- Good afternoon, everyone.
I'm Tomiko Brown-Nagin, the
Dean of the Radcliffe Institute
for Advanced Study, and it's
wonderful to see all of you
here this afternoon.
I'd like to begin by extending a
very special welcome to members
of the Harvard and Radcliffe
classes of 1979 and 1974,
who are celebrating their
40th and 45th reunions.
Let's all give them
a round of applause.
[APPLAUSE]
Our program today
features Harvard Professor
of Astronomy Edo Berger, the
2019-2020 Mildred Londa Weisman
Fellow here at Radcliffe.
Two weeks ago, we welcomed
Edo and his fellow Fellows
to campus.
They are a remarkable group
of 50 individuals working
across the sciences, humanities,
social sciences, creative arts,
and the professions.
They're pursuing
innovative projects
that will lead to
scientific breakthroughs
to progress on critical
societal issues,
and to works of art
and culture that
prompt deeper understanding of
our communities and ourselves.
Edo's Fellowship project
bears the enticing title,
"All that Glitters is Gold--
Gravitational Waves, Light,
and the Origin of the Heavy
Elements."
And he has bold plans
for his Radcliffe year.
He aims to show definitively
how gold and other rare elements
are created in the universe.
Now, I want you to take a minute
and picture the periodic table
of elements-- even if it's just
a faint memory from high school
science, as it
certainly is for me.
As it turns out,
how more than half
of the elements in the
periodic table are created
remains a mystery to scientists.
Edo's interdisciplinary
work draws
on a range of new and
well-established fields
to begin filling in
this surprising gap
in our understanding
of the universe.
Specifically, he studies
explosive and eruptive
astrophysical events,
and he's looking further
into the theory that collisions
between neutron stars--
the smallest and densest stars--
are, in fact, the
cosmic origin story
for gold and other
rare elements.
This is a fascinating topic,
and I'll leave it to Edo--
as I must-- to help us
understand how it all works
and what it might
mean for science
and for the general public.
Edo studied astrophysics at
the University of California,
Los Angeles and the California
Institute of Technology,
and he's never been
one to shy away
from big, unsolved questions
or cutting-edge methods.
For his dissertation at Caltech,
titled "Cosmic Explosions--
The Beasts and Their Lair,"
Edo won the Robert Trumpler
Award from the Astronomical
Society of the Pacific--
a recognition of research
that is unusually
important to astronomy.
Unusually important
is a through line
of Edo's research as
a graduate student,
as a postdoctoral fellow at
the Carnegie observatories
and Princeton University,
and since 2008,
as a member of the
Harvard faculty
and of the Harvard Smithsonian
Center for Astrophysics.
With more than 300
publications to his name,
I won't even try to
summarize Edo's research.
But I'll share something
that illustrates
his remarkable
productivity and impact.
In August of 2017, scientists
for the very first time
observed a neutron
star collision,
a field-changing moment
for Edo and his colleagues.
And in the 12 months
following that event,
Edo and his research
team published
some 19 scientific papers on
their revolutionary discovery
and the study of electromagnetic
radiation emanating
from the collision.
Edo is also a respected and
celebrated teacher and mentor.
He served as Director of
Undergraduate Education
in the Department of
Astronomy for several years,
and his graduate
advisees have gone on
to receive many prestigious
postdoctoral fellowships.
We are very honored to have Edo
here this year at Radcliffe,
and please join me in
giving him a warm welcome.
[APPLAUSE]
- Well, thank you,
Dean Brown-Nagin,
for a lovely introduction.
Good afternoon, everyone, and
thank you for joining me today
to hear exciting news
about the cosmos.
Before I begin, I would like to
thank the Radcliffe Institute
for this amazing opportunity to
spend my very first sabbatical
in this incredibly vibrant
and multidisciplinary and
supportive environment.
It's been fantastic so far, and
we're only three weeks into it,
so I'm expecting amazing things.
By the way, this talk will
include collisions, explosions,
bright lights.
So if anybody is
averse to that, now.
Is the time to leave OK, so
let's jump right into it.
So what I'm going to tell you
today is that on August 17,
2017, our research team-- and
you can see these pictures here
of members of my
research group--
used information
carried by light--
our traditional way of
studying the universe--
to discover gold and other
rare heavy elements whose
cosmic origin has been
a complete mystery up
to that point being forged
in a cataclysmic collision
between neutron stars, which
was seen for the first time
through the emission of
gravitational waves--
a brand new way of
sensing the universe.
My goal in the
next 45 minutes is
to unpack this
really long sentence
and to take you through the
scientific and personal aspects
and story of this discovery.
By the way, you can find
some of these people
here in the audience.
So if you have any pressing
questions after the talk
and you don't get a chance to
talk to me, please ask them.
This was a truly unusual
scientific discovery,
both in terms of its
scope and the fact
that it can be traced to a
very specific moment in time--
Thursday, August 17, 2017.
2 years, 33 days ago--
not that I'm counting.
As scientists, I
think we're incredibly
lucky if we get to participate
in a single scientific
breakthrough.
But this was a really
unparalleled story of firsts.
It is the first time that we've
detected gravitational waves
from colliding neutron
stars, the first joint study
of gravitational
waves and light,
the first direct evidence
for the synthesis of heavy
elements, and the first time
that we've combined disparate
fields of science and
physics and astronomy--
general relativity,
nuclear physics,
high-energy astrophysics,
and cosmology--
in a single study to
study a single phenomena.
In fact, this discovery was so
exciting and complex that we
felt compelled to write nearly
20 scientific papers about this
in the months ensuing
the discovery.
And in fact, just this week
we submitted another paper
on this object--
which I sincerely
hope is the last one.
[LAUGHTER]
Just to give you a sense
of how this was received,
our initial series of papers--
the first one was accepted
for publication in the journal
within five hours
of being submitted.
So it usually takes weeks
or months for papers
to go through the review
process and back and forth
and all of that, so this
was really, really amazing.
But to understand how we ended
up, on this sunny August day,
with such an incredible
confluence of events,
we have to step back.
In fact, we have to go
back a full century.
So the year is
1916, and a fellow
by the name of Albert
Einstein publishes
a revolutionary
theory of gravity
called the general theory
of relativity, or GR.
GR posits that gravitational
attraction between objects
is due to the
curvature of space-time
caused by their mass.
The more massive an
object is, the more it
curves space around itself,
and therefore, the stronger
its gravitational attraction.
The famous physicist
John Wheeler
put it succinctly
and elegantly, saying
that "Matter tells
space-time how to curve,
and space-time tells
matter how to move."
So what does it mean?
It means that the Earth--
jumped the gun a little bit.
[LAUGHTER]
We can go back to this.
It means that the Earth orbits
around the sun not because
of some unseen, mysterious
force between them,
but because the sun
curves space around itself
and the Earth is forced
to orbit and move
within that curved space.
Now, this is a much
more elegant explanation
of gravity than the
story of an apple hitting
Newton on the head.
You can see where
I'm going with this.
And I was actually reminded of
this-- going apple picking--
this past weekend.
By the way, these are
not pictures of kids
I pulled off the internet.
These are actually my
sons, Ilan and Noah,
who are 3 and i years old.
Now, a key prediction of
Einstein's general relativity
is the existence of something
called gravitational waves--
ripples in space-time that
are caused by the acceleration
of mass--
for example, when two
objects orbit each other,
as we see in this illustration.
In some sense, you can think
of these gravitational waves
as analogous to throwing
a rock into a pond
and watching the ripples
move away from where
the rock hit the water.
Now, interestingly,
Einstein himself
had a tortured relationship
with gravitational waves.
He initially stated,
in his 1916 paper--
I'm not going to
try a German accent,
but he said, "There are
no gravitational waves
analogous to light
waves, period."
Then in 1918, he
revisited the issue,
and he formulated the basic
notion of gravitational waves
that we still think about today.
But then in 1936, he again
published a paper casting doubt
on the existence of
gravitational waves.
So in a sense, Einstein did
what every good theorist does--
he hedged his bets.
And this way, whatever the
outcome, he could claim credit.
It was finally only
after Einstein's death,
in the 1950s and
'60s, that theorists
established that gravitational
waves, in fact, should exist.
They showed, mathematically,
that it was an inevitable part
of general relativity.
And these theoretical studies
argued that gravitational waves
carry away energy, therefore
causing these binary systems
of objects to spiral in
and merge into each other,
as shown in this actual
computer simulation of two
merging objects.
So you can see the gravitational
waves emanating from the system
as the objects get closer
and closer to each other,
until they merge
together in a final burst
of gravitational waves.
And once the objects have
merged into a single object,
the emission of
gravitational waves ceases.
As a result of this, it
means that the amplitude
and the wavelength of the waves
actually carry information
about the objects
that generate them--
information such as their
masses, their distances,
their spins, and
other parameters.
Information that,
in many ways, is
inaccessible to us
in any other way.
Now, I give this analogy of
throwing a rock into a pond
and watching the ripples.
This analogy only goes so far.
Gravitational waves are,
in fact, a compression
and decompression, a
stretching and compression
of the curvature of
space-time itself
propagating at
the speed of light
as shown in this
scientifically-accurate
animation.
So you can see space-time
itself actually rippling
as a result of the passage
of gravitational waves,
but in this compression
and decompression sense.
So if we form the circular ring
of free-floating particles,
gravitational waves
passing through the ring
would first compress
the ring in one axis,
and then stretch
it and compress it
along the other direction, in
this back-and-forth motion.
And that is a unique signature
of gravitational waves.
Now, as a result of this
stretching and squeezing
effect, here's a
vastly exaggerated view
of what would happen
to the Earth--
or what happens to the Earth--
when gravitational waves
actually pass through it.
So you can see the same effect.
It's stretching and
compression of the Earth
as gravitational
waves pass through.
Now, this happens all the time.
Even as we're sitting right
here in this auditorium,
there are gravitational waves
from outer space passing
through the Earth
compressing, decompressing it.
Anyone feeling
stretched, squeezed?
If you do, let me
know right away.
You should see a doctor.
So the strength of
gravitational waves,
or the strength of the signal
of gravitational waves,
is given by a quantity
called the strain, which
is essentially just a measure
of the change in length
of an object relative to
its undisturbed length
as a result of the passage
of gravitational waves.
So in this example
that I've shown before,
you can see that the ring is
stretching and compressing
roughly by its radius.
That means that h is about 1.
That's what h of
1 would look like.
In general relativity,
Einstein tells us
that h is the following
equation-- and don't worry,
this is the only
equation in the talk,
and there's no test at the end.
But I put it up
there because I think
it's really important
to see where the effect
and what the actual effect of
gravitational waves on Earth
comes from.
So let's look at it briefly.
G is Newton's
gravitational constant.
All you need to know is that
it's a very small number.
c is the speed of light.
It's a very big number.
And we take it to the
fourth power, which
makes it incredibly large.
d is the distance
to the source--
which, for astronomical
sources, is astronomical.
So G divided by c to
the fourth divided by d
is an incredibly small number.
Therefore, the only way,
the only chance we have,
or the only hope we have of
detecting gravitational waves,
detecting this effect of
stretching and squeezing,
is from objects that produce
an enormous amount of energy
and gravitational
waves to compensate
for these very small numbers.
And that requires objects
that are incredibly massive
and incredibly dense.
Even so, this is a
really small effect.
So a typical value for h for
real gravitational events
is 10 to the minus 21--
1 part in 10 to the 21, OK?
I'll come back to
this in a second
to orient you around
this very small number.
So what are these dense
and massive objects
that can generate the
gravitational waves?
The densest and most massive
objects in the universe
are neutron stars
and black holes.
They're remnants of enormous
supernova explosions
of massive stars.
Neutron stars weigh about 1 and
1/2 times the mass of the sun,
but their radius is
only about 7 miles.
That means that
they're so incredibly
dense that 1 teaspoon
of neutron star matter
weighs about 10 million tons.
Black holes are
even more extreme.
They weigh about 10 times
the mass of the sun.
The radius of their
event horizon--
the point from which light
can no longer escape--
is about 20 miles,
and the density
of the singularity at the heart
of a black hole is infinite.
So let's put this
in perspective.
Here's a map of Boston.
So we're sitting here, at
the Radcliffe Institute.
Here is Route 95 over here.
Here's downtown Boston.
So a black hole would fit
snugly within the Route 95 loop.
A neutron star is even smaller.
It would barely reach
downtown Boston.
And these are to
remind you objects
that weigh several times
the mass of our sun.
So these are really
incredible, extreme objects.
Now, in order to generate
gravitational waves,
these objects don't
just need to exist.
They need to exist
in pairs, They
need to exist in binaries
that can merge and collide
in order to generate
gravitational waves.
And as amazing as it
sounds, we actually
know from our own
Milky Way galaxy
that there are these pairs
of neutron stars that exist.
They're just incredibly rare.
So this means that if we want
to detect gravitational waves,
we have to build a
detector that is sensitive
enough so that we can sense
galaxies outside of our own.
In fact, we need to be able
to sense millions of galaxies
in order to detect these
gravitational waves.
Now, whether pairs of black
holes actually exist in nature
was an open question
until recently,
because black holes
are intrinsically dark,
and we had no other
way of finding
pairs of black holes
orbiting in our Milky Way
or in other galaxies.
OK, so now back to
this very small strain
of gravitational waves.
So even for neutron
stars and black holes
orbiting each other
in these pairs,
this is an incredibly
small change in length
that we're going to
have to measure--
1 part in 10 to the 21.
So what does this actually mean?
I think it's a really
hard number to capture,
so I want to just give
you some analogies.
So one analogy is that it's
equivalent to measuring
the distance to
the nearest star--
which is about 4.2 light years
away, or 25 trillion miles--
to a precision the
width of a human hair.
This is not something
that we usually do.
Another analogy-- this is
equivalent to the tallest
mountain on Earth being the
size of an atomic nucleus.
So imagine if instead
of Everest, we
had an atomic nucleus that was
the tallest mountain on Earth.
That's 1 part in 10 to the 21.
It's also equivalent
to measuring
the length of
4-kilometer steel tubes
to 1 part in 1,000
the size of a proton.
Now, that's a weird analogy.
Why am I telling you this?
Well, it's because
this is how we actually
detect gravitational waves.
So this is LIGO--
the Laser Interferometer
Gravitational Wave
Observatory--
which was conceived
already in the 1960s
when the idea of
gravitational waves
really came into fruition.
LIGO was funded in 1990.
Its construction was
completed in 2002,
and then it spent a
few years taking data,
but with very
limited sensitivity,
so it made no gravitational
wave detections.
In 2008, the National
Science Foundation
funded a large
upgrade to something
called Advanced LIGO, which
was an order of magnitude more
sensitive, and with the
idea of actually making
the first astrophysical
detections.
So the basic idea
at the heart of LIGO
is the laser interferometer.
It's essentially an
incredibly precise machine
for measuring the length
of two L-shaped arms
for the purpose of
measuring the stretching
and squeezing that's caused
by gravitational waves.
So the idea is the following-- a
laser beam is sent into the two
arms, which have
precisely the same length,
and it then returns
and cancels out.
But then when a gravitational
wave passes through,
one arm and then the other
are stretched and compressed,
changing their length by
1,000th the size of a proton.
And the laser beams therefore
come just a little bit out
of phase.
What LIGO does is to
incredibly precisely look
for these minute
out-of-phase signals.
That's why it took so many
years to build this facility.
It's incredible.
Now, in fact, there
are two LIGOs.
So there's one LIGO detector
in Hanford, Washington
and one in Livingston,
Louisiana and the reason
is that this way we can ensure
that the gravitational wave
signals are actually
real by correlating them
across very distant
geographical locations, OK?
So the gravitational
waves are very large.
They will pass
through both places.
But other effects
that are local are not
going to show up in both.
And to put this in
perspective, the waves hitting
the Coast of Louisiana
from the Gulf of Mexico
actually make a bigger
effect in the detector
than gravitational waves.
Somebody walking in
Hanford, Washington
makes a bigger effect on
the detector than the signal
from gravitational waves.
So we really need
these two detectors
to correlate the signal.
Now, LIGO is not alone.
It's part of a larger
and growing network that
includes, currently,
the operational Virgo
detector in Italy, KAGRA, which
is under construction in Japan,
and the planned LIGO clone that
will be constructed in India.
And this network also
allows us to roughly locate
the location of
gravitational waves
on the sky using triangulation.
So here's what these
detectors are designed to do.
They are designed to see,
or measure, how to strain,
the amplitude of the
gravitational wave
signal, and the frequency
increase as a function of time.
So LIGO will capture
the final second
before merger of black hole
and neutron star binaries
and measure the
strain and frequency
evolution of the gravitational
waves, which you can see here.
So this is a simulation of
two black holes orbiting
into each other.
And you can see,
here, on the graph--
this is the final second in the
life of these two black holes.
So LIGO is designed to just
capture this final second.
Now, what I want you to note is
that in those final few orbits,
just before those two black
holes collide into each other--
you can see this on
the axis over there--
they're traveling at more
than half the speed of light.
So this is quite a violent
collision between two
incredibly massive objects.
Now, this uptick in
frequency and amplitude
that you're seeing at the very
end right before the merger
is called a chirp.
And it will become
obvious in a few minutes
why that's the case.
OK, so now I'm going
to put Einstein
and gravitational waves and LIGO
aside for a couple of minutes,
and we're going to switch from
astrophysics to chemistry.
So what I'm showing you
here is the periodic table
of the element.
I think it's one of the
major scientific triumphs
of humanity in terms of
organizing and classifying
nature.
There are presently about
120 different elements known,
and the periodic
table beautifully
organizes them together based on
their properties and behavior.
Now, here's how astronomers look
at this incredible complexity.
[LAUGHTER]
We have hydrogen, helium,
and everything else.
And we call everything
else metals.
But this is not a joke.
This is really how
we refer to this
in scientific publications.
But slightly more
seriously, astronomers
are interested not only in
the properties of the elements
and how they react
with each other
and how they get classified,
but in how they actually
form in the universe.
And specifically, what
cosmic sites and what
processes are
responsible for forging
different types of elements?
So it was recognized in
the early 20th century,
partly through work
here at Harvard,
that hydrogen and helium--
the two lightest elements--
formed primarily
in the Big Bang.
Other light elements, such
as carbon, nitrogen, oxygen,
form in the cores of
stars like our own sun.
More massive elements up to
iron and cobalt and nickel
form in the cores and in
the supernova explosions
of very massive stars.
And then it was just
generally assumed
that everything else formed in
supernova explosions as well.
That's what the textbooks
told us as recently as 2016.
Every astronomy textbook
said these heavy elements--
we're not sure, but it must
be supernova explosions,
because what else is there?
Now, the periodic table--
the way it's presented is flat.
And what I mean by that
is that it tells us
about the properties
of the element,
but it doesn't actually
tell us how abundant
the various elements are.
But that's what I'm
going to show you next.
So this is a plot that
shows, on the y-axis,
the abundance of different
elements as a function
of their mass all the way from
the most abundant elements--
hydrogen and helium--
to the most rare.
So you can see that the
most massive elements,
including platinum and gold--
which sit right over there
in that shaded region--
are incredibly rare.
They're less than 1 parts per
billion relative to hydrogen
in the universe.
So they're not
just rare on Earth.
They actually are intrinsically
rare in the universe.
Now, the pathway for
forming these heavy elements
is something called
r-process nucleosynthesis.
I'll come back and use this
slightly "jargon-ish" word,
but it kind of rolls off
the tongue a little bit.
It's the rapid process--
r stands for rapid-- rapid
process of element formation.
So here, the idea is
that rapid means that
to build up these
very heavy elements
would require a very high and
very rapid influx of neutrons
into seed nuclei,
which are lighter,
before these neutrons
can decay into protons
and build up successively
heavier elements.
So the key question for us
is where, in the cosmos,
does the r-process happen?
Where does it happen?
That's where gold forms.
That's where platinum forms.
That's where uranium forms.
That's what we
need to figure out.
OK, so with that
question on our mind,
let's go back to gravitational
waves and to LIGO.
So in April 2014,
about a year and a half
before LIGO was scheduled to
start its first science run,
and kind of sensing
the potential
for the first gravitational
waves detections,
I signed a memorandum of
understanding with the LIGO
and Virgo collaborations
to get access
to their detections in
exchange for maintaining
confidentiality.
So it's kind of like a
nondisclosure agreement type
thing.
Now, I can tell you
with great confidence
that other than my marriage
certificate and mortgage,
and the Radcliffe Fellowship
offer letter, [LAUGHTER]
this is the most important
document that I ever signed.
So there's my
signature up there.
You can't forge it.
So we signed this document
and then September 18, 2015,
LIGO operations are
scheduled to begin.
But four days earlier,
during the final hours
of engineering shakedown,
the unthinkable happens.
OK, so what you're seeing here
is a gravitational wave signal.
And the reason it's called
chirp is that we can actually
convert this signal
to audio frequencies,
and you can see this uptick
in frequency and in amplitude
that causes this chirping sound
at the very end of the merger.
So what was this thing?
So this is GW150914--
the gravitational wave
event of September 14, 2015.
It's the first burst
of gravitational waves
ever detected by humanity.
It's really an incredible event.
It represents the
merger and collision
of two black holes
weighing 35 and 30 times
the mass of our
sun at a distance
of 1.5 billion light
years away from Earth,
as shown in this computer
simulation over here.
This is an actual simulation
reconstructing the signal
that LIGO detected.
So you can see this in-spiral
of the two black holes,
one slightly more
massive than the other,
and then this large burst
of gravitational waves,
followed by a merger into
a more massive black hole.
Now, if the detection itself
was not incredible enough,
we can calculate that
in the final second
before the collision, when LIGO
actually detected the signal,
this merger released more
energy and gravitational waves
than the entire visible--
or entire combined light--
of the visible universe.
So that's more
energy than hundreds
of billions of galaxies, each
containing hundreds of billions
of stars, all released
in a single second.
So I'll let you ponder
that for a minute.
So this was amazing.
This happened, really,
during engineering shakedown
[INAUDIBLE] the
science operations.
So just like that, a century
after the original prediction
of gravitational waves, 30 years
after work on LIGO started,
and after spending $600
million on building LIGO,
the leaders of the
LIGO experiment
won the 2017 Nobel Prize in
physics for their contributions
to LIGO and for the
direct observation
of gravitational waves.
But to me, watching
the Nobel Ceremony--
despite all the excitement.
It was a lot of fun.
The more pressing and
lingering question
still was, so
where in the cosmos
did the gold in the Nobel
Prize medal come from?
[LAUGHTER]
Where's it come from?
I'm stuck with this question.
OK, so for this, we have
to shift our attention away
from the collisions
of black holes
to the collisions
of neutron stars.
So when black holes
collide, they simply
merge to form a more
massive black hole--
simple, right?
When neutron stars
collide, we think
that much more
exciting things happen,
as shown in the
simulation that shows
just a few milliseconds
before, during,
and after the collision.
So you can see that this violent
collision between a neutron
star leads to the
ejection of neutron star
matter at speeds
approaching about one
third the speed of light.
And in this ejected debris,
which is rich in neutrons,
we can imagine r-process
nucleosynthesis--
which requires a
lot of neutrons--
could take place.
We can imagine gold being
forged in the debris from such
a collision.
But how can we
detect the signature?
How do we know?
How do we know if this is
really going to happen?
How do we know if
gold is forming?
So luckily, some of these
r-process isotopes and nuclei
are unstable, and
the radioactive decay
can power a flash of visible
and infrared light that
lasts for about one week.
So that's relatively short.
It's very much human timescales.
So that's shown here.
Brightness is a
function of time.
You can see the signal of
a fading flash of light.
And with very special
spectral characteristics
that we call wiggles
that are very
different from any
astronomical event.
So you can see, here, brightness
is a function of wavelength,
and you can see these
wiggles in what we
call the spectrum, which
are unique to this process.
And by the way, "wiggles"
is a technical term.
[LAUGHTER]
I'm not just making it up.
OK, so here was our game plan.
LIGO detects a
gravitational wave signal
from a neutron star collision.
They rapidly alert us
through our memorandum
of understanding that we signed
a year and a half earlier.
We then use special instruments
to scan the sky area where
the gravitational
wave event came
from for a visible counterpart,
and if we can find it,
then we take over every big
telescope around the world
and in space and we search for
the signature of the r-process.
It's essentially a plan
for world domination.
[LAUGHTER]
OK, so finally we arrive
at August 17, 2017.
It's 9:21 AM.
And I'm sitting in my
office in the midst
of a thrilling committee meeting
on astronomy graduate-level
curriculum revisions when
my cell phone goes off.
So I'm a polite person, so
I ignore the cell phone.
And then my office phone
rings, and I ignore that too.
And then I get a blast of text
messages, and at that point
I just kick everybody
out of the room.
And I check my text.
I check my voicemail.
I check my email.
And the message is clear--
LIGO and Virgo detected
a neutron star collision.
And here it is.
This is the chirp of GW170817--
so the gravitational wave
event of August 17, 2017.
The first neutron star
collision ever detected.
OK, time to activate
the game plan.
So at 9:21 AM, the neutron
star collision is detected.
LIGO tells us that it's about
125 million light years away.
It's located to a region
that's 100 times the size
of the full moon.
That's a big chunk
of sky, and that's
shown schematically in those
white contours in the image.
And that region of the sky will
be first viewable from Chile
about 10 hours later.
So then 7:13 PM, after
a full day of planning,
the sun finally sets in Chile.
We open the telescope dome.
We begin taking images.
In each image, as it
comes off the telescope--
and you can see these images
marked by these red regions
over there--
each image is
inspected to search
for a new source
of light that could
be the visible counterpart of
that gravitational wave event.
Now, at this point, the
tension was unbearable.
We're looking at these images.
We're excited.
We're exhausted.
And the question is, will
we be able to locate,
for the first time, the
light source associated
with a gravitational wave event?
And then an email pops up in
my inbox from my colleague,
Ryan [INAUDIBLE]--
[LAUGHTER]
--who's looking at images
at his home in Ohio.
And he says-- he's
a succinct guy.
He says, holy smokes.
[LAUGHTER]
Check out NGC 4993.
So that's the name of a
galaxy in the localization
region of this
gravitational wave event.
He says, attached
is tonight's image--
our image that we just took--
and then archival
preexisting image,
and he also says the galaxy's
located at a distance of 40
megaparsecs.
That's 125 million light years.
That's exactly the distance
of the neutron star collision.
You want to see the image
that he attached to the email?
OK, I'll show it to you.
OK, so here's the
image he attached.
And just to make sure
that in all the excitement
we didn't miss
what was going on,
he put a bright red
circle around the source,
just in case, right?
So you can see our image--
our discovery image--
showing this new
source of light,
and the archival
image which was taken
several years earlier of
that same part of the sky
does not show that source.
So it's really hard
for me to put in words
exactly what I felt
when I saw this,
but I remember being
essentially stunned.
It's the first time
that we've seen
light associated with a
gravitational wave event.
It just felt like this
kind of historic moment.
It's too bad we didn't have
a professional photographer
in the room ready
to take pictures.
But really, there was
no time for reflection.
We had to proceed to the
next step in our game plan.
So within two hours
of getting this image,
we already submitted a request
to repoint the Hubble Space
Telescope at this
location on the sky.
And I've been told that our one
paragraph justification for why
Hubble had to do this is the
shortest proposal ever written
to use the Hubble
Space Telescope.
[LAUGHTER]
And in the submission
comments, we noted--
kind of nonchalantly--
these observations are
extremely time critical,
so the proposal should be
evaluated as soon as possible.
And by the way, the information
is strictly embargoed.
Now, there's a
really funny story--
kind of funny in
retrospect, to be honest--
related to these
Hubble observations
if anyone wants to ask
me about this in the Q&A,
or maybe over wine or scotch.
But here's a beautiful
image that we got back
from the Hubble Space Telescope
showing this visible source
of light associated with
a gravitational wave
event embedded in its
beautiful host galaxy.
So we continued
observing the source
every single night, tracking
how it faded back to obscurity.
And I have to say,
in putting this movie
into my presentation,
I realized that one
of the fun things about
being a Radcliffe Fellow
is that there are
some filmmakers here
in the audience, and
they are probably not
terribly impressed
with this movie.
But I can tell you--
to me, this is the most
exciting movie I've ever seen.
[LAUGHTER]
So now I'm going
to show you the sum
total of all the brightness
measurement collected
for this visible light source,
all the way from ultraviolet
to infrared wavelengths,
using 45 different telescopes
around the world.
This is, as far as I know,
the largest collection
of telescopes that's
ever been pointed
at the same part of the sky
to study the same event.
And these data represent
many, many sleepless nights
for a lot of people.
So what you're going to see
is the source brightness
as a function of time in
days since August 17, 2017
where each day of data is
compressed into a half-second.
So here it is.
You can see the source
starting off relatively bright,
and then quickly fading away,
and it faded away more quickly
in the ultraviolet
part of the spectrum
than in the visible
part of the spectrum,
and finally in the infrared
part of the spectrum.
Now, here's a comparison
to those theoretical models
that I showed you of what
light from r-process material
is going to look like.
You can see it's quite
a nice agreement.
In fact, those
curves on the plot
are this model tuned to the
properties of this event.
Here is our collection,
our time series of spectra.
So this is brightness as
a function of wavelength.
And you can see
the source, again,
fading away with time
day after day after day
and developing those
characteristic wriggles that we
expect for r-process material.
Again, here's a comparison
to the theoretical models,
and you can see the same thing.
It starts off bright
and featureless,
and then it develops these
wiggles as a function of time.
So not to too fine a point
on it, we found a pot of gold
at the end of the rainbow.
[LAUGHTER]
OK, so there we were sitting
on a big discovery that
connected light with
gravitational waves, the origin
of the heavy elements,
and the question
was, when can we tell the world?
When can we tell the story?
And the thing, of
course, is that we're
bound by this memorandum
of understanding,
so we had to maintain
confidentiality
about these results until
the public announcement that
was made two months
later on October 16, 2017
at a joint NSF and NASA press
conference in Washington, D.C.,
and I was lucky to participate
in that press conference.
Here's me, looking very serious.
It's a big announcement.
Now, I'm going to tell
you a secret today,
since we're all friends here.
I actually broke the embargo.
Every single night after
putting our kid to bed,
I told my wife everything
that was going on.
[LAUGHTER]
And she's here in the
audience, so she can testify.
OK, so as soon as this
story was announced,
it was all over the
news, all over the media.
What I found particularly
fascinating is that business
outlets--
Business Insider, Financial
Times, Wall Street Journal--
were all reporting on this.
They never report about science.
They're all fascinated
by this story,
and I just couldn't
understand why.
[LAUGHTER]
It's amazing.
All right, so now I'm going
to answer the question that I
know is on everybody's mind.
This is what you've
been dying to ask
from the beginning of the talk--
how much gold are
we talking about?
[LAUGHTER]
How much gold did this
one collision produce?
OK, so imagine the Earth
made out of pure gold--
the oceans, the continents,
the atmosphere, the core,
everything-- pure gold.
This one collision
made enough gold
that's the equivalent of 10
times the mass of the Earth.
That's a lot of gold.
So I actually
checked this morning,
and I multiplied
all the numbers,
and at today's market rate,
that's about $3,000 octillion
That's 3 with 30
zeros after it--
give or take a couple
of octillion dollars.
But before you rush out of
this room prospecting for gold,
I want to remind you
that all of this gold
is located 125 million light
years away, which equivalently
is 125 million years ago.
So I think we're kind
of late to the game.
But I think more
and more importantly
than any imaginary
monetary gain is
we actually ended up
amending the astronomer's
periodic table.
I mean, this is on Wikipedia,
so this is the truth.
[LAUGHTER]
These heavy elements
now are coming
from merging neutron stars.
I think this was kind of
an amazing breakthrough.
So this is how we think
about the formation
of the heavy elements today--
although, we're not done.
So what's next?
LIGO and Virgo have detected
more than 20 black hole
and neutron star mergers in
the past six months alone.
In fact, their most
recent detection
was just four days ago.
It was a collision
of two black holes
5.2 billion light years away.
By 2021, LIGO and Virgo
will reach their design goal
of being able to
detect neutron star
mergers to a distance of 650
million light years-- so much
further away than
the one we saw,
and that means that we
will see more of them,
because we're seeing a
bigger volume of space.
So it's just instead
of just one or two,
we're going to see
10 or 20 or 30.
KAGRA, which is the
Japanese detector,
and LIGO India will become
operational in 2020 and 2025,
but there's already work on
next-generation detectors that
are being planned both on the
ground and actually in space
with the goal of essentially
seeing every single black hole
merger that ever happened
in the entire universe--
the entire history
of the universe--
and perhaps as many
as 100 neutron star
collisions per day.
This absolutely terrifies me.
We're never going to sleep.
How are we going to study
all of these neutron stars?
OK, so I want to end this
talk where just a few thoughts
for you to reflect on.
So after millennia of studying
the universe with just light--
first with the naked
eye, the Greeks
going out, looking at the sky,
and then with telescopes--
we now possess a
completely new way
of exploring an otherwise
invisible universe.
And that's a real revolution.
This doesn't happen
very often in science.
Secondly, this journey that
started with Einstein in 1916
has led us in a completely
unexpected direction tying
gravitational waves with
light and the origin
of the heavy elements.
To me, this is one of the beauty
of science and serendipity.
We start with one
thought, and we end up
discovering something
completely different.
Third, a similar neutron star
collision in our own Milky Way
more than 5 billion
years ago seeded the gas
from which the solar
system formed with gold,
and that gold is
now in your jewelry.
So just think about this when
you look at your wedding band
or necklace or earrings.
So to paraphrase Carl Sagan,
who said that we are all star
stuff, I'd like to add
that our jewelry is
colliding star stuff.
[LAUGHTER]
Thank you very much.
[APPLAUSE]
