(brittle music) (audience applauds) - Hello, everyone. So my name's Tessa. And as Hasson has mentioned, I'm a cosmologist and a
gravitational wave astronomer at Queen Mary University of London, just up the road in mile end. I'm very lucky to be invited here to talk tonight in this incredible venue. And I'm very lucky that
many of you consider gravity and cosmology
suitable entertainment for a Friday night. So thank you for coming along. I'm also incredibly lucky to
be in the field of astronomy at this present time. Within the past 10 years, we have seen a major revolution that has changed the face of astronomy. And it's incredibly
exciting to be part of that. And it's that revolution, that new window on the universe that I want to tell you about tonight. So, let's see. Good, okay, so I'm gonna come around here. Astronomy is arguably one
of the oldest sciences. We've been looking up at the night sky since we've been walking
around on the Earth. And of course, for thousands of years, that meant naked eye astronomy, just looking at the sky with our eyes. Thousands of years later, the first telescopes are
developed in the 17th century by Galileo and other
people working at the time. And within only a few hundred years of technological development, that takes us to our
modern-day telescopes. So at the top there, I have a photo of the Vera
Rubin Observatory in Chile. That's a state-of-the-art facility. It's just finished construction. We at Queen Mary are very
proud to be involved with it. And it's going to be taking data within the next two years or so. Down the bottom there, we have the James Webb Space Telescope, which many of you might have
heard about in the news. Perhaps you saw it launch
on Christmas Day, 2021. And you might have also seen some of the stunning first images released by the James Webb team in the middle of last year, okay? And if you haven't seen them, they're definitely worth Googling. They're just beautiful. But one thing links all of these parts of astronomy together, which is that they are all
made using light, right? In all these cases, we
are seeing the universe. We are seeing distant
galaxies or distant stars with our eyes. And this new era of astronomy I want to tell you about today is the first time we found a different way to observe the universe. And this will be using something we call
gravitational waves, okay? And this new window on the universe opened on the 14th of September, 2015, so really not that long ago. And this new way of sensing the universe, you can think of as being
akin to hearing the universe. So having these different
senses on the universe, seeing it and also hearing it. And in the course of today's talk, I'll explain why I refer to
it as hearing the universe. Having those two senses
open is enabling us to learn lots of new things in astronomy that we haven't been able to before. So what we'll do today is I'll tell you something
about gravity itself. Then we'll talk about what
these gravitational waves are. Then we'll understand why we can think of them as
the sounds of space time. And finally, I want to tell you
about one very special event that shows the power
of combining this sight and this sound together. Now, I had to put an extra
slide in my talk last night because something happened yesterday. And this shows you what an exciting field
this is to be working in. So just yesterday, just
before two o'clock, we had an alert of a
possible new detection of a new gravitational wave event. Now, I realize I haven't told you yet what gravitational waves are. And this image might not
make sense right now. I hope it will make sense
by the end of the talk. But, you know, this was so exciting. I couldn't resist just
not telling you about it. It's still early days, I have to say. The data, you know, needs more checks. So it's not confirmed yet. But this is what's happening. We're detecting these new
events, you know, regularly. Okay, but let's get back to our story. And the story is going to
start with Sir Isaac Newton. So if you studied
gravity at all at school, perhaps Newton is probably the
person you meet first, right? He's credited as really thinking
about gravitational forces in the first instance. And the story goes that Newton was sitting under
an apple tree in an orchard and an apple fell to the ground. And that got him thinking
about what makes the apple always be attracted to the Earth. Probably this is a bit of a fairy tale. Probably it didn't actually happen. But it's a nice picture. And Newton formulated
the first law of gravity. Now, there are only two
equations in this talk, and I thought I would start strong and get one of them out
of the way, straight away, because it's an equation that I think a lot of people
might remember from school, or maybe some of people who are still studying might meet it. So this is Newton's law of gravity. And Newton says that the
force between the apple and the Earth is given by
the mass of one object, so in this case, the apple, times the mass of the Earth, and divided by the square of the distance between their centers. And there's a number in there we call G, which just makes the
units work out correctly. Okay, so Newton's given us a recipe for computing gravitational forces. But he hasn't really
told us what gravity is. He's just given this force prescription. And what gravity really is, why there is a force of gravity, wouldn't really be touched
upon for a few hundred years until it was picked up by Albert Einstein, arguably one of the most
famous physicists of all time. Now, Einstein was
definitely a person to sit and think hard about
long-established scientific concepts because, of course, Newton's law was long-established by then. He has lots of really great
quotes about human curiosity. So if you're ever having a
down day, at school or at work, you know, go look up Einstein quotes. So he said something like this, right? "The important thing is
not to stop questioning. Curiosity has its own
reason for existing." He was a curious person. However, he did also say, "A table, a chair, a bowl
of fruit and a violin; what else does a person need to be happy?" So I think Einstein might have lived a slightly different
lifestyle from most of us. Okay, nevertheless, what's Einstein's big revolutionary idea? So here is a picture of the deep space, well, a deep patch of the universe. Okay, so every spot of light
in this image is not a star, but a galaxy, okay, a collection of millions of stars. Now, I should stress that
Einstein didn't have access to this kind of image. They weren't around in his day, which makes his kind of
realizations even more incredible. So when we look at this image, in between the galaxies, we have blackness, patches of empty space. And it's very tempting to think of that as just being nothingness, void. But what Einstein realized is that space itself, empty space, is not just a stage on which the rest of the universe happens. Space itself is part of the action. So space has dynamics. That means it can move, it can warp, and it can bend and stretch. And Einstein realized what
causes space to bend is the presence of massive objects. So we have a little
patch of space over here. Now, of course, you know, we are moving through
three dimensions of space. And actually in Einstein's theory, the correct thing to talk about is not just three dimensions of space, but he also folds time into there as a kind of fourth dimension. So we often say this word spacetime, which kind of means the
combination of space and time. But I can't show you a
patch of 4D spacetime. It's a bit hard to visualize. So this is my two dimensional
patch of space here. And what Einstein's theory tells us is that if we have massive
objects sitting in space, they effectively cause
dips, dents, or wells. They bend that space around them like a piece of fabric, okay? And the distortion they cause
is proportional to their mass. So this is something really heavy. It causes a really big
dent in spacetime or space. If we have something much lighter, this might be, you know, a moon, if that's a star like the Sun, just causes a much
smaller dent in spacetime. Okay. So that's Einstein's big revelation. As I say, this is just a 2D example. We do have a 3D example, which
makes it a little bit better, because of course our universe
is not just kind of full of sheets of fabric. So here's a little 3D graphic, which I think shows the situation
a little bit more clearly. So we've got this orb, this white mass, and it's moving round on a circle. And around it, there's this sort of grid of blue cubes. Those are meant to be representing this kind of 3D fabric of space. And you can see what happens. As the orb moves, the space
around it kind of bends. It gets sucked inwards almost, okay? That's just a slightly
better representation of Einstein's bending of space and time. Okay, so Einstein formulated
all of this mathematically. Now, don't panic, we are not going to go
into the mathematics of his theory today. But we can look at some of its
main equations in word form. And this is what he published as his general theory
of relativity in 1915. So the central equations of Einstein's theory say the following. On the left hand side, they've got a bunch of
mathematical objects which tell you about this
bending of space and time. And they equate that to a bunch of objects on the right hand side that describe what space contains, matter, or even light, and energy, if they're concentrated
enough, can bend space time. And so now we have a
picture of what gravity is. When we have a small body
like a satellite or the apple, it feels this tendency to
move towards the larger body because it's experiencing
that distortion in space or space and time. And it has a tendency to move towards the bottom of that
well, that dent in spacetime. And that's what we experience
as the force of gravity. Okay, so this is
Einstein's big revelation. And he doesn't have to wait too long for some experimental confirmation of it. So luckily four years later,
there's a solar eclipse due. So the moon is gonna come along and temporarily block out the Sun, as seen from certain
locations on the Earth. And Einstein spots this opportunity and he makes a prediction
for a particular effect from his theory during that eclipse. And what he predicts is
that the position of stars around the Sun is going to wobble. So, of course, normally
you can't see stars anywhere near the Sun. It's far too bright. It just blocks them out, okay? But during an eclipse, of
course the moon is in the way. It blocks out the Sun. And now, particularly if
you've got a good telescope, you can see stars around
the edge of the Sun for a few minutes. And what Einstein's theory
says should happen is that the positions of those
stars should appear to move. So the light is coming
from that distance star. You can see the blue line there. But it's getting bent as it travels through this
curved spacetime well of the Sun. We receive it here on Earth. But when we look at that light, our eyes just trace it
back in a straight line. So we see the star as appearing
where that white object is, where it says apparent position. And actually the star is
following the blue ray in the actual position. And of course, once the Sun has moved on, as it continues orbiting through the sky, the effect goes away, okay? So this wobble is just
there for a few minutes during the eclipse. So Einstein predicts all
of this using his theory, and it is confirmed. The results are a little
bit, you know, dodgy. It doesn't all work perfectly, but enough for the
newspapers to proclaim this a success for Einstein's theory. And this is probably my
favorite newspaper clipping of all time. "Lights All Askew In The Heavens, Men of science." And I'm afraid to say, at that time, it was
mainly men of science. I hope we can convince you
that is no longer the case. "Men of science more or less agog." Who uses the word agog these days? "Agog over results of
eclipse observations. Einstein Theory triumph," the best bit. "Stars not where they seemed
or were calculated to be, but nobody need worry." (audience laughs) Don't you wish the
tabloids did that today? "It's okay, guys. Don't panic, everything's gonna be fine." A book for 12 wise men. And so the idea at the time was the Einstein's theory was so complicated that only a handful of people on earth would
understand it, okay? Today we teach Einstein's
theory to physics undergraduates all around the world. Okay, so Einstein had
had his theory confirmed, or at least he'd had some
evidence for it confirmed. However, there's still a secret
hiding in Einstein's theory. And to talk about this secret, I need to introduce you to some objects I think of as the ghosts of stars. So when stars die, that is when they finish
burning all their fuel that keeps them alive as, you
know, like the Sun shining. When they finish burning all that fuel, they undergo a spectacular
set of processes in which they throw off a
lot of their outer layers and they leave behind a
very, very dense core. And there's a couple of
forms those cores can take. But we'll be interested in
two particular types today. One is black holes, and the
other is called neutron stars. Now probably people have some
kind of notion of black holes, may be coming from popular
science from "Interstellar," or something like that. So you might know that they are these sinister, scary regions of space which don't let anything out. It's more or less true. So a black hole is an ultra,
ultra collapsed region of space such that nothing can
escape, not even light. And what's happening in
this simulation here, and this is a simulation,
not a real image, is that you are seeing some gas that's about to fall into the
black hole and it's very hot, and so it's emitting light. And that's why in this case, you can sort of see the
black hole silhouetted against the gas. But once that hot gas has fallen in, you wouldn't see any light. Black holes are really, really massive. They start at a few times, roughly 10 times the mass of our Sun, and they go up to maybe 100
times the mass of our Sun, okay? So they're really, really big things. Now, several times in this
talk we are going to think in terms of what we call solar masses. And I realize that's not a unit
you use in your daily lives. When you're doing a recipe,
you're baking a cake, you don't kind of measure
out a solar mass of flour. But you might have at home
a kilogram bag of flour. So when expressed in kilograms, it's something like that, okay? I'm not going to even try and
read out that number for you. I don't even know what that
number's called, quite frankly, but that's how massive these things are. Neutron stars are slightly lighter. So they are still ultra, ultra
dense, dead stellar cores, but they're not quite
as heavy as black holes. They might be sort of two times
the mass of the Sun, okay? And these two objects are
relevant to our story today, because they're so big, so heavy. Sorry, I should clarify
that they are so heavy, but they're not so big. So they're actually quite compact objects. And that means they're
really, really dense, lots of mass packed into
a really small volume. And that means that, remember we said that massive
objects bend spacetime, that any special effects
in Einstein's theory are going to be really pronounced
to these objects, right? They've got so much mass
in such a small region. Okay, so that's why they are
the place that we look to when we look for kind of
effects of Einstein's theory. Okay, so to understand what
these objects do to space, we have a little demo for you. So stars very often live in pairs, what we would call a binary, and they'll spend millions of years happily orbiting around each other stably. And once both those stars have died, have used up their fuel, they can both collapse
to form these objects. So you can end up with
a pair of black holes or a pair of neutron stars, or you can have the mixed case. You can have one black
hole and one neutron star. And those dead stellar
chords will carry on orbiting each other. And so what we want to do now is demonstrate what's gonna
happen to our spacetime here when we have these objects
orbiting each other. Dan here is gonna help me out. And I should say we've also had some help with this demonstration from Steven Mould who created it originally. Now what we've got here is a drill with a special piece on the end. So it's got these two wheels here, which is gonna be our two black holes, or our two neutron stars. And Dan is going to to use
the drill to make them spin, so orbit in our space time. Now when we use a drill, the orbit's gonna be very fast, okay? It's gonna be too fast for us to see. So what we're going to do is
we've gotta strobe light here. And that's gonna enable
us sort of to make it kind of look like things
are in slow-mo, right? Just by using the frame rate so that you'll actually be
able to see what's happening to our sheet of spacetime here. Okay, it's gonna be quite noisy. So I probably won't
attempt to talk over it. But when you are ready, Dan, take it away. (drill squealing) Okay, can you see we get these
ripple patterns coming out, this, there we go. Oh, almost. There we go. So we've got our system orbiting and we're getting this spiral pattern coming out of the center and
propagating out to distances. (drill squealing) All right, fantastic. Thank you. (audience claps) Okay, so hopefully you all saw that kind of spiral
pattern start to emerge. Okay, here it is again. So we've got our orbits
going around each other and much like ripples, when you throw a stone into a pond, there's this pattern
that travels outwards. So these ripples, these waves
are gravitational waves. And they have that name because they are ripples in the fabric of space and time itself. So they are ripples in
the fabric of space. That is the same thing that
gives us gravity, okay? That's why they have that name. Okay, so that's what
gravitational waves are. Now, this little video
here isn't quite accurate because in this video, the objects just go
round each other forever. That's not what happens in nature. What happens is as those
objects orbit around each other, they take energy away from the system with these gravitational
waves they're emitting. And that means the objects will start to get closer and closer and they'll get so close that
eventually they will touch and then they will merge. So our two black holes will
basically cannibalize each other and form a single big black hole, okay? And there's something similar happens with the neutron stars. So I've got another video
showing you that, okay? So this time, they're
gonna eat each other. This is what we call in spiral, in spiral and merger, the technical terms. And I want you to watch what happens to the fabric of spacetime around them as they get closer and closer
to that moment of merger. Okay, so here we go. Can you see the waves are
starting to get bigger? Okay. So as they get closer and closer, the waves get larger and larger, and they actually get
faster and faster as well. And then they merge, they form that single black hole, and then everything settles down. Once you've got to a single black hole or neutron star sitting there, that doesn't emit gravitational waves. You need this special pattern of two objects going around each other to shake up spacetime in this way and produce gravitational waves. So this gives us two really useful things. We found the best place to go looking for gravitational waves. We said it was those really
massive objects, right, black holes and neutron stars. And actually now we've seen that there's a characteristic
signal we can go look for, that gravitational waves
should get louder and louder, faster and faster, and then they should die away in those last moments before
the black holes merge. So how do we detect them? Well, you can't detect them
with a regular telescope because they're not made of light. So you need a very, very
special piece of kit. So special, in fact there
are only five of these in existence in the world right now. There is a sixth one under
construction there in India. Okay, what do these detectors look like? Well, they're huge. Here are photos, aerial photos, of the two detectors in the USA. This pair are called the LIGO detectors. That stands for Laser Interferometer
Gravitational Wave Observatory. And they're on opposite sides of the USA. So this one's called Livingston. It's in Louisiana. The one in the top left is called Hanford. It's in Washington. Sorry, Washington State that is. And the reason they're on
opposite sides of the USA, that's not accidental. It's so that they're really
independent of each other. And so if they both see
the kind of same signal at the same time, that's an extra bit of verification for us that it was something that
came from outer space, it wasn't, you know, an
earthquake or something like that. Okay, and you can see these detectors have these huge great tubes coming out of them. We call them arms. And they're four kilometers long. And each detector consists of a pair of two arms at right angles. Usually we can't get the
second arm fully in the photo because they're too big. And where those arms meet, there's obviously some kind of building. There's something happening. Okay, so what's going on here? Here is a cut-down version
of what's going on. So what happens is,
somewhere in that building, we shine a very high-power laser at something called a beam splitter. That's the diagonal object in the middle. And that beam splitter
chops that laser beam into two identical copies. And we send those two
copies of the laser beam off down these four-kilometer-long arms. They bounce off mirrors, special kinds of mirrors at the ends, and they're brought back together. And we add those laser beams together. We say we interfere them, okay? So light is a wave, or at least you can often
think of it as a wave. And so the default setup
of the detectors is such that when we bring those
two laser beams back together and we add them, we can think of adding these two waves. And in the default setup, when there's no
gravitational wave present, everything's arranged such
that the peak of one wave sits over the dip, the trough, of another wave. And so the two waves when
added cancel each other out. And that means we don't get any light out at the
final detector here, okay? We call this destructive interference. What's gonna happen when a gravitational
wave passes through is that it will change the
length of these arms. So I am a gravitational wave detector. A gravitational wave is
going to come at me face on. And by the way, gravitational waves will
pass straight through you. They'll pass straight
through the detector, straight through the Earth, out the other side and keep on going. Okay, but what happens is that gravitational wave causes
the length of one of my arms to temporarily shrink just a little bit. Obviously, I'm gonna exaggerate here. And it will make the other
arm stretch a little bit. And then a short while later, it's gonna reverse the pattern. So it will shrink this
arm and stretch this arm. And that pattern will alternate
backwards and forwards. So this is the
gravitational-wave-detector dance. Okay, I'm told all the kids
are doing it on TikTok. Okay. Okay. So why does that matter? What happens when the lengths
of those arms change is that our two light waves
in our laser beams get shifted past each other. And now we can have a
situation where the two peaks, the red and the blue there, are in sync. And that means the light
beams add together. We call this constructive interference. And you get light coming out, okay? So light reaches the detector. And the precise pattern of this light, how it changes over time, tells us things about
that gravitational wave that passed through. Now, what blows my mind
every time is the sensitivity that these detectors have to have to detect gravitational waves. Because these waves are
coming from millions, if not billions, of light years away, by the time they reach us,
they're absolutely tiny. So these detectors can detect
a change in arm length, 1/10,000th of size of a proton. I mean, if you thought
atoms were small enough and a proton is about a factor
of 1,000 smaller than that. A proton is a particle that
sits in the nucleus of an atom, so 1,000 times smaller, and then 1/10,000th of that? I mean, it is just mind-blowing, okay? And that's why you don't feel gravitational waves passing through you, because if they do pass through you, the changes they would
cause in your body are just so much smaller than kind of all the other things going on inside your body anyway, okay? So you can't feel them. So these are incredible bits of kit, incredible bits of engineering, and it took decades of work. But we did eventually
have our first detection on the 14th of September, 2015. And we give these detections the sort of signature call signs. So they all have a name. They get called GW for
gravitational waves, surprisingly. And then the numbers after it give the date of the detection,
but sort of backwards. So you have to read it backwards. So it's the 14th of September, 2015, okay? I have another little movie, which is a simulation of this first event. What you're going to
see are two black holes. So this was a black hole binary merger. And you're going to see lots
of sort of orange patterns coming out of it. And those orange patterns are the patterns that the
gravitational waves make, okay? And you'll see they make some really kind of beautiful patterns. So these are not just artists impressions. These are the kind of real structure. Okay. Is this gonna load? Good. So here we zoomed out, but you can see the gravitational
waves already, okay? And here are our black holes. And the green regions
are showing the regions where space is most strongly distorted. Okay, and now look at
these kind of helixes. Okay, and away they go. So we've seen that
pattern several times now before this kind of really intense burst of gravitational radiation, and then they're settling down. Okay, that was the first detection. I don't think I can put into words for you how exciting it was to be, you know, sitting in a physics department and, you know, someone comes and knocks on your door and says, "(gasps) Have you heard the news?" Right? They've detected these
things for the first time. It's incredible. So here's what we know
about this first event. So as I said, two black holes, they have masses of 36 and 29 times the
mass of the Sun, okay? So that symbol M with the
sort of circle below it is our way of saying solar masses, okay, that unit we met earlier. So these are really big black holes. These were whoppers. They merge. And we are left with a final black hole, which we measure has a
mass of 62 solar masses. Now if anyone's good at
quick mental arithmetic, you will spot that 36 plus
29 does not equal 62, okay? 36 plus 29 is 65. So we've lost three solar
masses somewhere in this event. And what's happened is all that mass, three times in the mass of
the Sun has been converted into pure energy and dumped into spacetime over a period of about
half a second, okay? That is the energy of these things. And that's actually the energy it takes to generate gravitational waves. That's just how stiff spacetime is. Down the bottom there, we have the real gravitational wave data detected by our two detectors. So Livingston and Hanford
are the two LIGO detectors. And it's spread over about half a second. You might just be able to see the numbers at the bottom
of the screen there. And you see over here, it
just starts out as noise. It's just kind of
jumbling in the detectors. But then about 0.3 seconds, there starts to emerge a
wave, right, a pattern. And it's there in both
the red and the orange, and in sync the waves
get larger and larger and they get smaller and smaller spacing. So they get faster and faster. And then the signal dies away again, okay? And that's exactly what we
were looking for when we said, you know, that's our characteristic signal of gravitational waves. Now, here's the same data just presented slightly differently. We can talk about a wavelength
of these gravitational waves. So that means that the
spacing between two peaks. And it just happens that the spacing, the wavelength of these
gravitational waves, is about the same as
the wavelength of sound that the human ear can hear, okay? So sound itself is a wave. It's caused by pressure waves in the air molecules
that fill the room, okay? So if I go and smack the desk, I won't do it too loudly
near all of this stuff. I set up pressure waves,
which traveled to your ears. And that means I can actually convert this gravitational wave into a
sound wave that you can hear. Okay, so we're going to do this. I'm gonna play you these
sounds of spacetime. It's going to be quite quick. So what's gonna happen
is you'll hear it twice, and then you'll hear it twice more, but shifted up in pitch. And that makes it just a
little bit easier to hear. Okay, here we go. (sound waves rumbling) Okay, so there's this sort of general sort of rumbling noise. That's just the noise in the detectors. But then hopefully you could hear to start with this kind
of low thump, right? It sounds like someone kind of
punching a cushion, I think. And then when we played
it a little bit higher, there's this kind of whoop sound. It's quite quick, but it's there. So that's the sound of
this particular event. Was this still working? Yes. Okay. And that was our first detection. Now we didn't have to wait very
long to get a second event. Few months later on the
26th of December, 2015, so again, the call sign GW151226, affectionately known as
the Boxing Day Event. Somebody had their Christmas
severely disrupted by this one. This time the black holes were smaller. So there are about 14 and 8
times the mass of the Sun, versus the kind of 30
solar masses we had before. Now, smaller black holes
produce gravitational waves with a smaller spacing,
a smaller wavelength. And that means the sound
they make is higher in pitch. So I'll play you this one. What this video is going to do, it is going to compare the
two events we've talked about. So confusingly, it's gonna
play the second event first and the first event second. (laughs) Okay, so it's gonna reverse
the chronological order. Again, you'll hear it twice. And again, it'll shift up in pitch just to make it that
little bit easier to hear. Okay, so here we go. (sound waves whooping) Okay, now you definitely
heard those ones, right? Okay. So what we're understanding is that each gravitational wave event is a bit like an instrument,
an orchestra, right? It makes its own sounds. Hopefully, you could hear
that the second event kind of woo-oop, and the first event woo-p. Okay, so they have
these different pitches. You just want me to make more silly sounds while leaving you. I know, I know. So we carried on detecting these events for a couple of years more. And actually as of today, there are 91 confirmed
gravitational wave events. So this is our sort of
symphony of spacetime. They're all represented here. Everything in blue is a black hole. Everything in orange is a neutron star. And what you are seeing is
that there's two objects, they're linked together by a line. And then they have an arrow
pointing up to a third object. So those two at the bottom are
the two objects that merged. And then the one at
the top is the remnant, the one that's left after
they've cannibalized each other. And the vertical position
on this picture represents the mass again. So you can see we've got
this big block of black holes somewhere in the region of
sort of 10 to 50 solar masses. We even have a few remnants getting up to maybe 100 solar
masses, really big ones. And then we've got a much smaller handful of the neutron stars. And that's because the
neutron stars are quieter. And what I mean by that is
because they're less massive, the waves they produce
are less big basically. They're harder to detect. So it doesn't really mean there are less neutron stars out there. It just means we are not as
good at hearing them yet. Nevertheless, one of my favorite events, and indeed an event that
was really, really important in my career, it's actually the event
that got me started as a gravitational wave astronomer, is one of the binary neutron stars. And it is this fella down here, affectionately known as 170817. Yeah, it doesn't really work, does it? So this was the first
binary neutron star merger that we detected, the first case that wasn't black holes. So neutron stars, remember,
with these guys over here, these ultra dense, dead stellar cores, but they're not black holes. And because they're not black holes, that means light can
escape from a neutron star. And that is the big, big difference. So I have another little video to play you that shows the first
binary neutron star event, 17th of August, 2017. And what I want you to look
out for in this video is the ways in which this is different to the black holes merging where there was just a
gravitational wave signal. Now can we have the lights
down for this video, please? There's some music in this video. It's very kind of epic music. So I won't talk over it. I'll just let you enjoy this. (celestial music) (upbeat music) (beeps) Okay, the spacecraft at the
end is just the credits, I should say. It's not part of the event. Okay, what on earth happened there? So some of the things you might have seen, we had our two white objects. Those were neutron stars. It's a bit hard to say what color a neutron star actually is. And you saw them in spiral and merge. And as they did, there was
gravitational waves coming out. But once they merged, all this other stuff started happening. So you saw these huge jets come out of it. And this is something we
call a gamma-ray burst. This is the, you know,
cataclysmic amount of energy that is released when
those two objects smash into each other. It throws out these jets of ultra-high energy radiation, okay? Gamma rays are the most
energetic form of light that we have. So this stuff is lethal. So that powers out into
the surrounding space. And remember, that's light. So there's the gravitational
waves being emitted, yep. But now there's light coming out as well. So something a telescope
might be able to see. So first of all, we get
the gamma-ray burst. Then there's this kind
of blue fluffy pompom that seems to expand out the center. This is the kilonova. So what this is, is matter that's in those neutron stars, because again, not black holes. So there's still matter that can get out. It's matter. It's dust. It's very neutron-rich dust
being kicked up and thrown out. And actually these neutron
star collisions are responsible for making a lot of the heavy
elements in the universe. So if you've got a gold wedding ring or if you've got silver jewelry, a lot of the gold and
silver in the universe, probably most of it, is
made in these events, okay? And over millions, billions of years, this dust spreads out into
space and ultimately collapses to form things like stars and planets. So the kilonova itself also emits light, slightly less energetic, light that we can see
in the optical waveband. So things we could, in
principle, see with our eyes, although we can't see things this fainted. So that means this is an event that we can hear and we
can see at the same time. Okay, so here's the real data. This is a set of images taken on the left about half a day after the the event, and on the right, about
two weeks after the event. You are not looking at the
bright object in the center. That is the center of the galaxy in which this event took place. Instead, you are looking
at the small faint, well, fainter spot, where
the cross-hairs are. And it's there in the left-hand image, and then two weeks later it's gone. Now that's pretty rare in astronomy. Most things in the universe evolve on a timescale of millions of years. There are very few things that, you know, we can actually catch coming and going on a timescale of two weeks. It's very, you know, human-friendly. So that is the kilonova. That's that blue fluffy pompom thing. And that's then light from the event. But we also have the
gravitational wave detection. So let's play this one. This sound is gonna
start quite-quiet, okay? So if you think it's not playing, it is. But we just have you really quiet. So let's hope this works. (sound waves whooping) I could play those all day. Okay, so what do you notice? It was a much longer sound. It started really quiet, but it was a much longer sound. And that's because these neutron stars, the gravitational waves that they produce, although they're quieter, they are detectable to us for
longer if we can detect them. So we get many more, what we
call many more cycles, right? Many more orbits of
the binary neutron star before they finally merge. And so we get a longer sound. Okay, as I mentioned, this event was really important to me because seeing and hearing the
event at the same time meant we could do something really incredible that we hadn't been able to do before. And this was something I took part in. We could see what happens in a race between the gravitational
waves and the light. So what we want to do here
is effectively measure the speed at which
gravitational waves travel. And I call it a race because
these things are gonna set off at the same time from our
binary neutron star merger. They're gonna travel the
same distance to the Earth. So we see who gets there first. Now we have a little demo
for this as well, okay? And you'll notice folks that
I'm putting on ear defenders and safety goggles. You'll be fine, but
please do cover your ears. This is going to be loud, okay? I'll give you a signal
before we get going. But once I give you the signal, please do cover your ears. I'm now not gonna be
able to hear anything. - Wanna stand a bit further away? - Yes, I think that's a wise. So there's that. And I need the stop clock as well. Okay, and what we've got here is we're gonna have our neutron star merger. And at the far end, there
we've got two little flags. One that's gonna pop up to represent the
gravitational waves arriving, and one that's gonna pop up to
represent the light arriving. And it's gravitational waves in blue, it's light in yellow, okay? And I'm also gonna attempt
to time these things here. Just set that going. Okay, folks, please cover your ears. (balloon bursts) Off they go. (audience laughs and applauds) Oh, I wish I could do
that in every talk I give. Okay, so what happened? We had our neutron star merger, our two signals set off towards the Earth. Now they actually arrived
pretty close together. But if you were watching closely, the gravitational waves
arrived slightly quicker. I can't say my timing
accuracy was brilliant. But I got them here as about
two seconds earlier, okay? At least that's what I got. Okay, so if that happens, if one event arrives
earlier than the other, get the wrong one of those, that means it must have been
traveling faster, right? So we can use the information
about which we saw first to say which one's the fastest. So here comes the second
equation of the talk. And again, I hope it's something that's familiar to a lot of people. If you did high school physics, you might remember that
you can measure a speed as taking the distance traveled divided by the time taken
to travel that distance. Now, we don't quite use
this formula in astronomy. I'll be totally honest. It's a little bit more complicated, the actual calculation we do. But basically it's a
poshed-up version of this. Okay, so what have we got? We need to know the distance
to our binary neutron star. That's 40 megaparsecs. Megaparsecs, what's that? That is another unit
that astronomers define to try and make the mind-blowing numbers we work with every day manageable. So actually a megaparsec in meters is about 30,000,000,000,000,000,000,000
meters, okay? But when we define these units, we can just say it's 40 megaparsecs and it sounds like it's
a kind of, you know, just around the corner. If you prefer thinking of it this way, that's about the length of the UK, 1,000 kilometers, 600 miles, if you're imperial, multiplied by the number of
grains of sand on the planet. Yikes. Okay, that's the distance. And the time it takes, well, we know the speed of light already. So we know that part of the problem. We know it would take
light 130 million years to travel that distance. What we don't know is the
speed of gravitational waves. And in the real experiment, what we found is that the
gravitational waves arrive first. They arrive about two seconds
before the light does, okay? So their travel time, 130
million years minus 2 seconds. Okay, so what's the speed going to be? Well, as I said, we do know from other experiments on Earth that the speed of light in vacuum is 299,792,458 meters per
second, to be precise. The speed of gravitational waves comes out as 299,792,457.999999 meters a second. So they're basically the same. If we look at the last two digits, we've got 58 and we've got 57.9999999. Okay, in fact, it turns out that the
greatest difference in speed that is allowed by the
data is that percent. Again, I can't even pronounce that. Okay, it's tiny. And probably, probably, that tiny difference is
just due to the errors in our experiments, right? When we do scientific experiments, we can never do them perfectly. There's always some kind
of small systematic error. So it seems that
gravitational waves travel at or at least very, very,
very, very, very close to the speed of light. And that's actually a win for Einstein because that's exactly what he predicted. His general theory of relativity says, "The gravitational waves should travel at the speed of light," okay? So Einstein wins. Now you might say, "Why
do we bother doing this? You know, we know Einstein's
theory is correct. You know, why did you need to go and check that gravitational waves travel at the speed of light?" Actually, that's slightly
oversimplifying the situation. For me as a cosmologist, if Einstein's theory is fully correct, that means there's
something else very wrong with our current
understanding of the universe. So we take Einstein's theory
that seems to work really well every time we test it and we apply it to the
universe as a whole. And we can ask it the question, how fast is the universe expanding, okay? And Einstein's theory says
it should be, you know, expanding at a moderate
rate or a constant rate. And actually what we find
when we use other data to measure the speed of
expansion of the universe is that we find it speeding up. It is actually accelerating. And that's completely, you know, almost the opposite of what
Einstein's theory predicts. So there's a problem here. There's something going wrong. Either Einstein's theory is incorrect and we can't apply it to
the universe as a whole, which means there's still more we need to learn about gravity, or we have something else
in our universe, okay? And we don't know what
this something else is. So as a placeholder for our ignorance, we give it a name that makes
us feel better about it. And we call it dark energy. But if this dark energy, this strange new substance that causes the universe to accelerate, if it exists, it has to make
up 70% of the universe, okay? So this is just a simulation here showing galaxies and dark
matter in the universe. But 70% of that has to be
made of something else. So if I've given you the
impression at all this evening that cosmology and
gravity is kind of sewn up and done and dusted, please forgive me. That is certainly not the case. There are major, major questions that we are still trying to answer. Okay, folks, I'm nearly
done for this evening. Let me just wrap up by telling you about the gravitational wave detections that are about to start. So these detectors that I
showed you images of here, they've actually been offline
for about three years. And that was initially
due to the pandemic. We had to take them offline for the safety of the
people working there. And then they were having a
lot of engineering work done, as you can see, to make
them more sensitive. They were having upgrades. That's basically all done now. And our next observing run is due to start in five days' time, okay? So this talk is super well-timed. It starts on the 24th of
May, which is next Wednesday. Now, actually, as I said
at the start of the talk, things have started happening already. Right now, we are in engineering mode, which means we're not fully calibrated. We're not supposed to be seeing anything. But if something happens, and, you know, the engineers are there testing the detectors, but they're online, we can't help but detect it. Okay, so we have our
first candidate event. Maybe it'll be confirmed,
we have to see yet. But nevertheless, five
days' time, we switch on. So doubtless we will see more black holes, more neutron stars, and hopefully a few more
unexpected discoveries along the way, too. If you would like to
follow along, you can do. There is an app on your phone called, well, it's not on your
phone right now, sorry. There is an app for your phone if you wish it to be on your phone called Gravitational Wave Events. What this does is it
plugs into a set of alerts that are sent out worldwide if we find something interesting. So you can get a
notification on your phone. If you choose the settings correctly, it will actually play you one of these gravitational wave chirps. So your phone can go whoop when you've got a
gravitational wave detection. I mean, it's just pretty cool. Alternatively, of course, if you don't want your phone
going whoop all the time, you can follow the LIGO
Detectors on Twitter. Okay, I will leave it there folks. Thank you so much for listening and I look forward to some questions. (audience applauds)
