I'm trying to put my head around how
many multiple universes there are and
it's gotten to the point that it's
almost inconceivable - for example, the cat.
We could have a sleeping cat, we could
have an awake cat, we could have a
sleeping clap with fleas,
we could have an awake cat that's having
a fur ball, we can have an observer with
fleas and an observer who's having a
coughing fit. We can have
you standing over there as opposed to
standing over there and me holding this
in my left hand. Now if each one of those
splits like that for every single breath
that all of us are taking - I've kind of
got lost there. How does that - I mean,
that's really nice because you're like
12 of those.
 
It's actually much - worse than that one of my favorite
facts from your book is that the human
body has about 5,000 atoms that undergo
radioactive decay every second, so that's
two to the five thousand branches every
second and that's just me!
 
So how does that work with a multiverse in
a way that humans can comprehend that?
 
 
Well, imagine the real numbers between
zero and one. Divide them in half - so that you
have the numbers between zero and a half,
and the numbers between a half and one.
Then divide them in half again between
numbers between zero and 1/4 and 1/4 and
1/2 and 1/2 and 3/4 - you never run out of
numbers. 
 
How thin do the universes get? So
do they disappear they're so thin?
This becomes embarrassing at some point
but the answer is we don't know. 
 
Thank you!
The slightly longer answer is I wrote a
book and I talked about this - we don't even
know whether the total possible number
of universes is finite or infinite and
in my mind this reflects the fact that
we haven't been trying to answer this
question, we
haven't really been focusing our brains
on the foundations of quantum mechanics
to the extent that we should have in the
last 90 years. I can give you a good
argument that the number is infinite in
which case there's plenty of room. I can
also give you a good number a good
argument that the number is finite but
if the number is finite the kind of
numbers that it might be are things like
10 to the power of 10 to the power of
123 which is plenty of room for all the
divisions you need.
 
But then the universe gets
very thin it becomes something other
than another universe, it becomes a
another wave?
 
No, once you're in the
universe - so there's a couple things
going on here that I should also mention.
For one thing, breathing does not branch
the wave function of the universe, okay?
the wave function of the universe
doesn't just branch because you do
something or don't do something -  the wave
function the universe as we said before
branches when a quantum mechanical
system in a superposition becomes
entangled with the environment. That
happens a lot but it doesn't happen just
because something happened, right? The
wave function, the many worlds interpretation of quantum
mechanics does not say there's a world
in which every different possible thing
happens. It says the wave function obeys
the Schrodinger equation and the
Schrodinger equation is going to lead to
some kinds of universes and not to
others. So there's a lot of universes but
there's not literally a different
universe, a different branch of the
universe, for every possible thing you
can imagine. But nevertheless there is
something going on in your question - the
thinness of the branch you're in is not
something that you have any idea what it
is, like right now we're in a branch of
the wave function, how thick is it or how
thin is it? Yeah you have no idea, there's
no measurement you can do, there's
nothing you can do,
you don't feel thinner sadly -
you can branch the wave function
but it doesn't have any effect on how you feel.
But there is a process going on,
there are two processes going on, that
are not completely distinct - one is all
of these branches are separating from
each other, differentiating from each
other, the other is in each universe,
space is expanding, stars are using up
their fuel, galaxies are moving away from
each other, the universe is emptying out -
so even though all these branches are
becoming
different from each other, within each
branch the universe is kind of moving
toward an identical future. So when you
get far enough in the future even though
there's a lot of branches they all look
the same. So at that point way way way in
the future literally a Google years from
now it will be difficult to even talk
about different branches of the
wave function - that will no longer be an
appropriate description.
