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Alt 18.09.20, 00:52
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Auch wenn von ihm keine längeren Beiträge zu diesem Themenkomplex existieren, hat sich Steven Weinberg doch intensiv mit der Problematik befasst.

Zunächst „Kopenhagen“:

All this familiar story is true, but it leaves out an irony. Bohr's version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wave function (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from? […] The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wave function, the Schrödinger equation, to observers and their apparatus.
(Steven Weinberg)

Dann die Frage nach der Erklärung

If the time-dependent Schrödinger equation described the measurement process, then whatever the details of the process, the end result would be some definite state, not a number of possibilities with different probabilities. 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?
(Steven Weinberg)

Und zuletzt die Philosophie dahinter:

Where then does this radical attack on the objectivity of scientific knowledge come from? One source I think is the old bugbear of positivism, this time applied to the study of science itself. If one refuses to talk about anything that is not directly observed, then quantum field theories or principles of symmetry or more generally laws of nature cannot be taken seriously […] But scientists have the direct experience of scientific theories as desired yet elusive goals, and they become convinced of the reality of these theories.
(Steven Weinberg)

Weinberg fragt nach einer Erklärung – das ist natürlich keine instrumentalistische Position.



Sehr intensiv hat sich David Deutsch damit auseinandergesetzt, warum er eine rein positivistische Position, die keine Erklärungen liefert, für absurd hält.

Let me define ‘bad philosophy’ as philosophy that is not merely false, but actively prevents the growth of other knowledge. In this case, instrumentalism was acting to prevent the explanations in Schrödinger’s and Heisenberg’s theories from being improved or elaborated or unified. The physicist Niels Bohr […] then developed an ‘interpretation’ of the theory which later became known as the ‘Copenhagen interpretation’. It said that quantum theory, including the rule of thumb, was a complete description of reality. Bohr excused the various contradictions and gaps by using a combination of instrumentalism and studied ambiguity. He denied the ‘possibility of speaking of phenomena as existing objectively’ —but said that only the outcomes of observations should count as phenomena. He also said that, although observation has no access to ‘the real essence of phenomena’, it does reveal relationships between them, and that, in addition, quantum theory blurs the distinction between observer and observed. As for what would happen if one observer performed a quantum-level observation on another, he avoided the issue […]
Some people may enjoy conjuring tricks without ever wanting to know how they work. Similarly, during the twentieth century, most philosophers, and many scientists, took the view that science is incapable of discovering anything about reality. Starting from empiricism, they drew the inevitable conclusion (which would nevertheless have horrified the early empiricists) that science cannot validly do more than predict the outcomes of observations, and that it should never purport to describe the reality that brings those outcomes about. This is known as instrumentalism. It denies that what I have been calling ‘explanation’ can exist at all. It is still very influential.

(David Deutsch)

The overwhelming majority of theories are rejected because they contain bad explanations, not because they fail experimental tests.
David Deutsch

Science is objective. And in my view, we cannot take any experimental results seriously except in the light of good explanations of them.
(David Deutsch)



Zuletzt Zeh; wir erinnern uns an “such a concentrate of wildest nonsense”.

The dishonesty of the Copenhagen interpretation consists in switching concepts on demand and regarding the (genuine or apparent) collapse as a “normal increase of information” – as though the wave function represented no more than an ensemble of possible states.
(Dieter Zeh)

According to my attempts to understand them, reality is systematically denied in the Copenhagen interpretation in order to circumvent consistency problems […]. If there is no reality, one does not need a consistent description!
(Dieter Zeh)

I expect that the Copenhagen interpretation will some time be called the greatest sophism in the history of science, but I would consider it a terrible injustice if—when some day a solution should be found—some people claim that ‘this is of course what Bohr always meant’, only because he was sufficiently vague.
(Dieter Zeh)

Zunächst mal keine weitere Diskussion, lediglich die Feststellung, dass der Instrumentalismus – seit langem – keineswegs eine unumstrittene philosophische Position einnimmt, noch die instrumentalistische Auslegung a la „Kopenhagen“ eine Deutungshoheit beanspruchen kann.



Abschließend die zentrale Fragestellung

This poses the obvious problems of (i) when is an interaction between two systems to count as a measurement by one system of a property of the other? and (ii) what happens if there is an attempt to restore a degree of unity by describing the measurement process in quantum mechanical terms rather than the language of classical physics which is normally used? There is no universally accepted answer to either of these questions.
(Chris Isham)

The first charge against 'measurement', in the fundamental axioms of quantum mechanics, is that it anchors there the shifty split of the world into 'system' and 'apparatus'. A second charge is that the word comes loaded with meaning from everyday life, meaning which is entirely inappropriate in the quantum context.
(John Bell)



Wir stehen selbst enttäuscht und sehn betroffen
Den Vorhang zu und alle Fragen offen.
__________________
Niels Bohr brainwashed a whole generation of theorists into thinking that the job (interpreting quantum theory) was done 50 years ago.

Ge?ndert von TomS (18.09.20 um 07:27 Uhr)
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