GANG XIAO: Good afternoon.
My name is Gang Xiao.
And it's been a long tradition
in our physics department.
Once a year we sponsor and
schedule this distinguished
lecture series, called "The
Arthur Olney William, Jr.
Lecture."
We have had more than 20 such
lectures by physicists working
on cutting edge of sciences.
And today, I welcome all of
you to Williams Lectures,
given by Professor
David Shoemaker of MIT.
First, let me say a few
words about Arthur William.
He was actually a descendant
of two prominent Rhode Island
families, Roger
William and the Olneys.
His forebears include Steven
Olney, secretary of state
in the cabinet of
Grover Cleveland.
Arthur grew up in
East Providence.
He graduated from East
Providence High School
in the 1930s.
He went to MIT on a
scholarship after winning
a nationwide science contest.
Arthur graduated from MIT in
1934 and came back to Brown
and worked with Professor Bruce
Lindsay on the wave function--
atomic wave function
calculations.
Afterwards, he was professor
of physics at University
of Maine for three years.
And then came back as physics
professor here for 30 years.
He became the chair of
the department in 1955,
overseeing the expansion of
the department of acoustics
to solid state physics
and particles physics.
Professor William-- Williams
was a distinguished physicist
in the field of acoustics.
He received the Pioneers of
Underwater Acoustic Medal
from the Acoustic Society
of America in 1982.
And today, we
continue to benefit
from his legacy and generosity.
With that, I'd like to ask
Professor Rick Gaitskell
to introduce today's speaker.
RICHARD GAITSKELL: Thank you.
So good afternoon, everybody.
So I'm just here to introduce
David Shoemaker, who's
very kindly agreed to talk to
us about what I think and I
hope many of you
already understand
is probably one of the more--
or one of the most
exciting experimental sort
of cosmology/astrophysics
experiments
we have running
right at the moment.
It is seeing for the first
time gravitational waves.
And obviously, he's going
to spend quite some time
telling us about that.
I think another reason why you
sort of want to pay attention
to this story is because it
does represent the long road
that often these amazing
scientific breakthroughs are
required to go through.
I mean, the-- and I
hope I'm not going
to repeat too much of what
you're likely to tell us.
But it's back in the
'60s that Rainer Weiss,
who David went on to work with,
who is doing the experimental--
and proposing this
idea of interferometry
as a mechanism for what
ultimately became the ability
to detect over scales of--
well, I think it's 4 kilometers,
you're able to see less than
1/10,000--
a movement of less than 1/10,000
of the width of a proton,
not an atom, not the width of an
atom, the width of the proton,
the nucleus, inside.
And at the same time,
the theoretical--
the astrophysics,
this idea that there
would be a signal, as
Kip Thorne at Caltech
is really trying to figure
out what magnitude would
these gravitational waves have.
So as I say, that's the '60s.
Now, David was lucky enough to
work directly with Ray Weiss
and at MIT.
And he then had spells in
both Germany and France,
where in France I
believe he got his PhD
and the driving license, both
fairly high-risk occupations.
I've worked with a number
of French physicists.
God knows how I survived the
driving part of it actually.
But then David
moved back to MIT.
I would say when the late
'90s comes around again,
as I'm sure we're going
to hear the phase--
well, one of things again
you to realize about LIGO,
as you go to the '90s--
and although they've
managed to demonstrate
some fairly significant progress
in terms of building prototype
instruments and what
have you, you still--
year, after year,
after year they
are going to the funding
agencies, looking for approval,
looking to get funding.
It was quite a campaign I
believe during that period.
But by the time you
get to late '90s,
they're in this phase
where they're actually
going from the original LIGO,
which didn't see anything,
didn't have enough
sensitivity, that you then
have Advanced LIGO, which late
'99 was in white paper form.
And then that led ultimately to
approval of funding proposals
and actual construction of
the Advanced LIGO, which
is the instrument we're
going to be hearing about,
which took place from
2008 through to 2015.
And then, of course, amazingly,
having been finished,
they turn it on and
immediately a signal,
which is obviously a fantastic
way to begin a story.
And in that sense,
you can say it
was a hell of a road to travel,
to get all the way to 2015,
to start getting signal.
Already you're looking at
40 years plus of journey.
But also, of course, we're
right on the threshold
now of doing a whole new
style of astronomy, which is--
well, it's fantastic right at
this birth of this new field
to hear from David.
So without further ado, I
really should hand over to you.
Thank you so much
for being here.
[APPLAUSE]
[SIDE CONVERSATION]
DAVID SHOEMAKER: --an
interesting lecture
for you all.
I build instruments.
And so that's really
very much going
to be the focus of what I'm
going to talk to you about.
And I hope it will
lead you to have
some intrigue for the
fundamental physics
of measurement
that challenge us.
I also say a little
bit about astrophysics.
But that's not going to
really be the focus here.
100 years ago, roughly, Albert
Einstein, our favorite patent
officer, was looking at how to
synchronize electrical systems
across Europe.
Commerce was starting to grow
among the various different
European nations.
He also had a tendency to
close his eyes and dream
about falling in elevators,
and so forth, and so on.
It led him to be able to come
up with both special relativity
and then general relativity
not so very long afterwards.
He also made a prediction
for gravitational waves
as one of the consequences
of his formulation
of general relativity?
He wrote a paper in 1916
that contained an error, that
said that there would be
no gravitational energy
transmitted through
gravitational waves,
so they couldn't be discovered.
An anonymous reviewer
gave him a review,
saying that he forgot something.
And without ever apologizing,
he then published a second paper
in 1918, so just 100
years ago, which said,
in fact, there would
be gravitational waves.
But he also made a calculation.
We've looked through his
notes to try and find out
what he used as an
example to conclude
that it would be impossible
to see gravitational waves.
But I think by any
technology that one
could have imagined at
the time, that he came up
with this theory, it was true.
It would be a fool's
errand to try and detect
gravitational waves.
So a little bit of time passed.
Oops.
Yeah, there we go.
And by 1993, we'd
had a number of--
decades of people trying
to develop technologies
that could be used to do this.
The laser came along meanwhile
and offered us opportunities
for precision
measurement of length,
and so forth, and so on.
And by the time 1993
arrived, we could
propose to the National
Science Foundation
to establish observatories.
I'll be telling you why they
take this particular form.
The proposal does state clearly
that the initial detectors only
had a chance of
making detections.
And it was probably going to be
necessary to iterate on that.
I have to say, many times over
the National Science Foundation
has been a wonderful
partner for us
in making this research happen.
It's a great funding agency.
And they deserve a
lot of the credit.
So this is the
proposal cover art
for what we sent in in 1993.
And then they sent
us a bunch of money.
And so we built it for real,
in about the same environment.
Caltech and MIT
built this together.
We are partners in that.
And this is the
Hanford Observatory.
So then from 2000, 2015, we went
through a gestational period.
Virgo, a French-Italian
collaboration
also built an instrument.
It's almost close enough
to the Tower of Pisa
to see it from the observatory.
We did what we
call commissioning,
which is bringing these initial
instruments to ones which
would be performing at
the limit of sensitivity
that we said they should.
We observed for quite
a number of years.
And we saw no signals.
We actually got some interesting
upper limits out of it.
We also learned a great
deal about building
these instruments and
what not to do again.
So in parallel, the LSC,
LIGO Scientific Collaboration
and Virgo, we were working
on R&D for second generation
detectors.
We proposed ours in 2005.
The project start was in
2008, completed in 2015.
I have to say, that I was
the leader of that thing.
I brought it in on
budget, on time.
I am extraordinarily
proud of that.
The sensitivity target
was 10 times better.
It's important to
realize we sense
the amplitude of gravitational
waves, not the power.
And so the signal
falls as 1 over r.
If we can double the
distance to which we can see,
we see eight times as
many potential signals.
A factor of 10 gives us an
incredible leverage, some 10
to the 3 more sources in reach.
That was our objective.
Let me say a couple of words
about gravitational waves.
According to GR,
gravitational waves
propagate at the speed of light.
They're emitted from rapidly
accelerating nonsymmetric mass
distributions.
And the result is
a strain in space.
It stretches and squeezes space.
A very simple idea of how
you build something like this
is to take two massive objects
and cause them to orbit around
each other.
A basic equation for strain
looks something like this.
A delta L over L is created.
If I have a meter stick,
it changes by a delta L,
according to the length
of that meter stick.
The amplitude falls as
1 over r, as I said.
It is proportional to the source
of mass quadruple moment, which
is basically how
asymmetric the thing is--
the second derivative of that.
It's acceleration.
And then there is
this fearsome factor
of c to the 4th in
the denominator.
That's really what makes it
hard to do this experiment.
And it's reflective of
the stiffness of space.
Spacetime does allow you
to cause ripples to flow.
But it's very, very stiff.
If you make a number up for
the strain due to this rotating
dumbbell, well, it gets
bigger if the mass is bigger.
It gets bigger if
the r is bigger
because that makes a bigger--
mass quadruple.
It gets bigger if you
have a high frequency.
And it's still got
these terrible factors.
So the number that falls out.
If you say you want to see
something at least once a year,
then you probably have
to have a sensitivity
to strain of about
10 to the minus 21.
So a meter stick will
change its length by 10
to the minus 21 meters
when the signal goes by.
You can only really
expect to make
measurable gravitational
waves from the coherent bulk
motion of matter, on the order
of stellar masses in a highly
relativistic regime.
Otherwise, the
signal would just be
too small to be accessible to
any experimental techniques
that we currently know about.
What is that
experimental technique?
We use enhanced Michelson
interferometers.
Imagine you have a
ring of free particles
and there's a
gravitational wave that's
passing into the plane
of this projector screen.
And as a sinusoidal wave,
it will distort space,
making an ellipse with
one major axis and then
the other major axis, with
a frequency that corresponds
to that gravitational wave.
Ray Weiss's intuition--
he wasn't the only person
to think of this.
Good ideas come to many people.
He was the first person
to think through what
was hard about doing
it and doing it right--
is to place a Michaelson
interferometer
on that ring of free particles.
It's ideally suited for
measuring the difference of L1
minus L2 by looking at the
difference of phase of arrival
at this beam splitter.
Here's a somewhat
larger drawing.
And you imagine you shorten this
arm and you lengthen this arm.
it will change the return
phase from these two arms
and cause a fluctuation in
light at this output detector.
So it acts as a transducer,
turning gravitational waves
into a photocurrent.
It's a really beautiful
idea when viewed so simply.
Now, our arms are short compared
to our gravitational wave
wavelengths.
There are a bunch of
practical constraints
that lead us to
gravitational frequencies
on the order of audio
signals, 10 hertz, 100
hertz, or kilohertz.
And with the speed
of light, you can
calculate that that leads
to hundreds and hundreds
of kilometers of wavelength.
So the longer we can make our
arms, the larger the signal.
That arm length is limited by
taxpayer noise, by the way.
It's one of the noise
sources with which we deal.
So I said that if you want
to see a signal sort of once
every year or more
frequently, you
have to have a sensitivity
to 10 to the minus 21.
And to give you a
sense of that, that's
a delta L over L.
Protons, for me,
are not very
physically accessible.
To be a little bit better is,
the distance to Alpha Centauri,
it's about four
light years or so.
The thickness of a human
hair, it's about 10 microns.
We can resolve fluctuations
in that distance
by the thickness of a
human hair, the distance
to Alpha Centauri.
And when a gravitational
wave passes,
in fact, it changes
the distance between us
and Alpha Centauri by the
thickness of a human hair.
And that's what we can sense.
That's what we must be able to
sense to do this experiment.
The bottom line is
that we need to be
able to detect
effectively fluctuations
of this length on the order
of 10 to the minus 18 meters
in tens of milliseconds.
So this is the sort of
sketch of an interferometer.
I already snuck in
two input mirrors
here, which I didn't talk about.
They store the light for
a longer time in the arms.
It gives you more
photons to interact with,
a somewhat better sensitivity.
So you have to fit that into
a real optical schematic.
And you have to come up with
mirrors to couple light in
and couple light out.
Then you have to fit
it into a vacuum system
because if the light
that we use as sensing
sees a fluctuation in the
density of air along the way,
it's like shimmering
light over a hot highway,
it would completely
ruin the measurement.
And then you have to put
all of the components on.
It's an incredibly
complicated thing.
I have a really
neat animation here,
that's going to lead you
through the optical system.
And I'll try to voiceover
my way through it.
This is an actual physical view
of the Hanford Observatory,
up in Washington state.
And now we move over
into CAD/CAM space.
And we're going to rip the
roof off of this building
and look inside at
the vacuum system.
So these pipes are about a
meter and a half in diameter.
And we're going to run
down to the end of where
the laser in this Michelson
interferometers sits.
The animator cheated.
And he doesn't actually
show us the laser,
just has it start spontaneously
at this point here.
And now the light beam falls
through a pattern of mirrors.
This first optical system, it's
a triangular cavity made up
of three mirrors,
tick, and tick, tick.
And it acts as a
frequency reference
and also as a beam
stabilization device.
We transmit the light through
that very stable cavity.
Then we fall on a couple of
focusing mirrors that take us
from the kinds of
beams we usually see
at the output of the laser, to
the kind of beam that's matched
to a 4 kilometer long arm.
It now falls on that
beam splitter, which
is about a 40 centimeter plate
of very, very fine glass,
and through the
first input mirror
of this arm cavity, which
increases the laser power.
This animator chose
to pause for a moment
here and show us how the
light intensity is built up
by about a factor
of several hundred
by the light falling back
and forth on resonance
within this Fabry-Perot cavity.
It increases our sensitivity.
Now, what we're going to do
is take a path down the arm
as though we were a
photon and take a look
at the mirror at the far
end of this interferometer.
And there we see, housed in
a rather large vacuum system,
the object itself.
This is the mirror, which
is isolated very, very
well from the outside world.
And it moves to a
gravitational wave.
And then back to
the beam splitter
again, where we'll see
the beams recombine.
I may have just turned the
volume all the way down
with this button.
I wonder how I turn it
back all the way up again.
And now the output beam from
this interferometer comes.
We actually work
the interferometer
at the dark fringe.
I'm sure you know an
interferometer has
a kind of co-centers, so
it'll output in the function
of a path light difference.
We work at the dark fringe.
But here, it's
showing rather bright.
It goes through a couple
more filter cavities.
And then it falls
on a photodiode.
This is the thing which
converts light intensity
into fluctuations
in photocurrent.
Now, when a gravitational
wave passes--
this would have to be a rather
strong gravitational wave,
a little bit stronger
than 10 to the minus 21--
we see at the output
of the interferometer,
the fluctuations in intensity.
And here the animator
chose to have
us move back and forth between
the minimum in the output
of the interferometer.
In fact, we hit a little
bit off to the side
and move a tiny fraction of
a wavelength around there.
But in that photodetector
then, we see a time trace.
The electrical
current versus time
maps out the strain in
space as a function of time.
And if we're lucky, there's a
gravitational wave that passes.
So I'm going to tell you a lot
more about that measurement
process because it's the
stuff that I really love.
We're going to see a
number of these plots.
This is strain.
This is the delta L over
L measured in a 1 hertz
interval of integration time.
And here's frequency, 10
hertz, 100 hertz, kilohertz.
So these are frequencies that
you can hear with your ear,
if your ear that sensitive
to strain in space.
And what we see here is
the sensitivity curve
that we had for
our first observing
run with the new
interferometers.
I'm sorry, with the first
generation of interferometers.
And I'm going to show
you a number of things
that we did to reach the
better sensitivity that we
have with Advanced LIGO.
And the first one is up in
what we call our high frequency
region.
There's a much reduced
shot noise level.
What is shot noise?
The photons fall on
our photodetector one
by one, following
Poisson statistics.
And I think you probably know
now that if the fluctuations,
the fractional fluctuations,
in Poisson's statistics,
go with the square root
of the number of objects
that are falling, the
square root of n statistics.
It's interesting to
note that Einstein
was the first person who brought
this fact to our attention.
And so we can note that the
fringe resolution, our ability
to split this fringe
increases at the square root
of the power that falls
on our photodetector here.
And here we see
the strain limit.
It goes as 1 over L,
as I promised you.
And then in the
square root sign,
there's a power in
the denominator.
So that's one half
of the story here.
The other half of the story is
that as you turn the power up
to get better and
better sensitivity here,
you actually start to
shake the test masses.
The momentum transferred
by the photons
to these 40 kilogram
pieces of silica
that we saw at the far
end of the tube there,
it causes some shake.
And it has a spectral
density of one
over our f squared because
your mass has got inertia.
And it doesn't want to move.
But that means that
the strain limit
due to that effect of the
radiation pressure noise
falls as 1 over mf squared,
m omega squared, if you will.
And then now this gets
worse with higher power.
So you can obviously
see you can solve
these two equations to find this
frequency of best sensitivity.
It happens to be right here.
But you have something which
is called the standard quantum
limit.
And these Advanced
LIGO detectors
actually operate at the
standard quantum limit.
The next thing that
limits our sensitivity
is the thermal noise.
This is effectively
Brownian motion.
It's kT of energy
per mechanical mode.
This is a mechanical object.
It's sitting at room
temperature, 300 degrees.
And it's shaking.
Its internal modes are
being excited by the fact
that it's in equilibrium
with its environment.
Once again, it was Einstein who
taught us about thermal noise.
In a simple harmonic
oscillator, the RMS motion,
this goes with the
square root of kT
where kT is the Boltzmann
constant over some kind
of characteristic
spring for the system.
But it turns out that's
distributed in frequency
according to the fluctuation
dissipation theorem.
I pressed this button again.
How can I get the
sound to come on again
when the time comes, I wonder?
And the consequence is that you
have a spectrum of excitation,
which is--
it falls as a
function of frequency.
And it gets smaller and smaller
as you make lower and lower
loss mechanical systems.
So the objective is to
make a mechanical system
with as low loss as possible.
And we moved from a rather dumb,
sort of wire-suspended system
in the Initial LIGO to this very
fancy, monolithic fused silica
system, to reduce the thermal
noise at lower frequencies.
Curiously, the thing that's
glossiest in the system
is the tens of microns
of reflective coating
that's on the surface
of the mirrors,
because that's
much glossier here
than the bulk object
of this test mass.
And as a consequence,
we have to work
very hard to get the noise down
in this critical middle region.
And to get that to be
better is material science.
That's really
complicated science.
It's not, like, simple stuff,
like quantum measurements.
And so as a consequence,
we are working hard,
with a bunch of
people who understand
coating better than we do, to
try and make better coatings
yet than the ones we have.
But we already improved
them for Advanced LIGO.
And then there's
the thing which you
might have thought of first,
and that is seismic noise.
There's this constant motion
around the instrument.
You have to isolate this
object here, this test mass,
from being influenced
by these stray forces.
And that involves in our case
first using multiple pendulums.
You know, if you have a
pendulum and you move it very,
very slowly, the test
mass will simply follow
that point of suspension.
If you move it at the
resonance, then there
will be a their high level
of excitation probably.
And then if you move
it quickly, the mass
will sit relatively still.
That last fact is
what we used to do
several of the layers of
isolation of our masses.
But we also have
highly active systems,
where we have a
seismometer that's
measuring motion, developing
a feedback signal to a motor
and holding the table
still in inertial space.
With multiple
layers of that kind
of isolation, three layers
each, of six degrees of freedom,
we can move the lower frequency
limit down a performance
from about 40 or 50 Hertz,
as it was for Initial LIGO,
down to about 10 hertz
for Advanced LIGO.
And that allows us
to observe a lot
of astrophysical systems
for a much longer time,
do more astrophysics.
AUDIENCE: What [INAUDIBLE]
DAVID SHOEMAKER: It turns
out that the parent spectrum
for seismic noise is
typically 1 over f cubed or 1
over f to the 4th.
And then there are a
bunch of other systems
in series with that,
that have sort of 1
over f squared characteristics.
So you end up with f to
the n, where n is 10 or 15,
by the time you get to
the actual instrument.
And there's an ultimate
limit on the lowest--
RICHARD GAITSKELL: [INAUDIBLE]
since this minor [INAUDIBLE]
is the wrong word.
But I mean the fact
that [INAUDIBLE]
DAVID SHOEMAKER: It's
a practical matter.
It's actually the thermal
noise of the reference masses
in the seismometers
that determines
the lowest frequency
to which you can
do a proper job of isolation.
But my next slide
will tell you why.
There's no point in
going any further.
Which is, that there
is a limit on the Earth
due to the Newtonian background.
What is the
Newtonian background?
Seismic waves,
either body waves,
where there's a compressional
wave running along the surface,
or Rayleigh waves, which
look a little bit more
like ocean waves, both end up
making a dynamic time varying
change in the density of the
matter around our test masses,
which I've described
in this pretty drawing
here-- actually Jan Hans has.
And our test mass is attracted
to dense material, at least
in the Newtonian
vision of gravitation.
If you put a lead
ball here, the mass
is going to be pulled
toward that lead ball.
And because that lead ball
is moving all over the place,
our test mass is
moving in response
to these seismic waves.
We cannot filter that out.
That's a fundamental
interaction between the mass--
it's actually deforming
spacetime near our test masses.
And that gives us a noise
level on the Earth's surface,
just due to the practical
level of excitation,
which makes it useless
to work on getting
seismic, direct seismic
coupling, lower than 10 hertz.
So we expect to be limited by
this noise source around 10
hertz or so because we're
on the surface of the Earth.
We would love to be limited
by only this noise source.
I'll give you a
picture of what we are
limited by in just a minute.
If you want to go
lower in frequency,
you have to go
underground, where there's
less seismic excitation.
And if you want to go
really lower in frequency,
then go to space.
Get away from this noisy Earth.
Well, I think I have
one slide on that, too.
So I've given you a litany of
the basic noise sources that
limit our measurement.
[INAUDIBLE] against
strain versus frequency.
You'll recognize all these
various different terms
that I've recounted.
This is the sensitivity
of the instrument
that we were able to achieve
for the first couple of science
runs.
We just started the
third one today,
which added a slightly
better sensitivity.
And what you can see is that
sum of these naive noise sources
is lower than what we achieved.
And to get from one
to the other involves
a lot of commissioning, a lot of
working through very practical
problems.
What's fun is to look
at the next slide.
This is what we
actually look at when
we're trying to commission
these instruments.
There are all kinds of
additional noise sources
that we have to
deal with, having
to do with electronic noise in
systems, coupling from angular
motion to longitudinal
motion, so forth, and so on.
And they all pile up
at low frequencies
because of various
different couplings
to physical ground motion.
However, we do get
the kind of resolution
that's needed to make
gravitational waves
detections already, even
though we have more work to do.
I think it's kind of fun to see
is, moving from Initial LIGO
to the first runs
of Advanced LIGO,
we made about a factor
of three improvement
in the high frequency regime.
And again, coming
back to that cube.
That means that there's about
27 times more objects which
are within our reach, raising
our probability of inception
by a factor of 27.
What's really striking
is the improvement
that we made at low frequencies,
about a factor of 100
improvement.
You can cube that
to see how many
more sources at low frequencies
were available to us.
We really did make
a qualitative change
in our ability to make
gravitational wave detections
by moving from the
initial instrument
to the advanced instrument.
So with that, we were able
to make our first detection,
as it did happen
just-- it actually
happened-- we were doing an
engineering run for the week
before our observation run.
And our first event happened
during the engineering run,
even before the
observing run started.
What we see here is,
again, strain in space.
And the numbers are about
the ones I promised to you,
10 to the minus 21 versus
time/ two tick marks is 1/10
of a second.
And there are two traces here.
One of them is for our
Livingston Observatory, L1.
And that's in blue.
And the other one is H1,
which is the one at Hanford.
And it's shifted in
time because the signal
didn't arrive at the two
observatories at the same time.
The signal wasn't directly
overhead between the two,
but off to the side.
So it first arrives
here and then here.
It could be anywhere in
this circle of confusion,
by the way.
And it's inverted because the
two instruments are actually
like this, and not like
this, on the Earth's surface.
So you have to invert a sign.
But we saw this
beautiful signal,
which is visible without
any fancy sensitivity to--
you don't have to assume a
model for gravitational waves
or anything.
This is high pass and low
pass, like adjusting the treble
and base control on your hi-fi.
And the signal was visible
in the photocurrent
at the output of the instrument.
It surprised us,
I'll tell you that.
I think that I really
want to stress--
how many people here know what
a strip chart recorder is?
OK, 10% or so.
It's a thing-- yeah,
paper that's coming out.
It's being drawn by a motor.
And then you've got red ink,
that gets all over your hands
when you try to refill it.
And it's got a
galvinometer needle.
And it swings across the paper.
That's the kind of thing.
We measure h of t
in this instrument,
which gives it enormous power.
You should know about these
beautiful measurements, that
were made of the decay of the
orbit of two pulsars, made
by Taylor, Hulse, Weisberg.
And they could see that
that orbit was speeding up
and the two objects were
approaching each other.
And the loss of energy
that was implied
was exactly what was
predicted by Einstein
for gravitational radiation.
They won the Nobel Prize.
And they deserved it.
And that was the
best confirmation
of the first deviation
from Newtonian law.
The thing that is
special about LEGO
is it actually measures in real
time, the stresses and strains
in space.
So you get the
instantaneous amplitude,
better than the
time-averaged power.
And it has much
richer information.
You can read things
into these signatures
that you couldn't get
from that other approach.
A second detection, that
I want to highlight,
happened-- and I'm going
to stress this date.
It was on the 14th of August.
Our colleagues in
Italy had just started
to observe with their
instrument two weeks before.
And we were able to see
an event in all three
of these instruments.
This is the signal to noise
ratio as a function of time.
These are spectrograms.
These are ways that we really
like to look at our signal.
This is the sound--
this is the phone.
This is the curve of the
increase in frequency
as these two objects
approach each other
and then coalesce
into a single one.
Superposed on the actual
signal is the best fit model
for a given source.
And you see that it is appearing
very strongly in the two LIGO
instruments, somewhat more
weakly in Virgo, but visibly.
And this was the first time
that we'd had these three
instruments working together.
It allowed us to
do triangulation.
I said with two
instruments you have
a confusion circle about
where the source could be.
When you add a third
instrument, suddenly you
have a patch on the sky.
And that's a really great thing
for talking to astronomers.
Well, a wonderful thing--
oh, I have another
slide on this.
So LIGO can make this
sort of limitation
in the sky with two detectors.
But when you add
the third detector,
you narrow quite
considerably the range
of things you can see.
Yes, please?
AUDIENCE: [INAUDIBLE]
Livingston and Hanford
DAVID SHOEMAKER: It's
just a practical matter
of the commissioning,
which one had we
gotten the noise down more in.
They're nominally
identical detectors.
But they end up
behaving differently.
The environment is
different, but stuff happens.
Mirrors can be dirty,
that kind of thing.
AUDIENCE: How stable does
the temperature need to be?
DAVID SHOEMAKER: Say again?
AUDIENCE: How stable does the
temperature in the chamber
need to be?
DAVID SHOEMAKER:
Well, the chambers
are the largest ultrahigh
vacuum system in the world.
And so it doesn't really matter
how stable the temperature is
inside the tubes, and so forth.
The temperature does influence
mechanical stability.
But we have servocontrols
that hold things--
AUDIENCE: The relative
distance [INAUDIBLE]
DAVID SHOEMAKER: The
mirrors are hanging freely.
It doesn't matter if the
vacuum system moves around.
Servocontrol systems hold
the masses continually
to be at exactly
the right length.
And then we look for
deviations in that length.
OK, good.
And so we are able to,
with these three detectors
and this first detection,
reduce the uncertainty
in position, but also in volume.
We measure distance.
And that's a really useful
thing for astronomers as well.
So that was on the
14th of August.
An amazing thing happened
on the 17th of August.
I mean, I want to
describe this to you.
I hope I've said--
there's a button here
which says turn off sound.
But I don't want to turn it off.
I want to turn it on.
I don't know if my technical
helper is here and can tell me
how to turn the
sound back up again?
Let me see-- make
sure it's-- actually,
it may even be up because
it's DVI rather than--
so when I press on
this button here,
it shows on the screen as a
loudspeaker with an x through
it.
So let me tell you
what this is about.
This is going to
be a time trace.
And then there are three
different instruments here.
And these are each
spectrograms, very much
like these short spectrograms
that I showed you.
And when we play this--
I hope it will come out.
Otherwise, I'll
hold my microphone
next to the loudspeaker--
you'll be able to discern--
you think it will work?
[SIDE CONVERSATION]
DAVID SHOEMAKER:
Well, I'll make it go.
And we'll see what happens.
You will be able to
hear in the loudspeakers
the photocurrent in the detector
here, amplified and played
back.
And you'll be able to hear
the fluctuations in current
that have to do with the
inspiraling of this test mass.
[LOUD RUMBLING]
That's the background noise
of the instrument spectrum,
the quantum noise,
thermal noise,
a little bit of seismic noise.
And the detector, which was
working best at that time,
was Livingston.
And so you'll start to see the
curve of the spectrograph here,
starting to trend up in
frequency as these two neutron
stars are spinning
around each other.
They're passing each other,
50 hertz, 100 hertz or so.
Now, if you listen
carefully [INAUDIBLE]
I hope you heard in there the
sound of these two neutron
stars spinning around each
other as fast as the Waring
blender in your kitchen and
causing this wonderful curve.
And it's visible in Hanford and
at Livingston as two detectors.
You might ask yourself, why
doesn't it show up in Virgo?
The antenna pattern for a single
detector looks like a peanut.
Here is a sketch of the
two arms of the detector.
If the gravitational wave
comes from above or from below,
it can excite maximally the
difference of x minus y.
If it comes down
along one arm, then it
has-- because the forces are all
perpendicular to the direction
of propagation of the
wave, it has no influence
on this arm at all.
But it still has
influence on this arm.
So then the antenna pattern
is down by a factor of 2.
If it comes in at 45
degrees, it doesn't
excite either one of the arms.
GW170817 came in along the
vertex of the Virgo detector,
which is a really cute thing.
Now, it's important to stress
it did no harm for localization.
If you know that the detector is
working and you see no signal,
it tells you a lot about where
the signal's coming from.
Because we'd had that trill
detection three days earlier,
we knew Virgo goes working.
This thing is so cool.
So the other thing that
happened with this detection
was that there was a gamma
ray burst, as seen in Fermi
and in INTEGRAL, which was 1.74
seconds at the initial peak,
after the end of our signal.
And so the first
question is maybe
that was just
chance or something?
But if you look at
the position where
Fermi saw its signal
and the position
where LIGO/Virgo saw its
signal, the probability
of that taking place by chance
is greater than 5 sigma.
So it's a gold-plated
coincidence.
One of the very first
things you can say--
I told you earlier,
gravitational waves
travel at the speed of
light according to Einstein.
Well, Einstein was proved
not incorrect, once again.
We can say that over a
period of 140 million years--
this object is 140
million light years away--
the difference in arrival time
between photons and gravitons,
which follow the same not
simple path through spacetime--
there's all kinds of minor
gravitational lensing
along the way.
They're zooming along together.
They arrived within
1.74 seconds.
That's a part in 10 to the 15,
which is a nice little fact.
The next nice little
fact that comes
from this already
mini-multi-messenger
measurement is that some
binary neutron star mergers
are progenitors of short
gamma ray bursts, which
had been theorized.
Nobody thought it was
likely, or so for.
But there it is.
It's plain as--
I don't know,
plain as something.
Let me talk to you
about the time evolution
though, of what we
did with astronomers.
The first signal that
actually was received
was from Fermi and INTEGRAL.
Even though it arrived
1.7 seconds later,
they had faster electronics
for identifying a signal.
And then the two LIGO
detectors were able to chime in
and made their usual,
rather uncertain measurement
of where things were in
the sky because there
are just two detectors convolved
with that antenna pattern.
You add in Virgo, and
it shrinks way down,
not only in angular aspect,
but also in distance.
We sent those coordinates
to the astronomers, where
nightfall was falling in Chile.
And about an hour after
nightfall in Chile,
the first telescopes were
able to identify GC4993.
And as I'll show
you in a picture,
I think on the next slide, there
was a new spot of light there.
A star was born.
And that was the result of this
binary neutron star merger.
Whoops.
Yeah, OK.
I don't have a photograph of it.
But it was really
quite impressive.
And you could see it
with optical telescopes.
So already we could
say a number of things.
We can say-- we can give the
sum of the neutron star masses
very accurately.
It turns out
there's a degeneracy
having to do with the
angle which you observe
this binary system
and the masses,
which means that
you have to have
some uncertainty in the
individual masses of the two.
However, you can say
that they're well
within the ranges of what's
expected for a binary neutron
star.
There are a number of other
things that we could learn,
relatively little
radiated energy
compared with black holes.
But it was a
beautiful measurement,
that was consistent with
Einstein's theories.
We could start to say something
about the neutron star
equation of state.
We have the habit of looking
at binary black holes, which
give really a very perfect
and simple chirping sinusoid
until the two horizons merge.
And then you get the ring-down
of a final black hole,
which we not yet observed.
But in the case
of neutron stars,
you actually start to
slosh material around.
It's a much messier,
uglier system.
And what we were able
to see is to start
to see a deviation from a simple
point-like mass in spiral.
And from that, we could
make a plot of-- well,
these are effectively
stiffness measures
of the two different objects.
And we could say that it was a
more compact system than some
of the models that existed.
So we've already started
to put some limits
on the equation of state of
the nuclear matter in neutron
stars.
It's a tough measurement for us.
It happens at high frequencies.
When I showed that
curve that's rising--
that is to say, we're
losing our sensitivity.
And the coalescence
happens up here
in the high frequency regime.
It's a tough call for us to
make much better measurements.
We need better
instruments to do that.
We were also able to make
a first measurement that
involved gravitational
waves, the Hubble constant.
We could give the
distance to the object
pretty accurately by assuming
that general relativity is
right.
And that seems to have been
confirmed by our black hole
measurements.
And with that, we can add
a new ingredient, that
was otherwise dependent
upon linked measurements,
and make an estimate
of the Hubble constant.
Well, you may know
this plot well.
There is the Planck estimate
for the value of the Hubble
constant.
And there is the one that's
based on supernova, Shu's.
And they are actually
in more and more tension
as time goes forward,
as being disjoint.
And so it really
looks like there might
be some physics behind that.
So far our result, stand
alone, is very broad.
And it's about centered
between the two.
We would love to be able to
help answer the question as to
whether or not there's
some real physics there.
It'll take a couple of
hundred more measurements.
More generally, there was
this multi-messenger bonanza
that happened as a
result of GW170817.
In addition to
gravitational waves,
there is visible infrared light.
There are radio waves.
There are X-rays and gamma rays.
We sought neutrinos,
but none were
seen, which may be valuable
information for some.
At any rate, it was viewed
by over 100 detectors.
There was worldwide engagement.
And we published a paper
with 3,000 authors.
And that was about 1,000
gravitational waves.
This is about 2,000
observing astronomers.
And I think it's something
like a third of all
of the observing
astronomers in the world
who joined on that paper.
It's just amazing.
And I just did a
search the other day.
There are 1,600 papers
in the MIT libraries
that contain the
string GW170817.
It's a very rich event for
a great number of observers.
This is a summary of what
we've been able to see
so far, just kind of cute.
It's a series of
these spectrograms
along with the best fit waveform
and then the actual waveform,
which is a little bit hard
to see in this lighting,
for all of our 10
black holes which
have been published so far,
plus the binary neutron star.
And I think it's a really nice
way of seeing that there's
a real zoo that's being formed.
Here's another way of
looking at that same thing.
Each one of these
blue pairs here
are two progenitor black holes,
leading to a final black hole.
There are a series
of black holes
that were identified through
electromagnetic means.
We've just about doubled
the number of black holes,
with the few measurements
that we've made to date.
You can also look down here.
There are many, many
neutron stars that
have been observed in radio.
But they're good radio beacons.
Our two little neutron
stars are here,
sort of average beasts,
that made some new particle
or object.
Almost certainly a binary
black hole, given its mass.
And so this is kind
of a summary of what
we've been able to do so
far, with the first two
observing ones.
AUDIENCE: What's on the y-axis?
DAVID SHOEMAKER: Nothing.
It's enough space to
display all of our toys.
AUDIENCE: Not the
x-axis, the y-axis.
DAVID SHOEMAKER:
Oh, I'm sorry, that.
The y-axis is in
mass, in solar masses.
So, for instance, you've got a
20 plus a 35 makes a sort of 65
minus a couple of solar
masses for the radiated energy
and gravitational waves.
So far we've seen coalescing
binary systems by the tens.
There are a variety
of other things
that we continue to look for
and are eagerly awaiting,
bursts of various
different kinds.
A supernova would be
a fantastic thing,
if it's sufficiently symmetric.
To have that I
double dot show up,
you need to have
asymmetry in the system.
It'll probably have to
be in our local galaxy.
There are continuous sources.
Neutron stars spin and are
very, very good timekeepers.
But if there's an asymmetry,
a physical symmetry,
it would radiate
gravitational waves.
And that should cause
it to slow down, as well
as emit a sinusoidal
gravitational wave.
And then the sum of all
of these different things
going off at the
same time, it would
make some kind of radio
static that we can listen to.
There is certainly at
some very low level also
the cosmic gravitational
wave background,
which is being sought also in
the cosmic microwave background
through looking at
the polarization.
It's very unlikely that we'll
be able to detect this directly
with any of the instruments
which are currently underway,
but maybe in the future.
All of this work is
done with the LIGO
scientific collaboration
and the Virgo collaboration.
This map here
actually represents
just the dots of the LIGO
scientific collaboration.
Virgo adds a handful of
dots over here in Europe.
There are about 1,600 members
between the two collaborations,
120 institutions, 30 countries.
And I think that I've noted
to a couple of persons
here is the complete
absence here in Africa.
And we have to do
something about that.
So what does the
future hold for us?
This is our planned
observing timeline.
Down here, we have years.
You are here, LIGO, Virgo.
KAGRA is a Japanese
gravitational wave antenna
that's just in the process
of being completed and hopes
to join our observing run soon.
LIGO-India is something which
we expect to not be operational
until 2025 or so.
Do we know what's
making that high sound?
OK, good.
A thing to note is
our O3 observing run
started this morning at
9:00 AM, with instruments
that are a little bit better
than the previous ones.
I think I'll say something
about the rates that we expect.
And we'll be observing with
LIGO and Virgo, as before,
no competition,
all collaboration.
And we're looking forward
to KAGRA joining us.
We'll then make a few
more minor changes
in the improvement of the
performance of the system
to make a further measurement.
And you have to remember to cube
the ratio of these two numbers
if you want to know the
kind of rate increment
that we'll have as we go from
measurement to measurement.
We have then, now
just recently received
funding to make improvements
to our instruments.
Incremental improvements
we would say,
but ones that will
bring us up by something
like a factor of
2 or 2.5 from what
we'll be doing starting today.
You can cube 2.5
and get the notion
that we'll have sort of 10 times
as many events by that time,
out to 2026.
Yes, right.
So one year, O3 started
today, at 15 UTC,
which is 9:00 AM here.
1.6 times the sensitivity,
so about four times the rate.
One of the important
things about it
is that we're giving
public triggers.
I'll say more about
them in the next page.
And what does that
mean in terms of rates?
We expect that we
could have as much
as-- it's a little more at
the upper limit of what's
practical-- no, is
what's expected--
one binary black hole
per week and maybe one
binary neutron star per month.
So there will be
lots more objects
to lay out on that graph.
This O3 observing run--
in O1 and O2, we had
memoranda of understanding
with observing groups.
But we want to move away
from a closed system
into a more open system.
So we'll have open
public alerts.
Anybody who's interested,
amateurs or professionals,
can receive GCN
galactic notices,
which will let them know
where we saw something
and when we saw it.
The timeline for that.
We've also worked
very, very hard
on getting our alerts
out more quickly.
I noted to you that
Swift and Fermi
were able to get their
signals to the world
within, I don't know,
within a minute or so
of the actual
event taking place.
It took us about six minutes.
So we'd been driving that down.
By one minute, we might be able
to send out a preliminary alert
so that telescopes
can start to slew
and see the very beginnings of
the evolution of new objects.
And then we'll refine
the information
as time goes forward.
Here's what the overall
map of detectors
looks like on the
surface of the Earth.
We have the two instruments,
LIGO instruments.
We have Advanced Virgo, which
is already observing with us.
There's the Japanese KAGRA
detector that I mentioned,
that we hope will come online
in late 2019 or early 2020.
We hope it will join our
current observing run.
And then LIGO-India
is an interesting one.
We built three detectors when
we built our two for LIGO.
And we put one in
storage and started
looking for a good place
to put the third one.
And it looks like a great
place is going to be India.
The Indian government is
currently constructing.
They just finished the
procurement of all of the land,
and the contracts for
the vacuum enclosure,
and the concrete,
and the buildings.
And the objective is to have
this instrument observing
with the rest of
the world by 2025.
And what will that mean?
Why is that interesting?
The run that we just started
has this as a sky map.
The size of the
ellipse tells you
how uncertain our
estimate would be
for the position of a source
that would happen to lie there.
We have about one
signal per week.
And about 20% of the sky is
in 20 square degree boxes.
When we have five detectors,
with India and Japan,
then 60 degrees,
60% of the sources
would be in 10
square degrees, which
makes a really big difference
for giving pointing information
to observers.
And you can see how
the map is completely
transformed to the one where
we can really give pinpoint
information everywhere.
We're in the middle of
making this A+ upgrade,
which is what brings us from
that sort of 175 reach to sort
of 330 reach, over at the
right-hand edge of my timeline.
It's a rough doubling of reach.
And so we'll move into
a regime where we have,
oh, there could be as many
as 10 binary black hole
detections per day, also
significant increases
in these others.
And then probably more and
different kinds of objects
will come into our zoo.
We plan to be observing by 2024.
And we think we can
hold to that schedule.
It's now difficult for us
to turn our machines off.
The astronomers
will ask us, where
are the data, where the data?
Then we have bigger,
grander plans,
to move beyond what one can do
with a 4 kilometer baseline.
I said the longer you make
it, the more sensitive it is.
If you make the system move from
4 kilometers to 40 kilometers,
you increase the signal
size by a factor of 10.
But most of the
other noise sources
do not scale to increase.
In fact, some of them
may actually even go down
on that longer baseline.
And so it's a really good
thing from science perspective
to make the thing
10 times longer.
From the standpoint of
a funding perspective,
it's a really bad thing to
make it 10 times longer.
It only would be
something that would
be merited if we can
show that this field has
a long-term promise for a very
broad scientific community.
There are all--
I just said, "all of this."
So we have two concepts.
There's one in the
US, which is to make
the system 40 kilometers long,
instead of 4 kilometers long,
to build it on the surface,
and to continue to focus
on the same frequency range.
In Europe, there's something
called the Einstein Telescope.
They have a notion of putting
the instrument underground.
And this helps with
reducing seismic noise
and also that
Newtonian noise that I
mentioned because the seismic
noise is physically smaller.
The compression of the Earth
is smaller underground.
They have the notion of making
a triangular system, which
allows you to resolve the two 45
degree separated polarizations
of gravitational waves
in a single location.
And also allows you
to build it in Europe,
where every square
meter of the surface
is doing something
useful for mankind,
or mankind making an
exploitation of the surface
of the Earth, if you prefer.
At any rate, it's
a notion which is
compatible with Europe in a
way that this Cosmic Explorer
in the US--
put it on the Bonneville Salt
Flats, and no one would notice.
There's another way of looking
at the capability of reach
for the gravitational
wave antennas
that we've been talking about.
This is the redshift
to which you can reach.
This is 1, 10, 100.
This is the mass of the objects
involved, 10 solar masses,
a hundred solar masses,
a thousand solar masses.
The current instruments
are these events, LIGO,
and so forth, and so on.
They can see out a
redshift of a couple.
And are focused on these--
sort of a hundred
solar mass objects.
These much longer systems--
ET and CE are their nicknames--
can see out in principle
to redshifts of almost 100.
That is to say, they can
see the entire universe.
And at least in this
mass range, they
would see all of the collapsing
objects, binary black holes,
of a reasonable mass,
some tens of solar masses,
in the entire universe.
And you could map
out the history
of the universe in that way, a
little bit like 21 centimeter
cosmology.
So when could this new
wave of grand instruments
come into play?
We've had a couple
of examples now.
It looks like it
takes about 15 years
to get from hopes to something
that actually functions.
We can imagine in the
2030s, if we can move along
reasonably smartly.
We would, of course,
partner with a variety
of electromagnetic
observatories that we
expect to be in
operation at that time.
The worldwide community
is working together,
as they have on the
current observations,
also in the planning
for the future.
And there's a thing called
the Gravitational Wave
International Committee, which
is coordinating the activities
for building a really good,
airtight science objectives
for these instruments.
We've just submitted the
Astro Decadal review.
But the crucial thing
for these endeavors
is to make sure that there's
a broad enough community that
thinks these are useful.
So that they say, OK, instead
of building another $1.5 billion
telescope on top
of some mountain,
we will recommend
that those funds
be given to build a bigger
gravitational wave antenna.
Things like GW170817
help a lot in proving
the worth of this new field.
I think many of us in
our field were astonished
at how well that worked out.
I hope we can do a
few more of those.
There's a broad spectrum of
gravitational wave sources.
There's the cosmic
microwave background,
which could be read to measure
a primordial gravitational
background.
There's pulsar
timing, where you use
the timing of the various
different pulsars in space
as a way of seeing whether or
not a gravitational wave is
destroyed in space.
I spent a lot of
time talking about
ground-based interferometers.
You can also put
things into space.
As I said, if you want quiet,
that's a good place to go.
So there's a
project called LISA.
It involves not 5 kilometer
arms, but 2.5 million
kilometer long arms.
And it moves the
best sensitivity down
to kind of a hundredth
of a hertz or so.
There are a bunch
of scaling laws
I won't go into
here, unless you look
at million solar mass objects,
instead of 10, 100 solar mass
objects.
You slave the shield satellites
around the test masses
so they're not buffeted by
solar wind and the like.
The orbit scans the sky.
You have this triangular
kind of system,
which pinwheels around the
Sun, chasing the Earth.
And the telemetry is
between those two there.
And it looks like the launch is
going to be in the early 2030s.
It's something that's
on the ESA timeline.
And they have not
canceled the mission yet.
The kinds of astrophysics
you can do with it.
I won't spend any time on this.
But just note that they
see in spirals too,
that take a path like this,
through time-frequency space.
About some months before
the final coalescence,
they would start
to see a signal.
And they would watch it over
days and hours of cycle times.
And finally, the
coalescence would
take place, maybe at a hertz
or something like that,
with very high signal
to noise ratios.
You could do a whole bunch of
really exciting GR with it,
as well as map out
more of the way
that mass is distributed
throughout the universe.
So with that, I draw to a close.
This is just the
beginning of a new field.
We've got new instruments,
new discoveries.
There are synergies between
our gravitational wave
instruments and the
electromagnetic spectrum.
We think it's going to
be a great contributor
to the general world of
astronomy and astrophysics
over the coming decades.
Thank you.
[APPLAUSE]
SPEAKER: OK, so
questions, please.
DAVID SHOEMAKER: And
microphones are good
or you shout
really, really loud.
I think Yeah, OK.
SPEAKER: [INAUDIBLE]
the microphone to them.
Don't be shy, as they say.
AUDIENCE: There's
a question about
the supermassive black holes.
I did not quite
catch the distinction
between gravitational
waves from them
and from ordinary black holes.
DAVID SHOEMAKER: There
are some differences.
The most spectacular one,
besides simply the amount
of mass, is the fact that
they're surrounded by matter.
And that matters plays a very
important role in the way
that you come together.
And that-- I mean there have
been actually some concerns
about the final parsec.
Do these things actually get
up to relativistic velocities,
so that they
radiate successfully
before their interaction
of their matter slows--
gums things up in such a way
that you don't get a signal?
We hope that it is the case
that we get gravitational wave
signals.
And in that case, by looking at
the details of that coalescence
we can read information
out about that matter.
One other thing
that we hope to do
with the supermassive
black holes,
and the fact that they've
got other objects near them,
is to map out spacetime.
The notion is that you take a
lightweight black hole, of 100,
or a 1,000, or even
10,000 solar masses
and you use it as a
test map of particles,
swinging around this 10 to the
8th, 10 to the 9th solar mass
black hole, and map
out in real time
any fluctuations in the
gravitational potential
around it.
And maybe learn some more
things about the structure
of black holes.
[SIDE CONVERSATION]
AUDIENCE: So the mirror that's
hanging there, is not cold,
is not even
temperature controlled?
DAVID SHOEMAKER: That's right.
That's right.
AUDIENCE: So then as
a function of time,
there is going to be a thermal
expansion and then shrinking.
And then you mentioned
there is a quantum noise.
Where's the quantum noise from?
The room temperature shouldn't
be altered by a thermal event.
DAVID SHOEMAKER: Yes.
It's a good question,
which I could talk about
for a long time.
First off, there are changes
in the temperature of the mass,
and in particular
induced by the fact
that we're throwing currently
several hundred megawatts
of light power against it.
It has an absorption of about
a tenth of a part per million.
And so it's receiving
something like a watt of power.
And it's hanging in a vacuum.
So, of course, it gets hot.
Furthermore, because the
thermal activity of fused silica
is relatively low, and has
a fairly large coefficient
of expansion with
temperature, a mirror,
which starts out as a beautiful
4 kilometer long curve,
develops a bump, which
can refocus the system.
So we have to apply additional
heat, in a donut form,
to compensate for that,
to bring the mirror back
to the correct form.
Meanwhile, the
mirror is heated up
by a couple of degrees, maybe
10 degrees C or something
like that.
It is showing thermal noise.
And that thermal noise scales
with the square root of t.
So you move from
300 to 310 Kelvin,
which is in the
square root makes
no practical difference in the
level of the thermal noise.
It's true.
The thermal noise is
something that has
to do with the physical mass.
The quantum noise
is something that
has to do with the
photon sensing system
that is bouncing off of it.
So I hope that covered that.
AUDIENCE: So the last question,
if you do want to tell,
that's fine.
What's the coating
material of the mirror?
DAVID SHOEMAKER: Oh,
it's tantalum pentoxide,
with silica as the second index.
So that's a nice--
it turns out that silica is
a low-loss material, even
in the sputtered form, and has
a reasonably high q or low loss,
more like q is 10 to the
5 or something like that,
if you lost tension
to the minus 5.
The tantalum pentodixe
is a messy material
and has a loss more
like 10 to the minus 3.
And that's what really
dominates the thermal layer.
We're looking for
other formulations
of two-index systems, which
are lower, either intrinsically
lower in their losses or
where we apply the tantalum
pentoxide in other ways,
either as microlayers
or through in annealing
process, where
we heat the substrate
before we put it down
or while we're putting it down.
And otherwise, to
try to drive it down.
We're also looking at
crystalline coatings,
which can have potentially
very little loss.
But it's hard to
make them bigger
than something like this.
And they may have some
other problems as well.
But it's a real
knotty [INAUDIBLE]
and [INAUDIBLE] problem in
material science, about which I
know absolutely nothing.
AUDIENCE: You
mentioned the KAGRA,
the Japanese experiment is
starting to actually go up.
DAVID SHOEMAKER:
Thank you, yes, right.
So we operate currently
at room temperature.
And the KAGRA experiment
actually has--
it uses sapphire as
its test mass material.
It has a very high thermal
conductivity, so lower
deformation under heating,
and so forth, and so on.
And also sapphire
suspension wires,
which are rather
thicker than ours,
because the way that they
get heat out of the system
is by conducting up
those sapphire wires.
They hold the external
system at about 4 Kelvin.
And they think they
can maintain the test
masses at about 10 Kelvin.
And so there you've gone
from 300 Kelvin to 10 Kelvin.
Square root of that,
you've actually
made a significant change
in the thermal noise.
So that's an approach.
It's a pretty heavy-handed one.
I'm sure that we'll all get
to using cryogenic systems
sometime in the future.
But for now, we're going
to be holding these things
at room temperature.
And it's an interesting point.
I mean the 40 kilometer dream
machine that we talked about,
the Cosmic Explorer,
where I said
you just use the same components
and make it 10 times longer,
we use that to convince
ourselves and funding agencies,
that an investment
of that kind of
would be well rewarded with
the current technology.
We definitely will
also be pursuing
cryogenics in parallel, to
drive that noise further down.
AUDIENCE: You mentioned funding
for future observatories
and how maybe it would
have to be shifted away
from, if you will, more
standard observatories.
I'm wondering, just
along those lines,
is there a sign of
people writing proposals
at observatories that
have to do with trying
to be more responsive to,
say, LIGO announcements?
DAVID SHOEMAKER: The
simple answer is yes.
And we see that in a
number of different ways.
We see a great
deal of enthusiasm
among observers for
this new observing run.
A lot of them are
tailoring their
they're observing
time exploitation
around the chance of
being a participant
in, say, another one of these
neutron star observations.
The other thing that we
noted is that quite a number
of the white papers that were
submitted to the 2020 Astro
Decadal, just a
couple of weeks ago,
stress how conventional
astronomical systems are being
proposed, either ground-based
or space-based, to complement
the gravitational wave data.
And so it's clear that the
astrophysics community is
starting to either see
this as an opportunity--
and like any good
drug dealer, we're
delighted to see they're
becoming dependent upon us
and they'll want more.
And so it's a good
sign that we're
following through
with our promise
to do something which is
beyond, of curiosity and--
I like GR.
And I think it's really
cool to either show,
once again, that Einstein's
predictions are right,
or, even more exciting,
show that they were wrong.
But we have to
have a bigger base.
And the binary neutron star
proved to ourselves, as well as
to the larger community,
that really it is true.
AUDIENCE: Hi.
You spoke briefly about how,
as we get better instruments,
you're going to be
moving up towards larger
masses combining.
Is there any interest in
going smaller and seeing
smaller masses
combine, or is it just
we're looking at the bigger?
DAVID SHOEMAKER: Let's see.
The problem is that as
the masses get smaller,
the signal gets smaller.
And it also so happens
that the interferometry
that we currently
know how to do has
poorer and poorer performance
at higher frequencies.
We've got a number of sketches
on the blackboard of things
that you could do, if
you could make mirrors
with 10 times lower
absorption loss.
I mean, the other
kinds of problems
that happen with-- well, if you
could turn up the laser power,
you could improve the
sensitivity in that regime.
But there are all kinds of
really weird things that
happen, besides, for instance,
heating of the mirror
and deformation of the mirror.
There are also really
significant torques
applied to the mirror.
If the beam is not exactly
centered on the mirror,
it twists the mirror away.
When the system goes from not
being in lock to being in lock,
it gets a huge kick.
There are all these
really strange dynamics
that take place.
We don't know how to handle much
more power than we currently
have.
So it's hard to
make a system that
would be sensitive enough to
see much smaller systems coming
together.
You also need them
to come together
at near relativistic speeds
to get the kind of signal
that you want.
And once you take something as
gloomy, and gummy, and low mass
as the Sun, and you spin
two normal functioning suns
around each other, they smush.
They don't get up to a high
enough rate of rotation.
A binary neutron star
consists of two objects
about the size of Providence,
or something like that.
And they can get really close
and at a really high velocity
before they actually start
to interact physically.
And in that way, we get
a large enough signal.
So I think there
isn't much hope,
at least with the technology
that we have in mind now,
to see smaller objects.
It also might be
less interesting.
I don't know.
AUDIENCE: Are there any low
frequency electromagnetic noise
which affect the--
DAVID SHOEMAKER:
Any low frequency?
AUDIENCE:
Electromagnetic waves--
DAVID SHOEMAKER: Oh.
AUDIENCE: --interacting
with the metallic piece
of your detector?
DAVID SHOEMAKER: Yes.
And I guess-- it goes
all the way down to DC.
Charge on the test mass causes
electrostatic attraction
between the test masses
and the surrounding cage,
which is not as well
seismically isolated.
So it is a transfer function
from there to the test masses.
The entire thing is
inside a Faraday cage.
And so there aren't, in fact,
EM waves of large amplitude
that arrive.
But we do have--
one of the very important
features of the observatory
is to have a complete
environmental monitoring
system.
So we have radio receivers
of all different kinds,
and shapes, and sizes;
microphones; seismometers;
accelerometers; cosmic ray
detectors, which are-- it's
peppered throughout the system.
We also have weather stations.
It turns out that one of the
biggest effects that we have
difficulty dealing
with is wind buffeting
the building in which
the system sits,
causing tilts of
the floor, which,
despite our seismic isolation,
the system is coupled
up and through.
We have trouble with
electromagnetic coupling.
And the way that we
manage it is mostly
through a Faraday cage,
but then also monitoring.
And with each one of
our monitoring systems,
we strive to have
a monitor which
is more sensitive than
our interferometer
to that particular disturbance.
And in that way, we
can reject signals
that we see as coming from
nongravitational wave systems.
AUDIENCE: It is
curious that you don't
mention dark matter when talking
about gravitational waves.
DAVID SHOEMAKER: I
think I've learned
more about dark
matter in my visit
here today than I have
in my previous lifetime.
One person-- I don't know.
I'm an instrument builder,
not an astronomer,
or an astrophysicist,
or even a relativist.
And one of the
interesting things
that was raised as a
possibility, perhaps
there are dark
matter black holes.
Is there any
possibility that there
would be a different
signature, for instance,
to a ring bound, or something
like that, due to a dark matter
black hole?
It could be that we can
shed some light on things.
I think maybe the
thing that I could see
is the most direct
and obvious way
that we can contribute to
these particular puzzles is
to see a deviation from GR.
And that could give
an additional clue
for constructing a
theory, which might
help understand the dark
energy, dark matter problem.
But it's not something
that we think about
as an experimental goal.
And we don't see a
recipe, at least a priori,
to do something
to our instruments
or have a specific
analysis regime that
would allow us to pull that
kind of information out better.
AUDIENCE: [INAUDIBLE]
[APPLAUSE]
RICHARD GAITSKELL:
[INAUDIBLE] I'm so sorry.
[INAUDIBLE]
AUDIENCE: Oh, it
was just a question.
What are the prospects
for measuring polarization
in the gravitational wave?
DAVID SHOEMAKER:
Oh, very, very good.
And, in fact, we already put
some limits on deviations
from Einstein's
prediction of the spin 2
45-degree polarization
vector with
that first gravitational
wave event that
was measured in Virgo.
Because while the
two LIGO instruments,
they were designed to be
as parallel as possible
to confirm detections.
Whereas what you really
want to measure polarization
is 2 at 45 degrees.
Well, it turns out Virgo
is over here somewhere.
And is at a cockamamie
angle with respect
to the two LIGO instruments.
So as we now start
to see detections
in the three detectors,
Virgo has really
improved its sensitivity since
last time, by a factor of three
or something like that.
We'll be able to say more
about the polarization,
both as a tool to get more
information about the angle
of observation, but
then also to test
whether or not GR
is working right.
So we're on the path.
RICHARD GAITSKELL: Now, this
isn't the last question,
because we have a-- out
in the reception area,
we have pizza for
undergraduates,
graduates who didn't
manage to make
the Q&A at lunchtime
today, or somebody
who has more questions.
Please do make use of that.
I'm afraid [INAUDIBLE]
[SIDE CONVERSATION]
But please do.
I'm sure [INAUDIBLE]
[APPLAUSE]
