remember when you throw stones in the
lake and it caused ripples when I was
doing this as a girl I had no idea that
this would turn out to be the analogy I
would most often use to explain
gravitational waves to people who are
not scientists gravitational waves are
prediction of Einstein’s theory of
general relativity which is the modern
theory of gravity that replaced Newton’s
classical theory in the early 20th
century
Einstein’s theory is based on geometry
masses induce a space-time curvature and
that curvature is responsible for
attracting other masses now when masses
move ripples in the space-time curvature
are generated and propagates through
space it’s a bit like those ripples on
the lake but on a much larger scale and
what vibrates is space itself and those
space-time vibrations what we call
gravitational waves or G W is for short
and I want to tell you the story of how
not seeing gravitational waves led to a
groundbreaking scientific discovery the
GW sources that we detect are generated
in very powerful Astrophysical events
like stars colliding and we’re
interested in both the waves themselves
to probe Einstein theory and in the
events that create them and in that
sense observing GWS is a new type of
astronomy and it’s a very difficult type
of astronomy because GWS are extremely
elusive and hard to detect let’s take a
look at numbers to understand how big a
challenge it is when a GW which is earth
space is briefly distorted distances
change and all detectors try to register
that change and the way our detectors
work is the used light from a laser
source monitor all the distance
between suspending mirrors that are
several kilometers apart and GW is going
to change that distance of several
kilometers for a short time by 10 to the
minus 19 meter which is an absolutely
tiny amount it’s a billion times smaller
than an atom and it’s even ten thousand
times smaller than the nucleus in an
atom it is so small that it took decades
to get detectors that would be sensitive
there are three such detectors we called
them interferometers around the world
the two LIGO detectors in the US and the
Virgo detector in Europe Virgo and LIGO
operate together they form a network of
detectors and a big collaboration of
more than a thousand people and we
started building LIGO and Virgo twenty
five years ago or so and we have kept
improving them to make them more and
more sensitive until finally on
September 14 2015 the first
gravitational wave signal was detected
by the two LIGO detectors so well ago
was not operating at the time but it was
a milestone for the collaboration and it
was just amazing to see that first
beautiful signal received from a
fascinating source colliding black holes
more than a billion light-years away so
it took a long time before GWS were
detected and you know what they say that
the path is the goal and well it has to
be the case when you walk the paths for
so long before getting any real results
it certainly helps that you don’t walk
the paths alone collaborative work is a
great way to sustain motivation and
progress in the long run and probably
also helps that you don’t know
beforehand that the past is going to be
so long when I joined the effort in 1993
I was aware that detecting the first
gravitational waves
would take some years maybe even I don’t
know maybe ten years I didn’t anticipate
that having children and raising them
would almost start looking like a
short-term project in comparison now
let’s get back to our GW sources there
were a few detectives in September 2015
most of them from colliding black holes
those black holes are what remains of
massive stars once they have exhausted
the fuel that keeps them shining and
gravity is so strong around black holes
that nothing can escape not even light
that’s why they are black and when two
black holes spiral toward each other
GWS are emitted which drives the black
holes closer together emitting stronger
and stronger GWS until the black holes
collide and merge and those binary black
hole mergers as we called them they were
corundum Li and we have detected a
handful so far and we named them after
the date when the signal was recorded so
GW 1509 14 for the very first one on
September 14 2015 now of the six black
hole mergers reported so far there is
one that is very special to us namely GW
1708 14 so it’s a very recent one it was
detected three months ago on August 14th
and on that day not only were the two
LIGO detectors operating but viago was
too and it was the first signal seen in
three detectors the first one ever
detected by Virgo now why does it matter
so much that a signal is seen by three
detectors the reason is that it’s the
only way that we can locate the source
in the sky with some precision a single
GW detector cannot tell where a signal
came from because the detector sees most
of the sky
it’s more sensitive to signals that come
from directly above or beneath the
detector and there are some blind
directions have to keep this in mind but
the detector sees most of the sky at any
given time so it’s not like a telescope
that you have to point to a particular
direction it’s more like one of our ears
which can hear sound no matter where it
comes from with the consequence that
it’s kind of hard to tell where sound is
coming from
now speaking of sound have you ever
heard the sound of GWS
let’s try to hear one now because we
have two ears and a brain that is able
to compare the sounds received by each
of our ears we can get some sense of
where sound is coming from and we do
just the same with GW detectors by
comparing the signals received in
several detectors we can nail the sauce
position in the sky now with two
detectors the localization is very crude
but a third detector is a game-changer
it really narrows the position to a
small area and that’s why it was so
important that LIGO and viago
worked together and why it was so great
to make a triple detection so after GW
1708 14 we had three days to celebrate
and recover until another signal came
that was even more extraordinary GW 1708
17 came on August 17 and the first
remarkable fact about it is that it is
from a different type of source instead
of black holes colliding it was neutron
stars colliding a neutron star is also
the remnant of an extinct massive star
but one that was not quite massive
enough to collapse to a black hole and
the other remarkable thing is when
neutron stars collide you expect a very
powerful blasts
to follow a blast that you can hope to
see with telescopes provided that you
can point the telescopes quickly enough
to the right direction in the sky
because the light from the blast is dim
and it fades away rapidly now on that
day are three detectors were operating
so we were well equipped to localize the
source and ask our astronomer colleagues
to move into action trouble was the
signal was quite loud in the two LIGO
detectors but there was no signal to
speak of in Virgo no signal big enough
to be identified as a signal while
sobering as this was it turned out to be
very valuable information because it
could only mean one thing that the
source was close to one of those
directions that the Virgo detector was
blind to at that time and that
information was key to pinpoint a small
area in the sky where the source had to
be and asked astronomers to target that
area and they did in the night that
followed and sure enough for the first
time they were able to witness the
explosion that followed the neutron star
merger and the gravitational wave signal
and the amount of data and insight that
was collected on and from this one event
is just incredible one of the things
that we learned is that GWS
do propagates through space at the speed
of light just as Einstein had predicted
how could we check that well the first
light from the merger reached us 1.7
seconds after the GW signal which means
that gravitational waves and light after
traveling for 130 million years arrived
to us within two seconds which is pretty
good synchronicity so this was the story
of how by essentially not hearing GW
1708 17
there go allowed to locate the souls in
the sky so precisely that telescopes
could rush there and witness the
explosion that followed and gather an
incredible amount of information and
discovery that was the first multi
messenger one so a new type of events
observed with both gravitational waves
and light and I’d like to close by
suggesting that you ponder the various
time scales that are involved in this
story so a century between Einstein’s
prediction and G W detection a last time
to build sensitive detectors 130 million
years from the source to the earth and a
delay of one point seven seconds between
gravitational waves and light upon
you [Applause]
lake and it caused ripples when I was
doing this as a girl I had no idea that
this would turn out to be the analogy I
would most often use to explain
gravitational waves to people who are
not scientists gravitational waves are
prediction of Einstein’s theory of
general relativity which is the modern
theory of gravity that replaced Newton’s
classical theory in the early 20th
century
Einstein’s theory is based on geometry
masses induce a space-time curvature and
that curvature is responsible for
attracting other masses now when masses
move ripples in the space-time curvature
are generated and propagates through
space it’s a bit like those ripples on
the lake but on a much larger scale and
what vibrates is space itself and those
space-time vibrations what we call
gravitational waves or G W is for short
and I want to tell you the story of how
not seeing gravitational waves led to a
groundbreaking scientific discovery the
GW sources that we detect are generated
in very powerful Astrophysical events
like stars colliding and we’re
interested in both the waves themselves
to probe Einstein theory and in the
events that create them and in that
sense observing GWS is a new type of
astronomy and it’s a very difficult type
of astronomy because GWS are extremely
elusive and hard to detect let’s take a
look at numbers to understand how big a
challenge it is when a GW which is earth
space is briefly distorted distances
change and all detectors try to register
that change and the way our detectors
work is the used light from a laser
source monitor all the distance
between suspending mirrors that are
several kilometers apart and GW is going
to change that distance of several
kilometers for a short time by 10 to the
minus 19 meter which is an absolutely
tiny amount it’s a billion times smaller
than an atom and it’s even ten thousand
times smaller than the nucleus in an
atom it is so small that it took decades
to get detectors that would be sensitive
there are three such detectors we called
them interferometers around the world
the two LIGO detectors in the US and the
Virgo detector in Europe Virgo and LIGO
operate together they form a network of
detectors and a big collaboration of
more than a thousand people and we
started building LIGO and Virgo twenty
five years ago or so and we have kept
improving them to make them more and
more sensitive until finally on
September 14 2015 the first
gravitational wave signal was detected
by the two LIGO detectors so well ago
was not operating at the time but it was
a milestone for the collaboration and it
was just amazing to see that first
beautiful signal received from a
fascinating source colliding black holes
more than a billion light-years away so
it took a long time before GWS were
detected and you know what they say that
the path is the goal and well it has to
be the case when you walk the paths for
so long before getting any real results
it certainly helps that you don’t walk
the paths alone collaborative work is a
great way to sustain motivation and
progress in the long run and probably
also helps that you don’t know
beforehand that the past is going to be
so long when I joined the effort in 1993
I was aware that detecting the first
gravitational waves
would take some years maybe even I don’t
know maybe ten years I didn’t anticipate
that having children and raising them
would almost start looking like a
short-term project in comparison now
let’s get back to our GW sources there
were a few detectives in September 2015
most of them from colliding black holes
those black holes are what remains of
massive stars once they have exhausted
the fuel that keeps them shining and
gravity is so strong around black holes
that nothing can escape not even light
that’s why they are black and when two
black holes spiral toward each other
GWS are emitted which drives the black
holes closer together emitting stronger
and stronger GWS until the black holes
collide and merge and those binary black
hole mergers as we called them they were
corundum Li and we have detected a
handful so far and we named them after
the date when the signal was recorded so
GW 1509 14 for the very first one on
September 14 2015 now of the six black
hole mergers reported so far there is
one that is very special to us namely GW
1708 14 so it’s a very recent one it was
detected three months ago on August 14th
and on that day not only were the two
LIGO detectors operating but viago was
too and it was the first signal seen in
three detectors the first one ever
detected by Virgo now why does it matter
so much that a signal is seen by three
detectors the reason is that it’s the
only way that we can locate the source
in the sky with some precision a single
GW detector cannot tell where a signal
came from because the detector sees most
of the sky
it’s more sensitive to signals that come
from directly above or beneath the
detector and there are some blind
directions have to keep this in mind but
the detector sees most of the sky at any
given time so it’s not like a telescope
that you have to point to a particular
direction it’s more like one of our ears
which can hear sound no matter where it
comes from with the consequence that
it’s kind of hard to tell where sound is
coming from
now speaking of sound have you ever
heard the sound of GWS
let’s try to hear one now because we
have two ears and a brain that is able
to compare the sounds received by each
of our ears we can get some sense of
where sound is coming from and we do
just the same with GW detectors by
comparing the signals received in
several detectors we can nail the sauce
position in the sky now with two
detectors the localization is very crude
but a third detector is a game-changer
it really narrows the position to a
small area and that’s why it was so
important that LIGO and viago
worked together and why it was so great
to make a triple detection so after GW
1708 14 we had three days to celebrate
and recover until another signal came
that was even more extraordinary GW 1708
17 came on August 17 and the first
remarkable fact about it is that it is
from a different type of source instead
of black holes colliding it was neutron
stars colliding a neutron star is also
the remnant of an extinct massive star
but one that was not quite massive
enough to collapse to a black hole and
the other remarkable thing is when
neutron stars collide you expect a very
powerful blasts
to follow a blast that you can hope to
see with telescopes provided that you
can point the telescopes quickly enough
to the right direction in the sky
because the light from the blast is dim
and it fades away rapidly now on that
day are three detectors were operating
so we were well equipped to localize the
source and ask our astronomer colleagues
to move into action trouble was the
signal was quite loud in the two LIGO
detectors but there was no signal to
speak of in Virgo no signal big enough
to be identified as a signal while
sobering as this was it turned out to be
very valuable information because it
could only mean one thing that the
source was close to one of those
directions that the Virgo detector was
blind to at that time and that
information was key to pinpoint a small
area in the sky where the source had to
be and asked astronomers to target that
area and they did in the night that
followed and sure enough for the first
time they were able to witness the
explosion that followed the neutron star
merger and the gravitational wave signal
and the amount of data and insight that
was collected on and from this one event
is just incredible one of the things
that we learned is that GWS
do propagates through space at the speed
of light just as Einstein had predicted
how could we check that well the first
light from the merger reached us 1.7
seconds after the GW signal which means
that gravitational waves and light after
traveling for 130 million years arrived
to us within two seconds which is pretty
good synchronicity so this was the story
of how by essentially not hearing GW
1708 17
there go allowed to locate the souls in
the sky so precisely that telescopes
could rush there and witness the
explosion that followed and gather an
incredible amount of information and
discovery that was the first multi
messenger one so a new type of events
observed with both gravitational waves
and light and I’d like to close by
suggesting that you ponder the various
time scales that are involved in this
story so a century between Einstein’s
prediction and G W detection a last time
to build sensitive detectors 130 million
years from the source to the earth and a
delay of one point seven seconds between
gravitational waves and light upon
you [Applause]
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