
Imported from Wikipedia on 20090222
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 wavelike and particlelike 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.
Imported from Wikipedia on 20090410
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) 
Agnostic^{1}  Agnostic^{1}  No  No  No  Interpretational^{2} 
Quantum logic  Agnostic  Agnostic  Yes^{3}  No  No  Interpretational^{2} 
Manyworlds interpretation (Decoherent approach) 
Yes  Yes  No  No  No  None 
Stochastic mechanics  No  No  Yes  No  No  None 
Manyminds interpretation  Yes  Yes  No  No  No  Interpretational^{4} 
Bohmde Broglie interpretation ("Pilotwave" approach) 
Yes  Yes^{5}  Yes^{6}  Yes  No  None 
Transactional interpretation  No  Yes  Yes  No  Yes^{7}  None 
Copenhagen interpretation (Waveform real) PAP 
No  Yes  Yes  No  Yes  Causal 
Relational Quantum Mechanics 
No  Yes  Agnostic^{8}  No  Yes^{9}  None 
Incomplete measurements 
No  No^{10}  Yes  No  Yes^{10}  Interpretational^{2} 
 If wavefunction is real then this becomes the manyworlds interpretation. If wavefunction less than real, but more than just information, then Zurek calls this the "existential interpretation".
 Quantum mechanics is regarded as a way of predicting observations, or a theory of measurement.
 But quantum logic is more limited in applicability than Coherent Histories.
 Observers separate the universal wavefunction into orthogonal sets of experiences.
 Both particle AND guiding wavefunction are real.
 Unique particle history, but multiple wave histories.
 In the TI the collapse of the state vector is interpreted as the completion of the transaction between emitter and absorber.
 Comparing histories between systems in this interpretation has no welldefined meaning.
 Any physical interaction is treated as a collapse event relative to the systems involved, not just macroscopic or conscious observers.
 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".
