Weinberg either does not understand the multiverse theory or he intentionally misrepresents it. It is not the case that the universe splits when a measurement is made. That description begs the question because it doesn't specify what counts as a measurement. It's putting lipstick on the Copenhagen pig. A much better (though still not very good) description of the multiverse can be found in David Deutsch's book "The beginning of infinity" chapter 11. Universes do not "come into being" when measurements are made. The entire multiverse always exists. This is the pithy summary:
"Universes, histories, particles and their instances are not referred to by quantum theory at all – any more than are planets, and human beings and their lives and loves. Those are all approximate, emergent phenomena in the multiverse."
Indeed, time itself is an emergent phenomenon:
"[T]ime is an entanglement phenomenon, which places all equal clock readings (of correctly prepared clocks – or of any objects usable as clocks) into the same history."
Here is Weinberg's fundamental problem:
"[T]he vista of all these parallel histories is deeply unsettling, and like many other physicists I would prefer a single history."
Well, I'm sorry Steven, but you can't have it. That's just not how the world is, and wishing it were so sounds as naive and petulant as an undergrad wishing Galilean relativity were true, or Einstein wishing that particles really do have definite positions and velocities at all times. Yes, it would be nice if all these things were true. But they aren't.
> Weinberg either does not understand the multiverse theory or he intentionally misrepresents it. It is not the case that the universe splits when a measurement is made.
I don’t know if your point is that the universe splits more often than that or never. Note that Weinberg is referring to the usual MWI formulations, I don’t know what is the precise definition of the “multiverse theory” you mention. And actually he says that “the fission of history would not only occur when someone measures a spin. In the realist approach the history of the world is endlessly splitting; it does so every time a macroscopic body becomes tied in with a choice of quantum states.”
“when an experiment is observed to have a particular result, all the other possible results also occur and are observed simultaneously by other instances of the same observer who exist in physical reality – whose multiplicity the various Everettian theories refer to by terms such as ‘multiverse', ‘many universes', ‘many histories' or even ‘many minds'. “
The “plain English” description in the book you cite lacks any rigour and can be confusing if you know a bit of QM because it postulates that the universe splits already (but in what basis?) in the cases where single-universe QM works fine (the wave function can describe a pure quantum state which is a superposition). These “soft splits” can be undone and allow for interference (corresponding to the unitary evolution of the Schroedinger equation).
But, he says that “interference can happen only in objects that are unentangled with the rest of the world” and “once the object is entangled with the rest of the world [...] the histories are merely split further”. These “hard splits” are the branches in the MWI. And correspond perfectly to Weinberg’s description: “[the world splits] every time a macroscopic body becomes tied in with a choice of quantum states.”
In Deutsch’s multiverse the number of universes doesn’t grow because there is always an uncountable infinite number of them, but I’m not sure this makes the theory much better.
Unfortunately I don’t have time to continue this interesting discussion in the near future. At least you have to concede that in an infinite number of alternative universes you found my arguments convincing. That’s good enough for me.
> it does so every time a macroscopic body becomes tied in with a choice of quantum states
But again this just begs the question. How big does a body have to be before it counts as "macroscopic"?
> The “plain English” description in the book you cite lacks any rigour and can be confusing
Yes, that's why I said that it wasn't a very good description (despite being better than Weinberg's).
> These “hard splits” ... correspond perfectly to Weinberg’s description
No, they don't. "A macroscopic body" and "the rest of the world" are not synonyms.
> I’m not sure this makes the theory much better.
I agree with you. That's why I prefer the "zero-universe" approach, and consider our universe and the wave function to be in different ontological categories.
> Unfortunately I don’t have time to continue this interesting discussion in the near future.
:-(
> At least you have to concede that in an infinite number of alternative universes you found my arguments convincing.
"Universes, histories, particles and their instances are not referred to by quantum theory at all – any more than are planets, and human beings and their lives and loves. Those are all approximate, emergent phenomena in the multiverse."
Indeed, time itself is an emergent phenomenon:
"[T]ime is an entanglement phenomenon, which places all equal clock readings (of correctly prepared clocks – or of any objects usable as clocks) into the same history."
Here is Weinberg's fundamental problem:
"[T]he vista of all these parallel histories is deeply unsettling, and like many other physicists I would prefer a single history."
Well, I'm sorry Steven, but you can't have it. That's just not how the world is, and wishing it were so sounds as naive and petulant as an undergrad wishing Galilean relativity were true, or Einstein wishing that particles really do have definite positions and velocities at all times. Yes, it would be nice if all these things were true. But they aren't.