Wigner's Friend
en.wikipedia.org
en.wikipedia.org
I'm not a physicist and only dug deeply enough into the topic to understand the basic quantum algorithms for factoring and discrete logarithm, but it seems to me that there is very obviously no conflict. It's just that the "observer of the observer" observes a meta-reality that contains all possible realities observed by the "observer". There doesn't seem anything strange about this if you just don't insist on "measurement" having a special physical meaning and accept the universal wave function. But maybe there's a twist I'm missing.
The latter. QM is weird, but it's not paradoxical.
If a set of atoms can be in a superposition, so can be you. Because you are a set of atoms, duh.
It does not follow that "if a set of atoms can be in a superposition, so can you." Here's some ways it might fail:
1. In the objective collapse theories, quantum systems undergo collapse when they reach a certain size. I cannot be in a superposition because I am physically too big.
2. In the ensemble interpretations, a quantum state applies to a set of many identically prepared systems. Since there is only one of me, we cannot speak meaningfully of my quantum properties.
Admittedly neither of these are satisfying interpretations, but they have their proponents.
The deeper objection is that "you can be in a superposition" risks detachment from experiments. If we cannot talk about the result of an experiment, that we cannot do science.
> From the friend’s perspective, the qubit has been measured and has collapsed to either |0〉 or |1〉. From Wigner’s perspective, no such thing has happened—there’s only been unitary evolution—and in principle, Wigner could even confirm that by measuring |ψ〉 in a basis that included |ψ〉 as one of the basis vectors. But how can they both be right?
> Many-Worlders will yawn at this question, since for them, of course “the collapse of the wavefunction” is just an illusion created by the branching worlds, and with sufficiently advanced technology, one observer might experience the illusion even while a nearby observer doesn’t. Ironically, the neo-Copenhagenists / Quantum Bayesians / whatever they now call themselves, though they consider themselves diametrically opposed to the Many-Worlders (and vice versa), will also yawn at the question, since their whole philosophy is about how physics is observer-relative and it’s sinful even to think about an objective, God-given “quantum state of the universe.” If, on the other hand, you believed both that
> 1. collapse is an objective physical event, and
> 2. human mental states can be superposed just like anything else in the physical universe,
> then Wigner’s thought experiment probably should rock your world.
It seems more reasonable to me that the act of measurement actually does not change the state of the quantum system, but entangles the observer with the system.
It is the observer and not the pre-existing entanglement that is subject to change as a result of measurement.
If a man in a box at some stage measures the state of a cat in a box, then my perspective of the box is a superposition of: a man who has not yet measured the state of the cat; a man who measured the state of the cat, and found it alive; and a man who measured the cat and found it dead.
I can expand this superposition with arbitrary possibilities - such as whether or not the cat is yet to enter rigor mortis.
If without seeing the cat myself - or the man's reaction to seeing the cat - I watch as the man looks at the cat, my superposition is reduced to the possibilities in which the man has measured the cat.
This is not about the state of reality - it's about the localisation of information regarding the state of reality.
(Additionally, if I saw the man's reaction to his observation of the cat it would skew my superposition based on the likelihoods that he would react that way if the cat were dead/alive).
Spooky action at a distance is another one where I don't think the common interpretation holds up - perspectives on quantum systems should be absolutely free to contradict each other (in terms of absolute measurement) it is only when the light cones of the quantum systems meet that contradictions are excluded.
Wigner apparently called Dirac his "famous brother-in-law".
Other anecdote: Oppenheimer, when recommending Feynman included a quote from Wigner on Feynman: "He is a second Dirac, only this time human."
I enjoy these bits. I also enjoyed going through a graph of people as a kid in order to sleep. People as vertices and relations as edges.