## In what sense is QM "not understood"?

 Quote by Ken G with what we actually do observe. That's the measurement problem.

to observe something has to exist before.

 Quote by sigma.alpha to observe something has to exist before.
That the problem - you run into the Kochen-Specher Theorem - it can't have a definite value before observation unless something else comes into play - IMHO that something else is decoherence. It transforms a superposition into a mixed state - but a mixed state where the states are eigenstates of what you are measuring. This means it does exist as what the measurement shows it is before measurement but we simply do not know which state of the mixture it exists in - however it is in some eigenstate 100% for sure.

Thanks
Bill

 Quote by bhobba it can't have a definite value before observation unless something else comes into play Thanks Bill
which, "it" ?

 Quote by Ken G The two issues are so tightly connected there's no reason to separate them into two different problems. The "why we don't see superpositions" problem and the "why we don't see unitary evolution" problem is the same problem, called the measurement problem. It's a disconnect between the postulates of the theory, and what we see. The fundamental postulate is that time evolution is unitary, which says superpositions evolve into superpositions, or into mixed states if you project onto a subspace of a system interacting with its environment. But we don't see either superpositions or mixed states for individual particles-- we see definite outcomes.
My point is that we don't really know what it would mean to see a superposition, or a mixed state. To be able to reason from observations to what that implies about reality, you have to be able to do counterfactual reasoning: If X were true, then we would see Y. We don't see Y, so X is not true. What would we see if there were superpositions? It's hard to know.

 Quote by Ken G The "must" means, at the least, "in order to not have threads about why we don't understand quantum mechanics", and at the most, to obtain the next great theory of physics. It's not important if you don't mind having threads like this one, and you don't mind a theory that cannot (yet) be merged with gravity, or be applied to arbitrary energy scales.
It's vaguely possible that the philosophical problems about the meaning of quantum mechanics might come into play in resolving the problems with quantum gravity, but I don't see any reason to think so.

 You claimed, above, that what was strange was that you could get 100% certainty about what one particle would do based on observations of another particle. But that's not strange at all, that's Bertlmann's socks.
My point was that there is NOT anything weird about correlations, if they have a "Bertlmann's socks" type explanation. But quantum correlations aren't like Bertlmann's socks. In particular, Bertlmann's socks don't violate Bell's inequalities.

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 Quote by stevendaryl But if Bob happens to be measuring the spin of the other particle along the same axis A, he's guaranteed to get the same value as Alice. That correlation isn't a matter of "Alice's measurement disturbed the system being measured". It's the things about quantum mechanics that are certain that makes it mysterious, not the things that are uncertain. My point was that there is NOT anything weird about correlations, if they have a "Bertlmann's socks" type explanation. But quantum correlations aren't like Bertlmann's socks. In particular, Bertlmann's socks don't violate Bell's inequalities.
I know, that's why the weirdness does not appear when outcomes are certain. Bertlmann's socks are certain, Bell violations are all examples of indeterminacy. Classical physics doesn't have indeterminacy, and it doesn't have Bell violations-- that's what's weird about quantum mechanics. Since the measurements are still definite, but the mathematical description involves indeterminacy, this disconnect is called the measurement problem.
 I rely on this august group of physicists to disabuse me of my illusion. I begin by assuming the two most accurate and proven theories in physics are correct; QM and general relativity. 1. Relativity says that we exist in a 4 dimensional universe that we apprehend as a 3 dimensional universe. Einstein believed that this was an "illusion." 2. It is the force of electromagnetism that causes us to think the universe is 3 dimensional. This is the force that defines matter as we experience it in its various forms. It is also the force that defines time. The constant speed of C is what defines how much time elapses for us depending on our own speed through space and/or the gravitational force we are subjected to. It is also the force that provides time with its arrow. The sum of the speed through space and the speed through time must always equal C. As C is the limit of speed through space, it is impossible for speed through time to be negative without requiring speed through space to exceed C. 3. Essentially, we live in a 3 dimensional subset of the 4 dimensional universe that is "knocked down" by the reality of electromagnetism, which we are made of and live under. 4. The Schrödinger equation describes particles as waves that permeate all of space-time, i.e. existing in a 4 dimensional "block universe" that we find particularly difficult to conceptualize. 5. Decoherence occurs when these waves encounter electromagnetic forces that compel them to appear as particles in that particular 3 dimensional subset. Information theory has shown that additional dimensions add enormously to the amount of information that can be held by any bit. This is why waves in 4 dimensions appear to us in 3 dimensions as particles. The old "Flatland" metaphor illustrates this perfectly. 6. "Measurement" is simply one way of forcing these waves to decohere. Consciousness has nothing whatsoever to do with it. We rely on the force of electromagnetism for any measurement we make. 7. Once decohered, these waves appear to us as particles in our 3 dimensional subset universe. They continue to behave as waves in the 4 dimensional block universe. 8. The wave nature of matter is necessitated by the relativity of simultaneity. Each observer's reality is equally valid, even though it doesn't agree with other observers traveling through space at a different speed or subjected to different gravitational forces. This truth would simply be impossible if matter were particles. Waves allow matter to appear anywhere in the 4 dimensional block universe where it happens to get decohered through the force of electromagnetism. 9. This is also why quantum indeterminacy is a foregone conclusion once one accepts the relativity of simultinaity. Bottom line, if you believe relativity is correct, quantum "weirdness" is a necessary result. Without wave-particle duality and quantum indeterminacy, relativity would have to be wrong. With it, it works like a charm. Please help me understand why the above has been proven to be incorrect. I'm searching for experimentally proven facts to blow this "understanding" out of the water!
 I subscribe to the info interpretation of QM, so for me there is no such equivalence between "why we don't see unitary evolution" and "the measurement problem". The measurement problem is a result of (as Zeilinger puts it) the conservation of the irreducible bit of information that an individual system is. And your (Ken G) remark "The fundamental postulate is that time evolution is unitary, which says superpositions evolve into superpositions, or into mixed states if you project onto a subspace of a system interacting with its environment. But we don't see either superpositions or mixed states for individual particles-- we see definite outcomes.", come on, QM is not about the individual system. Yes, physics is, but the theory of QM (and of course its measurement problem also) only accounts for statistically relevant sets of measurements, not individual systems. You're treading dangerously close to the worst QM interpretation in history, the spurious idea about conciusness having some role in things.
 In the information interpretation of QM the individual system is a "bit" of information (actually several bits over several components of the description). QM is a theory which describes statistically relevant sets of observations of individual systems. Therefore QM describes a system as a "set" of information based upon a description. The irrational process(which could also be called a non-unitary process) in the measurement problem results from a change in the preparation of the system (where the change in preparation may be an evolution of the state or a change in the preparation encountered by the system due to its movement). Any change in preparation results in a demand for new information about the system because the new preparation implies it, a measurement (and it is important to distinguish this as an "in principle" measurement because no one has to actually observe anything). Due to the conservation of the irriducible bit of information that the individual system is, only so much information can the system be. The system becomes the information demanded by the new preparation. The rest is irrational, and yes this is a problem for physicists who would wish to explain away all of existence rationally, but for those of us whom enjoy the indeterministic side of life it is not a problem but rather a fair compromise with existence.

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 Quote by al onestone QM is not about the individual system. Yes, physics is, but the theory of QM (and of course its measurement problem also) only accounts for statistically relevant sets of measurements, not individual systems.
I agree, but then I favor the Copenhagen interpretation, which says the same thing ("there is no quantum world"). But the key point is, this is not a statement about quantum mechanics, it is a subjective choice about how to interpret quantum mechanics. I don't take a rationalistic perspective that physics is trying to describe reality as it actually is, but those who do must seem an interpretation where quantum mechanics does describe a closed system.
 You're treading dangerously close to the worst QM interpretation in history, the spurious idea about conciusness having some role in things.
There's nothing spurious about that idea unless it is mishandled (as it often is). It is true that there seems to be little benefit in attributing a dynamical role to consciousness in the equations of physics, but an attempt to do that is rather missing the point of the role of consciousness. The role of consciousness is simply that physics is done by physicists, who are conscious beings, and who invariably invoke their consciousness when they do physics. Hence, the role of consciousness is quite demonstable, the only question is when do we need to think about it and and when can we get away with ignoring it. The latter answer is "virtually always", but quantum mechanics may be one of the places where we begin to encounter problems ignoring it. (Especially for those who take an informational interpretation-- for where is the only place that information is actually a demonstrable concept?)

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 Quote by josephwouk I'm searching for experimentally proven facts to blow this "understanding" out of the water!
Here's one: relativity was laid out decades before quantum mechanics, and was regarded as a complete theory in the absence of any quantum indeterminacy. What's more, the person most responsible for formulating relativity (Einstein) was also deeply involved in the discovery of wave/particle duality (his Nobel prize was on the photoelectric effect, not relativity), yet this person never accepted wave/particle duality or quantum indeterminacy as fundamental truths of nature, at the level of the fundamental truths of relativity. Today, most physicists take the opposite view, that quantum intedeterminacy is a more fundamental truth than the tenets of general relativity, yet even so, the two are not viewed as mutually necessary, and indeed are generally viewed as contradictory at the Planck scale, though no observations exist at that scale.

 What I wonder about is how the founders of QM figured out that the mathematics we use in QM (operators, bras, kets etc.) was the right thing to use. They didn't just pull it out of thin air, they must have reasoned their way to at least some of it, eg. Schrodinger didn't just get out a pen and write down HΨ=iℏddtΨ out of nowhere. Why isn't that considered "understanding" it?