"Observ(er|vation)" is really one of the worst terms ever in this area. It really is just about the experimenter (plus apparatus) becoming entangled with the experiment -- or entering into a superposition with it, if you will. That's it. No spooky consciousness, etc. goes into it. One's consciousness simply enters into the superposition.
(If you can't tell, I fully subscribe to the Many World interpretation. AFAICT it's, by quite a long way, the most parsimonious explanation in terms of extra "theoretical stuff", even if it is "wasteful" in terms of the amount of actual stuff it posits.)
Many worlds doesn't really explain more than other interpretations of standard QM. You still have the problem of why the Schrodinger equation shows you that that a particle has some amitude both here and there, and can interact with other particles both here and there, yet when you experiment you only find it either here or there, never in both places.
MWI just posits that the measurement apparatus exists in a single world, while the particles exist in many worlds. But it can't explain this basic fact, like any other interpretation. What we need is an actual new theory that can actually measure what a measurement is (at what precise point the Born rule must be applied, or how can we do without the Born rule).
There are essentially 3 options for Wave function collapse.
Their are some hidden variables which means it’s deterministic and there is no amplitudes involved. The universe has some sort of random number generator which determines the outcome. They don’t actually collapse, which is what the many worlds theory essentially posits.
> They don’t actually collapse, which is what the many worlds theory essentially posits.
That's not consistent with classical physics, and it is not what MWI actually posits. The universe branching that MWI posits is not really different from wave function collapse, and decoherence also doesn't really solve things. You still fundamentally have a single position for any classical system, but multiple positions with different amplitudes for quantum systems, and some mysterious threshold where you pass from one to the other.
Probability also can't really explain this, since the classical model also applies to single particles after they have interacted with a macroscopic system.
MWI considers that observers inside one branch can't interact or notice observers inside other branches, and this is why we perceive the world as if objects have unique definite positions. But this still doesn't hold up for quantum systems, which do in fact perceive and can interact with all of the other "worlds", including interacting with themselves in other worlds such as in the single particle double-slit experiment. So MWI doesn't really get away from the duality in any rigorous way.
By not collapse I mean no specific outcome is selected. The waveform simply gets directly translated as the distribution of universes. With effectively infinite universes the odds of someone being in an unusual one are directly proportional to how unusual it is.
You can’t experimentally differentiate between MWI and single universe wave function collapse.
This is correct, MWI is a no-added sugars interpretation. Just pure and simple what the QM equations say.
There is another interpretation that can also be considered as close as possible to no-added sugars, the "ensemble interpretation". In "ensemble interpretation" the formulation of QM is understood to be applicable to ensembles of similarly prepared systems. In this interpretation QM is not really applicable to a single system by construction.
In MWI you accept it can be applied to a single system but you have to pay as a price that the system can branch into many different ones simultaneously (the "many worlds").
Thank you for the info. The relevant Wikipedia article mentions Leslie Ballentine and his textbook ("Quantum Mechanics, A Modern Development") quite a lot. I happen to have studied a couple of the latter chapters in this book and now you made me want to open it again :)
(If you can't tell, I fully subscribe to the Many World interpretation. AFAICT it's, by quite a long way, the most parsimonious explanation in terms of extra "theoretical stuff", even if it is "wasteful" in terms of the amount of actual stuff it posits.)