This is what I envisage an experiment testing for, it was the 2nd sentence out of 2 in the post you responded to:
You forget that the cat itself is an observer, thus Schrodinger’s Cat is not even a true paradox. As pointed out, it was conceived to show how silly Quantum is, but it did not succeed.
But cats are so good at acting disinterested that even subatomic particles can’t tell when they’re being observed. 
Regarding Einstein in the tub… I wonder if the guy was mistakenly thinking of Leo Szilard, a colleague and collaborator of Einstein’s who was in fact known to do a lot of his thinking in the bathtub.
It seems to have worked - Szilard is the one who conceived of the idea of nuclear chain reaction. However, he claimed to have had this insight while crossing the street, not in the bath.
Assuming this is not a whoosh, I think the speaker meant Archimedes.
This is the quantum mechanical measurement problem. unfortunately nobody can give a complete answer to that question still
What we do know about the measurement is:
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It’s irreversible. The function that takes the state of a system immediately before it was measured to the state of the system immediately after it is measured isn’t one-to-one. This contrasts to the time evolution of the wave function as described by the Schrodinger equation which is reversible.
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It is instantaneous orr at least happens over vanishingly small timescales. Attempts to measure “collapse time” show this.
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It almost certainly involves decoherence. Decoherence describes what happens when a large complicated system (i.e. a system for which it would be practically impossible to determine it’s precise quantum state) couples to a small, simpler subsystem (a subsystem who’s exact quantum state could be known). In decoherence the the subsystems state goes from pure state (i.e. a precisely known quantum state) to a mixed state (i.e. where the precise quantum state is not known).
Decoherence bears a striking resemblance to what happens when a measurement is made on the small subsystem. It is, for all practical purposes, irreversible and happens over time scales that make even the decay of some of the most unstable isotopes look tardy. And the resemblance goes even deeper than that in fact.
There is a big ‘but’ though: decoherence tells you why immediately after measurement the subsystem will only take on certain states (and each different state correspondences to a different measurement outcome), what it doesn’t tell you is why it takes on one specific state.
- Beyond that not much else.
In a sense, the whole point of the “cat” thought experiment is to illustrate that there is no way, according to QM, to investigate or answer such questions.
Not a whoosh. Szilard’s wiki page discusses his conception of chain reaction, but not the bathtub thinking. Several books do, however, including the Richard Rhodes history of the Manhattan Project.
Also, Googling “Leo Szilard bathtub” will bring up other references.
The wiki article on Schrodinger’s Cat cites several such experiments, includingthis one where the “cat” is on the order of a photon. I also think the various experiments supporting Bell’s theorem would count in favor of a superimposed state having some kind of reality, but those are more indirect.
Ah, OK. Yes, I think that’s right, in principle. However, the effect is so small as to be neglected for all intends and purposes; in realistic experiments, it’s dwarved by other sources of noise, but that doesn’t exclude the possibility of detecting quantum behavior.
I disagree. Consistent histories (which is essentially MWI) includes consciousness in the same way that MWI does: it is that which does the anthropic selection. Again, this is not “woo-woo”; consciousness is not affecting anything, but it is the anthropic selection by a consciousness that appears to exist in a classical collection of delta-functions out of the universe’s whole wave function, that creates the appearance of random collapse. If consciousness existed in interacting macroscopic superpositions, then the appearance of collapse would not be predicted by consistent histories or MWI or decoherence.
If you “simply consider measurement as an unanalyzed primitive of the theory, with observers simply being those kinds of things capable of performing measurements”, you still do not escape internal inconsistency; we know that measurement is done by “things” which are themselves no different from other things, so it is disingenuous to consider them unanalyzed primitives. Either the Schrodinger equation is correct or it is not, unless you posit that humans are not made of regular atoms.
While I sympathize with the fact that Bohr et al were really very intelligent and subtle thinkers, and that the popular distillation of their opinions is somewhat ham-handed, I don’t think it’s really relevant. When I refer to the Copenhagen interpretation, my intention is to discuss it as it is generally understood and taught. What the Copenhageners actually thought is a bit beside the point, although I would be interested in hearing whether they promoted a program that was internally consistent, because my current understanding is that they did not.
Well, whether or not consistent histories really is an Everettian, much less many-worlds, interpretation depends on who you ask, I think; Gell-Mann and Hartle would presumably be fine with this characterization, while I think Omnès (and probably Griffiths, too) would hold CH to be in the spirit, if not the letter, of Copenhagen (‘Copenhagen done right’). I think this just exemplifies that both really are not as far from one another as is usually thought. And in order to do any ‘anthropic selection’, there would have to be a choice between genuine alternatives, which is the case in DeWitt’s splitting worlds version of Everett, but arguably not with what Everett himself really had in mind, and probably (though I’m no expert there) also not in consistent histories according to most of its advocates.
And I’m sorry, but the way you bring consciousness into the framework still seems to me no different from the way it plays into every empirical framework, by virtue of the simple fact that every observation is an observation by a conscious being.
Well, it’s easy to discount opposing arguments on their weakest reading; that’s why one usually strives to argue against the most charitable interpretation of an opponent’s argument.
But that’s where we tend to underestimate the evolutionary process. There’s virtually no doubt now that many animals sense magnetic field lines via quantum effects exploited on a cellular/molecular level. Photosynthesis is now understood (probably) to require quantum coherence.
So it’s not just a matter of there being background noise but that there is some remote chance that just like with the examples above, reception isn’t a completely passive matter. But don’t worry, I haven’t gone all new-agey yet. But if I start any threads on the healing power of crystals or colonic cleanses . . .
That’s the general ‘inconsistency’ in QM: The Schrodinger equation describes unitary evolution, whereas the evolution of the wave function during the measurement process is non-unitary.
There is a clear difference in what is predicted from the theory, depending on the nature of the consciousness, which is in radical departure from the classical theory. I really don’t see how the trivial statement “every observation is an observation by a conscious being” is in any way relevant. I’m not even sure if, by making that statement, you are implicitly agreeing with me (and calling my argument trivial), or disagreeing.
This is pretty unfair way of framing this discussion. My goal is not to argue against a purposefully uncharitable interpretation; I am simply attempting to use agreed-upon definitions in discourse. It is impossible to hold a coherent conversation with someone if they decide that their idiosyncratic definition is the correct one. When I use “Copenhagen interpretation”, I’m referring to what I believe is the most common definition used. This is an interpretation that I find ill-defined and logically inconsistent. If there is a neighboring interpretation which you would like to call “my Interpretation of the founder’s intention of what the Copenhagen interpretation should be”, then I would be happy and interested to discuss it without being accused of being uncharitable, thank you very much. That said, even in what is in my understanding the most charitable interpretation of what the founders might have thought, I still, to the best of my knowledge, find it to be internally inconsistent and ill-defined. If you disagree, I would very much enjoy being disabused of my ignorance.
No this is the inconsistency of the Copenhagen Interpretation, not a general inconsistency in QM. In the MWI/CH/Everettian/Decoherence class of interpretations, the Schrodinger equation is correct at all times and the wave function’s evolution is always unitary.
You can’t side-step the non-unitary nature of measurement that easily.
I don’t know too much about the consistent histories interpretation, but it says in the Wiki article that each proposition about a system that makes up a history of that system is represented by a projection operator on the Hilbert space of that system. A projection operator is non-unitary, so by recognising a certain proposition is true (presumably due to the result of a measurement) and excluding all histories that don’t contain it, your description of the systems state changes in a non-unitary way.
The many worlds interpretation assumes that everything is governed by a universal wave function whose time evolution is always unitary, but there’s a big cost to this as the state of the universal wave function doesn’t correspond that well to the state of the things we observe around us.
Aside from that, everything’s hunky-dory. Change the ground-rules!
In both MWI and CH, the wave function of the universe evolves according to the Schrodinger equation. The “big cost” you speak of is the same in CH (represented by the projection operator you pointed out), and is simply a bookkeeping problem. It is a “problem” in the same way that solving the Navier-Stokes equations are a problem: there is no conceptual problem, merely a calculational one. It’s true that the universal wave function doesn’t correspond in an intuitive way to what we see around us. But physics doesn’t have to be intuitive. Decoherence explains the appearance of wave function collapse in both of the above (basically equivalent) intepretations: you have a universal wave function evolving, and from the point of view of conscious observers within the wave function, there is the appearance of collapse. You can construct rules about how and when that collapse will appear to happen, and that problem is just a mathematical one, not a conceptual one or a reflection of any internal inconsistency.
Really it’s sort of shuffling the ontology, many-worlds explain very well why the wave function can be reduced to different parts representing different empirical realities (but it’s not as if this is beyond the Copenhagen interpretation once it is updated to reflect the advances in knowledge). But what it can’t tell you why is that empirical reality is only described by that specific part only. Reducing the state of the universal wave function to the relative state that describes empirical observation is a non-unitary operation.