I wanted to talk about and why I think
string theory is probably right and I'm not
a string theorist, I'm a cosmologist
there is a debate and it's an extensive
debate within the scientific community
is probably more of a debate was given
is given more credence i think by
the public that than within science itself
but you know what is the correct
theory of a quantum gravity is it string
theory or is it one of the other suggested
alternatives and I'm in the string theory camp
and just wanted to sort of say why really
so string theory basically in a nutshell
postulates that all the building blocks of nature
so you normally think of these like being little particles
string theory says that well no they're not little particles
they're little tiny strings so those strings
can be either closed strings
when they're like a loop a loop of string or and/or
they're open strings that was like an
extensive string and a different modes
of vibration that those strings give you
the different particles that you
perceive in nature.
[Brady]: but a string is longer than a particle
so wouldn't a string need to be made of particles?
No, it itself is the fundamental object
and when you look at it from it from
far away it looks like a particle
because you take it so far away
you can't you know resolve the
distances that but yeah I mean
fundamentally it's a string you can't
break it down any further it would be
the fundamental objective of nature
it's this sort of old aristotelian idea
that you break things down further and
further because he he postulates that
you reach the atom right of course now
we get to the atom and then we go we
know you could go further you can break things
down even further
well actually there is a point where
that stops it stops at the string
that's it there's no breaking down of
the string
if you look at what why where the problems in
quantum gravity arise it's when
you have point like interaction so when
things that are interacting at the point
particularly with gravitons are
interacting with Points and I think that
coupled to gravitons when that
happens you get divergencies you get
infinities and these are infinities that
you can't handle sometimes you get these
sorts of infinities in other theories as well
within the standard model for example but
you can handle them right in something
called renormalization you can't handle
them within just gravity so what string
theory does is very loosely it sort of
resolves that by instead of things
interacting with point where they can't
interacting at point anymore because the string has extense
[Brady]: it gives everyone a
bit of leg room if you like yeah it does
but what's important to realize is the
it wasn't like that was the order of the
argument okay it wasn't like somebody
said okay well let's just give it an
extense and that's gonna fix everything
it's like you know let's imagine that
things made up of strings instead
of particles it's like why strings why
not just something different why you
know snowflakes whatever right
it wasn't it that these are plugged up
there was a natural progression of that
led to this this idea and I think that
thats what really convinces me
one of the things about
theoretical physics is that on the
one it's not like we just sort of come
up with an idea and then ask
experiments to come along in maybe a
hundred years after we've died to
verify its it's not like there's not an
anything goes attitude physics is a
it's an old field now as it is more
mature has been around for longer than
take biology so we've learned to
develop a lot of rules of the game that
we we can't mess with these rules
tell us what were allowed to do next
even before we even go to an experiment
because those rules came from experiment
themselves right so in particular that
the two big guiding principles that we
have are special relativity and quantum
mechanics and just those two things when
they come together they really restrict
what we're allowed to do and it leads to
very naturally to something like string
theory why do people some people not
like string theory well string theory
it's hard at the bottom line right it it
maybe it's predicted lots of things but
but in terms of actually like one of the
things that you wanted originally from
string theory was and a unique solution
of string theory a unique solution of
string theory that describes say our
universe is compatible with our universe
now
string theory hasn't achieved that but
what you actually get out of string theory are
loads of different possible scenarios
loads of different possible universes a
truly gargantuan number and ours could be
one of them ours is compatible with one of
them but there's no unique solution so you
actually get these landscape and a lot of
people don't like that and make that
they are going to make it you lose
productivity at the end of the day what
you what we really need is to want you
to understand the mathematics of the
landscape more we can start to make
concrete statements but it at this stage
we're not at that level and the maths is
tricky but it but because of the
trickyness of the maths string theory
has also brought lots of important
developments with it within maths as well
the reason I like it is because
as I said it is the sort of it is a very
natural progression of an idea that
leads towards it and it is the same
thing that led inevitably to the to the
Higgs actually the same logic that led
to the existence of the higgs when
people talk about why the Higgs exists
right you usually talk about
it gives mass to particles or
if you're being a bit more sophisticated you might say
are because this symmetry is broken and
all that's true but the real important
thing about a Higgs is that it allows
our theory of nature to obey the rules
of special relativity and quantum
mechanics in a very particular way
what you get when you when you
consider relativity and quantum
mechanics together you get really called
quantum field theory which is the most
spectacularly beautiful theory that we
produced it's remarkably well and
you know well-tested it's it's it's it's a
great theory right
what are you really doing when you when
you do quantum field theory well you're
looking at how particles are interacting
right you might be interesting how
electrons are interacting so you fired
two electrons in each of it and you ask
what's the probability that they they
scatter by a certain amount or they
scatter through a certain angle and when
you do that you ever use quantum field theory to
do that calculate something called an
amplitude and that amplitude tells you
something about those probabilities and
when those electrons are interacting
they quantum field theory tells us that they
exchange a virtual particle which is
the photon it's not a real particle
it's not a particle that you can grab and hold
it it's a virtual particle so it has
real effects it causes of the and
electrons are scattered that but it's
not a real thing you can scatter these
things and use quantum field theory to
calculate it down to the two and
extract probability and when you do that
for the electron you get a central
number
ok you get a nice their number which is
nice and small so that we retain the
tablet of control over our calculations
and we can we can everything sensible ok
the trouble arises when you start
introducing W bosons which are different
types of particles is another particle
and W boson have have mass and that
means that they have an extra
of components that they would that they
have that they wouldn't have
if they didn't have mass and they have this
extra component which is kind of like
there because it's got mass and this is the
longitudinal component and when you
scatter the longitudinal components of W
bosons something goes wrong
it's all fine at lower energies
at low energies its fine at high-energy these aren't
you calculate these amplitudes and the
numbers get big they get big big and
they translate ultimately to
probabilities exceeding 1 this is
completely breaking down the quantum
mechanics so this tells you immediately
that something's gotta give
alright so you're like what can I do so
you look what you what you can do is you
can look at the expression the formerly
you've got for your amplitude and you
can say right it's cool at low energies I
need to fix it at high energy so i just
change the formula
ok I just fix it i just changed the
formula but then you say what can I
change it in any old way to show that it
works well at high energies as well when
you can just do anything again
quantum mechanics and relativity
commonly tell you that only certain
things that you can do you can't just do
anything so they tell you to look for
where the the poles where that where
things blow up in the amplitude look for
the locations of these poles and look
for the properties of these poles these
poles are kind of where the amplitude sort of
spikes up if it spikes up you know
when you're approaching from one
direction of it spikes up in one
direction that good if it spikes out in the
other direction that's bad because what
what are these poles they tell you about
when you do these experiments about
producing a real particle not just a
virtual producing a real one and they
tell you about the properties of those
particles and if there's spikes that are in
the wrong direction that means you've
got a bad part particle
it's inconsistent with quantum mechanics a particle
that will lead to negative probabilities
and all kinds of madness you do this you
can do this in a in a way that's
consistent for the W boson right and
it sends out the particle that you
actually introduced into the game was
the Higgs so it's softens everything
off and everything's well behaved so
this is the lesson from Higgs now let's
play the game of gravity
this is where string theory comes in
in gravity you have gravitons right these particles
of gravity if you like and you can
scatter them as well you can ask what
happens
at low energies everything's fine
at high energies things start to get big
and start to blow open this makes a
nonsense of your probabilities again
so things are going around quantum field
to be broken down you know you've lost
control and this is this is what goes
wrong with gravity so you can say right
I've got this formula again for the
amplitude can i play the same game
okay can I just say right I'm gonna
rewrite the formula so that I improve
its behavior at high-energy while keeping the
same behavioral energy so they did it
and they say but then you can't again
you can't just try anything because you've got
to check where the poles
what kind of new things have i introduced
right are they consistent with quantum
mechanics and you try and you try and
you try and it's really hard and it
seems to be like there's nothing you can
do and then one day these guys Virasoro
and Shapiro come along and they come
up with an answer and it works it doesn't
introduce any crazy new particles that
introduces perfectly well-behaved me
particles and it's like well that works
right it seems to have to say it
reproduces what we want to see at low
energies that has manifestly good
high-energy behavior it's consistent it
doesn't introduce any pop illogical new
species of particle or anything like
that it's great
and in this if nearly 50 years since this
was found nobody has come up with an
alternative right now that I've
mentioned anything about strings of this
date I and this is a beauty of it this
is the beauty of it all this was done
before anybody mention the word string
right but now you look at the formula
that you've got and you say can I
interpret it and you look and you
realise that actually it's a formula
that describes not the scattering
particles but really the scattering of
strings so wasn't that somebody said
let's try strings that never happened
you know you had this formula you
thought let's fix the formula
ok let's try to guess what the how this is
going to behave and what it has to do
and it's just kind of almost a unique
answer for what it could do and then you
go low and behold it was string theory all
along well originally of course
historically that they were looking
at other things they were looking at
just trying to explain the strong
interaction but there's this logical
argument that you can apply to gravity
and you realize actually the out pops
strings
it's not like you try strings it outpops
strings of course they're certain
assumptions that making here like that you
you're doing very standard way to try to
extend the theory into the UV and
keeping everything get weak coupling
that means that the interactions are not
getting too strong so there's some
assumptions in what I'm saying but it's a very
natural progression of ideas that lead
you along one path and that's the
string theory and that's why i buy into it really
i think most people do buy it
so i think i think i think most people
do
there are a community which don't
why don't they maybe I don't know that's
that's a controversial thing to
to actually I don't know
ok so we mentioned a bit before about
and the fact it doesn't predict this
unique state of the universe it's all this
gargantuan number of possibilities that's an
some people might interpret that as a virtue because then
you've got so many possibilities you're
going to get one that's compatible with what
we see right that it could also be
described as a weakness i don't think
it's fair to say it's not really a
testable theory i think i think that's not
true and I think you can for example you
assume that the symmetries of special
relativity are present and we find any
evidence that they're not then that
would be a proof again string theory so
it's wrong to say that it's not then
that it that it's not a testable theory
another issue that people might have
with with strings for example of BM the
cosmological constant we know that we
see out there we see the universe
acceleration that requires a positive
cosmological constant just as parameter
in your physical theory the vacuum
in the universe should be positive the
you take everything out the
universe the leftover energy it should
be positive that would be compatible
with this accelerating universe that we
see
string theory doesn't really like that
it prefers either zero or negative
that's not to say it can't necessarily
give you the positive one there are
some ideas out there which tried to
construct these with in string theory
they have been criticized in some
quarters its cutting-edge ongoing stuff
right. But that might be one perceived
weakness of string theory. Trying to get that
positive cosmological constant out but
yet there are there are very serious
work out there doing that and indeed
the claim to have done it is there an
ultimate test for proof is there are
silver bullet out
there that you could say look string theory
was right all along well I i think i think that
would be yeah I think I think so at the
moment we talked about the formula that this
form of that sort of Virasoro
Shapiro came up with that does everything
it needs to do right now it's not been
proved that that's the unique choice
that you could have made that given all
the criteria you want to reproduce the
right things at low energies and it
doesn't introduce any pathology you know
that violate quantum mechanics and all
Is it unique?
We don't know. If somebody comes along
and prove that it is unique
mathematically which is it is a
mathematical exercise now and then I
cannot infer that was kind of like it
could only have ever been string theory
right and you didn't have to do any experiment
to prove that all the other experiments
you need to prove that are the
experiments that confirmed special
relativity and quantum mechanics so that
for me would would would be great really
the other thing that string theory
typically needs is super symmetry so and
if we found evidence of supersymmetry
then that will be good
the string theory because we haven't
found it is not going to be found at the
LHC about look at it but that's not to
say that string theory is wrong it just
means that supersymmetry kicks in at
a higher energy than the LHC can't see
have two types of particles in nature you have
bosons and fermions so for example the
higgs particle is it like a boson so
is the photon is a boson the
graviton is a boson they're characterized
by the fact that they have integer
values of their spin it's like an
intrinsic spin
and they have integer values there are
also fermions they have half-integer
spin so think about the electron and
what supersymmetry does it postulates
the symmetry between these two kinds of
particles so for every boson you have a
fermion for every fermion you have a
boson and and yea that's its kind of
doubles up the number of particles but
it it is a very natural extension of the
symmetries that we know of the than the
next step would be that you would
include supersymmetry so again it's the
natural progression of ideas and string
theory requires it because without it it
predicts the it's unstable. It is the natural
path of all the ideas that we built of
twentieth-century that it in a way
string theory is a natural place that you would go
next I don't think you'd need to rewrite anything
I think it's been the opposite if string
theory was found to just somehow not be
right then you have to start thinking
about rewriting all the textbooks i think it
would be the other way around and the other
thing I suppose that people people
criticize string theory for is that it's a
theory predicts the dimension out in the
universe to be 10 space-time
dimensions which you know you look
outside and there is 4, 3 dimensions
of space one-dimension of time so where are
the other six but this is this you can
easily you know you can imagine that the
wrapped up very small or that we live on a
brane membrane or something like that or
not
not really an issue
you know I just got it is crazy to think
that you know it's so big
yeah we're looking at something so small
really into it the fans that were
breaking into the treasure box
it's ok so there this is authorized i'm
here with case all right you know what
just open the book
yes come on and what you know Moses
knows I'm Brady argue with you