How Many Worlds?

Miguel Lerma wrote:
 In my opinion, as long as nobody comes up with any serious flaw 
 in the MWI that does not apply to the Copenhagen interpretation too, 
 the resistance against the MWI will not be essentially different 
 from the ancient resistance to accept that the Earth is round 
 and there are other planets and suns in the universe: purely 
 psychological.

Physical theories and interpretations are, themselves, psychological 
constructions - as are our reasons for accepting or rejecting them -
so labelling the resistance to a particular interpretation as 
"psychological" isn't especially damning.  Of course, if you're 
referring to some kind of *pathological* psychology, then that's
another matter, but a review of the literature doesn't seem to
support the notion that the widespread lack of enthusiasm for
MWI among philosophers of science is based on manifestly specious 
grounds.

R. Hughes notes that the MWI can be - and has been - subjected to
each of four types of criticisms: "(1) the internal consistency of
MWI can be challenged, (2) it's philosophical coherence can be doubted, 
(3) one can object to the lack of fit between MWI and other physical 
theories, and (4) one can critize it on general methodological grounds."
I'd refer you to Hughes's book "The Structure and Interpretation of 
Quantum Mechanics" for examples of each of these types of criticism 
that have been leveled at the MWI.

Most especially, there is a serious problem concerning the lack 
of a valid probability measure for the outcomes of events.  Those
who subscribe to the MWI typically imagine that the probability 
distribution for the various possible outcomes of a sequence of 
quantum events can be associated with the relative number of future 
"worlds" that contain one set of outcomes versus another, somewhat 
along the lines of statistical thermodynamics.  Unfortunately
it's not that simple, because there is no suitable principle of 
individuation to distinguish "worlds" in such a way that their 
relative numbers can be "counted" in any statistically meaningful 
sense.

After reviewing this and other problems, Hughes adds that the MWI
does not evidently provide us with any new understanding of the 
measurement process, because each observer is still faced with
the dualism that the interpretation sought to eliminate.  The
only difference is that any "apparent" transition not governed by
the Schrodinger equation "is now accompanied by an ontological 
cloudburst beside which the original modest dualism of von
Newmann looks unremarkable, if not pusillanimous".

Is Hughes out to lunch with his appraisal of the MWI?  Let's check
out the comments of Peter Gibbins in his book "Particles and 
Paradoxes, The Limits of Quantum Logic".  Just to give you the
flavor, Gibbins remarks that the MWI "is taken very seriously by
a few physicists and philosophers, and not at all by most others",
but please read his substantitive criticisms (page 111 and on).
He again points out the lack of a viable probability measure.

Maybe both Hughes and Gibbins are Luddite Flat-Earthers, far from
the mainstream of the most cogent current thinking on this subject.
Let's have a look at Euan Squires' appraisal of MWI in his book 
"The Mystery of the Quantum World".  After stating that "the general
view of the theoretical physics community has been to reject the
many-worlds interpretation...[although it] has become more
fashionable in recent years", and reviewing again the problem of
a lack of a suitable probability measure, Squires sums up his view
as follows:

  "My conclusion is that the many worlds interpretation
   of quantum theory only makes sense if some notion of
   selection,  generally at random and probably associated
   with consciousness, is added to the theory."

This seems to be in accord with Roger Penrose's comment in "The
Emperor's New Mind":

   "It seems to me a theory of consciousness would be needed
    before the many worlds interpretation can be squared with
    what one actually observes...  Claims have been made that
    the 'illusion' of [quantum jumps] can, in some sense, be
    effectively deduced in this picture, but I do not think
    that these claims hold up.  At the very least, one needs
    further ingredients to make the scheme work.  It seems to
    me that the many worlds view introduces a multitude of
    problems of its own without really touching upon the
    REAL puzzles of quantum mechanics."

That last comment seems consistent with Hughes's belief (also 
expressed by Squires) that the MWI entails just as much (if not
more) "dualism" as the dualistic interpretation it seeks to
replace.

But maybe Hughes, Gibbins, Squires, and Penrose are just fringe 
element cranks and Flat-Earthers who've succumbed to some silly 
'psychological' resistance to a perfectly flawless conception.
Let's see what Alistaire Rae has to say on the subject, in his
book "Quantum Mechanics" (3rd ed):

   "The idea of a near-infinite number of universes which
    can never interact with each other and whose existence
    can therefore never be verified seems to most scientists
    to be an extreme breach of this principle [of economy] and
    certainly greater than that implied by the de Broglie-
    Bohm hidden-variable theory."

Rae goes on to say

 "In recent years a combination of Many Worlds and subjectivism
  has been suggested.  This proposes that there is no collapse,
  the wave function evolves according to the time-dependent
  Schrodinger equation, and that is all there is in the physical
  universe.  However, we conscious observers are incapable of
  seeing the world this way; it is in our nature that we can
  be aware of only one result of any measurement process so,
  although the others are still out there, we are unaware of
  them.  One of the problems that this approach gives rise to
  is to explain how different conscious observers always see
  the same results, and indeed it has been suggested that this
  points to all our consciousnesses being linked to some
  'universal consciousness', one of whose jobs is to see that
  this agreement occurs!"

Those who, for psychological reasons, prefer not to embrace the 
"Universal Consciousness" theory of physics are apparently compelled
to accept the only other alternative compatible with the MWI, namely, 
solipsism (which can, admittedly, be seen as simply a narcissistic 
variation on the "universal consciousness" theory).

In our quest to find a scientist who is not afflicted with 
psychological influences, let's see what J. C. Polkinghorne has
to say in his book "The Quantum World".  He says that, in the MWI

  "Entities are being multiplied with incredible profusion.
   Such prodigality makes little appeal to professional
   scientists, whose instincts are to seek for a tight and
   economic understanding of the world.  Very few of them,
   indeed, have espoused the Everett interpretation.  It
   has, however, become more popular with what one might
   call the 'Gee-whizz' school of science popularizers,
   always out to stun the public with the weirdness of what 
   they have to offer...  Reality is not to be triffled with 
   and sliced up in this way."

Looks like Polkinghorne is no help.  How about the philosopher Huw 
Price:

  "...quantum cosmologists have become enthusiastic supporters
   of the no-collapse view...  Philosophers have been rather
   less enthusiastic, and there are a number of astute critical
   studies of the no-collapse proposal in the philosophical
   literature on quantum mechanics.  One [criticism] that is
   well-recognized by the view's proponents is... the issue
   as to why the many-branched reality it envisages should
   appear classical... but I want first to draw attention to
   a problem which seems not to be recognized at all by the
   proponents of the no-collapse view...  It concerns the
   notion of probability in a model of this kind."

Price then goes on to recount the problem discussed above, pointing 
out the lack of a suitable probability measure.  He also notes that 
the task of critiquing MWI is complicated by "the fact that the 
interpretation is ill-defined in a number of crucial respects (among 
them, the issue of what, if anything, actually 'splits')."

Maybe my selection of references is biased toward the stodgy end of
the spectrum.  Let's see if we can find any clear "non-psychological" 
thinking in David Wick's iconoclastic book "The Infamous Boundary":
"Many-worlds go, in my opinion, beyond the reasonable and into
science fiction...  Contemplating all possible worlds suggests
hubris but is philosophically respectable; claiming that they all
co-exist is not."  No help there.  Of course, one used to be able
to count on the popular writer John Gribbon to wax enthusiastic
about Many Worlds, but he lately has adopted more of an "advanced 
action" attitude, akin to Price's ideas (and John Cramer's TI),
so we can't even get any help from the author of "In Search of 
Schrodinger's Cat"!

My point in citing these authors is not to argue by authority, or 
to suggest that the issue be decided by a vote, but simply to point 
out some books in which several thoughtful people have described 
reasons for thinking that the MWI is not, as it stands, a viable 
and/or useful interpretation.