There was earlier on a slide which
showed that 5 percent of everything was
you know us the real world, that we know
 
How do you know its 5 percent
when you don't know what dark matter is made of?
Yeah, good question. So,
well there are a number of different ways of working out how much dark matter is out there
so there is the lensing method
so you can use this lensing method across
the whole sky essentially and map the distribution of dark matter
by how light is being bent through space and you can
compare how much bending you expect from how much visible material
so thats one way of getting how much dark matter there is.
another way actually the way dark matter was originally observed was by
looking at the rotation of stars around galaxies so
A  physicist called Vera Reuben in the 70s was looking at
andromeda and the other galaxies near to the earth and she measured
she measured the rotation speeds of the stars and what
you expect is that closer to the center of the  galaxy
you have lots of gravity. The stars should be orbiting very
quickly in order to not fall into the center essentially
and then as you move out through the galaxy, gravity gets weaker and
you would expect stars at the edge of the galaxy to be going around more slowly
and what she found out was that the speeds of the stars
didn't seem to depend on how far
they were from the center
they fell off a little bit but more or less they stayed constant at any distance
and that again to explain that
motion you need to invoke some invisible material that adds
an extra gravitational attraction to hold
the stars in the galaxy
and you can use that again to measure how much dark
matter there is in neighbouring galaxies to our own for example.
The other way of doing it is by looking at something called the cosmic
microwave background which is a very very
faint microwave radiation left over from the big bang
so its essentially the point at which
just about 380,000 years after the big bang
the universe before that point was like the sun
it was kind of a boiling massive super-heated plasma
at 380,000 years it expanded enough
and cooled down enough and that plasma condensed into
a gas and light, for the first time could travel freely
and that light has been basically travelling
through space ever since, and we now detect it as this very faint echo
and by looking at patterns that this
radiation across the whole sky leaves, you can calculate again
how much dark matter there is in the universe and all three of these methods
give you broadly similar answers
and the dark energy one you can work out by basically how
much acceleration is there in the
way that galaxies are moving away from our own.
So they are all complementary measurements and they basically all line up
so its a fairly solid picture,
particularly in terms of the dark matter
Anyone over there? The young man in the middle
if
so like how  you have a dimension
is to what that means that there isn't
a boson for gravity
aah very good question(so how could you explain gravity)
is there a boson for gravity is that what you are asking?
no..no basically if there is no boson for
gravity, how could you explain it?.ok, so thats a very good question
so there are two parts of the question.You asked about extra dimensions
does that mean there is no boson for gravity..well we think,
there is a particle for gravity and it would be called the graviton
and it would be a bit unlike
the other particles in that table
the problem with gravitons is that
assuming that they do exist and we sort of assume that they do
because  gravity is so weak
its very very difficult to actually measure
their effect so,
if you go down to particle level,
you have got these
these incredibly strong forces.You have got the electromagnetic force
you have got the strong nuclear force and you have got the weak force
and they totally overwhelm gravity
so if you are trying to
do some experiment where you are measuring
say, the gravitational attraction between
two particles, that attraction is so
fantastically tiny, that it's basically impossible to measure
in an experiment at the moment
if you wanted to find really sort of
really see evidence of gravitons
you have to build a gigantic particle accelerator
that was the size of the milky way, unfortunately
so, we think gravitons are out there
but we dont have any firm
evidence for
their existence but its kind of assumed that they are but they are very very
at the moment, impossible to detect directly
but sometimes actually these, extra dimensional theories
contain massive gravitons in them
and those are things you could observe in a collider, so if some of these
extra dimensions of space ideas are right
then there could be gravitons that are detectable by the LHC for example
so we might find out maybe if we are lucky.
Thank you..I have noticed
that there is a question upstairs.We will come to you as the very final question
because unfortunately we don't have a microphone upstairs
but is there anyone in the center block?maybe on the left there?
The gentleman right at the back there
I'm sorry to send you right to the top
but we will just watch Phil run up the stairs *laughter*
There we go
You said in the large hadron collider(LHC)
there are black holes created
mini black holes that disintegrate
into particles spontaneously..but we know there
are black holes in
the universe at the center of galaxies.
Why don't they decay spontaneously too?
Ahh thats a good question..they do actually, but just very slowly
so..umm
essentially what Stephen Hawking showed..i think it was in the 60s or 70s
was that a black hole,
if you are not feeding it and it's just sitting in empty space
gently radiates energy away so that it evaporates
over a very very long period of time
the bigger the black hole is, the slower the evaporation is
so essentially as a black hole evaporates, imagine you had a black hole
and you left it in space for trillions and trillions and trillions of years
very gradually, it would give off radiation and get smaller and smaller
and as it got smaller, the radiation gets higher and higher.
So basically a black hole kind of
dies very slowly, but then at the last minute it kinda goes schwoop!
very quickly and all of its energy disappears
and it would be a very violent process..so
the black hole that we..well..we haven't managed
so far there is no evidence that black holes are being created at the LHC
there is certainly no sign of them..but if you could
create one of these things because it's
basically as small as a black hole could be
it would evaporate instantaneously because of its tiny size
whereas a much bigger blackhole
would evaporate very very slowly and the level of radiation it gives off is extremely low
I want to ask you about the Alice experiment
It's a bit of a Cinderella experiment at CERN that doesn't sort of get mentioned
yeah poor Alice yeah..so..what's coming
out of the colliding heavy
ions together
you are looking for new states of matter
can you just update us on what is happening with that
and will that have informed our theories that we've just had, this
wonderful discovery, the neutron star merger
multi messenger astronomy
has that informed at CERN our theories
of how nuclear synthesis takes place
in the neutron star merger
Really, you are getting a little bit at the edge of my knowledge here to be honest
I can tell you about Alice but the neutron star..whether there is a link there I don't know
there maybe..but what Alice does
essentially is the heavy ion experiment
so for usually for about a month of the year
the LHC, rather than accelerating protons accelerated lead ions
so that the nuclei of the lead bang into each other
and when that happens
I cant remember what the atomic number of lead is..but it's a big number (82)
some 100
hundred and something( atomic mass:207) I can't remember exactly..but when those nuclei bang into each other you get a huge amount of mess essentially
so *laughs* the Alice collisions..I showed you some of the
collisions of CMS
CMS collisions have just 100s of lines in them
the Alice collisions just have 1000s of particles going everywhere
so what the objective of Alice is really trying to do is to study
a state of matter that hasn't existed in the universe
since the very short
fraction of a second after the big bang something called a quark-gluon plasma
 
so..the states of matter you maybe familiar with
you got
solids, liquids and gases
if you take a gas and heat it up even more eventually the electrons will dissociate
from the atom and you get plasma and that's
what the sun is made of mostly
if you heat a plasma even more, eventually
at very high temperatures, the nuclei
will disintegrate into protons and neutrons
if you keep heating them even more
the protons and neutrons melt effectively
and they turn into a sort of soup of quarks and gluons
which behaves a bit like a liquid
kind of this blob of extremely hot liquid
umm
and what Alice was trying to do is really to study the properties of this stuff, so you get
 
in these lead ion collisions you get little blobs of quark-gluon plasma that exist for a short period of time and then they
condense like kind of a
a liquid turning into a solid
and the particles come flying out of that blob
and it gives what you see in the detectors turning further properties of that very hot blob
i think i may get this number wrong
but Alice broke the record for the highest man made temperature
..ever..a few years ago
i think it was something stupid like 1.5 trillion
kelvin, but don't quote me on that
but it was of that order anyway..so they
are doing really extraordinary things..I don't know very
much about it and it's very exciting and it's kind of
it's really telling us something about
what the conditions of the universe would have been like shortly after the big bang
and it also helps us to understand this strong nuclear force
because the strong force is actually very
hard to study because it's
impossible actually to separate quarks
out from protons
and the only time this happens is  when you make this quark-gluon plasma where
you have this free quarks and gluons floating about
and looking at how it behaves compared to what the strong force
tells us or the theory of the strong force tells us allows us
to also understand a bit more about this force in the standard model.
Going back to the dark matter theory,
You didn't mention the modified
Newtonian gravitational theory, the MOND
Is that falling out of favour now?
It's not really been in favour for a long time I think
MOND is essentially
when dark matter was discovered, an attempt was made
 
to say lets not invoke a whole new type of physical material
what if its just the gravity that needs to be modified
we need to change the law of gravity
and maybe we can explain gravity rotation curves
and various other things just by changing Einstein's theory of relativity
general relativity essentially
and its always been a bit marginal but I think
I mean really all the evidence suggests that it doesn't really work
you cant use it to explain
all these different things that you see. Lensing, rotation curves,
 
the formation of structures in the universe..it's very hard to account
for all these different things using the modified gravitational theories
I think actually that measurement in the neutron star collisions
umm
was a really big blow to the MOND theory
the reason these..so this is
you may have heard about it.. it was in the news I think couple of weeks ago
LIGO, which was this gravitational wave detector
saw the sign of
two neutron stars banging into each other in a very distant part of the universe
and that causes these ripples in spacetime
which can be detected by these incredibly sophisticated
gigantic lasers essentially in the states
what was interesting about the neutron star collision was that it
didnt just have a gravitational wave associated with it
it had a light source associated with it
so the two or three things that LIGO
has seen before when black hole-black hole collisions
when black holes collided they don't give off any light they just go..glub
they give out lots of gravitational waves but no light
neutron stars that were not black holes, they are made of matter
very strange and very dense matter,when they bang into each other you get
you do get a lot of light produced..so I think when
LIGO detected this gravitational wave they said
look at the sky quickly and telescopes pointed at the place where
they thought the wave had come from and they saw a glow of light
and what they found was that the time
the arrival time between the light and the gravitational wave was basically
exactly the same, within a sec or few seconds
and this gravitational wave had travelled
across almost the entire universe and what that tells us
is that the speed of gravity and speed of light is basically the same
and usually in MOND theories they are not
so..
as my understanding at least there is kind of this
neutron-neutron star collision
actually is a big blow to MOND theories as well..so
they are probably i suspect there would probably be any theory where you can find any way of
explain your way why you havent seen any sign of it
but they are not really in favor at the moment
ok thanks
so are there any questions?the gentleman at the back
no women asked questions i would like women to ask questions if thats possible
*laughs* the man in the middle..i am sorry i just
wanted a laughing frenzy.Phil can you get the mic up there?
umm so you talked about anti matter so
what if they are an anti-force field
if it is possible what would
they be like?
umm..well anti force fields..you mean the umm..
so like the photon...
yeah
so the force particles do
have anti particles but they are actually themselves
so this photon is the only anti particle
so two photons can annihilate
and make an electron and positron for example..the same with  gluons
and the same with
the Ws and the Zs so they are
all their own anti particles
actually the W is one where its really obvious..let me go back
one animation
ok not too far
there we go..so
the W actually comes in two different types..the W+ and the W-
the W+ is positively charged
and the W- is negatively charged..so they are literally
antiparticles  of each other
but the photon is its own anti particle,the gluons
they are their own antiparticles as well so..
its are not quite the same but they do have
anti versions
The lady in the center..pass the mic along
I just want to ask a question about something
you mentioned at the start about cosmic energy fields  giving
mass to all particles..umm..
Is it that different particles have different fields?
or is it that theres..
the field is giving mass to all particles?
well..so there is a field
for every single particle..so for every
square in this table has a field there is..
an up quark field for example and an
up quark is just a ripple in the up quark field
and the same for the
I don't know..the muon neutrino field and the
muon neutrino is a little ripple in the muon neutrino field
so those are the fields that these particles exist in
but on top of that there is another field which is the Higgs field
and that field...
for one thing the ripple in the Higgs field  is the Higgs boson
this thing..but that field also imbued to
these other particles with the property of mass
so if you like, when you make a little ripple inside the up quark field
 
that interacts with the Higgs field around it and it
acquires mass through that interaction.
its not a very good analogy unfortunately but that is basically what is going on
5-ish minutes left
there is a question here..I will come up..you don't mind
can I ask you a question about dark energy? so dark energy is a
field of some sort
that opposes gravity..but is energy really a..
 
the right name for it or are we talking about something completely different?
there are different..well everything is energy to some extent
so any field is
a form of energy..there is energy stored in
the electromagnetic fields,the quark fields,the electron field and so on
so energy is probably fine because it basically describes pretty much anything you can think of.
umm there are a lots of different possibilities
for what dark energy could be
one of the first attempts
of explaining it was that..well i said that
all of these particles in the standard model have associated fields
and those fields even when there are no particles in them
so when there are no ripples moving about
these fields have a finite amount of energy
thats due to something called quantum
uncertainly..because of quantum mechanics
all these fields if you think about them a bit like
the surface of a pond..they are all jittering  slightly all the time
and that
jittering contains a certain amount of energy
so what people tried to do was calcuate
if you take a cubic meter space and i calculate
how much
energy is stored in all these jitters and all these fields we have got here
how  big is that energy?and could it be dark energy?
and what you find..you get a number that is something like a
ten to the hundred and twenty two times big
*laughs* so..if you basically  calculate using the standard..
well,the naive version of the standard model..some calculation to estimate
roughly how much energy is stored..you find a number that
is 10  with a 120 zeroes after it
too large..so if that were..
dark energy then the universe would have ripped itself to pieces immediately
essentially..so..
it may be that its the energy of the vacuum
which is what we are talking about here
and its just that we dont know how to really calculate it properly we are missing something
umm..it could be some additional field though
so..umm..there are various ideas that it could
be a field a bit like the Higg's field so..
an additional something called the 'inflaton'
field..or something else..which then..
something that could have been involved in the
expansion of the universe shortly after the big bang and it
has been hanging around ever and since and is still causing this..
expansion..there are some ideas that it even could be the Higg's field itself
or that the Higg's field could somehow be causing
expansion of the universe..there are loads of ideas out there but unfortunately
we don't have evidence for any of them just yet.
umm..I am sure I saw some other hands over there here we go
there is a gentleman..there..
Thank you
umm..I was wondering if the Higgs field
that is what is giving all the other particles mass,
How is it that the Higgs boson
has an associated mass with it on the diagram there
the Higg's gives itself mass is the answer to the question
*laughs* it is a bit unsatisfying but umm..
yeah, the Higgs boson also
interacts with the Higgs field and gets mass through its interaction
with the Higgs field
you have to take my word for it unfortunately
and there is a question just there as well..just..yes
thats great
Hi, can I talk a bit about the..
you said that the Higgs boson gives mass and its
mass is in like..
in general is quite related to the gravity
but apparently it's not so..
can you talk about that relation?
yeah..ok so umm
lets talk about gravity first..so what Einstein is
well we get Newton's law of gravity
what Newton's law of gravity says is that
gravity is created by mass so  if i have
two masses mass M1 and mass M2
then there is a force between them
which depends on how big those masses are and how apart they are
so,the bigger the masses the stronger the force
that is what Newton says
Einstein said something slightly different  which is that
it's not just mass but its all forms of energy and momentum
so if I take a bit of space..lets say
I don't know..a piece of the interior of the sun
that contains
electrons,protons and other things whizzing about..they all have mass
they have rest mass..but they also have
kinetic energy because they are moving and they have momentum
because they are moving about and they have a pressure
as well..so in general relativity
basically all forms of energy are sorts gravitation
not just rest mass
so that means if things are moving about quickly
they contain mass..they create mass
as well..or create gravity rather
so that is the relation between energy and mass
so when we were talking about the Higgs field
we are really talking about the rest mass of these particles so if
I take an electron and i stop it and look at it
it has this fundamental amount
of energy left in it when it is not doing anything
which is its rest mass...and that rest mass
comes from the Higgs field but..
not actually interesting..not all mass actually
not all mass ..only the rest mass comes from the Higgs field
so if you have say..a hot bit of the sun
a lot of the energy in that system that creates
gravity is not
to do with the rest mass of the particles in it
umm..actually interestingly..the proton and the neutron
if you..umm
I said their masses are about 2000 times mass of the electron..protons and neutrons are made of up quarks and down quarks
so here are these two things here..so..these have
a mass in..this is particle physics units
of about 2MeV
which is about 4 times
the mass of the electron..so we got
4 times the mass..we got two of these guys i the proton..so thats about
umm..
8 times the mass of the electron and then you add on this
and that  gives you another 8
so about 16 times the mass of the electron
but the proton is 2000 times heavier..so how does that work?
well thats because most of the mass
of the proton doesnt come from the mass of the quarks at all
it comes from
the energy stored in the binding of the strong force
that glues these quarks together inside the proton
so the Higgs field only really accounts for actually
very small amount of mass..most of the mass in our body
is the mass of the strong force
binding around inside these protons and neutrons
so..it explains a particular type of mass
when there is a link to gravitation but the
Higgs field itself doesn't actuually
do anything different to  say the electron
to the quarks..in terms of adding extra energy
that can create gravity
so we will take the final question from down here..the gentleman with the mic
hi uhh..you said that the Higgs boson
was predicted from umm..
mathematical equations
I was just wondering if lot of these theories are based on math
Why are they all correct? Isn't like
Isn't there like only one right answer
from these mathematical equations?..well these are.. I mean yeah
these are only models so..
maths is the language you use but they are not
derived from first principles..so any theory
in physics starts with some assumptions so..
for example if you take umm..
if you take general relativity..Einstein's theory on gravity
that starts from the assumption that
acceleration and gravity are indistinguishable
so thats his assumption..so if you use that you can then
with a few other things you can make progress
and derive the theory of gravity
but it's not something that just emerges from mathematics by itself..you could
write down lots of
different theories..and people indeed..people do thats what theorists spend all their days doing
actually is coming up with some different..so
the standard model starts off with some set of assumptions basically,
you are about to add in different sorts of
symmetries of nature and different sorts of particle fields
and then you can use the theory to calculate
how these things would behave but you could equally well..put in some
different fields like super symmetry or something else they are not
derived from some single fundamental principle
and then everything comes out of it
you have to start somewhere..i am gonna introduce these ingridients
and then work out what the consequences are
I guess what you are describing is sort of this like dream of
the final theory that Weinberg and others have talked about for a very long time
there may be some basic principle
that if we start with it we can workout everything and
that seems rather unlikely..I think you are always
gonna have to make some assumption at the beginning
and that assumption will determine what the theory looks like
so finally
you have been bursting to ask this question the whole time so if you could just uhh
pass the mic thank you.....Are we in the 3rd dimension
or the 4th dimension because of time?
right..so thats quite a good question.
So there are three dimensions of space that is up,
that way and that way and here is also time
so there are four dimensions but time is quite unlike
the three dimensions..so spatial dimensions
you can choose how you move through them..I can go this way..
or I can go this way or I can jump in the air
but I can't choose how I move through time but that kind of just seems to happen
I can get carried along through time
and people don't really understand
very well what time is and there are lots of ideas about
you know trying to explain..you know
trying to get time emerge from a deeper principle
but yes..the short answer to the question is yes..there are 4 dimensions including time
but we don't necessarily really know what time is.
Fantastic well..
thats a very slightly intruiging*laughs* but also
tantalising point to end this evening
iIjust wanted to end with a slight commment and..
as some of you may know and I may well get the day wrong here
but i think it was 1897 when
JJ thompson at the Cavendish laboratory
umm discovered the electron
as should correctly say the fundamental particle that later
became known as the electron which is a very long-winded way of saying
he had once called it the electron umm where did he come
to come to announce it? Well, he came right here to this very lecture theatre to announce
the electron discovery so, I would like to think Harry
that when you, and your colleagues
of the Cavendish laboratory and uhh..
the LHCB experiment solved the problem
of the Lepton universality, you'll come back here
to announce the discovery but uhh..
and in the meantime thank you so much for the fantastic talk and an excellent evening *applause*
