It’s depressing to note the continued anthropocentricism of the “many worlds” idea. Apparently the cat itself can’t be the observer that collapses the wave function because, well (drum roll….) it’s entangled with the isotope in the quantum system.
But this is silly as a reductio ad absurdum demonstrates. Let’s say Professor X walks into the room to open the box. Supposedly this will collapse the wave function. But… Prof X is now entangled in the system, so just like the cat, Prof X can’t be the observer. Maybe Prof Y is watching Prof X, but then… Prof Y is entangled too. And so it’s difficult to see how anyone or anything can be the “observer” that must be there to collapse the wave function.
As for the many worlds idea, just imagine the trillions upon trillions of new universes that must pop into existence every second of every day just from the actions of one species on one small planet in one modest galaxy. If there is intelligent life elsewhere (and intelligence as we define it, e.g. human-scale intelligence, seems to be the unstated requirement as far as being “the observer” is concerned) then we’re talking about trillions to the power of billions of new universes popping into existence every second, which is, well, just silly.
Quantum theorists sometimes do interesting math, but that doesn’t mean they think through the implications of some of the more risible aspects of quantum interpretations. And no, eternal expansion doesn’t solve this problem either…