- Thanks to Kiwi Co for supporting PBS. - It's astonishing to me really, that Einstein's theory
of relativity formulated from 1905 to 1915, not knowing anything about black holes, not knowing anything about
relativistic, astrophysics, not knowing about what the
true nature of galaxies was at that time. That that theory, as far as we know, completely predicts what these
wave forms will look like for the most relativistic
systems ever observe, ever witnessed by human beings. And two black holes
revolving around each other at over half the speed of
light and smashing together. So that's in itself stunning. And we do these measurements
to try to understand, is there something missing from the an understanding
of the wave form, that relativity can't do for us? 'Cause that could be
evidence for what's beyond, what's beyond Einstein. Is there a quantum theory of gravity? Right now we've not been
able to see any problems with relativity and finding,
what these signals are. - Which is amazing, I mean- - Amazing and disappointing. - Hey, I'm Diana and you are watching a different kind of Physics Girl episode. This one is gonna be a lot longer. So this is an interview
that I did with Mike Landry, who's the head of LIGO, Hanford. LIGO is the observatory that first discovered gravitational waves, which are one of the most
amazing, intriguing phenomenon. We made a video about that last
week with our visit to LIGO. But while making that
video, I grilled Dr. Landry for about an hour and a half
with all of my questions about gravitational
waves and some questions that you guys asked me on Twitter as well. And usually when I do
an interview like this, I just cut the most interesting 30 seconds but what happens if the entire hour and 45 minutes is interesting. I really wanted to share more of that. So grab your tea, start your laundry, settle in to hear some
of the weirdest questions about the universe for the next half hour. So we're driving along
the X arm of the detector. Is that right? - That's right. - Yeah. The X arm with
the observatory head and outreach coordinator. This is the VIP tour. (laughing) So I have a lot of questions for you. - Excellent. - Yes. What does it mean to stretch space time? How can you conceive of
stretching and squashing space and or time? - So an analogy is good, right? It's the medium that we live in space-time that's doing the oscillating, that's getting stretched
and expanded and contracted. And the things within
it go along for the ride and that's what we witness. So a media might be a
canvas, canvas of a painting. So if you actually had an invisible canvas and you want it to register that the medium itself was moving, if the medium was stretched,
you could see the paint, the painting get distorted. - Is this the halfway station? - Yes this the station.
- Already. - [Levi] Are we turning around here or are we going to the end? - [Landry] You can move right to the end. - Levi we're going to the end. We didn't get to do that last time. This is uncharted territory. - [Landry] Yeah. - What would be the feeling
like the overall perception of, you said you could maybe hear that because the, your eardrum is, - I understand your eardrum has an ability to see a pressure change of
one part in 10 to the six. - Oh wow, okay. - And so you could, if
you can survive that, I dunno, maybe that's a
question for a doctor, could your cells survive that. - Is it, would it literally feel like your body is stretching apart? Like parts of your body are stretching apart from each other? - I don't think people
know the effects, I mean- - Of what it would feel like? - Of what it would feel like? - Yeah, right. (calm music) In the control room of LIGO, right? This is where we are? - Of LIGO Hanford observatory. (laughing) To interrupt, do credit to LIGO
Livingston and Observatory. - Okay of course. Of
LIGO Hanford Observatory. Oh, Physics Girl Tour. Sorry. I'm very distracted. We're on the calendar. - How do we know that the first collision was two black holes? How, and then how do we know
if it's two neutron stars? What evidence do we
have that is black holes versus some other object? - Right. So essentially the
signal tells you what it is, that kind of cries out to
you, what it is so the first- - Well not if you're not a
gravitational wave astronomer. (laughing) - Well, so like if you just listen to the first binary black hole
merger, it's low infrequency and it's really in our
detector just for a short time, quarter, second or so. So it goes, whoop! - Okay. - And then if you listen to
the binary neutron star event, it's a hundred seconds long, and it trips up to much higher frequency. So it's kind of like, (mimicking wave sound) for a hundred seconds. (mimicking wave sound) - And so the high frequency
is that the frequency of those objects orbiting- - Exactly. So what you're hearing is the
modulations of space and time that are transduced, changed fight to, from space-time changes
to an electrical signal in our detector. So ultimately to analyze the signals, we use general relativity. So if you solve Einstein's equations for this inspiral phase, we make approximations
on the inspiral phase and on the out phase. But in between, at the merger, you're using supercomputers
and numerical relativity and stitching it all together to give you what the wave form is and
extract, what are the masses? What are the spins? What's
the distance to those stars? What's the orbital inclination? All of these different
parameters from relativity are teased out at the data. - Is it possible that
it's a different object? That it's something else that has the mass that you're measuring, or are there other things besides mass that give a signature of
this being a black hole or a neutron star? - Well, there's kind of two
different types of answers to that question. That's a really probing question. So on one hand, you're just saying like, what if it's something else
not gravitational waves at all? And there's different
ways in which we show it's not something else. One of the things is we
register about 300,000 channels per interferometer at the two sites. So 600,000 channels overall,
and at a given site, 30,000 of those channels
are environmental sensors things like wind speed and
tilt meters and seismometers and voltage meters, and RF
monitors, and magnetometers, cosmic ray detectors, all
in order so that you know, what the environment that the
detector lives in is doing. So if there's a lightning
strike in Burkina Faso, could that somehow
electromagnetically couple into both detectors at the speed of light. And so you have to know
that and we can show that, nope, that didn't happen. And so at one hand, you're
just eliminating alternatives. And then, - So, okay. So this is the question of whether, how we know it is a gravitational
wave in the first place. - In that case we know it's not something we may not know it's a
gravitational wave yet, 'cause it could be the detector itself. It has its own noise sources. But in those cases, you're trying to exclude the environment, which and lots of scientists asked about various types of couplings right after the first detection. How do you know? You've got to have extraordinary claims, need extraordinary proof, so you can make sure that you can exclude all of these different possibilities. But then secondly, you wanna know, well, what if the detectors
just somehow managed to glitch like this on their own. They vibrated a bit and maybe it's, something that you can't
detect with your sensors, it's in the photo detector or something. How do you know? So then you statistically
also assess the data. And so you basically
artificially time slide the data between the two detectors and analyze for statistical fluctuations
in the detectors to eliminate that possibility. And you can pretty quickly
show that, you know, any statistical fluctuation and detectors would take more than hundreds
of thousands of years of running to actually make such an event. And we don't even make such an event. So you're just putting a bound, you'd have to run the detectors for hundreds of thousands of years, to even have a chance to have
this kind of fluctuation. And ultimately you use general relativity and see that time frequency evolution, the way that signal changes
as a function of frequency over time behaves in both
detectors in the same way. And as predicted. - Okay. Can gravitational
waves hypothetically interfere in the way that light waves
can like get constructive- - Superpose. Yeah, they can. Yeah. But in principle,
in the far-field regime, they'll add, they'll superpose, have linear superposition
the way waves do. Close to those black
holes, that's not the case. General relativity is this
wildly nonlinear series. - Okay. Wait, wait, wait, hold on. - Close, that's not true. - Okay. Go on. (laughing) - Sorry. I just needed a
moment to let that sink in. It's the case. Okay. So you're saying like the
kind of linear superposition of waves that you typically
get from electromagnetic waves. You don't get that. Oh my goodness. That has-
- Far-field regime. - That's mind blowing to me, but also has like no
implications for everyday life. - No, that's right. (laughing) - That's one of those things that's like, we know the speed of gravity, gravitational waves
don't combine linearly, close to the collision of two black holes, like things that I'm like, what! But, (calm music) So what are some of the newer discoveries or like the physics we learned from these gravitational
wave observations? Yeah. Okay, let's that start there. - Okay. For this neutron
star collision in particular? - The very first detection
was of two black hole mergers. - That's right. - The two black holes merging. - Yeah, that's right. - But the paper that you wrote
was about this neutron star. - Yeah. That was the, this capstone paper or something
that described the entire, detection pathway from gravitational waves to ultimately x-rays and radio waves for binary neutron stars. - Gotcha. - So if we start there, I
mean, it's just one event, but a huge amount of physics
and astronomy was learned and astrophysics was learned
from that one object. For instance, prior to that,
people weren't quite sure what caused short gamma ray bursts, which had been seen since the seventies and are some of the most
violent defense in the universe. People see them with gamma rays. I've a couple of days
space-based detectors see them and a short gamma ray burst, some of them are caused by
neutron star collisions. Now we understood that within
a few minutes of the call, that rapid response team call
where people get on the phone, when they get an alert on
their cell phone saying, hey, we just had a
gravitational wave trigger. Let's go look and see if it's real or if it could be something
that's terrestrial. - And so one of the
things that we learned, or maybe knew, but at some point the idea was supported that the speed of light,
as far as we know, is the same as the speed of gravity. - Yeah. There's several
different scientific results that came out of these two neutron stars. And one was the first direct measurement of the speed of gravity. So when Einstein laid
down his field equations for general relativity in 1915 and solved them for gravitational waves, the prediction that spills out is that they travel at the speed of light. And so that had been
indirectly in the past. - Okay. - And limits set on whether
or not gravitational waves travel at the speed of light or not. But you had a direct
race in this measurement of finer neutron stars from 2017. So 130 million years ago, two neutron stars collided
together in a galaxy called NGC, 4993 discovered by
Herschel in the late 1700s. And for 130 million years, gravitational waves and light
had been coming at the earth. And we first measured gravitational waves and then 1.7 seconds later, NASA's Fermi space-based detector
saw a burst of gamma rays. - So last time I sort of came
to this realization like, holy cow, if the sun disappeared, I had, I thought about this before, where if the sun just like disappeared. So it would take you about
eight minutes to know, to see that the sun had disappeared. - Absolutely. - Because that's the speed of light. So you would get the knowledge,
you get the information, sun has gone, eight minutes later. - Yes. - But I didn't realize
that the speed of gravity is the same. So therefore, correct me if I'm wrong, but therefore the earth
would continue to orbit as if the sun was still
there for eight more minutes. - Absolutely. That's right. Yeah. That's the crux of
gravity having a finite speed. Is that information isn't
conveyed to the earth, that you've somehow vanished the sun. So it would continue on an arc and then it would go on a tangent. Whereas in Newton's theory of gravity, where gravity is instantly conveyed, it's just a one over R
squared and tells you nothing about the propagator, how gravity
moves around the universe. - Right. - The moment you vanished
the sun, it goes, the earth would go off at a tangent. Here in relativity, it
continues on that arc for eight minutes and then
goes off on a tangent. - Which is so cool. And also we had talked about like, if you had an observer that was halfway between the sun and the Earth's orbit, then the sun disappears. You see that after, after four minutes and then
another four minutes go by, then the earth goes off, but you still have to wait for that light to get back to you. So you it would be eight minutes, right? Until you saw the earth continue on. - Yeah. 'Cause you're
always looking in the past. - Yeah. Yeah, exactly. - Astronomical scale. - So you would, I guess
you would still see, you would see that the
earth like continuing on not knowing what's coming until suddenly you would get
the light from the earth. Well, then he wouldn't
be able to see it though most likely because there
would be no sunlight shining. - If you bounce radar off. - Right, right, right. Yeah, yeah. Of course we have radar at our
observatory halfway between, (laughing) the sun of the earth. Yeah. This is great. Is there a word for the
collision of two neutron stars? - So there's the Kilonovas? Is that what you're thinking?
- Yeah. - So the Kilonovas describes the object that forms after the collision and it's thought to be the site of the formation of much
of the heavy elements in the universe. Like this- - Is a Kilonovas. - Is a Kilonovas. Yeah. - So does it explode in the way that it, and in an analogous way to
the way a supernova explodes or does it just collide 'cause neutron stars are aren't they the leftover
cores of supernovas? - They are. They're one of the corpses of supernovas. - Right. - So you can have, there's
all these different types and supernovas pretty complicated. So there's different ways in
which you can get a supernova. So a Kilonovas is very different objects. It's these two neutron stars that, they're probably born in a
binary pair of half the stars and the night sky are binaries. And one will go supernova, and the other one will go supernova later and form two nutrients stars. And they revolve around each other. And people have seen those. - Oh, okay. - In our galaxy neutron
stars locked in orbit. There's something like
10 pairs of neutron stars known in our galaxy. The reason why people wanna
study gravitational waves is 'cause they're excellent probes of what's dark in the universe. You can't probe the interior
of a supernova collapse with light because the light
scatters on the way out, whereas gravitational
waves will stream freely out of the core of a
supernova and inform you and tell you about that. Similarly light from the big
bang occurs 300,000 years. After the big bang, that's when light, the universe is cooled enough, such as the cosmic microwave background can stream freely from matter. And so there's lots of
physics before that. If you actually detect the
gravitational wave analog of that, you'll sample the universe at 10 to the minus 30
seconds after the big bang. So that's why people look
for gravitational waves. The sun gets its energy
from thermonuclear fusion at the core of the sun, right? And so that's generating photons. And those photons take
about a million years to get to the limb of the sun. So it takes a million years because the light is scattering around 'cause the mean free path
for a photon is so small. - I knew that it was long. - Around in there. - I didn't know it was a million years. - About a million years. And then it takes eight minutes
for that light to get out. And so that is an example of why you wanna know
about gravitational waves for these dense objects, dense star. - My imagination is running
wild and thinking if we had, if we had dark matter and
dark energy collisions or violent dark matter
and dark energy events that gravitational waves could
potentially detect those. Where we could potentially detect them with gravitational waves. I'm not saying this right. We could potentially detect
the gravitational waves that come off of that, of those, if that happened. - It depends on what those things are, I mean, we call it dark
energy and dark matter. - But we don't know. - Because we don't know what it is. - Exactly, so- - That's a placeholder
saying, I don't know yet. But it's true that at some point people will find some
gravitational wave signature, which is completely unexpected from something nobody theorized. That's a hugely exciting thing. And it's happened before, when people turn on new
detectors in different sectors, not gravitational waves, but in different sectors like
Jocelyn Bell and Anthony Huish putting on a radio telescope that could detect short timescale changes. Bell showed these were, detected neutron stars. So people find things unexpectedly. - Which is very exciting. I mean, to think that like we've
turned on this new detector that detects a different kind of, it's a different kind of technology. And we may be able to see
things we never knew of, objects we've never heard
of never even theorized. - First detection was like that, right? That... heavy stellar mass black holes. So the mass of the black
holes that we discovered that was a surprise at some level. - Interesting. (calm music) You mentioned space, you mentioned a talk that space is very, is not very stretchy. It's stiff. It's more, I think you
said it's more like steel than like rubber or something. What does that mean? What does it mean for
space to be stretchy? Like we typically think
of something material as being stretchy,
something that we can touch and pull apart. How stretchy is space? - Not very, it turns out. - So what does that mean though? Like how do you quantify
stretchiness of space time? - Well, so- - I never thought I'd ask that question. Why are you laughing? - [Man] It's a good question. - Okay. - Well, so you can just actually look at Einstein's field equations and look for what is
effectively the Young's modulus, which tells you how stretchy, how expand your compressity. - Compressity. (laughing) - So don't show that.
Please don't show that. How compressible. (laughing) - I think that's one thing I
would have said, compressy. - If you look at what is
the in general relativity it's effectively the Young's
modulus of space time. - Yeah. - Which in materials,
Young's modulus tells you like how compressible something is. - Right. - You could say, oh, Q is the Young's modulus of space-time and let's compare that to steel. And that's why we need, black
holes and neutron stars, stars exploding the birth of the universe. These are the kinds of things that are actually gonna
create vibrations in space that will be large enough
in amplitude for us to see. When people ask, well, can you
make an experiment on earth to make the waves and then detect it? The answer's no, just
because these numbers come out to be really, really large. It's certainly the kind of
thing people wanted to do. Could you make like a rotor and say for a hypothetical detector, even ignoring what the detector might be, could you make a bar and
spin it around fast enough, a big enough bar in order to
detect gravitational waves in the lab next door,
which is sort of the way electromagnetic waves were discovered. Yeah. In one half the
lab, make a spark gap, sort of emission of electromagnetic waves and on the other side, you
have the radio receiver to detect them. And turn it on, turn it off,
chop it on, chop it off, and you can see these radio waves. If you look at how fast
you have to spin the bar that exceeds the tensile strength of steel and the bar rips apart, the atoms literally ripped apart. So there's just no way to
do that here on the lab, given how space, how stiff space is. - Right. Yes, we talked about how we will, we cannot create gravitational waves. Not even with geological activity. Which is violent on earth
as far as earth terms go. - Violent, but not even close, like many people asking you
to set off a nuclear bomb and measure the gravitational waves there. Still a paltry amount
compared to what you need to actually see them. - Good segue way because so a nuclear bomb converts matter into energy. So in the collision of two large objects, like two black holes, a large amount of mass
is converted into energy in the form of gravitational wave energy. How does that work? - Basically, again, we
appeal to relativity here. So you have the wave form from your data and you've used Einstein's
theory of relativity to calculate what the wave form should be for these two black holes. Take the first event, 29
solar mass black hole, 36 solar mass black
holes merging together. And so you have the inspiral phase. Then there's a merger phase
where the things combined and at some object that
looks like a black hole, but it's got lobes on it and it's spinning around very briefly. And then finally you have the
ring down phase of that object as it smooths out and becomes
a spinning black hole. - Oh wow. - Now, what's leftover? 36 and 29, that's 65 solar masses. You can tell from the waves on that, on the ring down that this
thing was 62 solar masses. So three in that process,
three times the mass of the sun were converted into
gravitational wave energy. - Yeah. Yeah. - Right? - So three times, and
that's what's coming out from the system is these
gravitational waves. They're churning up of, so there's this conversion of those three times the mass of the sun into gravitational waves. None of it's coming out light that makes it super exotic object. And at the time it was the most
powerful astronomical event known to human beings because of that conversion
three times the mass of the sun in a quarter second, that outshines all the
stars in the known universe for that quarter second, none of it's coming out in light, it's all coming out in
gravitational waves. - So Twitter has a bunch of questions, like a rapid fire, just
like some Twitter questions. Okay, man, these are not quick questions. What was I thinking? The questions are like, why
is mass the cause of gravity? And why is gravity only attractive? - Why is gravity attractive? Well it's an observation in the universe, but it's also not absolute. I mean, there's at least
one example of we know where gravity appears to be repulsive in that's in dark energy. And so what the right solution of that is, is still not well understood, but it does seem like
on the largest scales on these timescales, that
there's a component of gravity, which is repulsive that's, you
know, so what does that mean? It looks to us like there's some sort of
negative energy density or something driving this
expansion of the universe. - Is a repulsive form of gravity a thing that is taken into account in a theory of gravitational waves? - No, not at all. So we have, in order to understand these waves, we were using classical
general relativity. And so you don't have
to fold in any knowledge of dark energy or quantum gravity thing. So it's the same theory that
Einstein laid down in 1915, but people know a lot more about it since by doing more investigations,
exploiting the theory, finding solutions for different scenarios and applying numerical relativity. So we don't absolutely don't need it in terms of gravitational waves. (calm music) - I think I tried to
ask you this last time, but I don't know how to ask this question. And I feel like it's fundamental
to what it really means to what fundamental to
how this observatory and how this detector works. Why is it that when you stretch
the distance of the arm, you're not stretching the light as well, or I don't, what is, I don't even know. Do you know the right question
that I'm trying to ask? - I think it's hard. - [Levi] It's perfect. - Yeah, that's right. It is. That's a good way to put it. It's sort of like, how
do you make a measurement with a ruler if you're a
ruler is changing like. - Exactly. Yes. Yes. - So the light does change. It has to change just
the same way light gets red shifted when the universe expands. - Exactly. - Same thing. - Same question I'm trying to ask. - So yeah, the light is effective, but it's only effective for a small time in the detector relative to
the gravitational wave signal. So yes, it's an effect, but
it's a small effect for us. It's about, you know, a
1% per kilo Hertz effect. Meaning our detector is sensitive to many different frequencies,
is a broadband instrument. So the higher you go in frequency, the more of an effect happens. - I see. - And we know it happens
and it gets extracted in the calibration. - Interesting. Let me see if I can, I
feel like I'm starting, I maybe am coming to an understanding, but is it that you're sending out light and you're measuring how
long it takes the light to go that distance and then you're, then maybe a half a second
later the detector has stretched and you're sending a new light and seeing how long that takes to go, whatever distance the detector arm is. - Yeah, you- - Something more like that rather than like you've got
this light in the detector the entire time the
detector is being stretched. - So light builds in the arms
and resonates in the arms. And on average, a photon
has about 300 bounces. - Okay. - Some get out earlier,
some take longer to get out. It's a continuous process. But the time that those
photons spend is the time in which they could be
expanded or contracted due to the gravitational wave. That time in general is less than the time that the gravitational wave spends, jockeying the arms around. And so it's the combination of those two effects that matter. If we made a measurement at 36 kilohertz, then it's possible that
if you had a source right on the Zenith, directly
above you or directly below, that the effect of the
gravitational waves on the arms, these arms, which are really sensitive to differential emotion
would be completely undone by the effect on the light. And you're at a no
point in the instrument. And then you wouldn't see anything despite having this perfectly sensitive, wonderfully sensitive
differential, measurement tool. - Why is that? Hold on. So first of all, we've made it to the end of the detector arm, but
I've was very enthralled and almost maybe cutting
to a point of understanding this question I asked,
that I got distracted. But we're here. We're at the end of, this is
where one of the mirror is? - Yeah, one of the end mirrors. - One of the end mirrors. - In a chamber connected
to the vacuum envelope, which goes all the way back. - The vacuum envelope. Is that the- - That's the tube. Yeah. The entire set of tubes. (calm music) - I think I'm done. I mean, I'm not done,
but I think I've done. I to let Mike Landry go eventually. I mean this is the best day ever, maybe. - Wow. (laughing) You've got a lot of videos. That's great. - I mean it's really just like, it's not that often that I get to just sit and ask people questions
that about things, about the universe that are so fascinating and constantly learn new
things and get my mind blown about every five minutes. - That's great. - Thank you.
- Thank you. - So much.
- So much. - Yeah.
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