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The best known example is the "paradox" of the Schrödinger's Cat : a cat is apparently evolving into a linear superposition of basis vectors that can be characterized as an "alive cat" and states that can be described as a "dead cat". Each of these possibilities is associated with a specific nonzero probability amplitude; the cat seems to be in a "mixed" state. However, a single particular observation of the cat does not measure the probabilities: it always finds either an alive cat, or a dead cat. After that measurement the cat stays alive or dead. The measurement problem is the question: ''how are the probabilities converted to an actual, sharply well-defined outcome?'' Different interpretations of quantum mechanics propose different solutions of the measurement problem.
While this viewpoint was sufficient to understand the outcome of all known experiments, it did not explain why it was legitimate to imagine that the cat's wavefunction collapses once the cat is observed, but it is not possible to collapse the wavefunction of the cat or the electron ''before'' it is measured. The collapse of the wavefunction used to be linked to one of two different properties of the measurement:
The latter approach was put on firm ground in the 1980s when the phenomenon of Quantum Decoherence was understood. The calculations of quantum decoherence allow the physicists to identify the fuzzy boundary between the quantum microworld and the world where the classical intuition is applicable. Quantum decoherence was proposed in the context of the Many-worlds Interpretation , but it has also become an important part of modern update of the Copenhagen interpretation that is based on Consistent Histories ("Copenhagen done right"). Quantum decoherence does not describe the actual process of the wavefunction collapse, but it explains the conversion of the quantum probabilities (that are able to Interfere ) to the ordinary classical probabilities. The Hugh Everett 's relative state interpretation, often inaccurately referred to as the Many-worlds Interpretation , attempts to avoid the problem by suggesting it is an illusion. Under this system there is only one wavefunction, the superposition of the entire universe, and it never collapses -- so there is no measurement problem. Instead the act of measurement is actually an interaction between two quantum entities, which entangle to form a single larger entity, for instance ''living cat/happy scientist''. Unfortunately Everett was never able to "close the loop", and demonstrate the way that this system would result in real-world measurements, ones in which the probabilistic nature of quantum mechanics could appear. The many-worlds interpretation is a development of Everett's that attempts to provide a model under which the system becomes "obvious". Everett's interpretation posits a single universal wavefunction, but with the added proviso that "reality" is defined as a single path in time through the superpositions. That is, "you" have a history that is made of the outcomes of measurements you made in the past, but there are many other "yous" with slight variations in history. Under this system our reality is one of many similar ones. The theory of Measurement In Quantum Mechanics , once the particle is observed, other wave-function channels remain empty and thus ineffective, but there is no true Wavefunction Collapse . Decoherence provides that this ineffectiveness is stable and irreversible, which explains the apparent Wavefunction Collapse . SEE ALSO REFERENCES |
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