Wigner's friend
Wigner's friend is an extension of the thought experiment " Schrödinger's Cat " by Eugene Paul Wigner and refers to the measurement problem in quantum mechanics. The example illustrates an idealist or subjectivist interpretation of quantum mechanics and a succession speculation on such a. In the " Wigner's friend " thought experiment no decoherence effects are taken into account, which is why that build reasoning of Wigner itself was discarded in the 1970s and is in today's scientific and philosophical discourse as outdated.
Fundamentals and thought experiment
The axioms of quantum mechanics postulate in popular versions that quantum mechanical systems are up to the time of a measurement ( in most cases ) in a so-called superposition state. Once we measure we "see" but not blurred "overlay " value, but an exact discrete state value. This explain is one of the most important challenges for interpretations of quantum mechanics, called quantum mechanical measurement problem. Various conventional methods of treatment now identify the time of measurement with the time at which the system transitions from an "entangled " state of superposition in a "reduced" unique state.
A somewhat different view is to illustrate Wigner's thought experiment. Suppose not just a cat, but also an observer ( Wigner's friend ) is located in a facility which is the cat's death triggered by quantum experiment or does not. At the latest, as soon as an external observer ( Wigner ) learns the measurement result of it, there is a clear description of the measured system state - so much seems indisputable. It is unclear, however, whether the reduction previously occurred - namely, as Wigner's friend saw them (consciously ). If in fact are present instead of Wigner's friend a material device, the state would be entangled with that of the cat and the ( quantum mechanical ) release of their death. Such a device would be even in a superposition state. From the perspective of the second observer, the reduction takes place but only later. Since both observers are alike, this is paradoxical.
Philosophical discussion
Wigner himself concludes that it is the non-physical consciousness of the observer, that this is different from the material world (that represents at least an ontological dualism: there is at least one other type of entities in addition to material, if not idealism: all is immaterial ).
This boundary between tangible and intangible would be by Wigner those very limit of quantum mechanics to classical mechanics, which is called the Heisenberg cut. Such a limit is usually not formulated, but material systems and " conscious observer " treated basically the same. However, since the measurement problem is thus simply kept open and the conscious observer, at least the last stop between superposition and reduction and some theorists have anyway dualist or idealist or constructivist ideas, failed since von Neumann or a constitutive role in quantum collapse of some performers consciousness at all. assigned the production of reality. A well-known representatives of such theories, for example, Henry Stapp.
More common views are directed towards explaining about interaction with macroscopic objects reduction or decoherence. Where the expression " macroscopic" physically precisified (this is controversial ), so that seems - as critics of theories of " collapse qua consciousness" object - prevents except physical components are imported into quantum mechanics. In addition, is, according to many critics, the physically präzisierbare term " consciousness" notoriously unclear, for example, with regard to the criteria for determining when consciousness is present.
The described paradox could be obviously also avoid simply, if the thesis is submitted that the reduction of the system state does not occur until the time of notification. However, this should not happen in certain idealistic interpretations straight. Instead, alternative ontologies and epistemologies are proposed to analyze the example and to illustrate to him an idealistic theory.
An extreme example is the constructivist theorist John L. Casti system. For the second observer, the first observer belong just like the cat with the system of the wave function. For Wigner, the entire world is one of them. This is here the consciousness of the crucial role assigned to the reduction of the wave function. Conversely, " the things of the world " out there "is not much more than useful constructions ". A real world independent of consciousness did not exist. Speculation as that Castis have not yet found little support among experts in the philosophy of physics. But you have similarities to other positions of constructivist theorists to the philosophy of quantum mechanics.
Wigner's interpretation and constructivist conceptions closest perhaps is the class of relativistic interpretations. After that is the description of a system state relative to the descriptive system, analogous to that space and time concepts relative to the movement ( and gravitational ) of the descriptive system are ( a analogy to relativity theory). It is unclear exactly how this analogy is elaborated. Objectivity occurs as a variant of these theories (which represents about Rovelli ) one only when descriptive systems interact.
Among the various realist interpretations of quantum mechanics deserves Everett's many-worlds interpretation mention. The different system states are therefore not realized superimposed in this world, but distributed to different worlds. When we measure, we learn what it is our world. One of the problems of this interpretation is the explanation of the probability assignment for expected results. This problem seems to have Wigner's interpretation.
A complicated hybrid of the many minds theory. According to these different system states are different consciousnesses ( one and the same people ) distributed. These theories contribute more realistic features, but have some similarities with the theory sketch Wigner, " collapse qua consciousness" and some relativistic theories.
Wigner scenario seems more inviting to non- realistic answers. From the class of interpretations generally non- realistic type the elaborate opinion Fraassens van is worth, the less on the special status of consciousness is based on a general non- realistic epistemological foundations.
Fiction
- In Stephen Baxter's science fiction book Time Like Infinity (Eng. The braid of infinity ), there is a sect, " Wigner's friend " calls itself and with its interpretation of the paradox described justifies destructive deeds.