Quantum mechanics, like Popeye, is what it is, and so has no real need of “interpretation”.

What it says is quite clear. The probabilities of various possible values of measurements on an isolated system can be calculated as expectation values of certain specified linear operators on a Hilbert space with respect to vectors defined in terms of the previous interactions that “prepared” the system before its period of isolation. Perhaps that sentence wasn’t clear to one who isn’t already familiar with what it’s about, but the process of applying the theory is nonetheless well defined and produces specific clear predictions.

So it’s not unreasonable to just quash the plea for interpretation with a quick “shut up and calculate!”

But given the amount of nonsense that has been put forward over the years about collapsing cats and what it all “really means”, I think it may be worth exploring what it actually says.

Because of the isolation condition, the “state” that is represented in quantum mechanics by a unit vector or ray in a Hilbert space is a property, not just of the things being observed, but also of their relationship (or rather temporary absence of such) to other things including in particular any outside observers. As such it might better have been referred to from the outset as the “relative state”, and the use of such a term might have saved us from a lot of confusion.

As it happens, the term “relative state” was adopted by Hugh Everett in his 1956 thesis which proposed the relative states of any specific experiments as essentially the result of integrating out the irrelevant variables to get what statisticians call marginal distributions for the relevant ones.

Another point to consider is that my experience as an observer corresponds to just one particular history out of many that might seem possible to an outside observer. And the flaw in the argument of those who see a problem with Wigner’s friend is their mixing of two inequivalent perspectives with insistence that there must be a classically true or false “fact of the matter” regarding what the friend has or has not observed after it is known to the friend but not to Wigner himself.

So rather than imagining many universes that split in some ill-defined way at ill-defined events or times, let us think of a single universe having many possible histories with the experience of each observer corresponding to some aspect of one particular history. The state of the universe relative to that observer at any particular time corresponds to the set of possible histories that are compatible with all prior experience of that observer, and this is what collapses when an observation is sensed; so what splits is not the universe itself but rather our view of the set of all imaginable histories – from which each experience reduces (or “collapses”) the compatible set and so at the same time “peels off” those histories which no longer fit.