Physics/Quantum

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Imported from Wikipedia on 2009-02-22

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale (at the atomic level). Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certainties. Classical physics can be derived as a good approximation to quantum physics, typically in circumstances with large numbers of particles.

Interpretation of quantum mechanics

Imported from Wikipedia on 2009-04-10

An interpretation of quantum mechanics is a statement which attempts to explain how quantum mechanics informs our understanding of nature. Although quantum mechanics has received thorough experimental testing, many of these experiments are open to different interpretations. There exist a number of contending schools of thought, differing over whether quantum mechanics can be understood to be deterministic, which elements of quantum mechanics can be considered "real", and other matters.

The most common interpretations are summarized here (however, the assignment of values in the table is not without controversy, for the precise meanings of some of the concepts involved are unclear and, in fact, the subject of the very controversy itself):

No experimental evidence exists that would distinguish between the interpretations listed. To that extent, the physical theory stands, and is consistent, with itself and with reality; troubles come only when one attempts to "interpret" it. Nevertheless, there is active research in attempting to come up with experimental tests which would allow differences between the interpretations to be experimentally tested.

Interpretation	Deterministic?	Wavefunction real?	Unique history?	Hidden variables?	Collapsing wavefunctions?	Observer role?
Copenhagen interpretation (Waveform not real)	No	No	Yes	No	NA	NA
Ensemble interpretation (Waveform not real)	No	No	Yes	Agnostic	No	None
Copenhagen interpretation (Waveform real) Objective collapse theories	No	Yes	Yes	No	Yes	None
Consistent histories (Decoherent approach)	Agnostic1	Agnostic1	No	No	No	Interpretational2
Quantum logic	Agnostic	Agnostic	Yes3	No	No	Interpretational2
Many-worlds interpretation (Decoherent approach)	Yes	Yes	No	No	No	None
Stochastic mechanics	No	No	Yes	No	No	None
Many-minds interpretation	Yes	Yes	No	No	No	Interpretational4
Bohm-de Broglie interpretation ("Pilot-wave" approach)	Yes	Yes5	Yes6	Yes	No	None
Transactional interpretation	No	Yes	Yes	No	Yes7	None
Copenhagen interpretation (Waveform real) PAP	No	Yes	Yes	No	Yes	Causal
Relational Quantum Mechanics	No	Yes	Agnostic8	No	Yes9	None
Incomplete measurements	No	No10	Yes	No	Yes10	Interpretational2

1. If wavefunction is real then this becomes the many-worlds interpretation. If wavefunction less than real, but more than just information, then Zurek calls this the "existential interpretation".
2. Quantum mechanics is regarded as a way of predicting observations, or a theory of measurement.
3. But quantum logic is more limited in applicability than Coherent Histories.
4. Observers separate the universal wavefunction into orthogonal sets of experiences.
5. Both particle AND guiding wavefunction are real.
6. Unique particle history, but multiple wave histories.
7. In the TI the collapse of the state vector is interpreted as the completion of the transaction between emitter and absorber.
8. Comparing histories between systems in this interpretation has no well-defined meaning.
9. Any physical interaction is treated as a collapse event relative to the systems involved, not just macroscopic or conscious observers.
10. The nature and collapse of the wavefunction are derived, not axiomatic.

Each interpretation has many variants. It is difficult to get a precise definition of the Copenhagen interpretation. In the table above, two variants are shown: one that regards the waveform as being a tool for calculating probabilities only, and the other regards the waveform as an "element of reality".