ON A HIDING TO NOTHING:

Erwin Schrödinger, father of the wave-function formulation of quantum mechanics, was greatly vexed by the implications of extending quantum mechanics to macroscopic objects, dreaming up the notorious case of a cat being both dead and alive as a way to voice his discomfort. Hungarian-American physicist Eugene Wigner took things a step further: rather than putting a cat in the box, he imagined an entire laboratory, complete with an experimenter, sufficiently isolated from the environment such as to make a probe of its internal state from the outside impossible.

Together with the experimenter--the eponymous 'friend'--a single quantum system in the state [↑,↓] was present. Now, a tenet of quantum mechanics is that after an experiment has yielded an outcome, that outcome will be observed again upon repeating that experiment. Meaning that performing the ↑↓-measurement on a [↑,↓]-system and obtaining ↑, say, repeating the measurement must again yield the outcome ↑. Therefore, after the measurement, the system can't be in the state [↑,↓] anymore, and must instead be in the state [↑]. This is the origin of the famous 'collapse' of the quantum state upon measurement: somehow, the state must make a transition from [↑,↓] to [↑] (or [↓]) when the ↑↓-measurement is performed.

Wigner now asked himself how quantum mechanics ought to describe the process of measurement itself, i.e. of an experimenter performing an experiment in such a way as to be subject to the usual rules of quantum mechanics (essentially, isolated from the environment sufficiently well to prevent decoherence). This involves modeling the experimenter themself using quantum mechanics. For our purposes, it will be enough to consider an experimenter with two possible states (for each measurement): ["↑"] or ["↓"] for 'having observed ↑' or 'having observed ↓', respectively (and analogously ["←"] and ["→"]).

Paradoxically, it seems that there are two answers: if the observation 'collapses' the state, then after making their measurement, the combined system consisting of the friend and the system they measure should be either described as ["↑"↑] or ["↓"↓], the object-system in a definite state, and the experimenter having definite knowledge of that state. But, modeling both as quantum systems, this is not what happens--rather, as with the dead-and-alive cat, we obtain a combined system in the state ["↑"↑,"↓"↓]--where neither the experimenter nor the object can be assigned any definite state.

Wigner thought this strange enough to postulate an absolute boundary for the quantum description at the experimenter's conscious experience--it is this, he claimed, that causes the collapse (hence, the 'Consciousness Causes Collapse', CCC, interpretation). The quantum buck stops where the experimenter takes conscious account of a measurement result.

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