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Decoherence is an extremely useful and elegant mechanism for understanding quantum systems, but unfortunately it doesn't solve the measurement problem. At least not to the satisfaction of most practising physicists. The entanglement and trace operation does produce classical probabilities in the observed sub-system. However, it requires you to make a pretty arbitrary division between the observed system and the wider environment. Why is everything not just entangled with everything else up and up the chain until the entire universe is in superposition? The problem of where the quantum world ends and the classical world begins is still unanswered. This is one of the greatest open questions in physics, likely requiring a unification of general relativity and quantum mechanics before it's resolved.



> Why is everything not just entangled with everything else up and up the chain until the entire universe is in superposition?

It is.

I'm going hazard a guess that your next question is going to be: why do I not perceive myself to be in a superposition of states? And the answer is that you are not what you think you are. You think you are a human being, a classical physical object made of atoms, but you aren't. This is a very good approximation to the truth, but it is not the truth. The truth is that you (the thing engaged in this conversation) are a software process running on a human brain. You are a classical computing process, i.e. a process that can be emulated by a classical computing model like a Turing machine. The reason for this is that the kinds of things you do necessarily involves copying information (e.g. the process of reading this comment involves copying information from your computer into your brain) and quantum information cannot be copied. Only classical information can be copied. Your conscious awareness of the existence of physical processes is an emergent phenomenon of accumulating memories, i.e. copying information. Because of this you cannot become consciously aware of your quantum nature, and because of that you cannot demonstrate the quantum nature of any system (to yourself) unless it is isolated from you. You could in principle demonstrate the quantum nature of the rest of the universe if you could somehow isolate yourself from it, but that presents insurmountable practical difficulties.

> The problem of where the quantum world ends and the classical world begins is still unanswered.

Because the question tacitly makes the false assumption that there is a hard boundary between the two. There isn't.


Clearly you have your own unorthodox interpretation of quantum mechanics and you are quite emotionally attached to it. The argument that the universe is really X, but your brain just perceives it as Y because of Z can be made for any X,Y & Z. I don't find it compelling whatever the alleged reason for the disparity (in this case the no cloning theorem). There's also whole host of other issues, if everything is QM why has the brain evolved as a classical information processor, QM is unitary so you need to explain time/thermodynamics ...

Anyway, if you truly believe you have nailed both the measurement problem and consciousness in one fell swoop. Then I suggest you seek some like minded collaborators and a peer reviewed outlet for your ideas.

Returning to your original comment.

> It is astonishing to me that the physics community still bifurcates into two camps: those who think this is common knowledge, and those who are completely unaware of it (or think it's a crazy idea).

As a former member of the physics community I can tell you this is completely false. Everyone is aware of decoherence, it's covered toward the end most of undergrad courses in QM where the density matrix is introduced. I've never met anyone who thinks it solves the measurement problem.


> Clearly you have your own unorthodox interpretation of quantum mechanics

None of these ideas are original with me. All of them can be found in the literature. My only contribution (if I've made a contribution at all) is pedagogical.

> and you are quite emotionally attached to it.

Yes, I am quite emotionally attached to the truth. And yes, it does annoy when people promulgate the myth that QM is hard to understand or contains intractable mysteries when I know it isn't true. It particularly annoys me when people say this and simultaneously ignore the incontestable fact that entanglement and measurement are the same thing, in the same sense that space and time are the same and matter and energy are the same. Yes, all of these things are weird, and yet all of these things are true, and none of them are intractable mysteries or even hard to understand.

> I suggest you seek some like minded collaborators and a peer reviewed outlet for your ideas.

Like I said, these aren't my ideas. But I did submit my paper to Physics Today back when I first wrote it in 2001. It was rejected on the grounds that everything in it was already common knowledge.

> As a former member of the physics community I can tell you this is completely false. Everyone is aware of decoherence

But obviously not everyone is aware of its implications. Your challenging me on this is manifest evidence of that. And for 25 years I have been stumping card-carrying physicists with the EPRG thought experiment. Heck, it took me ten years to find anyone in the physics community who knew the answer! (I even had a chance to pose the question to Freeman Dyson, and he didn't know the answer!)


This is correct. As it is usually applied in practice in the literature, the measurement device decoheres the quantum system and becomes entangled with it --- and at the end of the calculation, Copenhagen interpretation is applied on the measurement device. It works fine in practice, but in the end it is philosophically just the Copenhagen interpretation.

If something is not accepted by the wider physics community, it's very likely the correctness of the thing in question is not that clear.


> in the end it is philosophically just the Copenhagen interpretation

No, it isn't. The wave function never collapses. This is easily demonstrated with a simple thought experiment which is described in the first two references that I link to.


I wrote about the interpretation usually used in published papers, and obviously decoherence alone does not solve the measurement problem as it only shuffles it to the bigger system, you need many worlds / Copenhagen / etc.

I don't bother watching youtube videos of what someone thinks quantum mechanics is about.


You don't have to watch a video. My first reference is to a paper.


Many worlds is just a way of describing the results of entanglement. It doesn't add any structure or dynamics to the wave function. Those worlds are precisely those slices the parent is talking about.


> At least not to the satisfaction of most practising physicists.

This is true, as the parent said - it's not accepted by many physicists.

> However, it requires you to make a pretty arbitrary division between the observed system and the wider environment.

This isn't true. The whole universe is in a superposition and you can reason with decoherence at any level. Literally everything in one "world" including the vacuum of space is entangled with everything else, though to a very small degree.




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