The analogous question is “when does branching occur?” Your predictions in MWI depend on what notion of “observation” you use, it is only less apparent in the formulation. To obtain some meaningful prediction, at least you have to specify what are the observables and what subsystem of the world corresponds to the observer and his mind states.
But I am not completely sure what you are speaking about. Maybe you can give a concrete example where MWI gives a unique answer while collapse formulation doesn’t?
Branching is not a physical phenomenon in MWI, it is a way humans talk about normal unitary evolution on large scales. It is not involved in making predictions, just in talking about them.
The typical example distinguishing MWI and Copenhagen is the following. Suppose I build a quantum computer which simulates a human. I then perform the following experiment. I send an electron through a slit, and have the quantum computer measure which slit it went through (that is, I tell the human being simulated which slit it went through). I then stop the electron, and let the simulated human contemplate his observation for a while. Afterwards, it is still possible for me to “uncompute” the simulated human’s memory (just as it is in principle possible to uncompute a real human’s state) and make him forget which slit the electron went through. The electron then proceeds to the screen and hits it. Is the electron distributed according to an interference pattern, or not?
If you think the answer to that question is obvious in Copenhagen, because the simulated human is obviously not an observer, then suppose instead that I replace the simulated human with a real human, maintained in such a carefully controlled environment that I can uncompute his observation (technically unrealistic, but theoretically perfectly possible).
If the answer to that question is also obvious, suppose I replace the real human with the entire planet earth.
MWI predicts an interference pattern in all of these cases. However, Copenhagen’s prediction seems to depend on exactly which of those experiments have “observation” in them. Can a quantum computer observe? Can a single isolated human observe? Can an isolated planet observe? Does collapse occur precisely when “there is no possible way to uncompute the result”? The last would give the same predictions as MWI by design, but is really an astoundingly complex axiom and probably is never satisfied.
The analogous question is “when does branching occur?” Your predictions in MWI depend on what notion of “observation” you use, it is only less apparent in the formulation. To obtain some meaningful prediction, at least you have to specify what are the observables and what subsystem of the world corresponds to the observer and his mind states.
But I am not completely sure what you are speaking about. Maybe you can give a concrete example where MWI gives a unique answer while collapse formulation doesn’t?
Branching is not a physical phenomenon in MWI, it is a way humans talk about normal unitary evolution on large scales. It is not involved in making predictions, just in talking about them.
The typical example distinguishing MWI and Copenhagen is the following. Suppose I build a quantum computer which simulates a human. I then perform the following experiment. I send an electron through a slit, and have the quantum computer measure which slit it went through (that is, I tell the human being simulated which slit it went through). I then stop the electron, and let the simulated human contemplate his observation for a while. Afterwards, it is still possible for me to “uncompute” the simulated human’s memory (just as it is in principle possible to uncompute a real human’s state) and make him forget which slit the electron went through. The electron then proceeds to the screen and hits it. Is the electron distributed according to an interference pattern, or not?
If you think the answer to that question is obvious in Copenhagen, because the simulated human is obviously not an observer, then suppose instead that I replace the simulated human with a real human, maintained in such a carefully controlled environment that I can uncompute his observation (technically unrealistic, but theoretically perfectly possible).
If the answer to that question is also obvious, suppose I replace the real human with the entire planet earth.
MWI predicts an interference pattern in all of these cases. However, Copenhagen’s prediction seems to depend on exactly which of those experiments have “observation” in them. Can a quantum computer observe? Can a single isolated human observe? Can an isolated planet observe? Does collapse occur precisely when “there is no possible way to uncompute the result”? The last would give the same predictions as MWI by design, but is really an astoundingly complex axiom and probably is never satisfied.