Actually, it was you in your blog who mentioned the choice of histories depends on the "questions we ask". — Kumar 3 hours ago

That's disappointing because the purpose of this blog entry you mention - if I know which one it was - was exactly to clarify all questions such as the very question above that you asked on this server. — Luboš Motl 3 hours ago

Are you claiming a highly scrambled chain of projectors satisfying the consistency conditions isn't "objectively worse" than a "quasiclassical" chain? — Kumar 2 hours ago

Every set of histories (set of chains of projectors) satisfying the consistency conditions is an equally good starting point to ask physical questions. This statement is the key postulate of this whole formalism which gave it its name, too. It is hard to find solutions to the consistency constraints that are "highly scrambled". The intense scrambling makes the choice of right histories immensely sensitive to many details. But if you can find some, be my guest. By the laws of Nature, they are equally good. — Luboš Motl 2 hours ago

In your latest question+opinion_expressed, you nicely unmasked the widely spread misconception that QM is obliged "not to deviate from classical physics and its intuition too much". However, no such duty of quantum mechanics exists. For many questions we may formulate, QM will give quantitative answers that are close to those of classical physics. However, in many other respects, QM gives totally different answers, kills determinism, kills "objective constraint for everyone which questions they can ask". QM is simply an independent theory and it's the right one, unlike classical physics. — Luboš Motl 2 hours ago

On the contrary, Kent-Dowker showed that among consistent histories, highly scrambled chains are actually generic and outnumber "quasiclassical" ones. — Kumar 2 hours ago

On the contrary, Kent-Dowker showed that among consistent histories, highly scrambled chains are actually generic and outnumber "quasiclassical" ones. — Kumar 2 hours ago

The consistency with the classical limit means that if one formulates questions that may be formulated in classical physics as well - where the Mercury will be observed on the sky in 150 years - then QM will give the same predictions as classical physics, within error margins. But QM still allows one to prepare systems in superposition states that aren't allowed in classical physics, study interference of particles (or any other objects) with themselves, choose some commuting sets of observables (which make all other observables not commuting with them ill-defined) in many ways, and so on.. — Luboš Motl 2 hours ago

Kumar, I didn't say that they were not generic. Of course that they are generic if one counts them one-by-one. What I wrote was that it is hard to find them. They're contrived in this sense. The only easy enough way to "construct" them is to define them implicitly, "define the history in such a way that an evolution equation for parameters defining it are satisfied etc.". But for such choices of histories, one doesn't really know what question he's asking in advance, so they're not important. If there are questions one really cares about, they will be expressed by quasiclassical cons. hist. — Luboš Motl 2 hours ago

Find them... OK, with respect to which concrete algorithm is it hard to find them? You have to specify the kind of search algorithm. If you give me any specific random highly scrambled chain of projectors satisfying the consistency conditions, one can always come up with a post hoc algorithm which "finds" it. — Kumar 2 hours ago

But just because the generic set of consistent histories is scrambled, confusing, bizarre, and irrelevant for everyone who really cares what will happen, doesn't mean that they have to be "banned". They're allowed. ... When I said it was hard to find them, I meant it was hard to write the explicit form of the chain of projector for any single example. They're just not given by any explicit chains of projectors we know and we use. They're given by contrived implicit conditions and self-constraining laws. The histories correspond to contrived questions that no observer would normally ask. — Luboš Motl 2 hours ago

As I have written about 500 times already but you have still failed to notice, all sets of consistent histories are equally legit, whether you add the adverb "ontologically" or any other ill-defined adverb in front of it. If the word "ontology" is added with the purpose to ban the plurality of allowed sets of consistent histories, then the whole philosophy of "ontology" is just scientifically invalid. To one extent or another, people use the word "ontology" to declare that they will only accept classical physics at the fundamental level. But the world around us isn't classical. — Luboš Motl 2 hours ago

Are you claiming quantum mechanics admits no ontology? — Kumar 2 hours ago

There simply isn't any "one ontologically correct" set of consistent histories that would make every other set "worse", This is a fundamental delusion of yours about quantum mechanics. Consistent histories isn't the first place where quantum mechanics shows this "ambiguity" about which questions may be asked. The whole complementarity is about it. One may talk about position eigenstates or momentum eigenstates or other bases. None of these bases is "more true" than others. — Luboš Motl 2 hours ago

If by "ontology", you mean the assumption that one set of questions and/or consistent histories is objectively "more real" than others, then - as I have explained 500 times already - the discovery of quantum mechanics has proved that "ontology" was a pseudoscience much like phlogiston. The refusal of "ontology" in this sense is what defines a theory to be non-classical because this understanding of "ontology" is what defines a classical theory (one that admits an "objective state of everything"). Quantum mechanics is not a classical theory, so it doesn't obey this sort of "ontology". — Luboš Motl 2 hours ago

If there's no "ontologically preferred" set of consistent histories, then why are "quasiclassical" choices preferred? If "quasiclassical" choices aren't preferred, do we exist? — Kumar 2 hours ago

As I have mentioned many times above, they're not preferred by the theory. They may be preferred by various physicists for a simple reason – they're easy to be explicitly written down. This has been discussed above, too. We're clearly running in circles. — Luboš Motl 2 hours ago

If "quasiclassical" choices aren't preferred, do we exist? Only with respect to some "quasiclassical" set of consistent histories do we exist. With respect to generic Dowker-Kent choices, we can't be said to exist. Neither does this discussion. — Kumar 2 hours ago

Are you subscribing to a "many chains" interpretation of QM, kind of like how Everettians subscribe to "many worlds"? — Kumar 2 hours ago

As said 502 times in the discussion above, we don't exist (and this discussion doesn't exist) in the classical sense because classical physics, together with its interpretation of existence, is a wrong theory of physics. I exist for you because the existence of me has been a multiplicative factor in one of your projection operators in your set of consistent histories, you measured it, and you got a Yes=1 answer, so my existence became a fact according to your logical framework. There are others who didn't even ask the question whether Lubos Motl exists. Some questions are inconsistent with it. — Luboš Motl 35 mins ago

In your question about subscription, what do you mean by "many chains"? Clearly, this is not a legit concept of physics. I must speculate to find out what you mean by this bizarre new interpretation and the only interpretation I can find is that your "many chains" is the same thing as "many worlds" and all of them objectively exist. But the consistent histories approach isn't a many-worlds interpretation in any sense. It doesn't try to answer the totally unscientific question "what really exists" all the time. CH interpretation is a theory in science dealing with predictions of observations. — Luboš Motl 33 mins ago