since there are so many different
notions of what is a multiverse I think
it it's a it's quite useful to sort of
categorize them and that has been done
yeah yeah different there's a hierarchy
of a multiverses and a guy called max
tegmark very well-known physicist he's
err classified into four levels so
varying levels of controversy so well
they say we'll begin with level one
multiverse so what tegmark cause a level
one multiverse and this is basically the
idea that there is stuff beyond the
observable universe so we think of it we
talk about the observable universe which
is basically it's around 10 to the think
26 meters distance something like that
that order of magnitude so big distance
scales that's as far as the eye can see
essentially that's the farthest away you
can go that light could have reached us
from in principle since the beginning of
the universe so pretty big distance
scale but what's beyond that that's a
legitimate question and this is the
notion of a level 1 multiverse the
universe is if you like the existence
beyond that that strike me is still
being a universe well I would agree but
in a sense they're completely
disconnected from us so in a sense that
they are distinct I think that region
beyond the the observable horizon that
we call in the cosmological horizon is
not very big but it could be enormous
the universe could be absolutely
gianormous one of the reasons that you
can make it big is due to something
called its eternal inflation so we know
that why our universe around as our core
of the universe we we think it got big
because at some point it just grew very
quickly due to something called an
influence on expanded out very very fast
got very very big and and what the
influence on the funny thing about this
in flat on field is what it does is is
it generically it won't do this ok
generically it will appear and it won't
cause a universe to expand very quickly
it won't have the right kind of
conditions for that to happen but every
so often it will okay so what this in
flat on field does it takes a little
random walk in the beginning of the
universe and eventually it finds itself
in just the right conditions to suddenly
make that little pocket of the universe
expand very quickly okay so if you
like I like to think about this as our
friend ed at Copeland he's a pretty
chilled out guy right I mean we would
agree with that very chilled out but a
thing about Ed is is they every soft and
I've known him for quite a few years now
and every so often something will rile
him doesn't happen very often but it can
happen and then he'll explode and he'll
explode the universe around it right it
doesn't have a very often but it can
happen so you could imagine ed going on
a little random walk through town okay
most things keep them nice and serene
but every so often something rouse them
in it and it blows up okay that's like
the inflow Tom field so that's kind of
how you explain how these quite unlikely
scenarios suddenly made our pocket the
universe big but the thing is the in
flatten or ed doesn't stop doing its
little random walks okay in different
regions of space it can do its little
random walks here and there and
eventually sometimes it can be it can
jump up back into the right region where
it's going to cause this explosion again
or maybe it never stopped exploding okay
so ad for example might calm down but
some in some regions of space he's being
maintained annoyed because of these
quantum fluctuations likely keeping them
annoyed okay well like the inflow town
it's being kept maintaining this
expansion of space so you have this
eternal process where regions of space
are just continue to expand our region
has stopped expanding at such a huge
rate okay the expansion is slowing down
someone but and but other reasons it
doesn't it just gives it accelerated
expansion blowing up at a crazy rate and
so this is making those regions of space
enormous and generally you expect that
some regions of space will be behaving
like this and are eternally inflating
this is why we call it a tail inflation
it and so whilst our region of space of
might have slowed down that's not true
generically now of course we now know
that our region is places it's starting
to speed up again but another story but
what this means is is that those regions
of space outside of our observable units
could be truly gargantuan and this is
where you get all the sort of crazy sort
of notions that we talked about in the
in the Googleplex video arising there's
also the idea of Boltzmann brains as
well it can arise from this but
that's probably another video so one of
the important things to realize about
level one universe is that the laws of
physics the values of the fundamental
constants they're the same here in our
corner as they are in those far reaches
level two is subtly different to that a
level two universe well you've got to
imagine now is is that there are bubbles
the universe is made up of bubbles our
corner is living inside of one bubble
where the physical constants are what
they are and so on and so forth there
are other bubbles where the physical
constants might be different where the
laws of physics look different there are
Planck's constant
you know the Newton's constant and you
know fine-structure constant all these
constants that underline you know
physics
okay these can change from from Bubble
to bubble now why might this happen well
we believe this could happen because of
string theory
string theory tells us that there aren't
just one possible universe ones possible
vacuum for the universe that you can
have there's an entire plethora of them
ten to the five hundred of them a huge
number okay this is drink theory when it
was first introduced it was hoped that
it would come up with some sort of a
unique and vacuum solution unique sort
of set of circumstances which coincided
with the universe we see and that's not
what happened it was the exact opposite
it came up with a huge number of
possibilities now of course our corner
of the universe is just one of those but
generically there's many more and what
will happen is you'll find these bubbles
forming so within one one bubble you can
have true quantum tunneling you can have
a bubble of another unit another type of
universe form with different physical
constants and different yeah this can
happen within the bubble through quantum
fluctuations and so but then that
expansion will occur with it inside that
bubble and the bubble outside it can
expand and the whole thing sort of you
know proliferate you get percolation
effects and you know the whole universe
just gets filled with these with these
bubbles of different vacua of different
where the physical constants are
different are they because I'm thinking
of bubbles in a bubble bath or like you
know stacked up against each other but
what you're talking about there sounds
more like Russian dolls well no I mean
yeah but bokes
okay it's both actually so the bubbles
can happen you know you could have a
bubble nucleate here there's a universe
inside it where the laws are that they
are everything's expanding so your
bubble might get bigger and so you don't
really notice you know or maybe that
bubble will overtake take the other one
there's dynamics involved in the whole
process some bubbles may have an even
even experience different dimensionality
different numbers of dimensions so if
you're in one bubble you might not
experience a four-dimensional universe
you might feel a five or six or seven
dimension or what what's it the
interface between bubbles
can I Drive my car from one bubble to
another well that would be quite
destructive because of course the the
laws there you know the things that's
holding your car together the atomic
forces that are holding the horn
electromagnetic forces that are holding
your car together the physical constants
that are controlling that are going to
change as you traverse the edge of one
of these bubbles and enter another
bubble okay so what your car looks like
if you get to the other side might be
something extremely different and you
know who knows I don't want to speculate
what it's going to look like right and
so I mean the edges of these bubble
walls are going to be sort of you know
highly energetic regions where basically
you're going to get sharp gradients in
the in the in how things that you know
things have changed it because you know
Planck's constant you know it's going to
perhaps change as you cross one of these
walls that's going to create a strong
gravitational effect in principle and so
these are the sort of things that are
happening so but the walls themselves
don't have to be sort of you know they
can be wide walls they don't have to be
sort of particularly thin they'll be
thin compared to the bubble as a whole
but you know it said there's a there's a
transition as you go from one to the
other and one of the things we're
actually looking for in the Cosmic
Microwave Background is evidence of
these bubbles evidence of bubble
collisions so they're looking for
circles in the sky that's what the core
it's because you know you might see two
bubbles colliding the imprint it would
have on the Cosmic Microwave Background
would be like a circle some sort of
circular shape within the temperature
fluctuations so we're actually looking
for these things but this is a level two
multiverse and I would say level one's
not particularly controversial level two
a little more controversial but still I
mean if you believe in string theory
probably by that level three on the
other hand is I would say controversial
and this is the idea that we've already
done a video about this is the idea that
you know in quantum mechanics you know
you have this sort of a idea of
probabilities so and whenever there's a
chance that ever there's any probability
ever there's any uncertainty for example
in a coin toss if we're going to sort of
try to picture it classically you know
picture it sort of microscopically it
doesn't really make sense to do that but
if you were to try to do that you might
think that whenever you toss a coin that
the universe splits in two
and it um in one way it there the coin
came up heads and me and the other way
the coin came up tails and that both
happen what physical space at two
different universes occupy the same
space like like right now as we
continually fork and split and split and
split and split and split and split you
know where are these new universes exist
well I think the idea is that we'll be
right on top of each other essentially
but you can't interfere from one to the
other there's no way of communicating
that I don't think that's really
necessary though this is this is I mean
there's a few things I don't like about
this firstly you know your kink
whenever you have a split well when does
the split occur which point does the
split occurred precisely when is that I
mean there's already time uncertainty in
this so in quantum mechanics so when
does the splits occur that's not really
clearly answered secondly and you can't
just if you do these splits then you're
increasing through the split
it's either slit you're increasing the
number of states suddenly you're
increasing the size of the space that
describes a quantum system and that
doesn't seem satisfactory either and but
the problem I have is is that this was
originally introduced or people like to
think about it because they're
uncomfortable with the idea of collapse
of the wavefunction the idea when you
open the box the cat is out of dead or
alive and not this mixture but we
actually understand why that is now it's
to do with a process called decoherence
where they the large environment outside
of the box starts to mix up with the cat
in such a way that the overlap region
between cat dead and cat alive because
of the large environmental system
outside just get made very small so if
we're okay with wavefunction collapse
because we understand it
through this decoherence I don't think
that we need a what's wrong with the
idea that the nature is probabilistic
why do we have to have that every
possibility is realized it's a quantum
mechanics is probabilistic that's the
way nature make sure what is and just
because and you know our classical sort
of intuition tells us that and you know
we enjoy a deterministic existence that
doesn't mean quantum mechanics has to
behave light doesn't mean nature's
behavior I just means that day to day
that's how it seems to behave but truly
it's probabilistic and that's fine ok
doesn't mean every possibility has to be
realized so that's level three so the
last of these is the level four and this
is the last one true the last one there
is no level five so the idea of a level
four this is very much tegmark thing and
very much controversial so the idea of a
level four multiverse is the following
we look at our universe we look at of
physical existence and we say it's
described by a mathematical structure
that seems reasonable that I can build
build up general relativity from the
rules of mathematics and they can and
they can describe the universe that I
see for example or I can build a
standard model from both math m and I
have this mathematical structure which
is the standard model plus general
activity plus all these things that
describes the universe I see and then
you say okay but there are other
mathematical structures then fact there
are many more mathematical structures
that you can think of there's math the
Wiis is just that we use to describe you
know the standard model of generality
it's just one corner of the possible
huge realm of mathematical structures
that you could imagine you're saying if
I just gave you a blank piece of paper
and said can Tonys make up some new
mathematics exactly and this this is
known as the subject of formal systems
that this it actually exists you know
you just need certain operations you
know does your mathematical structure
include multiplication for example it
doesn't have to ours does that we use to
describe the universe that we see but in
principle you could imagine a
mathematical structure that doesn't have
multiplication in it that's just as good
as any other okay so so there are other
mathematical structures and then you
have to ask the question well which ones
correspond to physical reality is it
just the one that we speak of or is it
all of them
or is it none of them well clearly not
none of them but you know it's like so
what so mark says he says well I can't
really answer that question unless I
have mathematical democracy so he says I
exploit this notion of mathematical
democracy it says that all mathematical
structures are physically real so all of
them so they have a physical reality to
them so this is the ultimate multiverse
now that all mathematical structures not
just general tivity not just string
theory all these things but anything any
kind of operation you can create in a
formal system anything that constitutes
a mathematical strut it is physically
real okay and this is quite a dramatic
thing and that's are you saying each one
has its own universe it is physically
real I mean the notion of a universe may
not even exist in some of them but it's
physically real so you see what I mean
it says it's like it may not even
contain gravity it may not even it's
kind of hard for us to imagine what what
they are because we're so used to
thinking about that you know the one in
which we're familiar with but you know
anything that is basically it's saying
that maths
is reality hey now why is this why does
tegmark like this well on one hand he
likes it because it's kind of final in a
sense that the set of all mathematical
structures is itself a mathematical
structure so it's the it's the end of
the road it sort of completes itself and
it also answers or appears to answer a
question at wheel oppose which is why
why of all the possible mathematics that
we could have to describe the universe
why this you know what's special about
this one is there anything special about
it and well it seems to answer that well
no there's nothing special about it that
all structures are real is this like the
bubbles is this saying that next door to
us is a bubble where multiplication and
I have a store I mean I wouldn't say the
bubbles nest because the notion of a
bubble line I even exist with some
mathematical structures so I mean it's
it's so abstract it's kind of it is
bordering on the well it is bordering on
the absurd right but I mean let's think
about it it even makes sense I mean on
one level it's not clear that the set of
my all mathematical structures is
well defined so that's a problem there's
also this is challenged by girdle's
incompleteness theorem which says that
there are some things which are which
are true but but can't be proven to be
true and you know is that if you have a
universe where that's the case if we
have a mathematical structure which is
like that cannot properly describe a
universe it there seems to be some sort
of weakness there and in fact ten
markers there lesson his statement to
suggest that actually only girdled
complete mathematical structures are
physically real and but again I think it
comes back to for me it comes back to
the same debate that one would have with
the with the level three multiverses
just because and they can exist and just
because you can still have mathematical
democracy but why why do they have to
exist can't we just have the exist with
a certain probability can we just build
that into it
why is it and and maybe we could have
some sort of measure amongst the the
multiverse of possible mathematical
structures and just say an odd happens
to be the one that was picked out with a
certain probability that's fine doesn't
mean all the others have to actually
physically exist so that's level four so
I as I said I think level one pretty
uncontroversial level two not too
controversial and level three and four
well make them what you will
but you know it's sometimes just fun
thinking about these things it's just
such a nice idea it's almost like it's
science fiction but serious science
fiction because the game reality to me
at least means that it has some testable
consequences you know something for
something to be real you have to be able
to interact with it in some way