They don’t use those words, lib, but either imply that we should not even consider alternatives or that I am trying to give the possibilities I mention the same weight as current theories.
So if the wave is not “real” how does it interfere with itself in something like the double-slit experiement?
To answer your question, here is an article from AAAS describing a quantum entanglement experiment where the detectors are seperated by a distance of 600 meters, and here is one involving a different experment where the particles are seperated by 144 kilometers. Note that, other than verifying the apparent nonlocal relationship between the two particles, this doesn’t prove, or even give any greater plausibility to one interpretation or another. The two slit experiment, by definition, has to be done locally–if you seperate the slits by a difference much, the distributions from each slit don’t overlap significantly, and the result is a purely classical distribution with no interference patterns. You can seperate the paths the photon can take by using an interferometer (using waveguides to give the photon different paths but still resulting in an interference pattern) and put your detector anywhere along the path, getting the same result. I suppose you could talk about the maximum distance between the two paths, but I don’t know that this is useful, since you can place a single detector anywhere along either of the paths and get the same effect–a distribution of classical appearance due to “collapsing the waveform” before it has a chance to “self-interfere.” Regardless, this “collapse” takes place (or appears to) at points that are not locally connected; that is, there can be no intervening exchange of information between the two paths, so “spooky action at a distance”, or some other ontological mechanism must cause this choice.
You’re assuming that these interpretations–Copenhagen, Many Worlds, Bohm, et cetera–are theories, but in fact they are not; the net effect of all of them is to posit a set of underlying actions which give the exact same result when we interact with the particle, which means we can’t distinguish between one and another. By definition we can’t “see” a waveform before it collapses, because the act of “seeing” it causes it to collapse. This is clearly a tautological definition of “waveform collapse”, which is therefore capable of neither proof or disproof, and why I keep saying that it’s a mathematical formalism with no indication that it means anything in a material sense. Similiarly, we can talk about nonlocal hidden variables (preconditions that the particles have all “agreed to” which lay out what they’ll do when the time comes) but there’s no way to read these conditions directly; we can only infer them from the result of an experiment, which gives us exactly the same information as the whole “collapsing waveform” business, or multiple worlds, or whatever. The only theory involved in this is the overall probability distribution of the Schrödinger equation, or equivilent (and more useful) matrix mechanics formulation, or any other way of accurately describing the results we see from interactions on the quantum level.
So there’s no invoking Interpreation A as being more likely than Interpretation B, because there’s no practical way to differentiate between them. This sounds kind of stupid because they’re clearly very, very different descriptions of what might be going on, but in fact if the only observable results are the same then you can’t say anything useful about which one is more likely than the other. As an analogy, consider a tan doe in a cage with two buck hares, one dark brown and the other white. If we put a cover of the cage and come back and find her pregant, we might think we can tell which buck is the sire by the color of the children. However, when the doe births, we get a litter of kittens that are partially brown, partially white, and partially tan, which means that either, or even perhaps both, bucks sired the offspring. Since our lab isn’t equipped with tools to perform any kind of genetic analysis to distinguish between the bucks, we’re left with assuming that either one has equal claim on being the sire.
Note that the Copenhagen interpretation, at least as presented by Heisenberg, Bohr, et al doesn’t make any claims to the physicality of either the probability waveform or the action of it collapsing. In fact, they’d say that we don’t know, can’t tell, and it doesn’t matter. There is, however, a conceptual problem with your “expanding wavefront” notion; it assumes a priviledged position for each event from which the wavefront propagates. The problem is that there are no isolated systems and these systems are interacting all the time, or at least any time we can observe them. Despite the assumption of a “classical (non-QM) detector” in the traditional statements of the Copenhagen interpretation, any real detector is itself a quantum system, and so is anything attached to it, so there’s no way to actually isolate a coherent system as gedankenized with Schrödinger’s cat. So either arbitrarially defined isolated systems of expanding uncollapsed waveforms have to some how correlate themselves to each other in a way that appears consistent (implying some kind of “vast spooky conspiracy”) or the probability wave concept is just a convenient fiction that happens to fit but entirely something else is going on which is just as inexplicable in classical terms.
You seem to be getting very frustrated with the explanations provided, and I suspect a large portion of that stems from the use of words that you have a pre-existing classical conception of, like waves, wavefronts, particles, “seeing”, collapse, et cetera. It’s unfortunate, perhaps, that these terms were implicitly redefined and used by the founders of quantum mechanics, because they drag along with them connotations which don’t apply to quantum mechanics. (Not their fault, because when they started trying to lay all of this out, they didn’t know any better either, and tried to force it to be classical somehow, too.) At best, these terms as applied to quantum mechanical phenomena are crude analogies or rough approximations that get better and better the further you get from the system (or the bigger the system gets). I’m tempted, presumptuously, to provide an explanation which defines entirely new terms, thus avoiding the semantic confusion that previous descriptions have offered, but that’s ultimately not going to give you any better answer to your questions (to which we have none); it’ll just give you a new set of jargon to be frustrated with.
This is true, and “parsimony” may also be the subjective eye of the beholder; one man’s “trivial” may be another’s “inconceivable.” In the case of the different interpretations of quantum mechanics, however, there’s really no measure by which to say one is even more simple than the other. They all come with consequences that are at odds with normal experience and expectations, and the only way to select one from another–at least, with what we currently know or can even speculate about–is that it seems more less uncomfortable or more asthetically pleasing than the alternatives. Lord of Ockham can take his razor and have a good shave, 'cause we can’t use it here, even as a guiding principle.
Stranger
I was getting frustrated because you were just repeating lots of what I already knew. Perhaps my questions were not phrased in the exact correct terminology you were looking for. For instance, given our current observations, this:
is fairly obvious. But if wavicles were to be shown to automatically be “observed” (or collapsed, or whatever it is exactly that you want me to say here,) when they had travelled a distance of, say, 100 kilometers, it would cast doubt on what and who is an “observer” and at what ranges Quantum Mechanics has a significant effect versus at which the locations and velocities were automatically pinned down.
We can’t say anything about when the “wavicles” (I like to think of them as “blobicles,” personally, just 'cause I think it sounds funny) collapse, or indeed, if they ever collapse, other than that when we actually observe/interact with one, it is always a discrete state, albeit in a position and momentum that we can only predict beforehand as a probability distribution. There’s no measuring a velocity or distance at which this occurs because any measurement involved any interaction, and we know that whenever we interact the state is already “collapsed” (if it hadn’t some time before).
If you already know this, then you already know the most fundamental concept in quantum mechanics. There are, to be fair, a lot of other useful things to understand as well, like the exclusion principle, which gives us a way to relate quantum behavior to everyday electrochemistry and chemical bonds in an essentially deterministic manner, and of course a lot of unique and confusing mathematics, but these merely refine your understanding of the limits of what happens at the quantum level rather than advance it.
Stranger
It sure is fun to watch people discuss quantum mechanics in a language designed to scream deviance at the monkeys in the next tree. (after Pratchett)
(I like the ‘stuff is made of atoms’ quote)
Cute, but not really true. Human languages can be used to communicate any number of very complicated subjects, and in ways that are both subtle and multivaried, which is great fun when watching a production of Hamlet or As You Like It. It’s not much good, though, when dealing with concepts of which we have no prior understanding or agreement. The language of physics is mathematics, and it says all that can be usefully stated about quantum mechanics insofar as the current posteriori knowledge about it. Discussing the interpretations in mathematics doesn’t make any more sense, or come to any more definitive conclusions than discussing it in English, or German, or Danish, or Swahili.
Stranger
I was reffering to the ‘observer’ thingy. 
I’m afraid this thead has now collapsed.