Hello, everyone, and
welcome to the MIT Museum.
My name is Jennifer Novotny.
And I'm our public
programs coordinator here.
And I'm very excited
that you guys chose
to spend the first
day out of the house
after the snowstorm with us here
for our Soapbox About Quantum
Quandaries And
Other Heavy Matters.
Just by a show of
hands, how many people
have been to one of the other
two evenings in the series?
Anyone?
Yes!
How about both of the other two?
Yes!
I'm so excited!
Well, thank you guys, even
those first timers, for coming.
I hope you guys enjoy what
this evening will entail.
We have two wonderful speakers.
But before we get
into their biographies
I'd like to invite up our
moderator for the series,
Professor David Kaiser.
And pass it off to the person
who will do the introduction.
Thank you so much.
It really is terrific
to see you all here.
Most of us thought
we'd be talking
to each other in the
corner, and no one else
would get the message
to show up tonight.
So thank you for coming.
I hope you stayed
reasonably warm yesterday.
And I'm so glad
you're here tonight.
So as Jennifer was
mentioning, this
is the third and final
one in our series.
We've had two, I think,
really fun discussions so far.
And I'm really looking
forward to tonight's as well.
So tonight we're going
to move beyond the realm
of the minuscule and
microscopic and look at sort
of the vastness of space.
And then bring it
back to wonder,
does that actually have
a tie-in to the smallness
and the microscopic?
So our two speakers today.
The first's name
is Lisa Barsotti.
Dr. Barsotti's a principal
research scientist here at MIT.
She's affiliated
both at MIT's Kavli
Institute for Astrophysics.
And also a member of
the LIGO collaboration.
As you will hear soon, if
you don't know already,
LIGO stands for the Laser
interferometric gravitational
wave observatory.
They're the group who
did find, as we'll hear,
remarkable evidence in favor
of gravitational waves,
and made that announcement
just over a year ago.
Lisa completed her
PhD in applied physics
at the University of Pisa.
And after a brief
post-doctoral study in Italy
she came to MIT 10
years ago in 2007.
And she, as I say, is a
member of the LIGO team.
She therefore played
a pivotal role
with approximately 999
of her closest friends
in helping to detect
gravitational waves.
These tiny ripples in
space time itself, which
have been predicted 100 years
earlier by Albert Einstein's
general theory of relativity.
Following Lisa we'll
hear from Tracy Slatyer.
Professor Slatyer is a Jerrold
Zacharias career development
assistant professor of
physics here at MIT.
Tracy completed her
undergraduate studies
at the Australian National
University, and her PhD
at Harvard.
She then spent three years
as a post doctoral fellow
at the Institute For
Advanced Study in Princeton,
before joining the
MIT faculty in 2013.
Tracy's work is at
the intersections
of particle physics,
astrophysics, and cosmology.
And her work has focused
on trying to understand
the elusive dark matter.
So we'll hear from Lisa
first, and then Tracy,
and then we'll have time for
lots and lots of questions
and discussion.
So Lisa, take it away.
Hello everyone.
Can I be heard?
Yes?
Good.
All right.
OK.
So I'm going to talk to you
about these new messengers
from our universe,
gravitational waves.
We hope that in
the next years we
will learn a lot
from our universe,
thanks to gravitational waves.
Before I enter into the
detail of gravitational waves,
let me step back for a
second and talk a bit
about how we have learned
about our universe so far.
Primarily we have used light.
So we have built telescopes--
I'm not sure if you
can see it very well.
You can.
We built telescopes and we
have got images like this.
This is the Crab Nebula
in our Milky Way.
It's about 6,000 light
years away from Earth.
This is the image that you
get from the Hubble Space
Telescope.
And the detector here
operates in visible light.
Now, strangely this is a image
of exactly the same thing.
And I hope you can still see it.
Now you see this
purple looking thing.
And this is a different
space instrument operating
in the infrared.
And then even more
crazy thing is
that this one is an image of
exactly the same thing again.
And this comes from the
Chandra X-ray Observatory,
an outer space detector that
operates in the x-rays, right?
So the message that
I want to give you
is that the combination
all of these images,
and so all of these
different instruments,
give us insights of what
this object actually is.
And it's like each image
itself is not enough to tell us
what's happening.
And so people to
study these things now
can tell you that this one
here is a neutron start,
is a very heavy star.
It's actually spinning
so it's a pulsar.
You would have no idea of this
is looking, for example, just
in the visible spectrum.
So that's how we have been
looking at the universe for 400
years ago far.
And this Galileo.
This is the instrument
that Galileo used.
There's are the two telescopes
that Galileo built. 1610.
Actually you can see them in
Italy, at a museum in Florence.
And this was the first time
that Galileo looked at the sky.
And those are the
instruments that I just
showed you the pictures from.
The Hubble Space Telescope,
The Spitzer Space Telescope,
and The Chandra's telescope.
So in 400 years it
started from just
looking at this small patch of
the sky with the visible light
to remarkable-- those are just
a few of the many instruments
that we have today for
looking at the sky.
Now, about 100
years ago Einstein
formulates The Theory
of General Relativity.
And there are many
ways of describing
the impact of Einstein's work.
But one is that he
essentially told us
that there is another
powerful messenger
to understand how
the universe works.
It's not just light,
but it's gravity itself.
One very elegant way
of called summarizing
Einstein's Theory of General
Activity is this quote here.
Mass tells space how
to curve and space
tells mass how to move.
The meaning of this is that
the space, as we used to think
is a static thing is
actually not static at all,
can be warped.
And if you have very heavy
objects, like our sun,
for example, the space
gets curve by the sun.
And then the orbits
of the other planets
are determined by
this warping of space.
There is even a more remarkable
intuition that Einstein had,
which is that if these heavy
objects are in acceleration
you don't just get
a static warping
but you get ripples
in the space time.
And that's what we call
gravitational waves.
Another very
interesting thing is
that entirely based on
Einstein's Theory of General
Relativity you will find that
it's possible that there are
regions of space where gravity
is so strong where nothing
can escape, not even light.
And we usually call those
places, like, black holes,
right?
And so now you start to
put everything together,
Einstein is telling
us that there
might be a region
in the sky where
there is no light coming to us.
So we can't use telescope to
actually probe that place.
But the good thing is there
are gravitational waves,
these ripples of space time that
that are produced when you have
the in acceleration that can
tell us about these things
that otherwise we wouldn't know.
And so now I'm going
to show you a movie.
It's a bit dark.
Black holes, black.
But I hope you can see it.
So this is a
simulation of what you
would see if you could
be right in front
of two black holes
colliding into each other.
And this is the
actual simulation
based on Einstein's theory
of general relativity.
So there and no
observations here.
You just do the
math, essentially.
And Einstein tells you
that objects like this
could exist in our universe.
So you see the black
holes are here.
You know, to make this
simulation more clear there
are all these stars around it.
And you see this warping of
the space time around them.
And if you look, like here,
you can see these waves,
these ripples propagating here.
So now the--
Great.
I want to say this is wonderful.
Actually, Einstein
himself thought
that gravitational waves
wouldn't have really any impact
actually in physics
For this simple reason
that they are too small,
like the effect, this warping
of space time is too small.
Somehow the space is very
stiff even if it can be warped.
And so these ripples
are very, very tiny.
And so Einstein thought,
in one of his latest works
he said that gravitational waves
won't have any practical impact
on physics.
Now, several decades after that
astrophysicists and physicists
started, actually, to put
together all these pieces
and said, well, wait a second.
We just need to build a detector
that can measure tiny things.
That's all.
And in order to figure out how
to measure this thing again,
let's use this video.
This is an artistic
representation of the things
that I just showed you earlier.
The two black holes
have collided.
Those are the ripples of space
time, the gravitational waves
that propagate through space.
According to Einstein,
they propagated
at the speed of light.
Another image is throwing
a rock in a pond.
And now, if you solve
Einstein's equation
what you'd find is that
every time there is a,
you know, gravitational
wave passed
through space, this
stretches and squeeze space.
And so on Earth you'd
see something like that.
Now, the effect was
very much exaggerated.
You saw the Earth kind
of shaking around.
Just to make it
clear, the effect
is, instead, incredibly tiny,
as Einstein actually predicted.
Nevertheless, let's
look a bit more
in detail what will happen.
So imagine-- OK.
This is me traveling
with the wave, right?
What I would see in front of
me is space stretching this way
and squeezing this way.
So you are traveling
this way, and you
imagine that this circle is
now determining the plane
perpendicular to
you, that you're
traveling top of the waves.
This is what you'd would see.
Space would stretch
in one direction
and squeeze in
another direction.
And then people looked at
this and thought, OK, now,
the stretching and squeezing
is very, very tiny.
Extremely tiny.
But I think I know how to
build an instrument that
can measure that.
And the instrument is
a micro-interferometer.
So you have a laser source here.
This is a beam splitter.
This is a mirror to split the
lasered light in two paths.
And those are two
objects that are
free to move [INAUDIBLE]
those are reflective mirrors.
So light bounces off the
mirrors and come back to you.
And now imagine that
the circle that I
showed you is on this plane.
And the gravitational waves
is coming through this,
and stretching, and
squeezing space.
This is what you would see.
This is your laser light.
Gravitational waves arrive,
and they stretch and squeeze.
And so that's the
equivalent to see
this mirror moving that way.
So here is a slightly more
detailed representation.
This is the electromagnetic
wave of the light of our laser.
And what you can do is to set
up the position of this mirror
such that there is no light
leaving the interferometer when
there are no waves.
And then as soon as
a gravitational wave
arrives, and stretching,
and squeezing space
you start seeing
some light here.
And so the whole point is that
the change in the interference
pattern of light, the output
part of the interferometer,
has encoded the property
of the gravitational waves.
And the gravitational
waves are themselves
carrying the information about
the source that produced them.
So that was the intuition.
The intuition was to use
laser interferometers
to measure this very, very tiny
distortion of space time caused
by gravitational waves
produced by acceleration
of heavy objects
in the universe.
And so this would be a new
messenger from our universe.
You could see things that
telescopes cannot see.
Now, I've been telling
you this very tiny
but let me quantify how tiny.
Imagine I'm an
object of one meter
and typical gravitational
waves come through me.
They will stretch and squeeze
me by 10 to the minus 21 meter.
This is one thousandth
the radius of a proton.
This is extremely,
extremely small.
And so since the original ideas
of using laser interferometer
it took four decades
to actually build
instruments so sensitive that
could measure this tiny thing.
What you really wanted
is a magic, right?
Where you could have
and try to measure.
That it's impossible.
You need very
sophisticated instruments.
These instruments are
the LIGO interferometer,
gravitational wave
observatories,
those are two giant
interferometers
in the United States.
This one is located in
Hanford, Washington.
The other one is in
Livingston, Louisiana.
This is the
[INAUDIBLE] Michelson.
The laser is here.
And the mirrors that sense
the distortion of space time
are four kilometers apart.
One is here.
And one is down there.
Between interferometry
in Louisiana
you can distinguish very well,
because one is in the jungle.
The other one is in the desert.
So these instruments are being
funded by the National Science
Foundation in the 90s.
In the beginning
of the 2000s they
started to be built and
installed this huge vacuum
system.
This is the largest
ultra high vacuum
system in the United States.
Because you're
measuring this tiny,
tiny effect you cannot allow
to have air in the arms
of your interferometer.
Otherwise the laser beam won't
be able to actually sense
the space time distortion.
So those are like
huge instruments.
The mirror that I've shown
you that moves around--
we are on Earth.
If you attach anything
to Earth nothing moves.
Gravity keeps your mirror down.
So you want this meter
to be free to move.
And so what we do we,
have a very sophisticated
seismic isolation system
that isolates your mirrors
from the ground.
So this is a giant object.
Both they LIGO detector started
taking data in the last decade
until 2010.
No detection was reported.
Look at the data.
There was more than one
year of coincidence data
of these detectors.
No gravitational waves observed.
In 2010 a major upgrade of
these two instruments happened.
Only what you see here,
essentially, remained in tact.
All the components
inside the vacuum system,
like the way you suspend the
optics, the power of the laser
that you use, all the technology
was improved substantially,
such that the
instruments were then
able to sense even more smaller
distortion of the space time.
Those two detectors
in 2015 came back
online after five years of
upgrade of the instrument.
Now, these instruments
have the peculiar thing
that they operating in
the audio frequency.
So when we saw the change
in interference patterns
in interferometer
you can actually
play that signal, put
the signal on a speaker,
and you could hear it.
So when the detectors
came online,
this was actually
after a few days,
both detectors were
operating properly.
This happened.
So now I'm trying to play this.
See if this is at
the maximum already.
So what you will hear is the
noise of the instrument first.
And then you will
hear a chirpy sound.
And if you can't hear
it I will do it for you.
So it will be more fun.
The boop boop, that's the
chirpy sound at the very end.
You listen to it twice.
Let's try again.
The first time you hear
it at the exact frequency
where the signal
appeared in the data.
And the second time is actually
shifted up in frequencies such
that it's easier to hear.
So this is the same signal.
This time it's spotted in time.
And this is the frequency
of the signal itself.
And this is how the
signal looked like here.
And this is the
component in frequency
where the frequency increases
in a very, very short period
of time.
You see, this is just
a tenth of a second.
So it's a very short
period of time.
So one you could say
40 years so development
of the technology for less
than a second of sound.
But the remarkable thing here
is that what you just heard--
let me say it from
the beginning--
it's the sound produced
by two black holes--
and in a second I will tell
you why we can say that--
two black holes that
collided in the universe
1.3 billion years ago.
And you heard them because of
the distortion of the space
time that they produced.
So this is like, every time
I see it I'm like, wow,
this is really incredible.
So the point is, thanks
to general activity
you can actually calculate
exactly what the signal should
look like, what the amplitude
of the gravitational waves
produced by collision of
these heavy objects look like.
And so by analyzing the
particular signature
of this signal we can say what
the masses of these objects
are.
In this particular case they
were two massive black holes.
One is 29 solar mass.
And the other one
is 36 solar mass.
So 30 times the
mass of our own sun.
Two heavy objects like this
that collided into each other
to form a the third
black hole about 60 times
the mass of our sun.
And we can tell all of this
thanks to general relativity,
because we can extrapolate very
well all of the information
from this signal.
Now, the fact that
we have two detectors
is crucial here for two reasons.
The first one is the effect
that you're trying to measure
is so tiny that you
want to make sure
that you don't have
some accidental effect
lock on to your instrument
that this causing that.
So what we do is we synchronize
the data from these instruments
and we look at the data.
And we make sure that you
get exactly the same signal
within the window that takes
the light to go from one side
to another, which
is 10 milliseconds.
So the window in which these
two signals need to appear
is very, very tiny.
The other nice thing about
having more than one detector
is that you can localize
your source in the sky,
because you know
gravitational waves
travel at the speed of light.
So if you have two
detector you know
it's arriving here at this
time, or here at this time.
And you can draw
an arc in the sky
and tell pretty much
where it comes from.
The more detectors you
have the more precisely
you can localize the source.
So in some sense,
gravitational wave detectors
will, one day,
work as telescopes
where they can actually tell
you exactly where the source is.
So this happened, I told
you, three days after we
turned on the detectors.
We almost didn't believe
it that this was possible.
This was September, 2015.
In December there was
another event similar.
Smaller black holes
but similar data.
So LIGO, so far, have detected
two collision of black holes.
Then in January,
last year we stopped
to upgrade the instruments more,
and try to make them even more
sensitive, and we just
started last November
to take data again.
And we will through the summer.
And then at the
end of the summer
you will hear if we have
detected more things.
But the message is
that this is just
the beginning of what we think
is a new era in astrophysics.
We won't have only light.
But we will have
gravitational waves, as well,
that tell us about
objects in the sky
that we wouldn't know
about just using light,
because they're dark.
And then more remarkably, we
will also learn more things
in conjunction with
having telescopes
and gravitational
wave detectors,
because there are very energetic
phenomenon in our universe that
emits a lot of lights.
But they also have gravitational
waves associated with them.
And as for
electromagnetic waves,
as for light, what we
will think the future
is, is something like this
where you have many detectors.
Those are the LIGO detectors
that I've just shown.
They are not alone.
There are more
detectors on Earth.
There is one in Italy,
where it actually started.
And there is one under
construction in Japan
right now.
And one will be built in India.
So we will have a
network of detectors.
They operate at the
audio frequency.
But the gravitational
wave spectrum
is much broader than that.
And so there are,
already in existence,
other instruments
that are trying
to probe gravitational
waves at lower frequencies.
One is the LISA
project that's trying
to measure gravitational
waves by building
an interferometer in space.
And there are many
other detectors
that are trying to probe
much lower frequencies.
And so this is really
the reason why we are all
very excited about this,
because we believe that this
is like Galileo
pointing his telescope
at the sky for the first time.
This is the first
time in the history
that we have turned on
different type of detectors
that are opening a complete
new window on our universe.
Thank you.
OK.
Testing.
Can people at the
back here me OK?
Great.
All right so I am likely set
among the theorists rather than
the experimentalists.
But I'm going to
tell you about one
of the big theoretical questions
perplexing fundamental physics
at present, which is this
question, what is dark matter?
So this is an ongoing
puzzle that we're
trying to put together.
People sometimes
say, oh, you know,
we know very little
about dark matter.
And it's easy to get the
impression that this is just
a complete mystery.
We actually have several
pieces of the puzzle already.
We know quite a bit
about dark matter.
And I'll give you some
of the evidence for that
in the next few slides.
So what do I mean when
I say dark matter?
We've had evidence.
The first evidence
stretches back to the 1930s,
but really the 1970s was
when the issue started
to really intrude
upon the consciousness
of astrophysicists and
cosmologists as a problem.
We know that about 80 percent
of the matter in the universe
is dark, in the
sense that it doesn't
interact with visible light.
As far as we can tell, it's not
made up of any of the particles
that we know about.
We do know that this
matter appears to have
mass, and hence, gravity.
Lisa just gave us
a beautiful talk
about this new field
of astronomy using
gravitational waves.
But we've been
using visible light
to try to understand the
gravitational properties
of objects for much longer.
So we know that dark
matter gravitates.
But we also know that it doesn't
appear to scatter, or emit,
or absorb light at all.
In fact, you could call it
transparent matter rather than
dark matter.
It would actually be
transparent rather than black
if you were to have a
block of it in front of you
and stare at it.
As I said, the rest is
really negative information.
We just know what
it's not made out of.
So the open questions are many.
What is this new
matter made from?
Are we looking at a
new particle here,
like the Higgs Boson that
was discovered at the LHC
a few years ago, only stable?
Are we looking at black
holes like the ones
Lisa talked about?
Yeah, are we looking
at this huge population
of black holes left over
from the early universe?
Whatever the dark matter
is, where did it comes from?
Why is it 80% of the
matter in the universe?
I've told you it doesn't
interact with light.
Can it interact with
ordinary particles at all?
If so, how?
So first I want to just give
you a bit of an introduction
to what we already know,
why we think this stuff,
this dark matter is
out there anyway.
And then I want to
say a little bit
about what people are
doing to search for it,
and where my research fits in.
So first to situate
us in the universe,
we live in the Milky Way Galaxy.
This is a time lapse photograph
of the Milky Way Galaxy taken
from a point on the
coast of Australia,
where I'm from originally.
That means that we live in
a huge spiral disk of stars.
We're about eight and
a half kiloparsecs,
which is about 25,000 light
years out from the sentence.
So I've drawn a cartoon
image on the right hand
side of where the Earth
is relative our galaxy.
We, like everything
else in our galaxy,
is rotating around that center.
Just as the planets
rotate around the sun.
Now, it was noticed in
the 1970s by Vera Rubin
and her collaborators--
Vera Rubin was a real
pioneer of dark matter.
She unfortunately
recently passed away.
These astronomers
decided that they
were going to measure the
speed at which visible objects,
stars, and gas
clouds were moving
around the center of our
galaxy, and of other galaxies.
Now, those of you who know
Newtonian Gravity, not even
General Relativity
at this stage,
know that if we
see a body orbiting
around an object like a sun, by
measuring the orbital velocity
we can infer the mass of the
object that it's orbiting.
You can look at the galaxy.
You can add up all the
stars and gas clouds.
And you have some idea of
how much it should weigh.
You'd also expect that
velocity of rotation
to decrease as
you go out further
from the center of the system.
But what they
found, in fact, was
that when they tried to
mention this the inner regions
of the galaxies looked pretty
much how they expected.
You know, the
rotational velocities
of these stars and gas clouds
around the center of our disk
galaxy looked pretty reasonable.
But as you went out further
from the center of the Milky Way
they expected these
objects to slow down.
And they didn't.
They were traveling
just as quickly
as the stars and gas clouds
closer towards the center.
So I'm going to show this
little cartoon down here,
which hopefully you can see.
So each of these yellow dots
represents a tracer particle
propagating in the gravitational
field of our galaxies,
you can think of it as.
So I want you to
look particularly
at how fast the outer
particles are moving.
The plot on the
left shows what you
would expect from
Newtonian Gravity,
or from Einstein's
General Relativity.
This far out from the center
of the galaxy we think
gravity is pretty weak.
It doesn't make very
much difference.
So you see, in the
Newtonian picture,
as I said, the two points close
in to the center of the galaxy
move fast.
Further out they're
just meandering along.
But what actually
happens, in reality,
is that these points
in the outskirts
are actually moving
quite fast, comparably
with the center of the galaxy.
So this is known as The Mystery
of the Flat Rotation Curve.
Flat in that they don't change.
That the rotational
velocity just
doesn't change very much
as you move outwards.
So that's just two things.
One, the fact that these
particles on the outskirts
are moving faster
than we thought
suggest that there's more mass
enclosed inside their orbits
than we originally
believed there to be.
Secondly, the radial
dependence here
suggests that it's not just
a matter of adding more mass
to the center of the galaxy.
There needs to be
more mass further
out from the center of the
galaxy than we'd expected it.
So it's mass that
isn't in the same place
as all the visible stars
and bright gas clouds
that we can see.
There appears to be
something else there.
At least, that's one conclusion.
The other possible
conclusion is, maybe
Newtonian Gravity
doesn't work very well
on the scales of galaxies and
other things of that size.
So this was one of the
first big puzzle pieces.
And it touched off a very
interesting and longstanding
debate in the community
about whether what
we were looking at here
was some new kind of matter
or some modification to gravity.
Either would be super exciting,
as a theorist probably
the modification to gravity
would have been even more
exciting than the new particle.
The evidence however
seems to be coming down
on the side of dark matter.
So let me tell you one of
the major pieces of evidence
that changed people's
minds in that direction.
So now we're going to leap
forward about 30 to 40 years
to just about a decade ago.
So I'll tell you the
experiment that was done.
But first I want to give
you a little cartoon.
Can we switch the lights down?
Because yeah, the colors on this
make this quite hard to see.
So they idea here
was, all right,
if there is some new form
of matter around our galaxy
there has to be some
reason why it's not
clustered like the other
matter in our galaxy,
why it doesn't follow
the spiral disk.
So maybe that suggests that it
interacts in a different way.
Ordinary matter, particles
collide with each other,
they lose energy, they
distribute angular momentum,
and they spin down into
this spinning spiral disk.
If this new matter,
this dark matter
hasn't done that
maybe it suggests
that it interacts differently.
Now, the difficulty with telling
apart a new form of matter
from gravity in somewhere like
our galaxy is pretty simple.
You know, the matter
is going to go
where the gravity is stronger.
So it's hard to
tell the difference
from a certain amount
of visible matter,
the gravity falls off with
distance at a different rate.
It's difficult to tell the
difference between that
and simply having more matter
that's more spread out.
But if you could separate
the two forms of matter
from each other then you could
try to break this degeneracy.
You could see the
gravitational effects
of the new form of matter coming
from a completely different
region of space to the region
where the visible matter is
detected.
And one way to do that
is to look at a system
where two galaxies or
two galaxy clusters
are colliding with each other.
Now, the particle physicist's
answer to everything
is, if you don't understand it
smash things together and see
what comes out.
So this is sort of a
very particle physicist
approach to the universe.
But in this case, we're
allowing the cosmos
to do the colliding for us.
If we imagine that we started
out with two systems both
consisting of some ordinary
matter, which is denoted in red
on this, and some dark matter,
which is denoted in blue,
when these two systems
collide with each other
we know what the ordinary
matter, which consists mostly
of gas clouds, will do.
Those gas clouds will
ram into each other.
They'll excerpt
pressure on each other.
They'll heat each other up.
And when they heat each other up
that matter will glow brightly
in x-rays, which can be seen
by the Chandra telescope
that Lisa talked about.
But the dark matter,
if it doesn't interact,
those blue clouds or dark
matter will just pass straight
through each other.
They wouldn't ram together.
They wouldn't slow down.
They wouldn't heat up.
And so after the
collision you could
imagine that we would have
these splots of dark matter
out to the sides while
the visible gas was
localized in between.
And indeed, when the members of
the Chandra x-ray collaboration
looked at the system of
colliding galaxy clusters,
called the Bullet Clusters, back
in 2006, they saw bright x-rays
coming from the visible gas.
But they also used a technique
called gravitational lensing,
which is the bending of light
by the presence of massive
of objects, to tell where most
of the mass was in the system.
And what they found was that
most of the mass in the system,
after the collision, was
in these regions indicated
with blue, these
sort of side bands.
So it's very hard
to explain this just
with modified gravity.
If all the visible matter
is in these red regions
then no matter how fast or
how slowly gravity falls off,
away from those
regions, you know,
the gravity should still be
coming from those red regions.
Whereas, instead it seems to be
strongest in these blue regions
off to the side.
So that suggest that
we really are looking,
not at some
modification to gravity,
but at some new kind of matter.
Another piece of evidence, --and
I'm not giving you the complete
picture here, but I'm giving
you some of the key pieces
of evidence-- we have
measurements from the very
early universe, from when
the universe was only--
we can take snapshots of
what the universe looked
like when it was only
300,000 or 400,000 years old.
This is sometimes called The Big
Bang After Glow, or the Cosmic
Microwave Background Radiation.
This is what it looks like.
This is a map from
the Planck satellite,
maybe just a few years ago.
This is a map the sky, so
with the Earth taken out.
We live inside a
celestial sphere.
So we can project that
into this projection,
just like we project the
globe onto a map projection.
These different
colors are showing
the temperature, the
different regions of the sky.
The scale of these
fluctuations is
at the level of one part
in a hundred thousand
of the background.
So this radiation is
almost completely uniform.
It just has these tiny
little fluctuations in it.
But these fluctuations can
give us a lot of information.
What do these fluctuations
actually reveal?
Well, when the US was 300,000
years old it was very hot.
Its temperature was about
3,000 degrees Celsius.
So it was hot.
It was very homogeneous.
The fluctuations in the universe
were at a pretty tiny level.
There were no galaxies.
There were no stars.
There were certainly no people.
It was just this hot soup
off matter, mostly hydrogen
and helium, and photons,
and hypothetically,
our new dark matter component.
Now, in this hot soup of matter
sometimes, just by chance,
you would get a
region where there
was a bit less matter than the
other regions surrounding it.
What happens if I have
that kind of over-density?
Well, gravity is attractive.
So gravity, as soon as I had
one of these over-densities
more matter would fall onto it.
And it would tend to grow.
But if your matter
at this point is
made of mostly
electrically interacting
particles, so protons, and
electrons, neutrons, and stuff,
when you cram a
lot of that matter
together the radiation pressure
will push it back apart.
So the universe consisted
of this hot soup
with these continually
shifting, changing
fluctuations in
density and temperature
driven, on one hand, by
gravitational attraction,
and on the other hand
by radiation pressure.
But to the degree that you
have dark matter present,
we've just hypothesized dark
matter doesn't talk to photons.
It doesn't interact
with photons.
It doesn't feel
radiation pressure.
So the dark matter
modifies these oscillations
by being a term that
only feels gravity.
Any dark matter
that's in the system,
it doesn't care about
radiation pressure.
Any dark matter that you
add to these over-densities
is just going to make them grow.
So the question
becomes, how would
the presence of dark matter
change a map like this?
Now, a map like this is a
little bit hard to parse by eye.
So what we often do
is we say, OK, we're
going to look at a simplified
description of this map.
And we're just going to
ask, how much power is there
in fluctuations on
different scales?
How much power is there in
very small scale fluctuations,
very localized fluctuations?
How much power is there
in large fluctuations
that span most of the map?
That is described in a
plot that looks like this.
This is called a power spectrum.
You don't need to understand
the details of it.
It's just telling you how much
power there is on small scales,
versus larger
scales in this map.
But what I want
to show you is now
how this curve
changes as I change
the amount of dark matter.
So there's a little purple
bar to the right hand
side, which was how
much total matter
you have in the universe.
At the moment it's just
the matter that we know
is there, the ordinary matter.
As we increase it the whole
structure of this curve
changes.
The amount of power that
you have in small scale
fluctuations versus
large scale fluctuations
versus intermediate
scale fluctuations
transforms fairly dramatically.
Now, it turns out that there's
one value of the dark matter
abundance, which
causes this curve
to fit the data really well.
Now, we don't have measurements
of this curve at the level
that LIGO does.
This is not one part
in 10 to the 20.
But we do have measurements
at percent level accuracy.
And that tells us that this
fits the data simple well,
provided that you allow there
to be a new component of matter
in the universe, something
that doesn't feel radiation
pressure, that only feels
gravity, that is about five
times more abundant than
all the visible matter
that we know about.
So again, this cuts out a lot of
possibilities for dark matter.
You might think, oh,
well, maybe dark matter
could be like all
burnt out stars.
Just ordinary matter,
just in a form
that's not showing up to
most of our telescopes.
Well, this tells you
that it's not the case.
I mean, this tells you the
whatever this dark matter
is, it has to be
something that when
the universe was 300,000
years old before any stars,
before any galaxies.
It was around.
And there was five times as much
of it as the ordinary matter.
There are a couple of
other puzzle pieces.
We know that the dark matter has
to be pretty cold, pretty slow
moving.
It turns out that we
need a significant amount
of dark matter that's
cold and slow moving
or we wouldn't have galaxies.
We would not be here, and
Earth would not be here,
based on the time that it is.
OK.
So I've told you how we go
to some of these points.
How we estimate
that dark matter's
80% of the matter
in the universe.
We know that it gravitates,
otherwise none of these
signals that I told you
about would've been visible.
Dark matter, for the purposes
of The Cosmic Microwave
Background, for the purposes
of this 80% measurement
is essentially
defined by saying,
we're looking for something that
doesn't feel radiation pressure
and that doesn't
interact with light.
And that's enough
to tell us already
that there are no particles that
we know about that can do this.
Most of the particles
that we know about do
interact electromagnetically.
The ones that don't,
they tend to be unstable.
They decay away.
We need this dark matter to be
something that is still around
in the present day.
So if it's unstable
at all it has
to have a lifetime much longer
than the age of the universe.
So it's not anything
we know about.
It's 80% of the matter
in the universe.
This is a little
bit of a problem
for fundamental
particle physics.
Just a small hole
in our theories.
So we'd like to
solve this problem.
So how do we go about it?
So there are three
traditional ways
to hunt for dark matter,
which are called, technically,
indirect detection, direct
detection, and collider
physics.
And the idea in the first
case, in indirect detection,
is that in many models if
dark matter's a new particle--
these all assume dark matter's
some kind of particle--
if two dark matter particles
collide with each other
they can produce
visible particles
that we could look at.
You may know about
antimatter annihilation.
This is exactly the same
idea, just with dark matter.
There's direct detection
where you say, well,
what if I bounce a
dark matter particle
off a visible particle?
Maybe I can look for the visible
particle to recoil, to jump,
even if I can't see the
dark matter particle.
Or in a collider, like
the Large Hadron Collider,
if you smash things
together at high speeds,
as we particle
physicists like to do,
then you might be able to
produce some dark matter
particles.
So these are the technical
names for these searches.
Indirect detection, direct
detection, collider physics.
A colleague of mine
also likes to refer them
as break it, shake it, make it.
Where in the indirect
detection case you break it,
you put in dark
matter particles,
visible particles come out.
Direct detection you shake
up the visible particles
by bouncing dark matter
particles off them.
And the collider,
you hope to make it.
Now, most of my work,
this is why I'm here
at the Astrophysicists
and Cosmologists Soapbox
focuses on this indirect
detection picture.
About the question
of, if dark matter's
some new particle, if it's out
there, and if this process can
occur, if two dark
matter particles could
collide with each other
and make visible particles,
or if a dark matter particle
could decay and make
visible particles,
then how could that
influence the
signals that we see?
Now, my work spans a pretty
wide range, from theory
through to things that sometimes
experimentalists would do.
Instead, on the
theoretical side,
my group's been thinking
a lot recently about,
OK, we know that dark matter
doesn't interact with photons.
But what if it interacts
via its own force?
What if it experiences forces
that the normal particles
that we know about
don't, at all?
You could imagine
forming [INAUDIBLE] bound
states of dark matter
like hydrogen atoms.
As soon as you start
talking about dark forces
it makes this feel very
fertile for Star Wars jokes.
But I'll spare you
from that this evening.
So on one hand I do this
pretty theoretical stuff where
we think about, OK, if dark
matter were to have these,
you know, interactions, what
would the consequences be?
I also think about
the early universe
effects of dark matter physics.
It turns out that if
only a very tiny fraction
of the dark matter
were to convert
its energy into
visible matter it
could greatly reshape the
early history of our universe.
It could heat it up to
much higher temperatures
than were otherwise expected.
It could take areas where
most of the hydrogen
is currently thought to be
completely neutral atoms
and turn the universe
into an ionized plasma.
So I think about that stuff.
And I also think about what
signatures from dark matter
annihilation might show up
in astrophysical observations
of our own Milky Way Galaxy.
Back to that very first picture.
So I'll just show you--
so I'm very happy
to take questions
about any of these topics
later, and to discuss them.
I'll just show you a
couple of cute things
that we've done
on the last frame.
So Lisa mentioned observations
in a wide range of wavelengths.
I've spent a lot of time over
the last few years looking
at gamma ray data, coming
from the Fermi Gamma Ray Space
Telescope.
I am not an experimentalist.
I just rely on them to give me
nice data that I can look at.
So this is the Fermi Gamma Ray
Space Telescope and launch,
which was in 2008.
And using data
from this telescope
we can make a beautiful map
of the sky in gamma rays.
So here blue is fainter,
red is brighter,
yellow is brightest of all.
That big yellow stripe
along the center
is the disk of our
Milky Way Galaxy.
It's that same band of
stars as you saw streaming
across the sky in
the first image
only now in photons that are
about a billion times more
energetic than visible light.
About a billion
times more energetic
than the light in this room.
Now, if we were looking for
a signal of dark matter,
I said at the beginning that
because of these rotation
curves we think dark matter
is distributed more broadly
than the ordinary matter.
And to be a bit more
specific than that,
we now think that
every galaxy is
embedded in a large
cloud of dark matter.
So we could look for signals
of visible photons appearing
to come out of
nowhere, but really
originating from this cloud.
So my collaborators and I
have done a fair bit of work
at digging into
this gamma ray data
to see if there's
any evidence or if we
can exclude the presence
of photons from that cloud.
It turns out that
so far so we don't
think we've found dark matter.
But we've found some pretty
interesting other things
along the way.
So I'm a particle physicist
with a side in high energy
astrophysics because we keep
finding such neat things.
For example, back in 2010 when
we looked at these gamma ray
data and we subtracted off
the model for the background
gamma rays designed
to let us see
if there was a signal from
this dark matter cloud.
What we found instead were
these large figure-eight
shaped structures
in gamma rays, which
are called the Fermi bubbles.
These have nothing to
do with dark matter,
just to make it totally clear.
We found them doing
a dark matter search.
They probably have nothing
to with dark matter.
What they might have
something to do with
is the black hole at the
center of our galaxy.
We think that these might be a
relic from huge jets erupting
from our black hole somewhere
several million years
in the past and creating these
expanding bubbles of hot gas
and high energy particles
that are producing gamma rays.
We then went on
in the next years.
We said, OK, once
we try to understand
these bubbles we can add these
into our background model,
subtract these off.
If we look at the very center
of our galaxy then we see--
so here's the center of our
galaxy before subtracting
our background models.
Again, when we strip off
these background models
we see this additional
gamma ray emission
around the center of our galaxy.
Now, this could be the signal of
the core of a dark matter halo.
The dark matter that has
fallen into the very center
of the galaxy under
gravitational attraction.
This signal turns
out to have a lot
of features which argue in
favor of that interpretation.
But we actually think, based on
more recent work-- which again,
happy to talk about but it goes
in a little bit more detail--
we found that actually it
looks like this blob which
looks so smooth in this
image, looks like it's made up
of hundreds or maybe thousands
of individual little point
sources of gamma rays.
So what we may have
found here is not
dark matter but some wholly new
population of bright gamma ray
emitting stars,
which is pulsars,
lurking around the very
center of our galaxy.
So while I'm primarily a
particle theorist primarily
engaged in trying understand
what dark matter is
and design searches that
would allow us to look for it,
it turns out that when you do
those searches for dark matter
you quite often find pretty
interesting astrophysics as
well.
So I'm also happy to answer
questions about that.
OK.
I'll wrap up.
I hope I've given you some sense
of why the question of what
dark matter is is a big
question, some hints
of the approaches that we're
using to understand it,
and a bit of what I do.
Thanks very much.
OK.
Well, thank you both so much.
That was really fun.
I had fun anyway.
I want to start off.
I have a question
for each of you.
And that is to step
back a little bit
from these amazing astrophysical
discussions and discoveries,
and ask each of you how you
got interested in the topics
that you now devote
most waking and not
sleeping hours studying.
So I'm going to hazard a
guess that neither of you
were born wondering if you
could measure the propagating
distortions in the
fabric of space time,
or equivalent topics.
What brought you
from very early age,
to getting excited about
these topics, to here at MIT.
Lisa, do you want
to take that first?
Yeah.
My story is while I was doing
physics, studying physics
at university, and I think
I was studying physics
for the same reason that
all my friends were studying
physic, which is to discover
the law of everything, right?
To discover why the
universe is what
it is, and like very deep
questions about the universe.
And then when I started studying
general relativity I thought,
wow, this General
Relativity, it's
not quite that, yet, right?
Because it doesn't really
explain everything but
it's pretty close.
But I was doing theory.
That's what I was doing.
And then one day
the professor who
was teaching the class
of General Relativity
brought us to see an
experiment that was trying
to detect gravitational waves.
And that's an experiment
that's very similar to the LIGO
Detector's.
It's called VIRGO, in Italy.
And I realized that Virgo
was really in my backyard.
It's kind of funny, because
it's a three kilometer
L-shaped object that
I've never seen.
But it was literally 7
minutes from my mum's place.
And I thought, this is amazing.
I can do, you know, cutting edge
research while staying home.
What's better than that, right?
And that's how I started.
And then at that
time, you know, we
were installing these detectors.
No one had ever built a
detector of this scale.
So we were all learning,
you know, in US and in Italy
at the same time.
We were trying to learn how
to operate these detectors.
And that became
very challenging.
That's where I am.
Thank you.
Tracy, how did you start
the journey that led you
to dark matter searches?
Well, so I got interested in
physics back in the high school
when I read Stephen Hawking's
book, A Brief History of Time,
mostly because I was
about 12 at the time
and all the adults
around me told me
that it was too
difficult for me.
And I took this as a challenge
and I went an read it,
and found it really interesting,
just the idea that essentially
that you could use
mathematics to describe
the universe in this way.
So then I think,
much likely so, when
I hit university I was
interested in, you know,
finding the theory
of everything.
Throughout undergrad
I was interested
in theoretical physics.
I didn't really know
what I wanted to do.
I came to grad school at
Harvard with the thought
that what I had to choose
between was doing strings
theory or being an
LHC phenomenologist.
When I started grad
school at Harvard
the plan was that the LHC would
switch on in the next year
or so.
And then it did switch on
and it broke, initially,
to the vast dismay of about
half my class at Harvard
who wanted to work on the LHC.
Now, at about the
same time I was
running into that problem common
to graduate students where
you get to the place you were
planning to go to grad school,
and find out that all the
professors who you were
planning to work with
are not taking students,
or on sabbatical, or leaving
to be hired by another place.
Yeah, I had been planning to
work with either Nima Arkani
Hamed or Lisa
Randall, And then I
found out that Nima
was moving to Princeton
and Lisa was going to be
away for the next year.
So it's like, hmm, OK.
This is not so good.
But very fortunately
for me there
was a new, young,
assistant professor
in the astrophysics
department at Harvard called
Doug Finkbeiner who had been
chatting with Nima and Lisa,
my punitive advisors
about he wanted a student
who knew something
about particle physics
to dark matter searches.
And Nima and Lisa both
told him, well, there's
this student who's been
coming to both of us
and going can I have
a project please?
And we've both had to say,
sorry, we're about to be away.
So they put me in
touch with Doug.
And he shared with me
some of the research
that he was doing and made the
point to me that while everyone
in the particle physics
community, thousands of people
were waiting with bated
breath for the LHC to turn on
and were ready to leap on
anything that might come out
of that.
In astrophysics there are
things that experiments
saw in our galaxy a decade
ago huge, striking signals.
Not two sigma
signals or anything.
Huge striking effects
that nobody understands.
I mean, like, the data's
been there for years.
And they're just
theoretical problems.
Nobody understands where
these signals can possibly
be coming from.
And part of that is that
the number of people working
on understanding these
things is just much smaller
than they are in some other
areas of particle physics.
And it seemed to me that
particle astrophysics offered
a chance to work directly
with data and in an area where
both you got to do
with the data more
directly, because
astrophysics experiments make
all their data public.
Whereas, particle physics
experiments generally do not.
So it was a chance
to work really
closely with data as a
theorist and potentially answer
some very important questions.
So that's now I got hooked
on dark matter and particle
astrophysics more generally.
Thank you.
That's great.
So we have plenty of time for
questions from each of you.
I think, because
we're recording I'll
repeat your question
in case Jennifer
can't get a microphone to you.
But feel free to raise your
hands and jump right in.
Yeah, please, right here.
It looks like a
microphone is coming.
This is a question for Tina.
Besides in the spiraling of
celestial objects in galaxies,
what other places do you
see dark matter observed?
Sorry, was that-- I'm Tracy.
I'm sorry.
Tracy.
T. I got confused.
Yeah, it's all right.
Yeah, what other places
would you see dark matter?
Great.
So right, so you see
evidence for dark matter
in a lot of galaxy
systems and also clusters.
In fact, some of the earliest
evidence for dark matter
was in clusters,
people just trying
to measure the
mass of the cluster
from gravitational
effects, and also
estimate the mass of the
cluster from adding up
the mass of all the
stars and gas in it,
and finding that
they got answers
that were different by
several orders of magnitude.
So that's one example.
Some of the places
where people want
to look for dark
matter signals--
[INAUDIBLE]
OK, I appear to have
lost my microphone.
I'll just project.
So the Milky Way has many small
satellite galaxies around it
which we think are clumps
of dark matter left over
from early times left over from
when the galaxy is forming.
So these galaxies
have a small number
of stars associated with them.
You can detect these galaxies by
looking for the stellar orbits.
But because their mass is
so dominated by dark matter,
and more or less,
looks like these stars
are orbiting around nothing.
So I mean, the sort of the
simplest version of the picture
is if I see out in
space a star that's
just going in circles, that
tells you it has to be orbiting
around something, right?
I mean, it's not going
to do it on its own.
So the more complex
version of that
is evidence for a large
amount of dark matter in dwarf
galaxies.
And because they
have so few stars
and so little ordinary
matter there's
good hope for seeing
these indirect,
break out signals of dark
matter in those systems.
More generally--
I mean, we think
that there's this cosmic
web of dark matter
that pervades the
whole universe.
There's dark matter
outside galaxies
as well as inside galaxies.
There are little clumps of
dark matter all over the place.
We get indirect evidence for
dark matter from the fact
that our simulations involving
dark matter provide pretty good
matches to the
observed large scale
structure of the universe.
And in systems like
The Bullet Cluster,
or other non-equilibrium
galaxy systems
you can quite often
see nodes of mass,
regions where there appears to
be a lot of gravitating mass.
But there's very
little visible matter.
So there's a huge
range of systems
where you see gravitational
evidence for dark matter.
Thank you.
Other questions?
Yeah.
Up front.
Why aren't planets
partly dark matter.
So I'll just repeat the question
quickly, if I heard you right.
Why aren't dark planets
considered significant parts?
What's that?
Normal planets.
Why aren't they part dark
matter, part of matter.
So that's an
interesting question.
So why aren't ordinary planets
made of partly dark matter,
partly ordinary matter?
Did I get that right that time?
Yeah?
Good.
Thank you.
So well, dark matter is
indeed a subatomic particle
then there's dark matter
streaming through this room
right now.
You know, it is some
fraction of the Earth.
There could be some dark
matter trapped gravitationally
in the Earth, which would
contribute to its mass.
But likewise, for the
sun, likewise the stars.
Every system will have some dark
matter in it just by accident.
And some dark matter
can potentially
get trapped in those systems.
But the reason why most of
the mass of the Earth, or sun,
or something like it,
is ordinary matter
is because we
believe, and we have
evidence from things
like the Bullet Cluster
that dark matter interacts
only very weakly with itself.
It doesn't have an equivalent
of the electromagnetic
interaction.
And it's the
electromagnetic interaction
that binds objects
together into atoms,
and binds atoms into
molecules, and is
responsible for a lot
of the intense clumping
that we see in ordinary matter.
So I have a question
for Lisa while you all
think of your other questions.
So we now know this date,
September 14, 2015 when
the first signal was detected.
Of course, it wasn't announced
for many months afterwards.
I'm curious if you
can share with us what
it was like when you first
learned of this signal,
because I have many
friends of the project,
and it sounds kind
of like James Bond.
Or it sounds like this was
kept under such extraordinary
secrecy that you had
to really, you know,
like carefully vet
who already knew,
and who you could talk to, and
a raised eyebrow seem to be
that you were in the know.
That's my impression as an
outsider from the group.
Can you share with us, what
was like when you learned
about this potential signal?
And a sketch of the process
the group went through
for the months between
September and February,
when it was made
public to the world?
Yeah.
It was actually one of the
most incredible experiences
of my life, I have to say.
So I told you the LIGO had
just started taking data.
Literally, we had just
turned on the instrument.
And well, the event happened
at night in the United States.
So we have pipelines that
look at the digital online
in real-time.
So one colleague in
Europe was the first one
that was working on the pipeline
and checking every morning.
And so what he did,
he wrote an email
to the collaboration saying,
are you guys doing some tests?
Or are you actually--
you know, this is
something that we
do to test all the instruments,
we can inject signals, right?
Intentionally.
So he was asking, is that
what you're doing right now?
So I woke up in the
morning and because I'm
the chair for the LIGO
collaboration for their round
planning I was checking
my phone instantaneous.
Like the first thing
before almost waking up
I would check my phone
to check for news.
I so I saw this email,
and I was like, oh,
can't be a test because we were
not ready for doing that test.
That's kind of incredible.
But we didn't know how to
produce a signal like that
at that time,
because the system,
the infrastructure for actually
injecting this [INAUDIBLE]
was not ready.
What we do is essentially
shake the mirror a bit, right?
But we were not quite
ready to produce that,
because our activators were
not strong enough for doing it.
So I was 100% sure that
it wasn't a test, right?
And so then very quickly, you
know, I came to the office
at MIT.
And we start talking about it.
But we collectively,
especially people
working on the instruments
who knew that we were not
ready for doing this
test we very quickly
started to think
that this was real.
And this signal is huge.
Would
Like I didn't have time
to go through my talk
but the ratio between
the signal and the noise
combined between these
two instruments is 23.
It's like, it's
not a tiny thing.
It's a huge signal.
So people have spent
the following months
trying to see if this
could be an artifact.
There was no way that
the signal is so big,
that no matter what we
try to do to make it
disappear it doesn't go away.
Then because, I don't
know if you have heard,
so the history of
gravitational waves discovery
is a complicated history.
In this '60s gravitational
wave discovery was announced.
And it was not true through
another type of detectors.
In 2014 the very
nice experiment that
looks at the imprints
of gravitational waves
on the Big Man
reported a discovery.
And that turned
out to be not true.
So we wanted to be 200%,
500, 1000% percent sure
that the signal was real.
And so we all worked
for months to check
the data, all the additional
sensors that we have to monitor
local disturbances
at the observatory,
make sure that there was no
correlation with the data
we were seeing.
I told you in my
talk, the signal
needs to be exactly the same
in a very, very tiny window.
So that's already a very, very
strong way to reject artifacts.
But nevertheless, we
checked everything else.
And then another
interesting thing
is that we spent a lot of time
making sure that this could not
be a malicious type of event.
And there I learned a lot,
because before this analysis
I thought it was
actually possible
that you could fake
a signal, like, ah,
your group of people who were
very close to the instrument
could do that.
It's actually not true.
It was really impossible.
So we went through a
very long list of checks.
And that was an
interesting discovery.
I find it really sort of
simultaneously amazing
and amusing the
fact that you were
so early that you didn't even
have your signal injection
pipeline done.
Actually it made
it more believable.
You know, normally
when things happen
in the calibration run you're
like, oh, maybe I trust this.
Maybe I don't.
But in this case it actually
removed a potential source.
In the previous generation
of detectors actually
this was a standard practice.
You do these injections.
Obviously they are recorded
in every possible chance.
So it's not like you
can, oh, oh, wait.
I injected.
I didn't know that.
That's essentially
impossible because we
record every single step of
the signals along the chain.
So the point is,
in 2009 or eight,
in the previous run we
used to do that regularly.
But then we changed the
detectors and our activators,
because they're less noisy
are also less powerful.
And so you can't actually
push the mirrors that way.
So that was also a
thing that we learned.
I've heard a story
that at one point
you were doing this
blind signal injection,
and most of the collaboration
doesn't know whether--
I mean, to test your
analysis pipeline.
So most of the collaboration
can't know whether or not
it's a real signal, and you
actually going a fair way
writing the paper before--
Yeah, so that was
an exercise in 2010.
So you don't know if this
particular injection's
a blind injection.
But you know that you are
in a blind injection period.
And so the distinction's subtle
but [INTERPOSING VOICES].
In this case we were not
in a blind injection period
for the main reason
that we could not
make any type of rejection.
So you know, most
experiments don't do this.
It's really impressive
that LIGO did
this set up of basically
injecting a fake signal in,
and then running it through
the whole [INTERPOSING VOICES]
pipelines.
So we say that these black holes
were our test of the pipeline.
Right.
Yes.
It's even better when you
see a signal immediately,
and so you never need to
do the fake signal test.
I do actually have a
question for Tracy.
May I ask?
Yes.
So a few days after the
announcement in February
there were several, at
least maybe one paper,
the title was a very
powerful one, which was,
Did LIGO Detect Dark Matter?
And there was this
theory that, you know,
if you, OK there is this
population of black-- well,
we detected one at that time,
and people started to say OK.
If there is one there must
be a population of this type
of black holes out there.
And maybe those are
primordial black holes,
so they see fit the
requirement that Tracy told us
that, you know, you
need to have them
at the very beginning
of the universe.
So what's your take on this?
Yeah, so this is a
really interesting idea.
So like I said, I mean, the
main alternative to dark matter
being some kind of new
particle is that maybe it's
some kind of black
hole left over
from the very early universe.
There are sort of two regimes
the could work for this.
One is really, really
tiny black holes.
Black holes smaller
than our moon.
So if you want to use
that explanation then,
I mean, that works in
the sense that it's not
ruled out by any
experimental constraints.
But you have to
explain how you made
black holes the size of the
moon in the early universe.
The way we get black
holes in the late universe
is that stars explode, and
collapse on themselves,
and make black holes.
Key word being
stars and stars that
are significantly
bigger than the sun.
So that's not going to make
a moon sized black hole.
But maybe you could get them
out of some early inflationary
period, which Dave studies.
But then there's this
totally different mass range,
which is the mass change of
like 30 solar mass black holes
that LIGO found.
The question is, cold
they be dark matter?
It seems maybe
possible but tough.
I mean, the idea's fine.
But there are a number of
experimental constraints
on black holes in this
mass range making up
all the dark matter.
If you have enough
black holes scattered
around the region
around our galaxy
to generate all the
dark matter to be
responsible for those
modified rotation curves
that we saw back
in my talk then you
would generally expect
them to have other effects.
Like as they moved through
systems of stars that were only
weakly gravitationally
bound to each other,
those systems would
just fly apart
when something that was 100
solar masses flew through them.
Now, the loop hole is that
depending on which analysis
you look at, everyone
agrees that black holes
more than 100 solar
masses in mass
are ruled out as making
up all the dark matter.
And everyone agrees
that black holes
below about five
solar masses in mass
are ruled out as making
up all the dark matter.
That's a difference
such-- that's
if one of these black holes
were to move between us
and a distant object,
the gravitational bending
of light around the
black hole would
cause the object to
like wink brighter
for a short period of time.
And there have been searches for
those little winking effects.
And they didn't find anything.
Or I think they saw
one event, which
might have just been a fluke.
So everyone agrees that
between about moon mass, and 5
solar masses, and from 100
solar masses up, it's ruled out.
And then there are
some papers that's say
that this region between
them is also ruled out.
But it's a much smaller
number of papers.
And the constraints are
sort of much more tenuous.
So maybe if like 100%
of your dark matter
is like 10 to 100
solar mass black holes,
with not many at lower masses,
and not many higher masses,
then maybe they can be
all the dark matter.
It's sort of marginal.
It would appear to be in some
tension with some constraints
but maybe those constraints were
calculated wrong or whatever.
I wouldn't say it's
totally ruled out.
But it seems a bit hard.
And then you'd need
to explain, like,
why are my black holes all
in this mass range and not
heavier and not lighter?
But it could be.
It would be amazing if true,
if LIGO also detected that.
Yeah, now when we started--
this is not part of the LIGO--
this paper didn't come from
the LIGO collaboration.
It came from outside.
But it would be pretty amazing
if like LIGO in one event
managed to
simultaneously come up
with two huge questions
in fundamental physics.
Got a question right back here.
Yeah?
Yeah, so I noticed that when you
saw those signals you told us
it was two black holes, And told
us the solar mass of each one.
And I believe you said the
distance from here to the--
if I remember that
right-- so I'm
wondering if all that
information you gleaned
from the signal, not
any other source.
Give us some idea of how
you got that specifity of--
Yeah.
Let me just quickly repeat the
questions for the recording.
It's a great question.
So the question was,
as Lisa had reported,
the chirp, the now famous
signal that LIGO detected,
from that chirp Lisa said the
team could determine the masses
of each of the objects.
These black holes, were each
roughly 30 solar masses.
And also their distance from us.
So 1.3 billion light years away.
So the question was, how
do you know all that?
You have one chirp or two
that line up so beautifully.
How do you get from there
to these very concrete
quantitative statements
about the source?
That's very good question.
So the short answer
is it all comes
from Einstein's theory
of general relativity,
because you can actually model
these systems very accurately.
And so what happens
is the way we actually
search for signals
like this in our data
is by building a template.
So you run your code, and you
produce signals like this.
And you go from, you
know, 1,1 solar mass.
2,2, 1,2.
All the degrees of all of
these parameters, right?
And from that you're
actually matching.
It's a match
filtering technique.
So you match your data
with the template.
And the one that match
better is your signal.
And then the amplitude off--
this is old general relativity.
The amplitude of the signal
that you see gives you distance.
We call it like
a standard siren,
in this sense, because it's
an accurate measurement.
And then the fact that
we have two detectors--
I was trying to
explain in my talk.
So you know, like the
source is up here, right?
And you know that the
gravitational waves
travel at the speed of light.
And there is a fixed distance
between the two observatories.
So the light from the
source is right here.
And then you can measure
when it arrives there.
And you can imagine you
have, you know, like a wire,
like this, and then you
move this around, right?
And then you can
constrain where it
is in the sky just basing on the
timing of the signal arriving
on Earth.
And so it's quite remarkable
but because of these things,
from that fraction
of a second thing,
you can learn all
of these things.
Once we will have more detectors
online, or just the two LIGO
detectors in the United States.
But one in Italy.
The one in Japan.
Then the one in India.
The accuracy at which we can
actually point in the sky
would be way better than now.
And that's crucial because what
we think we will do is to say,
OK, we have seen a trigger now.
And we are doing that now.
We tell telescopes,
point your telescopes
in that part of the sky,
because if this event also
had light associated with it
and not just gravitational waves
we'll see those two
things at the same time.
But now we're telling the
telescope, pretty much
over there.
And quite a wide
fraction of the guide.
So hopefully in a
few years we can just
say way more localized area.
And so that's when, we call it
The New Era of Multi-Messenger
Astronomy, will start.
There's a question all
the way in the back.
And then I'll come to you next.
Yeah?
Yeah, I was wondering
the data from Voyager
mission is that any
useful for [INAUDIBLE]
The emissions from what?
I missed it.
Voyager 1 and 2 missions.
Oh, good.
So the probes that were
sent that have now,
basically, exited
the solar system.
[INAUDIBLE]
Yes.
Yes.
So the questions is, does any
data from the Voyager 1 or 2
probes help with any of
the kinds of investigations
that you've talked
about tonight?
So I haven't seen much
direct use of that data.
But I know that--
I'm just trying to remember.
I think there was
some information when
one of the voyager probes
passed out of the solar system
a few years ago on
measurements of what happened
to the cosmic rays out there.
So in the kind of analysis
like this one that's still
on the screen behind
me it's really critical
to understand how to
subtract the photons that
are coming from ordinary
non-dark matter processes.
The main process that
photons at this energy
are produced by in our
galaxy is by high energy
charged particles, cosmic
rays, interacting with the gas
and producing gamma rays.
So it's a sort of constant
challenge in this work
to try to get better
models of the cosmic ray
distribution in our galaxy.
So like, indirectly,
just understanding
what the cosmic ray
distribution looks
like outside our solar
system could potentially
feed into that
background modeling.
There was a much earlier use.
I believe it was of the
Voyager probes, a few years
after they'd been
launched, to actually
try to test the fundamental
theories of gravity itself.
That was Voyager,
I think, right?
So there were ways to
constrain alternatives
to Einstein's general theory.
So not just Newtonian gravity,
not just general relativity,
but a family of
alternatives that
would look kind of like,
but quantitatively distinct
from general activity.
And measurements, I
believe, with Voyager
are certainly similar
interplanetary probes.
By the late '70s we
had to look and say,
if it's not exactly
Einstein's theory
then it's something
awfully close.
The other theory is that
modified gravity have
to be very tightly constrained.
And that was a very early
dividend from those--
That's true.
Yeah.
So these theories
of modified gravity,
that I was talking about,
they were constrained
by these solar system bounds.
You needed to have
a theory which
reproduced Newtonian, or
Einsteinian gravity extremely
well at solar system scales,
and then looked totally
different at galactic scales.
And then looked like Newtonian
or Einsteinian gravity
again at cosmological scales.
So it needed to have
some special properties.
That's right.
So last question here.
Yeah?
So I was thinking if
gravity waves travel
at the speed of
light, and you can
use them to encode information.
Could you make gravity waves
and use them to communicate
with [INAUDIBLE]?
That's a great question.
So the question was, if
gravitational waves move
at the speed of light, which
they certainly seem to,
and if they carry all this
information, as Lisa said,
she learned all this information
from the particular signal
of those waves they detected.
Could one produce
gravitational waves
and use those as a kind
of telegraph signal?
Could you send messages
encoded in gravitational waves?
I would love to see the
grant proposal for that.
That would be a
little expensive.
I believe there has been,
actually, in the past.
A grant proposal,
not a detection?
So, not really.
I mean, in principal it's a
cool idea, because you know,
gravitational wave, they have
this nice property that they
don't interact with things.
And so they can't get scattered.
And so you would be
quite advantageous.
But in order to produce
gravitational waves,
like the one the LIGO described,
for these tiny, tiny 10
to the minus 21 stream
distortion the amount of energy
that you need is huge.
And when I say huge
I really mean huge.
So the space time
is extremely stiff.
So in order to be able to
produce this wave you would--
you won't win, in
the sense that you
have to put so much energy
in that at that point--
You need to get a couple of
ten solar mass black holes
and smash them together.
So the waves that--
It's not a very efficient
process, let's put it this way.
It's a wonderful idea,
and it's the sort of thing
that I think Tracy, maybe
I, and some of her students
would enjoy playing with.
Because it's, theoretically,
certainly well--
one could pursue that.
But the practicality aren't.
I mean, the collision that Lisa
described basically dissolved
in a tenth of a second.
Three times the mass of our
sudden into raw energy and it
showed up as this incredibly
minuscule little wave
that they somehow
miraculously discovered
with their enormous four
kilometer long detectors.
So it's an effect that
really should work.
But it's like Lisa was
saying, if you imagine space
as a kind of spring it's
just the stretchiness
is so limited just it's hard to
set that's slinky into motion.
But it could happen.
It could be done.
I can't do it.
Well, I want to thank Lisa
Barsotti and Tracy Slatyer.
I thank you all for
coming out the day
after our latest blizzard.
And come back to the
museum other times.
Jennifer, do you have other
closing announcements?
One quick one.
So we're handing
out a quick survey.
So if you guys wold be so
kind as to fill it out.
If you fill it out and put
your name at the bottom,
and if you fold it in
half, and turn it into us
we're going to do a quick
drawing for a pizza pie cutter.
So were supposed to
celebrate Pi Day yesterday.
But we're celebrating it today.
We're calling it
a rounding error.
And so the winner of our drawing
will get a nice pizza pie
cutter.
So if you'd be willing
to take a couple minutes.
It's half a sheets.
It's really fast.
And it just helps us better
understand events like these,
and hopefully help get you
guys to want to come back
to more in the future.
So while you're
filling it out let
me also encourage
you to come visit us
for other events
at the MIT Museum.
In April we are hosting the
Cambridge Science Festival.
The Cambridge Science
Festival runs from April 14th
through April 24th.
And there are 10 days.
About 180 different
activities, all science-y
related throughout Cambridge
for future scientists,
current scientists,
people who like science,
people who don't
even like science.
So you should check it out.
One of event we're hosting is a
premiere of a play called Both
And, which is a play
about quantum physics
and explaining the basics
of quantum physics.
And so that's a collaboration
between us here,
as well as The Central Square
Theater as part of The Catalyst
Collaborative at MIT.
And so we are
premiering that here
during the Cambridge
Science Festival.
If you want to see us
between now and then though
we have quite a
few other events.
So consider joining
our email list.
We'll take a couple more
minutes for surveys.
But thank you all
for joining us.
And hopefully we'll
see you in the future.
