Transcriber: Delia Bogdan Reviewer: Robert Tucker
I'm David Kaplan, you've heard.
I'm going to talk for 18 minutes or less,
on dark matter,
which is good, because we know very little about dark matter.
I will tell you what we do know,
and try to make it compelling, that it's interesting,
and it's something worth studying.
There are things that are just around,
like volcanoes you just see,
and they're there for the taking.
And dark matter had a much longer arduous story
to get to a point where people understood there's something out there,
and it's not us.
So I say on light and dark matter,
and what I really mean is on light matter and dark matter.
Things that we see -
and the thing is that most people don't understand matter either,
and so I'm going to say a little bit about what matter is
at the deepest particle level,
so you can get a sense of how we think about these things.
That galaxy -
that's an image of a galaxy, M33 or Messier 33,
and what's interesting about it is
you can look at that and other spiral galaxies
and measure, in a way, the velocity of the stars traversing the galaxy.
The way you measure velocity of something very far away is:
if emits light, then you can look at the light,
and if the light is a slightly different color or frequency than you expect
- so it's emitting light from it's atoms,
and you know the frequency is coming from those atoms and yet they're shifted -
then you can tell if the light is coming
from something moving towards you or away from you.
And in the case of this galaxy,
you can measure the velocity of all of the stars or chunks of the stars
that are coming towards you or away from you as it spirals around.
It's just at a perfect angle for doing that sort of thing.
And if you look at the stars right at the edge of the galaxy,
you measure roughly 100 km/sec, which is quite fast.
And you can measure what's called the red shift and the blue shift
of the light coming from them.
Then, if you look at the hydrogen gas,
which is actually at a much greater extent,
farther away from the galaxy,
you can look at the emission lines of the hydrogen
and ask how fast is the hydrogen going?
Now we know, we learned, or some of us learned,
in first year physics,
that Newton's law of gravity says
that as you get farther away from something,
the gravitational force is weaker.
And if it's weaker,
you can't be going very fast if you want to stay going in a circle.
So the stuff that's farther away from the center of the mass
should be going much slower than the stuff that's closer to it,
otherwise it'll go flying off
without enough strength of attraction from the central body.
And, in fact, [for] many galaxies
these velocity measurements have been made,
and what they find is: the stuff farther away is going faster,
or at least as fast as the stars inside,
and we don't know why.
So a guess would be that in fact most of the matter
is not that little bright spot that you see there,
but there's matter all over the place.
And in fact the guess is that you take a spiral galaxy,
and in that disc is all the stuff you can see,
and there's this amazingly large sphere around it or throughout it,
which is matter that you cannot see.
And we don't know what it is.
A more closely direct way of seeing what the mass is in a galaxy,
or even a cluster of galaxies,
is something called gravitational lensing.
What you can do is,
if you're lucky enough to have two bright objects
in a region of a sky that you're interested in,
one which is much farther than the other,
then you can look at the much farther objects
through the closer object.
And because, as Einstein's general relativity predicts,
and in fact that we've seen,
light bends around gravitational centers.
Mass or even energy attracts even light.
And so the light rays coming from something far away
can be bent by something that's much closer.
And we call that gravitational lensing,
because the thing closer is acting like a lens.
And so therefore you can get a very strange thing
where you're looking at a distant galaxy
and you actually see multiple images of that galaxy,
because the light rays could have come in many different directions.
And you see these funny images of the same galaxies,
sometimes shaped slightly differently.
And that then gives you a measure of how much stuff is in there,
in the intermediate region.
And what we often discover
is that that stuff is much, much greater in mass
than anything we can see.
So then you can go off and imagine:
Well, let's say there's so much stuff that we can't see,
and we'll call it dark matter for now.
Dark really just means it does not interact with light.
And so what people do
is make very large-scale computer simulations of the Universe,
which was very young,
and evolve it in time,
in which they've injected into the Universe a large amount of stuff
that doesn't interact with each other,
doesn't interact via the electromagnetic force, light,
and yet it interacts gravitationally.
And a very smooth, young Universe evolves into something like this.
And the guess is that what happens is
is dark matter shapes a structure in the Universe,
and the densest parts,
which are represented by the brightest dots there,
are where our galaxies and clusters of galaxies form.
It's where the "us", the visible stuff, falls in and creates bright spots.
Everything else, little streams and filaments in between, we can't see.
So what is that stuff? What is it?
That's our job: our job is to figure out what it is.
So, here's a very natural guess:
it's a bunch of rocks.
Rocks don't shine, planets don't shine, especially small planets,
so maybe it's just that.
So they've looked for rocks and planets and things,
and one way of looking for them
is looking at a bright spot through the galaxy,
and waiting for these little planets to pass by
and cause a deviation of the light you see
because of that gravitational lensing effect.
And so they've looked.
They're called MACHO searches for "Massive Compact Halo Objects."
The halo is the dark-matter halo.
And they've looked and tried to estimate
how many of these things are in our galaxy.
And it's way, way too little to explain it.
There is a bound on what that could be.
The chunks have to be smaller than about a tenth the size of the Earth.
And there's no known mechanism right now for producing so much of that stuff,
say Winnebago-size rocks
that completely populate our galaxy and beyond the visible part,
and explain the dark matter.
Another way of knowing that there's something funny about this stuff
is if you evolve the Universe backwards in time
in your head or on paper or on a computer simulation,
you have a time when the energy is very high,
when temperature is very high, and stuff is smushing into each other.
And at that time you can use thermodynamics
to figure out how many of the light elements
are created in the early Universe.
How much hydrogen versus helium-3 versus helium-4 and other elements
are created,
and then you could look out in the Universe now,
and count how much of that stuff is out there.
And that turns out to be an amazing measurement
of the number of protons
versus the number of photons, particles of light,
at that time in the Universe.
And we count all that,
we come, we look today,
and we discover that there are not enough protons and neutrons
to make up all of the matter in the Universe.
They don't explain the flatness of the Universe,
they don't explain the structure of the Universe.
There's some other kind of matter that's not made of protons and neutrons.
Protons and neutrons
are the majority of mass in your body right now.
There are electrons there too, but they are essentially worthless.
Protons and neutrons are basically all matter that you have ever experienced,
that is, all of mass.
And the electrons keep us attached,
in a part, so that we don't collapse into neutron stars.
(Laughter)
It's convenient, obviously.
So we need to have some description of what this stuff is.
And we now think the stuff,
which is the majority of matter in the Universe,
is not made of atoms.
It's made of something else.
And it doesn't seem to be compact, chunky objects.
So a simple possibility is that it's a new kind of particle.
Protons are particles, maybe there's a new kind of particle that is just that.
Let me talk a little bit about particle physics,
and how we think about particles.
I just produced a film about the Large Hadron Collider.
And I did it because I knew it was going to be an exciting event,
and it would be in the press,
and nobody would know what the hell anybody is talking about,
when it's being described.
And I thought this would be an opportunity to talk to people about particle physics.
A particle physics laboratory like the one in Geneva,
called the Large Hadron Collider,
is a place where they take protons, parts of atoms,
they send them at roughly the speed of light in two directions,
and they smash them into each other at incredibly high energy.
And when two protons smash into each other at those high energies,
and it's a good hit,
hundreds of particles come out.
And if you think about it,
the protons that go in, they each have some mass,
and the hundreds of particles that come out
have much, much, more mass.
So it's as if mass was created out of nothing.
Well, energy was there,
and energy was transmitted from two particles to many particles,
and, in fact, the equation that Einstein wrote
so we could put it on T-shirts, E=mc²,
(Laughter)
is reflected by particle physics,
in the sense that we're changing kinetic energy into mass,
into many massive particles.
And it's weird, because that means all that stuff
was not in the protons to begin with.
So where did they come from?
What are we looking at?
When you look through a microscope at something tiny, you do it using light.
Or at least a microscope in the visible spectrum.
And the smallest thing you can see depends on the wavelength of that light.
And so when the light bounces of something much smaller than it's wavelength,
you can't resolve it, it's a blob.
So you need higher frequency light.
Higher frequency light is higher energy.
It turns out, we know, quantum mechanics tells us,
that all particles are waves,
they all have wavelengths.
And if you want to go to very tiny, tiny things,
you have to use extremely high energies.
And you get incredibly tiny wavelengths and very high frequencies,
and then you can look at something tiny.
But what are we looking at?
We're not looking at a thing.
We're looking at nothing.
We're looking at the vacuum of space.
And when you collide two particles at extremely high energy,
what you are doing, is inserting energy into the vacuum
and seeing what it reflects out.
And those hundreds of particles are a description,
are a testament to what information is stored in the vacuum of space.
And that's the way we can produce particles
that don't exist now, but may have existed in early times in the Universe,
when there were particle colliders all over the place,
namely very high energies,
where everything was smashing into each other,
much higher energies than we could produce here on Earth.
That's particle physics.
And here are all the particles we've ever seen.
One of the amazing things about this chart
is that it was designed, not by a physicist,
but by the editor of our film, Walter Murch,
who also edited "Apocalypse Now."
So he's no physics training, but he's ready for an apocalypse!
And he - I taught him all the particles, he's done a ton of reading himself,
but he came up with this himself.
It's a beautiful chart.
This is every fundamental particle we know
that the vacuum can produce.
Everything.
In the center is the Higgs boson.
It was discovered recently,
and it completes what's called the Standard Model of particle physics.
So you look, you say OK, the red things are all quarks,
a couple of those make up protons and neutrons.
The e is the electron in the atom.
Everything else is unfamiliar to you, perhaps,
except for that gamma, which is blue, which is a particle of light.
Everything else, mostly is stable, but hard to see,
or it decays almost instantly.
The only candidate for what could be dark matter
are the green guys,
the three on the right, called neutrinos.
They turn out to be too light and too few
to make up the dark matter in the Universe.
They are in the Universe, they do make up an invisible part of it,
but it's much too small.
And so we don't know what it is.
That's my time left? Amazing! OK.
(Laughter)
I'm going to give you three possibilities of what dark matter is.
We looked for MACHOs, so then we thought maybe WIMPs.
WIMPs are "Weakly Interacting Massive Particles."
These are particles like neutrinos.
They just add to the theory, they interact very weakly,
but they are much heavier than neutrinos.
Here is a particle physics diagram for a process
where two particles we know about, the standard model, SM,
crash into each other
and produce two particles which may be new.
In this case dark matter.
That's reflecting off the vacuum
and producing something that the vacuum can't produce,
which is a dark matter particle, or two.
And the process can go both ways.
And when the Universe was very young,
and things were smashing into each other at extremely high energies,
one could produce the other.
So everybody lived together in great harmony,
standard model particles plus dark matter particles,
whatever they may be.
And if this is true,
this allows us to predict how much dark matter there is now,
based on how often that collision occurs,
and how fast the Universe was expanding at early times.
And, at some point,
the dark matter particles are too far apart from each other
to keep smashing into each other,
and they stop disappearing,
and there's a relic density of dark matter particles left over,
whatever they are.
And that depends on
detailed particle physics properties
of those things.
And so the process, standard model to dark matter,
turns off.
First, standard model particles don't have enough energy
as the Universe cools
to create dark matter,
and then eventually dark matter cannot find each other
to create standard model particles, and there's a relic.
And the parameters of the theory
will predict the number of dark matter particles
and the amount of stuff that's out there.
And what's great about this possibility is:
if you turn time on its side,
you see there's another process you can do,
which is dark matter can bump into standard model particles.
And that means you can create an experiment
where potentially dark matter is detected.
Another possibility is: since dark matter is out there,
there may be dense regions where dark matter comes together,
can annihilate and create cosmic rays that we can see.
This is indirect detection of dark matter.
And finally, the LHC, an experiment like that,
could potentially produce dark matter.
The problem being that dark matter is invisible,
and so you won't see anything.
But if you produce it in association with something else,
you can produce stuff you can see on one side,
and nothing on the other side,
and either decide energy is not conserved, or momentum is not conserved,
or that something has been produced which is invisible.
Indirect ways of seeing it, but potential.
And not knowing what the dark matter is
we have to think of all possible ways.
Another possibility is antisymmetric dark matter,
dark matter which matches what happened in the early Universe.
Antiparticles and particles lived in harmony,
but there was slightly more matter than antimatter.
And as the Universe cooled and they annihilated away,
matter was left over.
But what if dark matter was the same?
There were particles and antiparticles,
and eventually all that was left was dark matter.
Where we are with dark matter is the following:
we can't see it.
That's it.
(Laughter)
(Applause)