Arguably the single greatest question in quantum mechanics is the
measurement question. Does the wave function really collapse, or is it merely that
the interference terms are submerged into the environmental quantum
states? I would argue that the second possibility makes the many worlds version
of quantum theory unavoidable , there being no principle that makes one
decoherent branch more "real" than another.
The collapse model of quantum theory is generally associated with the
Copenhagen "interpretation" " promoted by Bohr, Heisenberg and many others. But
in reality the question of no collapse or collapse is not an
interpretational question at all. It is a question of correct or incorrect physics. The
Copenhagen school has obscured this fact, which, in my opinion, given it
more respect than it ever deserved. This model of quantum theory lacks any
anchor in physical reality. It's a mystical , philosophically befuddled
view of physical reality. I would argue that its prominence in the early
versions of quantum theory have delayed the advance of our understanding of
the physical world for many decades.
This fact has become very evident with the rise of the theory of
Decoherence. Decoherence at last provides a real physical process to explain how
quantum measurement works. And many experiments have been done , even
involving the reverse process of re coherence which as will be made clear
shortly, provide significant support for the validly of this idea.
Nonetheless, more physically based models which provide for a real collapse
have emerged. These are the Penrose gravity based collapse and the
Ghirardi-Rimini-Weber non linear models. In my own view the effective re
coherence of simple quantum states count against these models on purely logical
grounds but these models are by no means ruled out. .
The fact that this question is often framed as interpretational , rather
than in the context of correct and incorrect physical models, can be laid
at the door of the Copenhagen school, which replaced real science with
befuddled philosophy for far too many years.
The debate between collapse and no collapse is really the debate on the
fundamental question of time symmetry. All Collapse models violate time
symmetry invariance. The no collapse models on the other hand generate an arrow
of time statistically, just as we see in classic physics. This in my view is
another reason the no collapse model makes more sense.
A look as a thought experiment can make this point clear. Measure the spin
of electrons in the Z axis. This will give us two possible results.
[ U>_Z = (1/sqrt[2])*( [U>_x + [ d>_x>)
{ d>_Z= 1/sqrt[2])*( [U>_x - [ d>_x>)
We then take every electron in the state [ U>_Z and measure it's spin for
the X axis. We would of course get 50% up and down spins. ( This would of
course be true if we used the [d>_Z state also)
Now however we time reverse the entire measurement process. Of course we
can't really do this but this is a thought experiment. ( There have been
experiments where much simpler systems have been reversed, which does lend in
my view support for the no collapse model. )
Based on the collapse models the quantum states are completely destroyed.
Therefore if we measure the spin of the electron in the Z axis after this
reversal we should get a mix of up and down spin results. However, based on
the no collapse model, the quantum state was never lost, merely submerged
in the environmental states. So the no collapse model predicts that we
would get 100% up spins in the Z axis.
This in the same as if we could time reverse all the trajectories of gas
molecules in a container so that they once again become concentrated in a
smaller volume. Of course this experiment is not possible either.
It should be mentioned here that there remains many unsolved problems in
the Decoherence model. The selection of actual basis states remains a
mystery and based on the work of Dowker and Kent we know there are many states
robust under Decoherence which are unphysical , at least in our corner of the
multiverse. I have postulated that this states may represent other stable
vacua points in the string theory landscape , but of course this is pure
speculation.