[Turek] Bill, what is the most persistent objection you hear to the
fine-tuning argument?
[Craig] Well among students who are not informed about these things,
the most probable objection is that the universe is not fine
tuned for our existence, that in fact physics does not exhibit the
sort of fine tuning that I've just described to you.
That is
patently false, however.
The multiplicity and the variety of
constants and quantities that have to be fine tuned in this
way is so great that the probability that any future
physics will ever eliminate it entirely is highly unlikely.
So
among physicists who are informed, the most prominent objection is
the multiverse.
They recognize that if our universe is the only one that is,
then the odds that it would be fine tuned in the way that it
is are just impossible to face; the chances are practically
infinitesimal, and therefore what they do is they multiply
their probabilistic resources by imagining that there is a
multiverse, an array of worlds of which our universe is just a
member, and moreover all of these worlds are randomly ordered in
their constants and quantities so that by chance alone, if
this ensemble of worlds is infinite, then finely-tuned
worlds will appear somewhere in the ensemble, and lo and behold,
here we are; we happen to be in this one, but nothing to be
surprised about given the multiverse of randomly tuned
worlds.
So this is the major alternative to a designer today,
the multiverse hypothesis.
[Turek] And what would be your first
comeback to this wild speculation for which there's
no evidence?
[Craig] Well apart from what you just said, Roger Penrose of
Oxford University has offered a really powerful argument against using the
multiverse to explain fine tuning, and it's basically this:
that in order for observers to exist, you don't have to have a world
that is fine tuned like our universe is.
You can have a
universe which is no larger than our solar system where you
have an island order and a sea of chaos outside of that, and
that sort of universe is vastly more probable than a
finely tuned universe like ours.
In fact, the most probable
observable universe would be an even smaller universe, one that
consists of a single brain that fluctuates into being out of
the quantum vacuum, with illusory perceptions of an
external world.
So if you appeal to the multiverse hypothesis, you
have no way of knowing whether or not you are an ordinary
observer like us, or whether or not, in fact, you are a Boltzmann
brain, as they're called, with illusory perception of a world that
does not exist.
[Turek] I like what Paul Davies says about this,
he's an agnostic astronomer, he says that the multiverse is a dodge,
because no one would be positing multiple universes if
the evidence for design wasn't so strong.
[Craig] You know now
that's a fair objection is that it's ad hoc in that it's postulated simply
to explain away the fine tuning, but there's no
independent evidence to believe that such a thing exists, or as in
the case of theism, we have many independent arguments, like the
kalam cosmological argument and others, that there is such a transcendent
creator of the universe so that it isn't ad hoc in the way that
the multiverse hypothesis is.