another Schrödinger's Cat question

[tim314]

Since it was something of a digression from this thread, I decided to repost this in a new thread instead of bumping that one.

@Spatial Rift 47

Actually, it’s quite clear what an observation is: any kind of interaction that communicates information between two systems. The only confusion arises because, as Cecil so eloquently put it in the poem linked to above, “The act of observing disturbs the observed.” So yes, the cat can observe whether or not the gas canister has broken, but this in turn creates a new system: the cat-and-canister system, with which we presume nothing else has interacted. If something other than my eyes interacted with it, it would indeed collapse the wavefunction, but a new one would arise: the cat-and-canister-and-something-else system. If, however, I open the box, then I collapse the wavefunction, but from someone else’s point of view I have also created the cat-and-canister-and-me system, whose wavefunction is intact.

@Alan Smithee

So let me get this straight. You’re sitting in the lab and you open the box. From your perspective the cat has either died or not. But from my perspective, standing in the hallway, the entire particle-canister-cat-lab-you system is indeterminate until I open the door and ask you “How’s the cat?” Could the waveform collapse differently for me than it did for you? Would this place us in different universes?

Alan Smithee’s question is essentially the same thing I’m wondering about (and Spatial Rift 47 provides one answer, but I don’t know if it’s the generally accepted one).

Does a system have different wavefunctions relative to different observers? That is to say, is the wave function of a system a matter of perspective? Specifically, is the wave function of the system collapsed for observers who are themselves entangled with the system, while uncollapsed for observers who are not entagled with the system? Is there any sort of general concensus among physicists with regard to these questions?

(I mention entanglement because I’m thinking that “observing” a system means something along the lines of “interacting with the system so as to entangle its wave function with your own.” If that’s not right, or there’s a better way of defining what an observation is, please let me know.)

[Chronos]

This is not a question of physics, but of philosophy of physics. Physics only concerns itself with the results of experiments. There are some interpretations of quantum mechanics in which the state of a system depends on the amount of information available to any given observer, and there are other interpretations where the state is fixed, despite different amounts of information available. Despite this, statements about and predictions of the results of experiments will be consistent with each other, which is really all that’s relevant for physics. In other words, do the calculations, and don’t worry about which interpretation is correct.

[Squink]

Wigner