# Experimental test of local observer-independence

In a recent paper published on arxiv, what was formerly a thought experiment has been realized (with minor tweaks) and, while some say this indicates there is no objective reality, I rather think it means something else.

There are various “interpretations” for quantum mechanics, that are all at odds with each other. Two are fairly well-known: the Copenhagen interpretation holds that, when a quantum state is observed it, and the corresponding wave function, “collapses” such that what was a superposition between two states with two probabilities (or probability amplitudes) more becomes a single, well-defined, measured state. The many worlds interpretation holds that when a quantum state is observed, the universe splits and both possible states continue to exist, but as separate universes that cannot interact.

Both interpretations, as well as a bunch of others, correspond to observable reality in as far as neither can be empirically disproven. Both have also inspired quite a bit of science fiction, though the latter has been an easier source of inspiration than the former.

The question of whether an observer is independent of the thing being locally observed, though, is crucial in both interpretations: both ask what happens if a quantum state is “observed”. Specifically, they don’t ask what happens “when a local observer becomes entangled with a quantum state”.

Wigner’s Friend experiment hints at this: imagine you have Schrödinger’s cat in a box, in a superposition of alive and dead, unobserved and unperturbed. The Friend goes ahead and observes the state of the cat, but doesn’t tell Wigner what the state is. From Wigner’s point of view, the cat not only remains in a state of a superposition between alive and dead, but the Cat-Friend system is in a superposition of “cat dead, friend saw it dead ; cat alive, friend saw it alive” until the Friend tells Wigner what the observation was.

In the Copenhagen interpretation, the question is when the cat’s superposition collapsed. In the many worlds interpretation, the question is how many worlds there are now, and when that happened.

However, in an interpretation in which the Friend, upon observing the cat, becomes entangled with it, their own wave function merging with the cat’s, rather than the cat’s wave function collapsing, the superposition observed by Wigner makes sense. Once Wigner observes the state of his Friend, the Friend telling him what they observed, the local wave functions again merge and Wigner becomes entangled with the Cat-Friend system.

This interpretation, which I don’t think is new, also works better with Heisenberg’s uncertainty principle and his observation that any system that is observed is influenced by the observer – i.e. there is no such thing as a passive observer if the observer is to observe anything.

In this interpretation, the whole universe is a single, multi-dimensional wave function in which various (particle) states are entangled, but some are not. The whole thing can be seen as a single wave function with probability amplitudes along every conceivable axis. The Cat system is an isolated part of this wave function and can be interacted with independently, but such interaction by the Friend system (which itself is an isolated part of the wave function) entangles the Friend part of the wave function with the Cat part of the wave function, changing the state of the universe to include the observation the Friend has now made. Nothing collapses, no new universes are created, but if we were to write out the universe in Dirac notation, the part of the universe that designates the Cat-Friend system would now be in a state in which the Friend observing the Cat as being alive while it is, in fact, dead, (or vice versa) can no longer occur: friend-observation and cat-state are now dependent on each other. The factual part of this state has not reached Wigner yet, so Wigner’s part of the function is unaffected until the information reaches him, when Wigner becomes entangled with the Cat-Friend system creating a Cat-Friend-Wigner cluster within the universe.

The experiment by Proietti *et al*. actually raises the question not just of observer independence, but also of free choice and locality. Locality really isn’t at issue, I think, and free will is better left to philosophers, but the existence of an independent observer has always been in doubt.