- #26

vanesch

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Density matrix... in what basis ? Because depending on the answer, the density matrix will describe something that 1) looks like a statistical distribution in classical phase space (what you imply), or 2) will look completely different !reilly said:That's to say, with a density matrix approach to the QM of any brain, the drivers and processes will largely operated in the large-sample, classical domain. And then, classical? Quantum? it's all the same.

Write the density matrix in the basis (|dead cat> + |live cat>) and (|dead cat> - |live cat>) for instance... Now, you can say that one OBVIOUSLY has to use the basis {|dead cat>, |live cat>} but I hope that you see the circularity of that argument...

Again, you seem to think that I fight the potentially probabilistic aspect of quantum theory. I'm not.

I'm pointing out that, along the chain, you need to say WHAT it is that you consider classical, because it is in THAT basis that you have to write your density matrix and to put the non-diagonal elements to 0, and that is a matter of convention (except in decoherence schemes, but these are inherently MWI). How do you get out your "classical" basis of your brain, when you have your Schroedinger equation ? And if you use projection, in what basis do you project, in order to get out probabilities (or for that matter, the density matrix) ?

The density matrix is the singly most abused quantity to "solve" the measurement problem (because it always leads to a circular reasoning), because of the easy sneaking in of the "classical basis".

I went to the observer's brain, because for all other apparatus, it is "obvious" that a measurement instrument is going to measure in the basis it is designed for (which is in fact absolutely not obvious... except, again, if you go to a decoherence approach, which is part of an MWI view). Tell me, what "basis" is our brain supposed to measure ? The "classical" states ? Why ?