hey all you screenwriters professor
Brian Greene yeah that elegant universe
string theory world science festival
Brian Greene is going to show you the
correct way to time-travel ahead on
science goes to the movies welcome to
science goes to the movies a look at the
stories of science and how they change
our culture I'm faith Bailey welcome
today we have Brian Greene professor of
math and physics at Columbia University
and chairman and co-founder of the World
Science Festival Brian thank you so much
for being here thank you the idea of
time travel appears in stories as
ancient as the Sanskrit epic the
Mahabharata a Japanese tale called the
era Shima taro believed to have been
written in the year 720 tells a story of
a fisherman who spends three days in an
undersea palace and returns home to find
three hundred years have passed here in
America 1819 brought us Rip Van Winkle
but in all these stories characters only
skip forward in time and then in 1895 HG
Wells writes the time machine the first
travel backwards in time story to be
embraced by many now in the Newtonian
universe backward time travel is
impossible but then along comes I'm
Stein with his curved space-time
universe and the boundaries of time
become a little more porous and now we
have an explosion of time-travel stories
okay so Bryan Newton versus Einstein
Einstein's curved space-time does not
exclude the possibility of time travel
why is that well Einstein shows us that
time is far more flexible than we would
have thought based on a Newtonian world
which is really based on our intuitive
picture of how the universe is put
together and Einstein's vision time
becomes flexible
it can warp it can bend and that
definitely allows time travel to the
future
that's non-controversial and it may
although it's much more controversial
allowed time travel to the past as well
when you say it definitely allows time
travel to the future meaning that's that
can that can happen that is a that is
within the walls of physics as we
currently understand them if you want to
see what the earth is like a million
years from now Albert Einstein lays out
a blueprint for what you need to do to
get there would you like to know what
yeah so there are a couple of ways you
can go out in space travel near the
speed of light and turn around and come
back
your clock will tick off time very
slowly compared to clocks on earth so
you come back maybe one year has gone by
but a million years may have gone by on
the earth clocks or if you don't like
traveling near the speed of light hang
out near the edge of a black hole again
your clock will tick off time very
slowly so when you come back to earth
much more time will have elapsed which
means you will have leapfrog into
Earth's future that is time travel to
the future and there's no one who
doesn't agree with this who knows what
they're talking about so is it all
gravity and speed its speed and gravity
that's the key new feature that Einstein
injects into our understanding of time
when Einstein figured that out how many
hundreds of years after Newton a couple
hundred hundred years was it because
Einsteins mind was capable of that
thought experiment or was it because
there was technology around that Newton
could never have accessed no no it was
the mind of Einstein there was no
technology really driving what was going
on here
it was Einstein's recognition of a
deeper understanding of reality it came
from ultimately mathematics I mean can
we say Einstein was a lot smarter than
Newton no actually if you want to put
them head-to-head I think I put Newton
ahead what yeah because Newton came into
the world and there wasn't a base of
scientific understanding from him to
jump off from he had new shoulders of
giants
that's right although he said that he
did but that was just you know being
quite gracious he really invented modern
way of looking at the world the
mathematical insights the nature of her
and look there's no slouch don't get me
wrong right but in terms of
revolutionising the way we engage with
the reality Newton second to none the
character Merlin in th White's the once
and future king was the first and Amy
Adams in the 2016 film arrival is the
most recent character to live his or her
life in a time loop
remembering the future all right Brian
our Merlin and Amy Adams poor things
stuck in a closed timelike curve not
necessarily so these are characters who
somehow have access to information
knowledge about the future as you say
they can remember the future must as you
and I just simply remember the past that
doesn't mean however that they
themselves are stuck in a time loop
they simply have epistemic access to
information that ordinary people would
never have access that they can have
knowledge of things that we wouldn't
think possible and that's an interesting
way of warping the traditional
experience of time but it doesn't mean
that they're gonna live their lives over
and over and over again in a time loop
which is what a closed timelike curve
would yield so a closed timelike curve
capital C capital T capital C is a thing
it's a thing with in Einstein's general
relativity we're shockingly there are
solutions to the equations in which you
can have a particle moving through time
but somehow time is so warped that it
comes back to its starting point not in
space but in time we're all used to
being able to go into loop in space I
could get up right now and I could walk
in a loop and keep on going
nothing confusing about that I'm coming
back to my starting point in space over
and over again big deal but imagine
going around and around and coming back
to your starting point not in space per
se but in time that is mind-bending that
is literally time-bending yeah I I don't
get it it isn't what is so we have space
and we have time but Einstein told us
there's space
time yeah he never three different
things space time and space time no he
really took two things space and time
and melded them together into one thing
a unity that he called space-time so how
could you how could you come back to the
same place in time and not be coming
back to the same place in space well you
could come back to the same point in
space as well but that wouldn't really
slap your noggin right it's the fact
that you're coming back to the same
moment in time that really is the part
that that blows your mind and yes so the
the ordinary characteristics of space
that we're familiar with Einstein and
some said those kinds of features
actually apply to time too and that's
completely different Newton just says
look here space and time just elapses
universally it's the same for you and me
regardless of where we are what we're
doing and Einstein says no no motion
through space can affect the passage
through time hanging out near the edge
of a black hole a certain location in
space affects how you experience time
nobody would have thought there was that
kind of deep connection between space
and time until Einstein comes along
that's general relativity that's general
so the time is relative to where you are
in space yes exactly
that is so not and slapping yeah super
oh you're very brave all right so what I
embarrass myself at say a dinner party
if I suggested that mathematically a
closed timelike curve is possible
because the whole theory of quantum
gravity isn't actually whole parts of a
comprehensive theory of quantum gravity
are missing yeah no that's that's a
vital point whenever we talk about
Einstein's general relativity or even
Newtonian ideas of physics we're in the
realm of so-called classical physics
which means we're not taking into
account the insights of quantum physics
which is the physics of very small
things and many physicists believe that
were we were to take the ideas of
quantum physics and meld them with
Einsteins ideas of gravity general
relativity then some of these features
that we're talking about might go away
so for instance the thought is quantum
mechanics might prevent the possibility
of time travel to the past from being
executed is it possible in in light of
the fact that there is such a thing of
as quantum physics I mean there's always
been quantum physics yes wasn't named
right right can one be a classical
physicist today well you can you say oh
I prefer the class yes no there there
there are many interesting problems in
physics where the quantum implications
are so modest that you don't need to
take them into account so if you're
looking at the orbit of the moon around
the earth and the earth around the Sun
quantum physics hardly makes any
difference at all so if you want to
predict a solar eclipse you can do it
using classical ideas but yes you're
right quantum physics underlies
everything and ultimately does need to
be taken into account and sometimes
makes a huge difference is is that the
idea that out there somewhere is a
theory of everything that combines them
well theory of everything is a curious
phrase that I try to stay away from ok
that seems to suggest I'd be able to
predict what you had for breakfast and I
don't think we're going to have a theory
that there it is right so so there's a
prediction
but the the notion of unifying all our
understanding of physics general
relativity and quantum mechanics into
one whole one seamless hold that is a
major goal of physics today and that
would be a unified theory of the
fundamental forces and is that where
folks like you who know about string
theory come in yes the idea of string
theory is trying to make that unified
theory that's exactly the point Einstein
was the first to really articulate this
idea of a unified description of
nature's forces he worked for 30 years
to find the unified theory and on his
death bed was still scribbling away but
never found the theory and we think
underscore think we don't know for sure
that string theory may be the theory
that he was looking for but never found
will you be scribbling away on your
deathbed yeah I don't think it'll be
equations what love poem well there you
go okay you know I once read that
Stephen Hawking he worked on an escape
plan for characters like Merlin and Amy
Adams who would be stuck in time-travel
loops it was called the chronology
protection conjecture and and I love
that professor Hawking is so dryly funny
that in his 1992 paper presenting the
conjecture
he wrote it seems that there is a
chronology protection agency which
prevents the appearance of clothes
timelike curves and so makes the
universe safe for historians so
Hawking's paper sounds like a great
opening to a movie but is is there
really something that is the chronology
protection conjecture it could be right
i mean if you can travel to the past and
the future there are all sorts of
potential paradoxes that come up you've
seen them right and in in in movies
right that's talked about here and in
Back to the Future right Marty goes back
if his traveling to the Past prevented
his parents from ever meeting then how
would he ever be born to be there to
prevent them from meeting so it's a
paradoxical situation there so how do
you resolve that well there are there
many proposals Hawking's is one
Hawking's proposal is the way you
resolve it as you can never travel to
the past the laws of physics nobody
can't everything can't ever happen and
that would indeed make everything safe
there'd be no paradoxes history would
simply unfold in one way and that would
be it
now I should say that's not the only way
that you can protect the timeline there
are other proposals that people put
forward like like maybe if you go to the
past you cannot change the past even if
you wanted to prevail Hitler everybody
wants to go if you want to do that you
you try to pull the trigger and the laws
of physics somehow prevent that sequence
of events from ever taking place I
should say there is mathematics behind
this where people have set up the
possibility of time travel to the past
in controlled environments like just a
game of pool and they've found that you
could never have a pool ball go back
into the past and prevent that pool ball
from going into the hole by which it
traveled to the past
there can't be paradoxical situations at
least according to certain mathematical
calculations okay
how do you do it how do you do an
experiment like that this is a thought
experiment okay it's a thought
experiment an actual pool table with a
whole bunch of graduate shooting around
it and saying okay no no this is pure
mathematics but it shows you the power
of a mathematics to describe the
universe it can give you insights into
the nature of how reality unfolds that
otherwise would just be creative musings
lacking any real anchor so in all these
time-travel movies the or TV shows the
screenwriter always creates you know
time police or some other force
protecting time like in all those time
patrol stories by Powell Anderson or the
movie looper where time travel was
illegal and only organized crime
syndicates dare to use it and of course
in every fifth episode of Doctor Who the
doctor is running afoul of the Time
Lords but from what you say Brian there
there really is a time protection agency
maybe and it's called math yeah but
underscore there may be yes it could be
that math prevents this kind of time
travel to the past but the math is
suggestive right now it's not definitive
the door to travel to the past in my
opinion is still a tiny bit open and the
future will tell whether we can slip
through it door to the future totally
open yes door the past a little bit of
crack do you just honor like a personal
Brian Green level do you believe it's
possible my beliefs are shaped by
experiments and mathematics no basis for
judgment as yet do you wish you could
yes I think anything that changes the
way we grasp reality is the most
exciting experience that we can have so
yeah that would be amazing I'd love to
talk about some of the time-travel
tropes that that seem to draw on real
physics let's start with the most
obvious the the overarching rule of
fictional time-travel is create no
paradoxes and and paradox is this word
that screenwriters love to toss around
what does it exactly mean in science in
a paradox in science is a situation in
which you come to a result that's
logically inconsistent you're saying
this is true and this is not true a
logical inconsistency which we call a
paradox and we often throw around the
word paradox in science a little bit
loosely bottom line is science cannot
have any paradoxes it needs to be
logically consistent logically coherent
that's our no paradox paradox free
science makes no sense if it embraces
paradox
so does paradox involve something wrong
something yes yes exactly so every
supposed paradox that science has ever
faced when we looked at it more closely
it may have forced us to change our
understanding but we resolve those
paradoxes we find a way of making those
features of reality consistent that's
what we do but we're the paradox police
if you want to use that language alright
let's talk about Wolfgang Pauli
Wolfgang Ernst Pauli was born in April
of 1981 ear in quantum physics and
unbeknownst to him Wolfgang Pauli has
made major contributions to modern
sci-fi cinema in movies like Back to the
Future among countless others characters
travelling backwards
must always steer clear of their younger
selves and sometime trouble flicks go so
far as to say that if you touch your
younger self while time traveling you
will both immediately explode all
because of the Pauli exclusion principle
okay Brian first off what is the Pauli
exclusion exclusion principle and
probably more importantly what is your
advice to confronting your younger self
during time-travel yeah so so the Pauli
exclusion principle is a real principal
in quantum physics where it says that
certain kinds of particles they have a
name they're called
fermions doesn't matter but certain
kinds of particles cannot exist in the
same quantum state which means they
can't be in the same place and have all
of the same qualities the math forbids
that from ever happening
if you if you want to apply this to time
travel very loosely I guess I've never
seen these statements the thought is
well here are some particles that make
up me and those same particles are
making up my other self and therefore if
they come together there's something
that will prevent them from touching
because in some sense the particles will
be in the same quantum state that's a
very very loose interpretation that's a
long way to go I mean obviously the the
particles of the time travel version of
me need not be in exactly the same
quantum state as the earlier incarnation
of those particles and therefore I can
shake hands with my earlier self and be
quite safe oh okay
so your advice is maybe hug it out if
you meet your younger self see what
happens here's my my advice on time tell
if you can travel to the past do not
worry at all because I believe that the
timeline is fixed and if you travel to
the past it means that you were always
part of that moment that you are now
entering there are not two versions of
January 1st 1999 one that happened with
you there and one that happened without
you there that doesn't make any sense to
me at all what about the multiverse yeah
well the multiverse gives you another
way out of the time travel paradoxes I'm
glad that you raised that so it could be
that when you travel to the past you
don't travel to the past of your own
universe you travel to the past of a
parallel copy of your universe and
therefore if you prevent your parents
from meeting that's not such a big deal
because it only means that you won't be
born in that universe but your origin in
the original universe is completely safe
it isn't every universe your universe if
you exist in it well there are these
parallel realities and some you will be
in and some you will not and that's
perfect self mind esteem crushing if
there's a multiverse don't you what
exists in all what even goes the other
way around because you know if if there
are these various copies of you in some
of these universes maybe not all of them
who are you my more now Who am I there
like who's the real you or are they all
the real you
or has your identity been smeared out
across all these universes does this
this stuff would keep me up at night and
give me the shakes but this is your this
is your métier just are you totally
sanguine do these kind of questions this
is sort of the bread and butter of what
we do and the the deeper we head into
the mathematics sometimes the stranger
the ideas that emerge and it doesn't
mean that they're all correct but it
does mean that they're worthy of
consideration and that's what we give
them the the 2004 film the butterfly
effect starring Ashton Kutcher took its
title from a phrase and MIT meteorology
professor Edward Lawrence's 1963
groundbreaking mathematical paper with
the very boring title deterministic non
periodic flow Lorenz detailed how a tiny
change can dramatically alter pretty
much anything over time and like the
flap of a butterfly's wing the effect of
Lawrence's work was nearly imperceptible
at first but by the 1980s many
scientists across genres began to
recognize the ways Lorenz's work
challenged long-held views of the world
so Brian the time travel movie the
butterfly effect was devoted to seeing
Ashton Kutcher look hot well time
traveling but but time travel as an idea
became more commonly known as the chaos
theory is that right chaos theory yeah
Cass there is a is a important
development and our understanding of
physics and reality where as you rightly
described small changes today tiny
infinitesimal changes today can over
time yield fantastically rich changes in
the future and that means that it's very
hard to make specific and detailed
predictions in these systems because
it's so hard to lay out the state of
things today with that kind of accuracy
to not have the uncertainty of the
butterflies wings flapping does that
only apply when you're thinking about
quantum things or with anything no no
small changes can make a big difference
even in classical systems ok so Brian I
have to admit this this whole this whole
event has just been
warming you up for the big question as
as a physicist and as a mathematician
what is time
that's a hard one I don't know I know a
lot about the nature of time from
Einstein we've learned qualities of time
that you wouldn't have thought possible
many of which we've discussed here today
but if you ask me what time actually is
it's very hard to give an answer in some
sense it's that quality of reality that
allows for change we notice that time is
elapsed by things being different but to
go further than that and really grasp
time the way we can grasp other features
of reality is is an open question could
it be for instance that time itself is
made up of something more fine more
fundamental could there be atoms or
molecules of time itself that only when
those atoms and molecules arrange
themselves in the right way they yield
the conception of time that you and I
have in our intuitive brains that's a
possibility when we're sitting here even
using the word time does that implicitly
always mean space time because time is
yes in X trickable with space that's
right that's right but could this be an
emergent idea that makes sense on human
scales but perhaps the concept of time
breaks down if you try to invoke it in
the microscopic quantum domain that's a
possibility Wow so Brian there's this TV
show this is us and people love it it's
it's a it's a big soggy soap opera but
it but it's also a kind of backwards
time travel show in that these
characters the past and the present
world of this family is all happening
for them at once and in time is not
linear for this family so your father
and your husband and your scientist and
and your human do you think maybe
there's I can't think of a really
another word for it but maybe a depth to
time that we can't entirely comprehend I
think so
I would think that the next great
breakthrough in our understanding of the
universe is going to be a radical new
conception of time
time has woven its way into our theories
of reality since we started since the
time of Newton but I think that we're
missing something deep and I think that
there's a richness a quality at a time
that we've yet to really touch and when
we touch it our understanding is going
to go through a radical change the
notion of writers and artists trying to
get to the essence of of whether time is
like who we are in a timeline are we are
we who we used to be our way our future
are we all of it at once intersects with
what science is trying to get at at what
time means yes absolutely you know
somebody once described to me that what
life is about is you have this temporal
tale that keeps getting longer and
longer and you carry it with you
throughout your life and to be able to
look back at these different versions of
you that existed at different moments in
time and to recognize that in an
Einsteinian world those versions of you
still exist they've not gone away they
are part of reality at the moment at
which they existed and that's really
what we all are that's what life is
I wish we were stuck in a closed
timelike curve so I could keep talking
unfortunately that's all we have time
for for this show Brian thank you so
much for joining us today and we are so
lucky to have Professor Brian Greene
staying with us to shoot another show on
next week's show we'll be joined by the
fabulous timeless comic Lewis black mr.
black and professor green we'll be
debating the existence of Mac jokes
really seriously black and green math
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