Steven French at Aeon Magazine:
This question forms the basis of what came to be known as the ‘measurement problem’. One influential answer emerged from the mind of one of the greatest mathematicians of all time, János (or ‘John’) von Neumann, who was responsible for many important advances, not only in pure mathematics and physics but also in computer design and game theory. He pointed out that when our spin detector interacts with the electron, the state of that combined system of the detector + electron will also be described by quantum theory as a superposition of possible states. And so will the state of the even larger combined system of the observer’s eye and brain + the detector + electron. However far we extend this chain, anything physical that interacts with the system will be described by the theory as a superposition of all the possible states that combined system could occupy, and so the crucial question above will remain unanswered. Hence, von Neumann concluded, it had to be something non-physical that somehow generates the transition from a superposition to the definite state as recorded on the device and noted by the observer – namely, the observer’s consciousness. (It is this argument that is the source of much of the so-called New Age commentary on quantum mechanics about how reality must somehow be observer-dependent, and so on.)
more here.
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