Editing Consistent histories (section)

==Interpretation==

The interpretation based on consistent histories is used in combination with the insights about [[quantum decoherence]]. 
Quantum decoherence implies that irreversible macroscopic phenomena (hence, all classical measurements) render histories automatically consistent, which allows one to recover classical reasoning and "common sense" when applied to the outcomes of these measurements. More precise analysis of decoherence allows (in principle) a quantitative calculation of the boundary between the classical domain and the quantum domain. According to [[Roland Omnès]],<ref name="Omnès1999">{{cite book |first=Roland |last=Omnès |year=1999 |title=Understanding Quantum Mechanics |publisher=Princeton University Press |isbn=978-0-691-00435-8 |lccn=98042442 |url=https://archive.org/details/understandingqua00omne |url-access=registration |pages=[https://archive.org/details/understandingqua00omne/page/179 179], 257}}</ref>
{{blockquote|[the] history approach, although it was initially independent of the Copenhagen approach, is in some sense a more elaborate version of it. It has, of course, the advantage of being more precise, of including classical physics, and of providing an explicit logical framework for indisputable proofs. But, when the Copenhagen interpretation is completed by the modern results about correspondence and decoherence, it essentially amounts to the same physics.

[... There are] three main differences:

1. The logical equivalence between an empirical datum, which is a macroscopic phenomenon, and the result of a measurement, which is a quantum property, becomes clearer in the new approach, whereas it remained mostly tacit and questionable in the Copenhagen formulation.

2. There are two apparently distinct notions of probability in the new approach. One is abstract and directed toward logic, whereas the other is empirical and expresses the randomness of measurements. We need to understand their relation and why they coincide with the empirical notion entering into the Copenhagen rules.

3. The main difference lies in the meaning of the reduction rule for 'wave packet collapse'. In the new approach, the rule is valid but no specific effect on the measured object can be held responsible for it. Decoherence in the measuring device is enough.}}

In order to obtain a complete theory, the formal rules above must be supplemented with a particular [[Hilbert space]] and rules that govern dynamics, for example a [[Hamiltonian (quantum theory)|Hamiltonian]].

In the opinion of others<ref>{{cite journal | last1=Kent | first1=Adrian | last2=McElwaine | first2=Jim | title=Quantum prediction algorithms | journal=Physical Review A | volume=55 | issue=3 | date=1997-03-01 | issn=1050-2947 | doi=10.1103/physreva.55.1703 | pages=1703–1720| arxiv=gr-qc/9610028 | bibcode=1997PhRvA..55.1703K | s2cid=17821433 }}</ref> this still does not make a complete theory as no predictions are possible about which set of consistent histories will actually occur. In other words, the rules of consistent histories, the [[Hilbert space]], and the Hamiltonian must be supplemented by a set selection rule. However, [[Robert B. Griffiths]] holds the opinion that asking the question of which set of histories will "actually occur" is a misinterpretation of the theory;<ref>{{cite book |first=R. B. |last=Griffiths |title=Consistent Quantum Theory |publisher=Cambridge University Press |year=2003 }}</ref> histories are a tool for description of reality, not separate alternate realities.

Proponents of this consistent histories interpretation—such as [[Murray Gell-Mann]], [[James Hartle]], Roland Omnès and Robert B. Griffiths—argue that their interpretation clarifies the fundamental disadvantages of the old Copenhagen interpretation, and can be used as a complete interpretational framework for quantum mechanics.

In ''[[Quantum Philosophy]]'',<ref>R. Omnès, ''[[Quantum Philosophy]]'', Princeton University Press, 1999. See part III, especially Chapter IX</ref> Roland Omnès provides a less mathematical way of understanding  this same formalism.

The consistent histories approach can be interpreted as a way of understanding which sets of classical questions can be consistently asked of a single quantum system, and which sets of questions are fundamentally inconsistent, and thus meaningless when asked together. It thus becomes possible to demonstrate formally why it is that the questions which [[EPR Paradox|Einstein, Podolsky and Rosen]] assumed could be asked together, of a single quantum system, simply cannot be asked together. On the other hand, it also becomes possible to demonstrate that classical, logical reasoning often does apply, even to quantum experiments – but we can now be mathematically exact about the limits of classical logic.