Why I do not believe in the many worlds interpretation (yet)
The main feature of the m.w.i. is the absence of a 'collapse' of the
wave function and the believe that the
various 'branches' of a wave-function are equally real [*].
One consequence is that the m.w.i inevitably needs to assume the
reality of the one wave function Ψ of
the whole universe, since it is impossible to separate the wave
function of macroscopic bodies from the
environment. But this leads immediately to two problems: First, it
seems impossible in principle
observer to determine the wave function of the universe (as I argued
here and here).
The second problem is that one needs to make assumptions about quantum
gravity [x] and in particular
that incorporating general relativity will not alter quantum theory as
we currently understand it.
Notice that in quantum mechanics Ψ depends on the time parameter t,
implicitly assuming the existence
of classical clocks. Where do such classical clocks come from in m.w.i.?
If we consider quantum field theory, it is formulated in a classical
(flat) background geometry. Since one
cannot explain the existence of the classical entities corresponding to
this background within m.w.i., it
requires a complete theory of quantum gravity -
and of course we do not
know yet what it is. (But if it
turns out that superstring theory is correct, the multiple universes of
the string landscape and the many
worlds could perhaps be a perfect match.)
The question of the
preferred basis has been discussed
at length in the literature and the usual assumption
of m.w.i. proponents is that decoherence will provide for
a preferred basis. I do not need to repeat the
arguments here but would only add that decoherence usually considers a
quantum system coupled to its
environment. But the very existence of such an environment is dubious
in my opinion, if one has to consider
the wave function of the universe to begin with.
In the words of H.D.Zeh [x]: "If the Quantum Universe is thus
conceptually regarded as a whole, it does not
decohere, since there is no further environment. Decoherence is
meaningful only for subsystems
Universe, and with respect to observations by other subsystems."
While H.D.Zeh understands this as an argument in favor of m.w.i., I
think it shows the opposite: One can
ultimately not use decoherence to solve the problem of the preferred
base, without additional assumptions.
Another issue that is often discussed is the fact that m.w.i. considers
all 'branches' of the wave function as
equally real, no matter how small the
particular, this would include a large number
worlds (e.g. worlds where crucial quantum experiments have failed,
as noticed by John Bell).
And since quantum theory (without 'collapse') is invariant under time
reversal one has to conclude that the
collection of all possible worlds shows no preferred direction of time,
which leaves me with the question where
2nd law comes from. Of course, this is a difficult problem for
other theories and interpretations as well,
but I think it is much more severe for m.w.i.
Finally, I should mention that Ian Durham has posted
an argument against m.w.i., but I have to admit that I
do not fully understand it yet 8-)
So where does all that leave us? While m.w.i. considers Ψ, the wave
function of the universe, to be the
ultimate reality, the Copenhagen interpretation assumes that wave
functions are a description
of reality and
acknowledges that (at least currently) one needs more than one
I think this is an approximation I can live with for now.
[*] I recommend this
video lecture with Sidney Coleman talking about the interpretation
of quantum theory if
you have enough time (about 1h). Also, I recommend this
list of several posts about quantum theory and m.w.i.
as an introduction to this topic.
[x] H.D. Zeh, The Physical Basis of The Direction of Time, chap. 6, in
particular p. 174 of the 5th ed.