Quantum research over the last 20-30 years into decoherence/open quantum
systems has matured to the point that decoherence is now considered an
undeniable feature of our world and most certainly plays a role in explaining
the measurement problem.
Some of measurement type problems decoherence solves:
- Decoherence: How a coherent superposition of 2 different measurement options
can locally evolve into an incoherent mixture of 2 different possibilities
with probability outcomes given by the Born rule (i.e. wavefunction
- Einselection (Environmentally induced superselection): How classical states
seem to have a preferred basis i.e. $\left|alive cat\right>$ or $\left|dead
cat\right>$ cat but not $\left|alive cat\right>\pm\left|dead cat\right>$
(essentially into the basis represented by the Schmidt decomposition of the
interaction Hamiltonian between the quantum and classical system).
Now 1 explains wavefuction collapse which is half of the measurement problem,
but what decoherence by itself doesn't answer is how/why in a particular
measurement, such as of alive cat + dead cat, does nature pick the specific
outcome it does (say alive cat). This is where the MWI (with decoherence)
differs from the other decoherence based interpretations.
To explain this difference, consider flipping a coin. I flip a coin and you
have no idea what the outcome is until the coin comes up heads. So what decided
it should be heads? In a classical (deterministic) world the initial conditions
and the rules of mechanics decided what the outcome would be, so ontologically
(i.e. the real state of reality "out there") the outcome was already
determined, but epistemically (i.e. the state of the system based on your
knowledge) was the only thing that was uncertain. So a probabilistic
description of reality (in a classical world) is only needed due to subjective
lack of full knowledge.
Now the quantum world is different, because it is demonstratively
non-contextual, meaning that outcomes in general cannot have definite values in
advance. So in a quantum world probabilities are not just a demonstration of a
lack of knowledge, but are an intrinsic feature of reality itself. Wojciech H.
Zurek (one of the main physicists who discovered decoherence and one of the
biggest advocates of non-MWI, decoherence based
interpretations of QM)
describes such intrinsic uncertainty
"epiontic" i.e. such uncertainty
describes both the ontic and epistemic state.
So now if I flip a quantum coin and it comes up heads, what determined this
outcome? The MWI makes the analogy with classical probability and says it is
entirely due to the lack of full knowledge on the part of the observer,
specifically concerning which universe the observer would end up it. Whereas
the more mainline view (e.g. the view closest to the Copenhagen anti-realism)
is to maintain that there was no definite outcome in advance, i.e. nature is
intrinsically uncertain and random and additional universes are unnecessary to
simply to maintain a form of ontic determinism.