NJ 1965, pp. 75-101. (Reprinted in Hilary Putnam: Mathematics, Matter and Method. Cambridge University Press, Cambridge, Mass. 1975, pp. 130–158)
  • C. Rovelli, F Laudisa: Relational Quantum Mechanics Entry in Edward N. Zalta (Ed.): Stanford Encyclopedia of Philosophy . Template: SEP / Maintenance / Parameter 1 and neither parameter 2 nor parameter 32005.
  • H.-J. Treder: Schrödinger's cat and Wigner's friend (on the mobility of the Heisenberg cut). In: Annals of Physics. vol. 500, Issue 3, 1988, pp. 255-256.
  • B. van Fraassen: The Charybdis of Realism: Epistemological Implications of Bell's Inequality. In: Synthesis. 52, 1982, pp. 25-38.
  • EP Wigner: Remarks on the Mind-Body Question. In: IJ Good (Ed.): The Scientist Speculates. 1961, pp. 284-302. (Reprinted in: EP Wigner: Symmetries and Reflections. Bloomington, Indiana 1967)