Is our universe the only universe Brian Greene
[Music]
[Applause]
a few months ago
the nobel prize in physics was awarded
to two teams of astronomers for a
discovery
that has been held as one of the most
important astronomical observations
ever and today after briefly describing
what they found i’m going to tell you
about a highly controversial framework
for explaining
their discovery namely the possibility
that way beyond the earth
the milky way and other distant galaxies
we may find that our universe is not the
only universe but
is instead part of a vast complex of
universes
that we call the multiverse now the idea
of a multiverse is a strange one i mean
most of us were raised to believe that
the word universe means
everything and i say most of us with
forethought
as my four-year-old daughter has heard
me speak of these ideas since she was
born and last year
i was holding her and i said sophia
i love you more than anything in the
universe and she turned to me and said
daddy
universe or multiverse
but but barring such an anomalous
upbringing it is strange
to imagine other realms separate from
ours most with fundamentally different
features
that would rightly be called universes
of their own and yet
speculative though the idea surely is i
aim to convince you that there’s a
reason for taking it seriously
as it just might be right i’m going to
tell the story of the multiverse in
three parts
in part one i’m going to describe those
nobel prize-winning results and
highlight a profound mystery which those
results revealed
in part two i’ll offer a solution to
that mystery it’s based on an approach
called string theory
and that’s where the idea of the
multiverse will come into the story
finally in part three i’m going to
describe a cosmological theory
called inflation which will pull all the
pieces
of the story together okay part one
starts back in 1929 when the great
astronomer edwin hubble
realized that the distant galaxies were
all
rushing away from us establishing that
space itself is stretching it’s
expanding now this was
revolutionary the prevailing wisdom was
that on the largest of scales the
universe was static
but even so there was one thing
that everyone was certain of the
expansion must be
slowing down that as much as the
gravitational pull of the earth slows
the ascent of an apple
tossed upward the gravitational pull of
each galaxy on every other
must be slowing the expansion of space
now let’s fast forward to the 1990s
when those two teams of astronomers i
mentioned at the outset
were inspired by this reasoning to
measure the rate at which
the expansion has been slowing and they
did this
by painstaking observations of numerous
distant galaxies allowing them to chart
how the expansion rate has changed over
time
here’s the surprise they found that the
expansion
is not slowing down instead they found
that it’s speeding
up going faster and faster that’s like
tossing an apple upward and it goes up
faster and faster
now if you saw an apple do that
you’d want to know why what’s pushing on
it similarly
the astronomers results are surely well
deserving
of the nobel prize but they raised an
analogous question
what force is driving all galaxies to
rush away from every other
at an ever quickening speed well the
most
promising answer comes from an old idea
of einsteins you see we are all used to
gravity being a force
that does one thing pulls objects
together
but in einstein’s theory of gravity his
general theory of relativity
gravity can also push things apart how
well according to einstein’s math if
space is uniformly filled with an
invisible energy sort of like a
uniform invisible mist then the gravity
generated by that mist would be
repulsive
repulsive gravity which is just what we
need
to explain the observations because the
repulsive gravity
of an invisible energy in space we now
call it dark energy but i’ve made it
smoky white here so you can see it its
repulsive gravity
would cause each galaxy to push against
every other
driving expansion to speed up not slow
down
and this explanation represents great
progress but
i promise you a mystery here in part one
here it is when
the astronomers worked out how much of
this dark
energy must be infusing space to account
for the cosmic speed up
look at what they found
this number is small expressed in the
relevant units it
is spectacularly small and the mystery
is to explain this peculiar number we
want
this number to emerge from the laws of
physics
but so far no one has found a way to do
that
now you might wonder should you care
maybe explaining this number is just a
technical issue a technical detail of
interest to experts but no relevance to
anybody else
well it surely is a technical detail but
some details
really matter some details provide
windows onto uncharted realms of reality
and this peculiar number may be doing
just that
as the only approach that so far made
headway to explain it
invokes the possibility of other
universes an idea that naturally emerges
from string theory
which takes me to part two string theory
so
hold the mystery of the dark energy
in the back of your mind as i now go on
to tell you three
key things about string theory first off
what is it well
it’s an approach to realize einstein’s
dream
of a unified theory of physics a single
overarching framework that would be able
to describe all the forces
at work in the universe and the central
idea of string theory
is quite straightforward it says that if
you examine
any piece of matter ever more finely at
first you’ll find molecules and then
you’ll find
atoms and subatomic particles but the
theory says that if you could probe
smaller much smaller than we can with
existing technology
you’d find something else inside these
particles
a little tiny vibrating filament of
energy a little tiny
vibrating string and just like the
strings in a violin
they can vibrate in different patterns
producing different musical notes
these little fundamental strings when
they vibrate in different patterns they
produce
different kinds of particles so
electrons quarks neutrinos photons all
other particles were united into a
single framework
as they would all arise from vibrating
strings it’s a
compelling picture a kind of cosmic
symphony
where all the richness that we see in
the world around us
emerges from the music that these little
tiny strings can play but
there’s a cost to this elegant
unification because years of research
have shown that the math of string
theory doesn’t quite work it has
internal inconsistencies unless we allow
for something
wholly unfamiliar extra dimensions
of space that is we all know about the
usual three dimensions of space and you
can think about those
as height width and depth but string
theory says that on fantastically small
scales there are
additional dimensions crumpled to a tiny
size so small that we have not detected
them
but even though the dimensions are
hidden they would have an impact on
things that we can observe
because the shape of the extra
dimensions
constrains how the strings can vibrate
and in string theory vibration
determines everything
so particle masses the strengths of
forces and most importantly
the amount of dark energy would be
determined by the shape
of the extra dimension so if we knew the
shape of the extra dimensions
we should be able to calculate these
features
calculate the amount of dark energy
the challenge is we don’t
know the shape of the extra dimensions
all we have is a list of candidate
shapes allowed by the math
now when these ideas were first
developed there were only about five
different candidate shapes so you could
imagine analyzing them
one by one to determine if any yield the
physical features we observe
but over time the list grew as
researchers found other candidate shapes
from five the number grew into the
hundreds and then the thousands
a large but still manageable collection
to analyze since
after all graduate students need
something to do
but then the list continued to grow into
the millions
and the billions until today the list of
candidate shapes has soared to about
10 to the 500.
so what to do well some researchers
lost heart concluding that with so many
candidate shapes for the extra
dimensions each giving rise to different
physical features
string theory would never make
definitive testable predictions
but others turn this issue on its head
taking us to the possibility of a
multiverse here’s the idea
maybe each of these shapes is on an
equal footing with every other each is
as real as every other in the sense
that there are many universes each with
a different
shape for the extra dimensions and this
radical proposal has a profound impact
on this mystery
the amount of dark energy revealed by
the nobel prize winning results because
you see
if there are other universes
and if those universes each have say
a different shape for the extra
dimensions then the physical features of
each universe will be different
and in particular the amount of dark
energy
in each universe will be different which
means that the mystery of explaining the
amount of dark energy
we’ve now measured would take on a
wholly different character
in this context the laws of physics
can’t explain
one number for the dark energy because
there isn’t just
one number there are many numbers which
means
we have been asking the wrong question
instead the right question to ask is why
do we humans
find ourselves in a universe with a
particular amount of dark energy we’ve
measured
instead of any of the other
possibilities that are out there
and that’s a question on which we can
make headway because
those universes that have much more dark
energy than ours
whenever matter tries to clump into
galaxies the repulsive push of the dark
energy is so strong that it blows the
clump apart
and galaxies don’t form and in those
universes that have much less dark
energy well they collapse back on
themselves so quickly
that again galaxies don’t form and
without galaxies
there are no stars no planets and no
chance for our form of life to exist
in those other universes so we find
ourselves in a universe with a
particular amount of dark energy we’ve
measured simply because
our universe has conditions hospitable
to our
form of life and that
would be that mystery solved
multiverse found now some
find this explanation unsatisfying we’re
used to physics giving us definitive
explanations for the features we observe
but the point is
if the feature you’re observing
can and does take on a wide variety of
different values across
the wider landscape of reality then
sinking one explanation for a particular
value
is simply misguided an early
example comes from the great astronomer
johannes kepler
who was obsessed with understanding a
different number
why the sun is 93 million miles away
from the earth and he worked for
decades trying to explain this number
but he never succeeded and we know why
kepler was asking the wrong question
we now know that there are many planets
at a wide variety of different distances
from their host star so hoping
that the laws of physics will explain
one particular number 93 million miles
well that is simply wrong-headed
instead the right question to ask is why
do we humans
find ourselves on a planet at this
particular distance
instead of any of the other
possibilities and again that’s a
question we can
answer those planets which are much
closer to a star like the sun would be
so hot
that our form of life wouldn’t exist and
those planets that are much farther
away from the star well they’re so cold
that again our form of life would not
take hold
so we find ourselves on a planet at this
particular distance simply because
it yields conditions vital to our form
of life and
when it comes to planets and their
distances this
clearly is the right kind of reasoning
the point is when it comes to universes
and the dark energy that they contain
it may also be the right kind of
reasoning
one key difference of course is
we know that there are other planets out
there but so far i’ve only speculated on
the possibility that there might be
other universes so to pull it all
together we need a mechanism
that can actually generate other
universes and that takes me to my final
part part three
because such a mechanism has been found
by cosmologists trying to understand
the big bang you see when we speak of
the big bang
we often have an image of a kind of
cosmic explosion that created our
universe
and set space rushing outward but
there’s a
little secret the big bang leaves out
something pretty important
the bang it tells us how the universe
evolved
after the bang but gives us no insight
into what would have powered the bang
itself and this gap was finally filled
by
an enhanced version of the big bang
theory it’s called inflationary
cosmology which
identified a particular kind of fuel
that would naturally generate an outward
rush
of space the fuel is based on something
called a quantum field but
the only detail that matters for us
is that this fuel proves to be so
efficient that it’s virtually impossible
to use it all up which means in the
inflationary theory
the big bang giving rise to our universe
is likely not
a one-time event instead the fuel not
only generates our big bang
but it would also generate countless
other big bangs
each giving rise to its own separate
universe with our universe becoming but
one bubble
in a grand cosmic bubble bath of
universes
and now if we meld this with string
theory here’s the picture we’re led to
each of these universes has extra
dimensions the extra dimensions take on
a wide variety of different shapes
the different shapes yield different
physical features and we find ourselves
in one universe instead of another
simply because it’s only in our universe
that the physical features
like the amount of dark energy are right
for our form of life to take hold
and this is the compelling but highly
controversial picture of the wider
cosmos
that cutting-edge observation in theory
has now led us to seriously
consider one big remaining question
of course is could we ever
confirm the existence of other universes
well
let me describe one way that might one
day happen
the inflationary theory already has
strong observational support because the
theory predicts
that the big bang would have been so
intense that as
space rapidly expanded tiny quantum
jitters from the micro world
would have been stretched out to the
macro world yielding a distinctive
fingerprint
a pattern of slightly hotter spots and
slightly colder spots across space which
powerful telescopes
have now observed going further if there
are other universes
the theory predicts that every so often
those universes can collide
and if our universe got hit by another
that collision
would generate an additional subtle
pattern of temperature variations across
space that we
might one day be able to detect
and so exotic as this picture is it may
one day be grounded
in observations establishing the
existence
of other universes i’ll conclude with
a striking implication of all these
ideas for the very
far future you see we learned that our
universe
is not static that space is expanding
that that expansion is speeding up and
that there might be other
universes all by carefully examining
faint pinpoints of star lake coming to
us from distant galaxies
but because the expansion is speeding up
in the very far future
those galaxies will rush away so far and
so fast that we
won’t be able to see them not because of
technological limitations
but because of the laws of physics the
light those galaxies emit
even traveling at the fastest speed the
speed of light
will not be able to overcome the ever
widening gulf
between us so astronomers in the far
future
looking out into deep space will see
nothing but
an endless stretch of static
inky black stillness
and they will conclude that the universe
is static and unchanging
and populated by a single central oasis
of matter that they inhabit
a picture of the cosmos that we
definitively know
to be wrong now maybe those future
astronomers will have records handed
down from
an earlier era like hours attesting to
an expanding cosmos teeming with
galaxies but
would those future astronomers believe
such
ancient knowledge or would they believe
in the black
static empty universe that their own
state-of-the-art
observations reveal i suspect the latter
which means that we are living through a
remarkably
privileged era when certain deep truths
about the cosmos are still within reach
of the human spirit of exploration it
appears that it may
not always be that way because today’s
astronomers by turning
powerful telescopes to the sky have
captured
a handful of starkly informative photons
a kind of
cosmic telegram billions of years in
transit
and the message echoing across the ages
is clear sometimes
nature guards her secrets with the
unbreakable grip
of physical law sometimes
the true nature of reality beckons from
just beyond
the horizon thank you very much
brian thank you um the range of ideas
you’ve just spoken about
are dizzying exhilarating incredible
how do you think of where cosmology
is now in a sort of historical side are
we in the middle of something
unusual historically in your opinion
well it’s hard to say
when we learn that astronomers of the
far future
may not have enough information to
figure things out the natural question
is
maybe we’re already in that position and
certain deep
critical features of the universe
already have escaped
our ability to understand because of how
cosmology evolves
so from that perspective maybe we will
always be
asking questions and never be able to
fully answer them on the other hand
we now can understand how old the
universe is we can understand
how to understand the data from the
microwave background radiation that was
set down 13.72 billion years ago and yet
we can do calculations today to predict
how it will look and it matches
holy cow that’s just amazing so on the
one hand
it’s just incredible where you’ve gotten
but who knows
what sort of blocks we may find in the
future you’re going to be around
for the next few days maybe some of
these conversations can continue
thank you my pleasure thank you thank
you brian
you