Ronald Held wrote:
> I am giving a talk on the Multiverse to Star Trek fans in several
> weeks. I would appreciate any advice and suggestions, since as of now,
> I have an outline based on Tegmark's four levels.
> > 

One thing I would avoid is presenting the multiverse (of any level) as the 
latest "gee-whiz, science has discovered that..."  It is interesting 
speculative metaphysics.  Good fodder for SciFi fans but not yet science. I 
cringe when Scientific American or the L.A. Times or some other popular 
publication takes scientific speculation and hypes it as though it were a 
new revelation of science.  It happens most often in medical stories, but 
also in physics and astronomy pieces.  In the end I think it debases 
science in the popular mind as just more advertising hype and spin noise.

ISTM there were several independent threads that lead to different ideas of 
the multiverse:

1) The apparent inherent randomness of QM inconsistent with "hidden 
variables" which made the "collapse of the wave function" mysterious and ad 
hoc.  The led Everett to argue for a relative-state interpretation which 
implied  the QM-multiverse.  However, theories of einselection based on 
decoherence and perhaps weak anthropic selection may undermine the 
QM-multiverse.  In this theory there is really only one universe but each 
of us consists of multiple branches on which different values are projected.

2) Completely independent of (1), development of theories of cosmogony 
based on quasi-classical quantum gravity showed the universe could arise 
from "nothing".  These theories naturally have the consequence that 
arbitrarily many other universes could also have arisen.  Any natural 
process is repeatable.  This theory of multiverses allows that universes 
exist with different values of those physical constants that seem arbitrary 
in our current theories (a set that could change).  These universes all 
exist in the same sense this universe exists, they may even have common 
points (e.g. in singularities).  Again some anthropic selection principle 
must be invoke to explain *this* universe.

3) The extremely abstract nature of physics has led naturally to the 
speculation that only the mathematics matters.  We seem to know nothing 
about elementary particles and the interior of black holes beyond the 
mathematics they satisfy. So perhaps it is only the relational properties 
of information coded into the mathematics that is the ur-stuff of the 
world.  In that case all possible information structures may be considered 
equally "existent".  This also fits with the idea that reality can be 
simulated.  If reality consists in the relation of information, it doesn't 
matter how that information is embodied or maybe that it is embodied at 
all.  Therefore reality may *be* a simulation - as on the holodeck of the 

4) Finally, what seems to have motivated Tegmark and also perhaps Bruno and 
  others is Wheeler's question, "What makes them fly?"  Why is one set 
equations instantiated in the world and others are not?  Why is there some 
special property of "existing" that some possibilities have and others 
don't.  Tegmark sidesteps the question by answering that they are all 
instantiated; so "to exist" is simply to be the subject of propositions 
that form a non-self contradictory set.  Starting from this he then must 
try to recover some explanatory power by limiting what we experience by 
appealing to some anthropic principle.

Brent Meeker

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