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- Why Steven Weinberg finds that today there is no interpretation of quantum mechanics that does not have serious flaws.
I think Weinberg is quite clear about this:vanhees71 said:I don't know, what precisely it is what Weinberg thinks is missing, because in his book he just says that which interpretation is "correct" is an open question, but I did not see clearly what he thinks is missing from what to be expected of a physical theory to describe beyond what QT successfully describes.
On p.87 of the second edition of his quantum mechanics book, he says,
and on p.88:Steven Weinberg said:what is known as the Born rule [...] entails a departure during measurement from the dynamical assumptions of quantum mechanics. [...] If the time-dependent Schrödinger equation described the measurement process, then whatever the details of the process, the end result would be some definite pure state, not a number of possibilities with different probabilities.
After having discussed decoherence, he says on p.92:Steven Weinberg said:This is clearly unsatisfactory. If quantum mechanics applies to everything, then it must apply to a physicist’s measurement apparatus, and to physicists themselves. On the other hand, if quantum mechanics does not apply to everything, then we need to know where to draw the boundary of its area of validity. Does it apply only to systems that are not too large? Does it apply if a measurement is made by some automatic apparatus, and no human reads the result?[...] There has emerged in recent years a clearer picture of what actually happens in a measurement. This has been largely due to the attention given to the phenomenon of decoherence. But as I will try to show, even with this clarification, there still seems to be something important missing in our present understanding of quantum mechanics.
For the instrumentalist approach (apparently your view of the matter), he states on p.92f this drawback:Steven Weinberg said:There seems to be a wide-spread impression that decoherence removes all obstacles to this class of interpretations of quantum mechanics. But there is still a problem with the Born rule [...] The “derivation” given above, based on Eq. (3.7.8), is clearly circular, because it relies on the formula for expectation values as matrix elements of operators, which is itself derived from the Born rule. So where does the Born rule come from? There are two main approaches to this question, that are often called instrumentalist and realist, each with its own drawbacks.
Then he states what seems to be his main concern - for him, essential for a flawless foundation is a foundation that does not need to refer to measurements (which do not exist independent of the human technology):Steven Weinberg said:But if these probabilities are taken to be the probabilities of obtaining various results when people make observations, then this approach brings people into the laws of nature. This is not a problem for those physicists who, as did Bohr, view the laws of nature as no more than a set of methods for ordering and surveying human experience. They are certainly that, but it would be sad to give up the hope that they are something more, that the laws of nature are in some sense “out there” in objective reality, the same laws (aside from language) for whoever studies them, and the same whether or not anyone is studying them. [...] The problem arises precisely because we want to be able to understand scientists along with everything else scientifically, and for that very reason, we need to keep humans (scientists, observers, or anyone else) out of the laws of nature, which by definition are unexplained.
On p.95f he discusses coarse-graining, your way of handwaving the classical world into the quantum picture, and then criticises on p.96:Steven Weinberg said:Only if the laws are expressed in impersonal terms, whether particle trajectories or wave functions or something else that does not refer to people making observations, can we hope to come to a scientific understanding of what is going on when people do observe nature or make a measurement.
After that he discusses in detail the drawbacks of the realist approach, criticism that you probably share, so there is no point in reiterating these here.Steven Weinberg said:The problem here is not that the choice is not unique, but rather, that it can only be made by people. [...] So the Born rule in the decoherent-histories approach seems to bring people into the laws of nature, as is apparently inevitable for any instrumentalist approach.
These considerations fully justify (in his eyes) his conclusion on p.102 that
Steven Weinberg said:today there is no interpretation of quantum mechanics that does not have serious flaws.
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