Tom Caylor writes:
> I just don't get how it can be rationally justified that you can get 
> something out of nothing.  To me, combining the multiverse with a 
> selection principle does not explain anything.  I see no reason why it 
> is not mathematically equivalent to our universe appearing out of 
> nothing.

I would suggest that the multiverse concept is better thought of in
somewhat different terms.  It's goal is not really to explain where the
universe comes from.  (In fact, that question does not even make sense
to me.)

Rather, what it explains better than many other theories is why the
universe looks the way it does.  Why is the universe like THIS rather
than like THAT?  Why are the physical constants what they are?  Why are
there three dimensions rather than two or four?  These are hard questions
for any physical theory.

Multiverse theories generally sidestep these issues by proposing that
all universes exist.  Then they explain why we see what we do by invoking
anthropic reasoning, that we would only see universes that are conducive
to life.

Does this really "not explain anything"?  I would say that it explains
that there are things that don't need to be explained.  Or at least,
they should be explained in very different terms.  It is hard to say
why the universe "must" be three dimensional.  What is it about other
dimensionalities that would make them impossible?  That doesn't make
sense.  But Tegmark shows reasons why even if universes with other
dimensionalities exist, they are unlikely to have life.  The physics
just isn't as conducive to living things as in our universe.

That's a very different kind of argument than you get with a single
universe model.  Anthropic reasoning is only explanatory if you assume the
actual existence of an ensemble of universes, as multiverse models do.
The multiverse therefore elevates anthropic reasoning from something of
a tautology, a form of circular reasoning, up to an actual explanatory
principle that has real value in helping us understand why the world is
as we see it.

In time, I hope we will see complexity theory elevated in a similar way,
as Russell Standish discusses in his Why Occam's Razor paper.  Ideally we
will be able to get evidence some day that the physical laws of our own
universe are about as simple as you can have and still expect life to
form and evolve.  In conjunction with acceptance of generalized Occam's
Razor, we will have a very good explanation of the universe we see.

Hal Finney

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