August 3, 2021 • 43 mins
Daniel explains how pulsars can be used as a galaxy-sized physics experiment.
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Speaker 1 (00:08):
The deepest questions about the nature of the universe have answers.
Answers that are out there right now, waiting to be discovered.
The nature of dark matter, the secret to quantum gravity,
the mystery of dark energy. The answers lie in wait
for us, but not just the answers, also the clues
(00:28):
to reveal them. Right now, the clue to unravel these
mysteries is out there. Probably the information we need to
crack the code is washing over us in the form
of messages from space we don't yet know how to decode.
Think about how much we have learned from listening carefully
to the sky answers to questions that previous generations didn't
(00:52):
even know to ask. So that makes us wonder, of course,
what answers are arriving here on Earth right now, waiting
for someone clever enough to know how to listen. Hi,
(01:18):
I'm Daniel. I'm a particle physicist, and I desperately want
to hear the answers to questions about the universe. I
know that these answers are out there, and I know
that humans will figure them out in fifty years and
a hundred years, in a thousand years, people will know
the answers to deep questions about the universe that we
are totally perplexed by. Little children will read books explaining
(01:42):
to them secrets that it takes Nobel Prize winning discoveries
to uncover about the universe. They will hear about them.
They will be bored. They will throw a tantrum. I
am desperate to read those books. I am desperate to
know those things about the universe. I know that the
answers are out there. I know there are are ways
to figure them out. But science moves slowly and steadily,
(02:04):
with brief flashes of insight, sometimes revealing the nature of
the universe, and we just have to wait for it
to happen. But not just wait, we can also push
it forward. So welcome to the podcast Daniel and Jorge
Explain the Universe, a production of I Heart Radio in
which we do our best to push it forward by
encouraging your curiosity about science, by encouraging everybody's curiosity about science,
(02:28):
by asking the big questions about the universe and thinking
about how we might possibly answer them. On this podcast,
we talk about everything in the universe, the tiniest little particles,
the most super massive black holes, and all the signals
we are receiving from these cosmic and tiny objects that
are telling us those secrets of the universe. My friend
(02:50):
and co host Jorge can't be here today to join us,
so I'm on my own telling you about all the
things we can learn from the universe. If we just
knew how to listen, and is no shortage of questions
about the universe, we'd like answers too. Of course, there
are the known unknowns, the things that we know we
don't know, and we also know how to figure them out.
(03:12):
There are so many things in science where we know
exactly what it is we need to do. We know
exactly what question we want to answer, and we just
haven't done it yet, either because of time or expertise,
or frankly, just money. Think about all we could accomplish
if we poured more money into science. We know how
to send rovers to other planets. We know how to
(03:34):
send satellites to land on the moons of Jupiter. We
know how to build big space telescopes. We can do
these things, and we know that if we did them,
we would essentially just be buying scientific knowledge. We know
that the answers to some of the questions we have
about whether there are life in the oceans, on those moons,
and what is going on in the deepest, darkest reaches
(03:56):
of the universe. Those a answers are out there waiting
for us if we just buy them. We are like
children walking around in a scientific candy shop, keeping all
of our money in our pockets and not purchasing those tasty, tasty,
delicious scientific treats. So that's why on this program and
everywhere in my life, I'm always advocating for increasing the
spending to government agencies. We could buy so much knowledge
(04:20):
about the universe. We could crack open some of these
mysteries if we just spent a few more pennies. All right,
maybe not pennies, maybe a few more millions of dollars,
but on the scale of government spending, it really is pennies.
But in the face of those budget realities, we have
to get clever. And one of my favorite things that
astrophysicists do is that they don't try to build experiments themselves.
(04:43):
They just go out and look for them. Like in
particle physics, if I want to know what happens when
I smash two protons together, then I say, let's go
build an accelerator that does just that, that smashes the
particles together and see what comes out. We are creating
the conditions that we want to The astrophysicists don't usually
have that luxury. For example, if you're curious about what
(05:05):
happened to be a smash two galaxies together, well you
can't go out and build a galaxy sized collider. Fortunately, however,
the universe is vast, and it is crazy, and it
is filled with all sorts of amazing stuff, including, if
you look hard enough, almost every experiment you would want
to do, including colliding galaxies. The same thing goes for
(05:27):
colliding things like black holes. Who doesn't want to shoot
one black hole at another one to see what happens?
Does one black hole eat the other one? Do they
eat each other somehow? Is it some crazy cosmic dance?
We now know, of course, that when you do that,
you emit enormous quantities of energy in gravitational waves. We've
actually seen these things using gravitational wave detectors. So while
(05:49):
we didn't have to build the experiment to shoot black
holes at each other, we did have to build the
experiments to see the gravitational waves. But what if we
didn't have to build the experiment and we didn't have
to build the detector. What if nature said it all
up for us. What if all we had to do
was listen? And so today on the podcast we'll be
(06:10):
talking about just that, a crazy new idea that might
just work. So on today's program will be answering the
question can we use the entire galaxy as a gravitational
wave detector? I know that sounds preposterous, but astrophysicists like
(06:30):
to think big, and when they do, sometimes they make
it work. So I was wondering how much people have
heard about this topic already, if this was something everybody
was talking about or only people in the physics community. So,
as usual, I asked for volunteers out there on the
Internet to answer random physics questions with no preparation. It
gives me an idea for what people out there already
(06:50):
know and what they think about when they hear this question.
If you'd like to participate for future episodes of the podcast,
please don't be shy. It's easy, it's fun, and we
love hearing from you. Please just write to me two
questions at Daniel and Jorge dot com. So think for
a moment, Do you have an idea for how you
could use the entire galaxy to detect gravitational waves. Here's
(07:14):
what our volunteers had to say. I don't know. I
had no idea. I don't know how detectors like Ligo
measure gravitational waves. I don't know the mechanism. But objects
in our galaxy are quite spread out and comparted to
the size of the galaxy, each of them is quite small.
(07:34):
So I don't think our galaxy could be an effective
gravitational wave detector. I don't know. The ones on Earth
I believe use lasers and obviously their own distance apart.
I sort of seen lectures one time about using dust
or hydrogen gas or something in the universe. Whether that's
gravitational waves or something else, I don't know. I'm not
(07:54):
quite sure how you do that one. Yeah, if you
think about how they measure gravity waves, uh, you're essentially
like measuring the separation between two mirrors in one direction
versus another direction. That's what the Liego observatory does, so
you get the disoscillation signal. So I imagine you can
do something similar with the entire galaxy. Um, you can
(08:15):
just look at the Milky Way galaxy and see if
if things get squished in one direction relative to another,
for like supermassive gravitational waves, Like maybe you can look
at the red shift at one end versus the blue
shift at the other and see if there's a difference
that sort of correlates as you move around. Yes, yes
we can. We just have to observe how gravitational waves
(08:39):
are affecting all the bodies in the galaxy. I don't
see why you couldn't. In fact, that would be probably
a good way to measure antitecht gravitational waves. So if
you used an entire galaxy, and let's say you're one
edge of the galaxy versus the opposite edge of the galaxy,
and you were using the same principle, you should be
(09:02):
able to pick up the movement of a gravitational wave
through that galaxy. Since you're asking that question, I would say, yes,
you can use the whole galaxies of gravitational wave detector.
If you have a large gravitational waves traveling through the galaxy,
(09:22):
you will impact some stars before others, and will increase
and decrease the length the distance between two far apart
stars quite significantly. So it's probably a very good gravitational
wave detective. All right. So our listeners are smart, they
know something about gravitational waves, and they have the idea
(09:44):
that the gravitational waves must be somehow affecting things in
the galaxy, and then we could see that effect somehow.
But nobody quite figured out exactly how we see those effects.
These ideas there about red shift and blue shift and
optical lensing and ripples against things. But it's a tricky topic,
and we're gonna dig to it and explain to you
(10:05):
exactly how to use the entire galaxy as your physics experiment.
But first, let's just remind ourselves what we're talking about observing,
what we want to see out there, what we're trying
to study, what we're using the entire galaxy as a
detector of. Are these crazy things called gravitational waves? What
is a gravitational wave anyway? Well, you know what a
(10:28):
wave is. A wave is when something moves up and down,
Like you put your hand in your bathtub and you
slap the water and you get a wave of the water. Right,
The water goes down and then goes up, and then
it goes down and then goes up. Or you see
waves in the ocean here in southern California and you
can serve them, and you might wonder like, well, why
are there waves? Right? Why does the water in the
bathtub go down in waves rather than all going down
(10:50):
at once? And that's because of a really important property
in physics called locality. When your hand hits the water,
it only affects the water that it touches, doesn't affect
the water water on the other side of the bathtub
or in bathtubs all over the universe. Right, Physics is
local and information takes time to propagate, So the water
on the other side of the bathtub doesn't know that
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you've hit the water on this side until it gets
that information, until the wave arrives there. It's the same story.
For example, you pluck a guitar string, right, which part
of the string starts to vibrate at first? Well, the
part that you plucked, and the rest then moves as
that information moves down the string. So then why do
you get a wave because the part that you pluck
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moves down and then it comes back up, and then
it goes back down and then it comes back up
and the wave are those ripples moving down the string.
If information propagated instantaneously, the whole string would move at once.
But the reason it doesn't, the reason you get a
wave is because information doesn't propagate instantaneously. So let's get
back to gravity. Gravitational waves are the waves in space itself, right,
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How do you make waves in space? Why does that
even mean? Well, remember that we know now that space
is not just like the backdrop. It's not just like
where things happen in the universe. It's a thing in
and of itself. It has properties. It can do things
that emptiness or nothingness can do. For example, it can bend.
(12:18):
If you put a big mass in space, what happens, Well,
space bends around that mass. It changes the curvature of space.
And a lot of you are probably thinking like a
bowling ball on a rubber sheet, right, And that's a
helpful analogy to sort of shape your mind out of
the idea that space is nothingness, that it can have curvature.
(12:38):
But it's also confusing because it's suggesting that space is
curving in some other dimension. In that example, space is
a two dimensional rubber sheet, and it's curving in some
third dimension because the bowling ball is pulling it down right, Well,
that's not what happens in our universe. Our space is
already three dimensional, and when it curves, what we mean
is that it changes the relative of distance between two points.
(13:02):
Means that things that might have seemed further away are
now closer because space is sort of scrunched. And that's why,
for example, photons bend around massive objects, because photons always
take the shortest path available to them, and that shortest
path looks like a curve. If you aren't aware of
the bending of space, and we have no way to
detect the bending of space, we can't see it, we
(13:24):
can't feel it other than watching things trace it out.
So that's why the Earth moves in a circle around
the Sun, because the space is bent in a way
that makes that its most natural motion. Alright, so space
can bend, But how does it ripple. How do you
get a wave in space? Well, the same way you
get a wave in your bathtub. Say, for example, you
(13:44):
deleted the Sun from our solar system. What would happen. Well,
you guys know that gravity doesn't move instantaneously, so we
wouldn't notice for eight minutes. That's a gravitational wave. That's
information about the shape of space propagating through space. It's
just like when you plug a guitar string, the information
that the string was plucked moves down the string. If
(14:06):
you delete the Sun, the information that space is no
longer bent moves through space, making it flat rather than curved.
And if you have two black holes, for example, orbiting
each other, then they are making huge gravitational waves. Because
anything that has mass and accelerates makes a gravitational wave.
It's changing the curvature of space through time, and that's
(14:28):
what creates gravitational waves. And so black holes, for example,
orbiting each other and eventually collapsing into a single mega
black hole, they send out these ripples as they orbit
each other. They're changing the shape of space around them
because they're very massive, and they're bending space, and the
way they're doing is changing because they are orbiting each other.
So that's where those gravitational waves come from. That's where
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the ripples in space come from. And so how do
you detect the ripples in space and time? I said earlier,
And we can't see or feel the bending of space.
All we can do is detect the change in relative distances.
If the space between me and you contracts, then the
distance between us contracts. You might think, well, whold lot
of second. If we have a ruler between us and
(15:12):
space between us contracts, won't the ruler also contract. There's
a chance of that. So to avoid that, we build
rulers out of light. We send light pulses back and forth,
because if the space between me and you get smaller,
then light will cover that distance more quickly. And if
you have a mirror, I can shoot my laser at
your mirror and measure how long it takes for that
(15:33):
light to come back. And so if a gravitational wave
passes between you and me, and I'm doing that all
the time, I'll notice because all of a sudden, the
space between us will be shorter and then longer as
the gravitational wave passes. And this is not a theoretical idea,
This is real. This is something we have actually seen.
People have built these detectors they're called Ligo and Vitigo,
(15:55):
and they have these mirrors underground and the mirrors are
very very stable. They try not to shake them at
all because any shaking those mirrors makes it impossible to
see gravitational waves, which after all look a lot like
the shaking of the mirrors. So they have these mirrors
hyper stabilized. They're hanging from cords, and those cords are
attached to something else, which is buffered, which is attached
to something else, which is protected from shaking. It's like
(16:17):
nine layers of protection against any sort of activity trucks
driving by, or people slamming screen doors, or any kind
of thing that would make a signal that looks like
a gravitational wave. The first one we saw was in
ten It won a Nobel Prize. And now we've seen
more than a dozen of these things from black holes
nearby that have collided and sent these pulses, and we
were kind of surprised by how common these things were.
(16:40):
So now you might be thinking, great, we have a
gravitational wave detector. We've seen these things. Why would we
need to build a gravitational wave detector at the size
of the galaxy. Well, gravitational waves come in lots of
different colors, basically, just the same way light has lots
of different frequencies. Light is electromagnetic radiation, and it can
have a frequency that puts it in the visible so
(17:02):
it has different colors, or it can have very long
frequencies like down in the radio, or it can have
very short frequencies and be an X ray or a
gamma ray. So all of these are different kinds of
electromagnetic radiation and we need different kind of instruments to
see them. You can't see the same thing with an
optical telescope that you can see whether radio antenna or
an X ray telescope. Well, the same thing is true
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for gravitational waves. Gravitational waves come in all different frequencies.
Space can ripple it lots of different frequencies, and very
high frequency ripples are different from very low frequency ripples,
and the kind of things that we can see with
LIGO are in a sort of narrow range of gravitational waves.
It was designed to see gravitational waves from solar mass
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black holes that were colliding with each other, and so
it's sort of only sensitive to that little spectrum. Imagine
if we had only ever built optical telescopes and we
couldn't look at the sky in the UV or in
the X ray or in the IDEO, we would be
missing a huge slice of the picture. So what we
need to do our build gravitational wave detectors that can
(18:07):
detect various different frequencies of gravitational waves so we can
listen to the messages from the universe and learn all
of its crazy secrets. So we'll talk about how to
build gravitational wave detectors that can detect very very low
frequency gravitational waves and tell us all about what's going
on in the early universe and the collisions of supermassive
(18:28):
black holes. But first, let's take a quick break. All right,
we're back and we are talking about building a gravitational
(18:48):
wave detector the size of the galaxy actually made out
of the galaxy, and we reminded ourselves what gravitational waves
are and how they have been seen so far by
gravity aational wave observatories on Earth that use delicate mirrors
balanced underground miles apart to detect very very small deviations
(19:09):
in the distances between those mirrors. These deviations are like
one part in ten to the twenty into the extraordinary
experimental accomplishment that they can do these things. It took
a huge amount of work to make those mirrors insensitive
to all sorts of things that would shake them, that
would look like the gravitational waves, and when they finally
did see them, it was a really nice, very clear
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signature because they knew exactly what they were looking for.
They knew what kind of gravitational waves black holes should
make as they fall into each other. What happens is
that the black holes start slowly moving towards each other
as they get closer and closer, and they start spiraling
faster and faster. So the frequency of the gravitational wave
increases as the black holes get closer together. And so
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they call this like a chirp because it goes faster, faster, faster, faster,
faster and higher entire and hire and higher and high,
and that's a hi, as my voice will go. So
they knew sort of what they were looking for. They
did all these numerical relativity calculations to figure out just
what it looks for. But you know, those black holes
were generating gravitational waves long before we saw them. It
took years for these black holes to actually merge. What
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we saw was just the last little bit as the
frequency moved into a range that Ligo could see it.
Ligo was designed to see gravitational waves from black hole mergers,
but only the last few seconds of them. Right there
were years, probably a gravitational waves that we couldn't see.
So why can LIGO not see gravitational waves that are longer?
(20:37):
The problem is seismic noise. The Earth itself is shaking.
We live on the surface of the Earth, which is
part of the crust, and the crust is always sliding,
and that makes it very difficult to see little ripples
in space and time. We can see them if the
ripples are fast enough, sort of faster than the Earth
typically shakes, but anything at a lower frequency, the sizemic vways,
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the shaking of the Earth itself makes basically impossible to
see those things. The Earth is shaking more loudly than
those gravitational waves, and it's not just the frequency, it's
because of the amplitude. Also, the intensity of the gravitational
wave signal gets stronger as you get near the end
of the black hole merger. As the black holes are
getting close and closer together, the gravitational waves get stronger.
(21:22):
So the gravitational waves from the early part of the
story we're missing because they're longer frequencies that our detectors
can't see over the seis mc noise of the Earth,
and they're much quieter, which makes it harder for us
to see them. So how do you see these longer
frequency gravitational waves. Then, well, the problem is that you're
buried in the Earth. One idea is, don't be buried
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in the Earth. Take it to space. Right. So one
science fiction sounding project that's actually very real is a
project called LISA, which is a laser interferometer in space. Right.
It takes the same concept of having mirrors where you're
bouncing lasers back and forth, and it puts it out
there in space. That's much more technologically difficult and expensive,
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of course, but it does solve this problem of the
Earth background noise. There is no seismic noise out there
in space. So LISA would be much more powerful, much
more sensitive, and able to hear gravitational waves at longer frequencies.
But again that's expensive and that's far off in the future,
and so until then people are thinking, do we need
(22:28):
to build our own gravitational wave detectors or can we
find one already existing in the galaxy? Can we use
the galaxy itself as a gravitational wave detector? And the answer,
of course, obviously is yes, because if we're doing an
whole episode about it, I wouldn't get to this point
of the episode and then just say no goodbye, see
you later. The way we do it is us an
(22:49):
ocean of very precise clocks that naturally exist in the galaxy.
Of course, I'm talking about pulsars. Pulsars are the end
point of a stall are you know. The star forms
when gas and dust swirl together and compactify and eventually
get dense enough that fusion happens. Then hangs out for
a few billion years, burning all of that fuel, pushing
(23:11):
back against gravity, preventing it from a collapse. But eventually
that fuel gives out and it can no longer provide
the heat and the radiation to prevent gravity from compacting
it even further. And depending on the mass of the star,
it can end up in various scenarios. It might turn
into a black hole if it's very massive, might turn
into a white dwarf basically a hot lump of stuff
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if it's not that massive. In the middle is a
category of object called neutron stars. Here there's enough gravity
to compact ify it to squeeze it down really really dense.
We're talking about a significant fraction of the mass of
the Sun in an area like the size of Los Angeles.
It's incredibly dense, it's incredibly weird matter. Also, it's called
(23:53):
a neutron star because it's been taken and squeezed so
much that the electrons and the protons and he had
ms are squeezed together and turn into neutrons. Usually it
goes the other way. You have a neutron hanging out,
it turns into a proton and an electron. But here
because the pressure that's basically been reversed, and you've got
an object which is mostly neutrons and in some really
(24:15):
weird intense state. In addition, these things are spinning really
really fast because they have all the angular momentum of
the original stuff that made them. But now they're a
really small space. And because angular momentum is conserved, you
can't just get rid of it. It doesn't just disappear.
Then it has to spin faster as it gets smaller,
just like a figure skater pulling in her arms and
(24:36):
you get more compact, you need to higher velocity to
match the smaller radius to have exactly the same angular momentum.
All right, So we have a spinning object, the neutron star,
some fraction of these have also really powerful magnetic fields,
and those magnetic fields operate on the particles on the
surface of the neutron star and can generate beams of energy.
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They push the protons and the neutrons and they generate
these massive beams of energy which follow the magnetic fields.
So you have this spinning object with a very powerful
magnetic field, with a beam of energy coming out the
top and the bottom, the magnetic north and the magnetic south.
What happens to the spin of this object is not
aligned with the magnetic axis. What if the beam is
(25:19):
not shooting straight up, so it's always going in the
same direction, but sort of off to the side a
little bit, then what happens is that that beam sweeps
around it points in a different direction. Right. Imagine holding
a flashlight and spinning around. If you're holding it straight up,
the flashlight doesn't change as you spin, But if you're
holding it to the side, then you're gonna be blinding
different people as you spin around. Right. That's what a
(25:41):
pulsar is a very intense beam of light pointing outside ways,
so that as it sweeps around, that beam passes over
different things. And from Earth we see these things when
that beam passes us. So there's a lot of pulsars
out there in the galaxy that we can't see because
their beam never passes us. But the ones where the
beam does sweep over the Earth, we see that as pulses.
(26:02):
We got a pulse every time it sweeps by. The
incredible thing is that they're very very regular. Here you
have an object of incredible mass, trillions of tons of
stuff spinning at very high speeds up to like hundreds
of hurts, right like an incredible amount of stuff, spending
many times per second, and doing it very regularly. It's
not like every point two seconds and then every point
(26:24):
three seconds and every point four seconds. These things are
more precise than some atomic clocks. They're like the most
precise natural clocks out there we have found, and the
universe is filled with them. They are all over the galaxy.
So you might imagine then how we might be able
to use them to measure the distortion of space. If
you are on Earth and you're surrounded by a bunch
(26:45):
of pulsars, and even watching these pulsars for a while
so you know them. You know how long it takes
between pulses for a given pulsar then what you can
do is see if that changes. Think about what happens
as a gravitational wave passes over the Earth. It changes
the distance between us and those pulsars. What that means
is that the pulses would take longer or shorter amounts
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of time to arrive here on Earth. So if you
know how often the pulses should be arriving and you
see a deviation, you see your residual from what you expect,
then that means something happened. The distance between you and
that pulsar has changed. And so a while ago people
figured out how to use all of these pulsars, these
precise clocks to calculate what would happen if a gravitational
(27:32):
wave past us, and it wouldn't affect all pulsars the
same way, right, because gravitational waves have this sort of
quadruple effect. They squeeze in one direction at the same
time they're pulling in another direction. So we can't look
at an individual pulsar and say, oh, there was a
gravitation wave. What we need to do is have a
whole network of pulsars, have them all around us in
(27:54):
every direction, so that a gravitational wave has a very
distinct signature, so it looks different from other random weird
blips we might see, or changes in our instrument or
anything else that might affect the timing but isn't due
to gravitational waves. Is a classic trick and experimental physics
is to make the thing you're looking for look unique,
(28:14):
so that when you see it, you know you saw it.
And so there were a couple of folks named Hellings
and Downs, and they did this analysis and they showed
what would happen if a gravitational wave passed over the
Earth and between us and a whole network of pulsars,
And what would happen is a predictable pattern in the
way that the pulses arrive on Earth. You can google
(28:37):
this and check it out if you're interested in learning
more details. But there's a particular signature we would expect
to see in the pulses from pulsars and the timing
of those pulsars arriving here on Earth if a gravitational
wave passed. And remember that we're not targeting fast gravitational
waves the ones that liego can see. Those are things
(28:57):
where it's like a hundred hurts in the frequency. There
is fast ripples in space and time. We're interested in
slow ripples in space and time. We're interested in very
long gravitational waves. Were interested in like the beginnings of
black holes coming together, and not just little itty bitty
black holes like the ones that Lego has seen. We're
interested in super massive black holes right because we think
(29:21):
that those black holes also combined. We talked earlier about
galaxy colliders shooting one galaxy at another. Well, that actually
happens in the universe. I don't know who's controlling it,
or if anybody ever is, but galaxies do merge. We
see evidence for this in lots of galaxies. We can
tell that some galaxies have recently undergone a merger because
they're sort of chaotic, and we can see other galaxies
(29:43):
that have had mergers billions of years ago. We think
that the Milky Way, for example, has remnants of other
galaxies that it's eaten. So if galaxies have supermassive black
holes at their center, then what happens when two galaxies merge?
What happens when one eats another one, which you get
is the merger of super massive black holes. These things
(30:04):
are black holes, not like just a little bit bigger
than our Sun. These things have masses like ten million
or sometimes billions of times the mass of our Sun.
It's staggering. It's hard to even get your mind around. Now,
imagine two of them and they're coming together, and they're
eating each other. They're forming one huge Grandma black hole right. Well,
(30:25):
that is going to admit a lot of gravitational waves,
and in the very beginning, very early part of that,
while the galaxies are still merging, while those black holes
are just beginning their dance, there's going to be very
low frequency, long gravitational waves that take a long time
to propagate, in a long time to measure as they
(30:46):
move through the universe. And so that's what a pulsar
array could be sensitive to. It could see gravitational waves
from the collisions of super massive black holes from the
beginning stages of those collisions while the two galaxies are
still beginning to form together. And we also don't understand
that the size of supermassive black holes. We know that
(31:08):
there's a relationship between galaxies and supermassive black holes that
typically the larger the black hole, the larger the galaxy,
But we don't understand how these supermassive black holes got
so big. We look back in the very early universe
and we see that there are already black holes like
a billion times the mass of the Sun, only a
(31:28):
billion years into the history of the universe, and in
our calculations, that's just not enough time to make that
bigger black holes. The deep mystery how these supermassive black
holes got so supermassive, and so one way to figure
this out is to see them merging. Is to understand
what happens when these two things combine. Is to look
at the early parts and say, oh, okay, this came
(31:50):
from too slightly smaller black holes, or maybe three, or
maybe something else entirely is going on. That's why we
are desperate to listen to these messages and to understand
what's going on with these very low frequency black holes.
So we talked about how to listen through low frequency
black holes by building a system of pulsars all across
the galaxy and watching as the signals from those pulsars
(32:14):
shift in frequency as a gravitational wave passes, and we
talked about what might be generating those gravitational waves. I
want to tell you all about an experiment that claims
to maybe have seen some of these low frequency gravitational
waves by using a pulsar array. But first, let's take
another break. All right, we're back and we are talking
(32:45):
about using the entire galaxy as a gravitational wave detector.
We reminded ourselves that gravitational waves are these ripples in
space and time. Sometimes they are generated when two small
black holes merged become a larger black hole, but they
can also be generated by super massive black holes as
galaxies merge and their central masses do a dance to
(33:08):
find out who's going to be in charge of the
new galaxy, and you can use pulsars to watch these
things happen. Pulsars are very regular clocks that send us
pulses at a very precise intervals, and as a gravitational
wave passes between us and them, shortening or extending the
distance between us and them, it can change the frequency
which those pulses arrive and give us a clue that
(33:29):
gravitational wave may have passed us. And this is not
a brand new idea, which means people have been doing
this for a while now. There's a group called Nano
grab that's been doing it for the last fifteen years.
They have a set of about forty five pulsars that
they've been listening to very regularly. They picked them and
they watch them with radio telescopes, and they observe the
(33:50):
frequency at which these pulses arrive here on Earth. And
after twelve and a half years they think they see
something interesting. They see something which they can't explain. They
see deviations in the patterns of these pulsars, right, And
that's exactly what you would expect to see if there
was a gravitational wave. You would expect that the pulsars
(34:11):
wouldn't be sending you their pulses at the very precise
atomic clock level, calibrated pulses that we're used to, but
that there would be these deviations. Now, nanogravi is not
his own experiment. It's of course using pulsars that are
already out there in the universe, and it uses telescopes
that already exist on Earth. For example, the Green Bank
Observatory that we always talked about in the center of
(34:32):
the radio quiet zone in the United States where you're
not allowed to own a telephone or turn on your microwave,
they used the Aristobo radio telescope before it's unfortunate collapse,
and they use all of these things together to try
to monitor all of these pulsars. Now, in January of
one they release their preliminary results, and what they see
is not consistent with no gravitational waves. Right, it's not
(34:55):
what you would expect if everything was normal. Unfortunately, it's
also not it's consistent with gravitational waves. We talked about
how if there were gravitational waves, you would expect to
see sort of a regular pattern. You would see pulsars
in one direction from Earth looking closer to you, and
pulsars in another direction, looking further because gravitational waves squeeze
(35:16):
space in one direction and lengthen it in another direction.
So that's not what they see. What they see can't
be explained by gravitational waves, but it also can't be
explained by anything we know. And that's exciting, right, because
every time you open up a new kind of eyeball
or build a new kind of ear to listen to
the universe's messages, we hear a surprise. Because sometimes we
(35:38):
go out there trying to answer one question and we
get evidence to answer another one, one we didn't even
know existed. We all remember stories of accidental discoveries. In fact,
pulsars themselves were an accidental discovery. Somebody was out there
looking to study quasars in the distant universe and hear
their radio messages and accidentally discovered pulsars. So it would
(35:59):
be pretty funny if pulsars then in turn gave us
clues about something else in the universe and we didn't
even know to look for. There are several of these
groups doing these studies watching pulsars. It takes a while
because we're talking about very low frequency events. We're talking
about gravitational waves. They could take years, decades, centuries to
(36:19):
propagate across the universe. Not that they're moving slowly, but
that their frequency is very very long, So the information
moves quickly, but the ripples in space themselves are moving
at a very slow speed, just like you can have
light traveling at the speed of light. Having very low
frequency waves like radio, and the kind of things they
can look for are not just super massive black hole collisions,
(36:42):
although that is super fascinating. We're also interested in general,
in what is the gravitational signal out there. We recently
did an episode about the cosmic gravitational background because we
suspect that the universe is filled with these low frequency
gravitational waves. We know that every thing that has mass
and accelerates creates gravitational waves. That means that as the
(37:04):
Earth goes around the Sun, it generates gravitational waves. It
means that every time you run to the store to
get a pint of ice cream, you generate gravitational waves.
And so there should be gravitational waves everywhere. There should
be sort of hard to make out. It's not like
we can pick out individual things unless they are very
dramatic events, like to nearby black holes colliding. But in general,
there should be sort of like a low level hum
(37:27):
of gravitational waves in the universe, some of them from
inspiring supermassive black holes and some of them from neutron
stars being formed or other black holes being created, or
supernovas should be generating gravitational waves. It should be everywhere.
So we should be able to sort of pick up
this low frequency gravitational waves as it sort of slashes
(37:47):
through the universe. And if we see something in those
low frequency gravitational waves that we don't expect, we might
learn something new about the universe. For example, we said
that one way to generate frequency gravitational waves that you
could detect with a galaxy size pulsar array come from
inspiraling black holes. Well, that might be true. What it
(38:10):
might be that the way these black holes merge, these
supermassive black holes merge, is different from what we expect.
There might be something else going on, and that might
help us understand how they get so big and how
galaxies form. Because when black holes pull each other together,
mostly what's going on is the force of gravity that
dominates everything. But black holes have other properties as well. Right,
(38:32):
black holes can spin because when something falls into a
black hole doesn't lose its angular momentum. So if something
falls into a black hole with angular momentum, then the
black hole itself has to spin. Anglo momentum doesn't go away,
same way electric charge doesn't go away. If you have
a black hole which is electrically neutral and you throw
an electron into it, what happens, Well, now you have
(38:53):
a black hole with a charge. So there's a famous
theorem called me no hair theorem that tells us that
black holes can have only those properties mass, spin, and charge,
and any other information about what's going on inside the
black hole is hidden from you, and that's not because
you can't give it a charge. It's because it's sort
of unstable that the process is going on there will
(39:14):
seek to balance it out. If, for example, you throw
a cork into a black hole, well, a cork feels
the strong nuclear force. It's a colored object, where color
is the equivalent of electric charge for the strong nuclear force.
What happens if you throw that into a black hole,
but there's so much energy there that it will pull
other corks out of the vacuum and eventually balance itself out.
(39:35):
That's why most things around us don't have a strong
nuclear charge, because those charges are inherently unstable. So that's
something that's going on with black holes, and black holes
are able to neutralize all those forces except for spin,
charge and mass. But what if there are other forces
out there? We know that there's a lot we don't
know yet about the universe. We know that there are
(39:56):
huge questions that are unanswered. There might be entirely new
forces out there. What if, for example, dark matter feels
a force, not a force that we're familiar with, but
a force that only dark matter can feel with itself.
Imagine if there was like a dark photon out there
that interacted with dark matter particles that had a dark charge.
In that case, it might be that supermassive black holes
(40:19):
have more than just spin and mass and electric charge.
They might also have a dark charge, in which case
that could affect the way that these supermassive black holes
fall into each other. It could have a powerful force
that changes the way they're interacting and how fast they're
falling into each other, and that could change the way
these gravitational waves look. These very low frequency gravitational waves
(40:42):
as they're beginning their dance would look different if there
are different forces in play, because it would change the frequency.
Remember that the frequency of the gravitational wave is determined
by how fast the black holes are moving around each other,
which depends entirely on the forces between them. So we
could use these gravitational waves as a probe to look
for new physics, new beyond the standard model, things that
(41:05):
we do not yet understand. So, while it would be
exciting to see gravitational waves and have them be exactly
what we expect, have them be just the kind of
gravitational waves we expect from neutron stars and supernovas and
inspiring supermassive black holes. It might be even more exciting
if these pulse are arraysed detect gravitational waves that we
don't understand that need new explanations, they need new ideas,
(41:29):
because they are clues that there are things going on
out there in the universe that we don't know about.
And in the end, that's the biggest goal. That's the
reason we listen to the nights guy, That's the reason
we do science because we want to find something new.
We want to gain a broader understanding of what's out
there in the universe. We want to break the cognitive
shackles of being here on Earth and be creatures of
(41:52):
the universe. We want to understand everything that's out there,
and we want to use all of our tools to
find it. Unfortunately, we are trapped here on the Earth
and we can only use the signals that get here.
But at the very least we should pay careful attention
to those signals. So even if they are little hints
from pulsear timings, that's something weird is going on in
the space between us and the pulsears, revealing that something
(42:13):
else weird is going on between super massive black holes
as they dance. These are the kind of clues that
we need to unravel, these subtle little stories that tell
us the deepest secrets of the universe. So write to
your politicians and tell them we should fund more science
because we want to learn more about the universe. But
until then, we will come up with clever and cheaper
(42:33):
ways to listen to those signals from the universe and
hope to unravel those mysteries. Thanks for listening to this
crazy story of ingenuity and creativity in astrophysics. Stay tuned,
and if you have a topic you would like to
hear us talk about, please don't be shy. Or if
you have any question at all about physics or something
you read, I answer all my emails, so right to
(42:55):
us two questions at Daniel and Jorge dot com. Can't
wait to hear from you. Thanks everybody, Yeah, thanks for listening,
and remember that Daniel and Jorge explained. The Universe is
a production of I Heart Radio or more podcast For
my Heart Radio, visit the I Heart Radio app, Apple Podcasts,
(43:18):
or wherever you listen to your favorite shows.
The deepest questions about the nature of the universe have answers.
Answers that are out there right now, waiting to be discovered.
The nature of dark matter, the secret to quantum gravity,
the mystery of dark energy. The answers lie in wait
for us, but not just the answers, also the clues
(00:28):
to reveal them. Right now, the clue to unravel these
mysteries is out there. Probably the information we need to
crack the code is washing over us in the form
of messages from space we don't yet know how to decode.
Think about how much we have learned from listening carefully
to the sky answers to questions that previous generations didn't
(00:52):
even know to ask. So that makes us wonder, of course,
what answers are arriving here on Earth right now, waiting
for someone clever enough to know how to listen. Hi,
(01:18):
I'm Daniel. I'm a particle physicist, and I desperately want
to hear the answers to questions about the universe. I
know that these answers are out there, and I know
that humans will figure them out in fifty years and
a hundred years, in a thousand years, people will know
the answers to deep questions about the universe that we
are totally perplexed by. Little children will read books explaining
(01:42):
to them secrets that it takes Nobel Prize winning discoveries
to uncover about the universe. They will hear about them.
They will be bored. They will throw a tantrum. I
am desperate to read those books. I am desperate to
know those things about the universe. I know that the
answers are out there. I know there are are ways
to figure them out. But science moves slowly and steadily,
(02:04):
with brief flashes of insight, sometimes revealing the nature of
the universe, and we just have to wait for it
to happen. But not just wait, we can also push
it forward. So welcome to the podcast Daniel and Jorge
Explain the Universe, a production of I Heart Radio in
which we do our best to push it forward by
encouraging your curiosity about science, by encouraging everybody's curiosity about science,
(02:28):
by asking the big questions about the universe and thinking
about how we might possibly answer them. On this podcast,
we talk about everything in the universe, the tiniest little particles,
the most super massive black holes, and all the signals
we are receiving from these cosmic and tiny objects that
are telling us those secrets of the universe. My friend
(02:50):
and co host Jorge can't be here today to join us,
so I'm on my own telling you about all the
things we can learn from the universe. If we just
knew how to listen, and is no shortage of questions
about the universe, we'd like answers too. Of course, there
are the known unknowns, the things that we know we
don't know, and we also know how to figure them out.
(03:12):
There are so many things in science where we know
exactly what it is we need to do. We know
exactly what question we want to answer, and we just
haven't done it yet, either because of time or expertise,
or frankly, just money. Think about all we could accomplish
if we poured more money into science. We know how
to send rovers to other planets. We know how to
(03:34):
send satellites to land on the moons of Jupiter. We
know how to build big space telescopes. We can do
these things, and we know that if we did them,
we would essentially just be buying scientific knowledge. We know
that the answers to some of the questions we have
about whether there are life in the oceans, on those moons,
and what is going on in the deepest, darkest reaches
(03:56):
of the universe. Those a answers are out there waiting
for us if we just buy them. We are like
children walking around in a scientific candy shop, keeping all
of our money in our pockets and not purchasing those tasty, tasty,
delicious scientific treats. So that's why on this program and
everywhere in my life, I'm always advocating for increasing the
spending to government agencies. We could buy so much knowledge
(04:20):
about the universe. We could crack open some of these
mysteries if we just spent a few more pennies. All right,
maybe not pennies, maybe a few more millions of dollars,
but on the scale of government spending, it really is pennies.
But in the face of those budget realities, we have
to get clever. And one of my favorite things that
astrophysicists do is that they don't try to build experiments themselves.
(04:43):
They just go out and look for them. Like in
particle physics, if I want to know what happens when
I smash two protons together, then I say, let's go
build an accelerator that does just that, that smashes the
particles together and see what comes out. We are creating
the conditions that we want to The astrophysicists don't usually
have that luxury. For example, if you're curious about what
(05:05):
happened to be a smash two galaxies together, well you
can't go out and build a galaxy sized collider. Fortunately, however,
the universe is vast, and it is crazy, and it
is filled with all sorts of amazing stuff, including, if
you look hard enough, almost every experiment you would want
to do, including colliding galaxies. The same thing goes for
(05:27):
colliding things like black holes. Who doesn't want to shoot
one black hole at another one to see what happens?
Does one black hole eat the other one? Do they
eat each other somehow? Is it some crazy cosmic dance?
We now know, of course, that when you do that,
you emit enormous quantities of energy in gravitational waves. We've
actually seen these things using gravitational wave detectors. So while
(05:49):
we didn't have to build the experiment to shoot black
holes at each other, we did have to build the
experiments to see the gravitational waves. But what if we
didn't have to build the experiment and we didn't have
to build the detector. What if nature said it all
up for us. What if all we had to do
was listen? And so today on the podcast we'll be
(06:10):
talking about just that, a crazy new idea that might
just work. So on today's program will be answering the
question can we use the entire galaxy as a gravitational
wave detector? I know that sounds preposterous, but astrophysicists like
(06:30):
to think big, and when they do, sometimes they make
it work. So I was wondering how much people have
heard about this topic already, if this was something everybody
was talking about or only people in the physics community. So,
as usual, I asked for volunteers out there on the
Internet to answer random physics questions with no preparation. It
gives me an idea for what people out there already
(06:50):
know and what they think about when they hear this question.
If you'd like to participate for future episodes of the podcast,
please don't be shy. It's easy, it's fun, and we
love hearing from you. Please just write to me two
questions at Daniel and Jorge dot com. So think for
a moment, Do you have an idea for how you
could use the entire galaxy to detect gravitational waves. Here's
(07:14):
what our volunteers had to say. I don't know. I
had no idea. I don't know how detectors like Ligo
measure gravitational waves. I don't know the mechanism. But objects
in our galaxy are quite spread out and comparted to
the size of the galaxy, each of them is quite small.
(07:34):
So I don't think our galaxy could be an effective
gravitational wave detector. I don't know. The ones on Earth
I believe use lasers and obviously their own distance apart.
I sort of seen lectures one time about using dust
or hydrogen gas or something in the universe. Whether that's
gravitational waves or something else, I don't know. I'm not
(07:54):
quite sure how you do that one. Yeah, if you
think about how they measure gravity waves, uh, you're essentially
like measuring the separation between two mirrors in one direction
versus another direction. That's what the Liego observatory does, so
you get the disoscillation signal. So I imagine you can
do something similar with the entire galaxy. Um, you can
(08:15):
just look at the Milky Way galaxy and see if
if things get squished in one direction relative to another,
for like supermassive gravitational waves, Like maybe you can look
at the red shift at one end versus the blue
shift at the other and see if there's a difference
that sort of correlates as you move around. Yes, yes
we can. We just have to observe how gravitational waves
(08:39):
are affecting all the bodies in the galaxy. I don't
see why you couldn't. In fact, that would be probably
a good way to measure antitecht gravitational waves. So if
you used an entire galaxy, and let's say you're one
edge of the galaxy versus the opposite edge of the galaxy,
and you were using the same principle, you should be
(09:02):
able to pick up the movement of a gravitational wave
through that galaxy. Since you're asking that question, I would say, yes,
you can use the whole galaxies of gravitational wave detector.
If you have a large gravitational waves traveling through the galaxy,
(09:22):
you will impact some stars before others, and will increase
and decrease the length the distance between two far apart
stars quite significantly. So it's probably a very good gravitational
wave detective. All right. So our listeners are smart, they
know something about gravitational waves, and they have the idea
(09:44):
that the gravitational waves must be somehow affecting things in
the galaxy, and then we could see that effect somehow.
But nobody quite figured out exactly how we see those effects.
These ideas there about red shift and blue shift and
optical lensing and ripples against things. But it's a tricky topic,
and we're gonna dig to it and explain to you
(10:05):
exactly how to use the entire galaxy as your physics experiment.
But first, let's just remind ourselves what we're talking about observing,
what we want to see out there, what we're trying
to study, what we're using the entire galaxy as a
detector of. Are these crazy things called gravitational waves? What
is a gravitational wave anyway? Well, you know what a
(10:28):
wave is. A wave is when something moves up and down,
Like you put your hand in your bathtub and you
slap the water and you get a wave of the water. Right,
The water goes down and then goes up, and then
it goes down and then goes up. Or you see
waves in the ocean here in southern California and you
can serve them, and you might wonder like, well, why
are there waves? Right? Why does the water in the
bathtub go down in waves rather than all going down
(10:50):
at once? And that's because of a really important property
in physics called locality. When your hand hits the water,
it only affects the water that it touches, doesn't affect
the water water on the other side of the bathtub
or in bathtubs all over the universe. Right, Physics is
local and information takes time to propagate, So the water
on the other side of the bathtub doesn't know that
(11:11):
you've hit the water on this side until it gets
that information, until the wave arrives there. It's the same story.
For example, you pluck a guitar string, right, which part
of the string starts to vibrate at first? Well, the
part that you plucked, and the rest then moves as
that information moves down the string. So then why do
you get a wave because the part that you pluck
(11:32):
moves down and then it comes back up, and then
it goes back down and then it comes back up
and the wave are those ripples moving down the string.
If information propagated instantaneously, the whole string would move at once.
But the reason it doesn't, the reason you get a
wave is because information doesn't propagate instantaneously. So let's get
back to gravity. Gravitational waves are the waves in space itself, right,
(11:57):
How do you make waves in space? Why does that
even mean? Well, remember that we know now that space
is not just like the backdrop. It's not just like
where things happen in the universe. It's a thing in
and of itself. It has properties. It can do things
that emptiness or nothingness can do. For example, it can bend.
(12:18):
If you put a big mass in space, what happens, Well,
space bends around that mass. It changes the curvature of space.
And a lot of you are probably thinking like a
bowling ball on a rubber sheet, right, And that's a
helpful analogy to sort of shape your mind out of
the idea that space is nothingness, that it can have curvature.
(12:38):
But it's also confusing because it's suggesting that space is
curving in some other dimension. In that example, space is
a two dimensional rubber sheet, and it's curving in some
third dimension because the bowling ball is pulling it down right, Well,
that's not what happens in our universe. Our space is
already three dimensional, and when it curves, what we mean
is that it changes the relative of distance between two points.
(13:02):
Means that things that might have seemed further away are
now closer because space is sort of scrunched. And that's why,
for example, photons bend around massive objects, because photons always
take the shortest path available to them, and that shortest
path looks like a curve. If you aren't aware of
the bending of space, and we have no way to
detect the bending of space, we can't see it, we
(13:24):
can't feel it other than watching things trace it out.
So that's why the Earth moves in a circle around
the Sun, because the space is bent in a way
that makes that its most natural motion. Alright, so space
can bend, But how does it ripple. How do you
get a wave in space? Well, the same way you
get a wave in your bathtub. Say, for example, you
(13:44):
deleted the Sun from our solar system. What would happen. Well,
you guys know that gravity doesn't move instantaneously, so we
wouldn't notice for eight minutes. That's a gravitational wave. That's
information about the shape of space propagating through space. It's
just like when you plug a guitar string, the information
that the string was plucked moves down the string. If
(14:06):
you delete the Sun, the information that space is no
longer bent moves through space, making it flat rather than curved.
And if you have two black holes, for example, orbiting
each other, then they are making huge gravitational waves. Because
anything that has mass and accelerates makes a gravitational wave.
It's changing the curvature of space through time, and that's
(14:28):
what creates gravitational waves. And so black holes, for example,
orbiting each other and eventually collapsing into a single mega
black hole, they send out these ripples as they orbit
each other. They're changing the shape of space around them
because they're very massive, and they're bending space, and the
way they're doing is changing because they are orbiting each other.
So that's where those gravitational waves come from. That's where
(14:51):
the ripples in space come from. And so how do
you detect the ripples in space and time? I said earlier,
And we can't see or feel the bending of space.
All we can do is detect the change in relative distances.
If the space between me and you contracts, then the
distance between us contracts. You might think, well, whold lot
of second. If we have a ruler between us and
(15:12):
space between us contracts, won't the ruler also contract. There's
a chance of that. So to avoid that, we build
rulers out of light. We send light pulses back and forth,
because if the space between me and you get smaller,
then light will cover that distance more quickly. And if
you have a mirror, I can shoot my laser at
your mirror and measure how long it takes for that
(15:33):
light to come back. And so if a gravitational wave
passes between you and me, and I'm doing that all
the time, I'll notice because all of a sudden, the
space between us will be shorter and then longer as
the gravitational wave passes. And this is not a theoretical idea,
This is real. This is something we have actually seen.
People have built these detectors they're called Ligo and Vitigo,
(15:55):
and they have these mirrors underground and the mirrors are
very very stable. They try not to shake them at
all because any shaking those mirrors makes it impossible to
see gravitational waves, which after all look a lot like
the shaking of the mirrors. So they have these mirrors
hyper stabilized. They're hanging from cords, and those cords are
attached to something else, which is buffered, which is attached
to something else, which is protected from shaking. It's like
(16:17):
nine layers of protection against any sort of activity trucks
driving by, or people slamming screen doors, or any kind
of thing that would make a signal that looks like
a gravitational wave. The first one we saw was in
ten It won a Nobel Prize. And now we've seen
more than a dozen of these things from black holes
nearby that have collided and sent these pulses, and we
were kind of surprised by how common these things were.
(16:40):
So now you might be thinking, great, we have a
gravitational wave detector. We've seen these things. Why would we
need to build a gravitational wave detector at the size
of the galaxy. Well, gravitational waves come in lots of
different colors, basically, just the same way light has lots
of different frequencies. Light is electromagnetic radiation, and it can
have a frequency that puts it in the visible so
(17:02):
it has different colors, or it can have very long
frequencies like down in the radio, or it can have
very short frequencies and be an X ray or a
gamma ray. So all of these are different kinds of
electromagnetic radiation and we need different kind of instruments to
see them. You can't see the same thing with an
optical telescope that you can see whether radio antenna or
an X ray telescope. Well, the same thing is true
(17:24):
for gravitational waves. Gravitational waves come in all different frequencies.
Space can ripple it lots of different frequencies, and very
high frequency ripples are different from very low frequency ripples,
and the kind of things that we can see with
LIGO are in a sort of narrow range of gravitational waves.
It was designed to see gravitational waves from solar mass
(17:47):
black holes that were colliding with each other, and so
it's sort of only sensitive to that little spectrum. Imagine
if we had only ever built optical telescopes and we
couldn't look at the sky in the UV or in
the X ray or in the IDEO, we would be
missing a huge slice of the picture. So what we
need to do our build gravitational wave detectors that can
(18:07):
detect various different frequencies of gravitational waves so we can
listen to the messages from the universe and learn all
of its crazy secrets. So we'll talk about how to
build gravitational wave detectors that can detect very very low
frequency gravitational waves and tell us all about what's going
on in the early universe and the collisions of supermassive
(18:28):
black holes. But first, let's take a quick break. All right,
we're back and we are talking about building a gravitational
(18:48):
wave detector the size of the galaxy actually made out
of the galaxy, and we reminded ourselves what gravitational waves
are and how they have been seen so far by
gravity aational wave observatories on Earth that use delicate mirrors
balanced underground miles apart to detect very very small deviations
(19:09):
in the distances between those mirrors. These deviations are like
one part in ten to the twenty into the extraordinary
experimental accomplishment that they can do these things. It took
a huge amount of work to make those mirrors insensitive
to all sorts of things that would shake them, that
would look like the gravitational waves, and when they finally
did see them, it was a really nice, very clear
(19:31):
signature because they knew exactly what they were looking for.
They knew what kind of gravitational waves black holes should
make as they fall into each other. What happens is
that the black holes start slowly moving towards each other
as they get closer and closer, and they start spiraling
faster and faster. So the frequency of the gravitational wave
increases as the black holes get closer together. And so
(19:54):
they call this like a chirp because it goes faster, faster, faster, faster,
faster and higher entire and hire and higher and high,
and that's a hi, as my voice will go. So
they knew sort of what they were looking for. They
did all these numerical relativity calculations to figure out just
what it looks for. But you know, those black holes
were generating gravitational waves long before we saw them. It
took years for these black holes to actually merge. What
(20:16):
we saw was just the last little bit as the
frequency moved into a range that Ligo could see it.
Ligo was designed to see gravitational waves from black hole mergers,
but only the last few seconds of them. Right there
were years, probably a gravitational waves that we couldn't see.
So why can LIGO not see gravitational waves that are longer?
(20:37):
The problem is seismic noise. The Earth itself is shaking.
We live on the surface of the Earth, which is
part of the crust, and the crust is always sliding,
and that makes it very difficult to see little ripples
in space and time. We can see them if the
ripples are fast enough, sort of faster than the Earth
typically shakes, but anything at a lower frequency, the sizemic vways,
(21:00):
the shaking of the Earth itself makes basically impossible to
see those things. The Earth is shaking more loudly than
those gravitational waves, and it's not just the frequency, it's
because of the amplitude. Also, the intensity of the gravitational
wave signal gets stronger as you get near the end
of the black hole merger. As the black holes are
getting close and closer together, the gravitational waves get stronger.
(21:22):
So the gravitational waves from the early part of the
story we're missing because they're longer frequencies that our detectors
can't see over the seis mc noise of the Earth,
and they're much quieter, which makes it harder for us
to see them. So how do you see these longer
frequency gravitational waves. Then, well, the problem is that you're
buried in the Earth. One idea is, don't be buried
(21:45):
in the Earth. Take it to space. Right. So one
science fiction sounding project that's actually very real is a
project called LISA, which is a laser interferometer in space. Right.
It takes the same concept of having mirrors where you're
bouncing lasers back and forth, and it puts it out
there in space. That's much more technologically difficult and expensive,
(22:07):
of course, but it does solve this problem of the
Earth background noise. There is no seismic noise out there
in space. So LISA would be much more powerful, much
more sensitive, and able to hear gravitational waves at longer frequencies.
But again that's expensive and that's far off in the future,
and so until then people are thinking, do we need
(22:28):
to build our own gravitational wave detectors or can we
find one already existing in the galaxy? Can we use
the galaxy itself as a gravitational wave detector? And the answer,
of course, obviously is yes, because if we're doing an
whole episode about it, I wouldn't get to this point
of the episode and then just say no goodbye, see
you later. The way we do it is us an
(22:49):
ocean of very precise clocks that naturally exist in the galaxy.
Of course, I'm talking about pulsars. Pulsars are the end
point of a stall are you know. The star forms
when gas and dust swirl together and compactify and eventually
get dense enough that fusion happens. Then hangs out for
a few billion years, burning all of that fuel, pushing
(23:11):
back against gravity, preventing it from a collapse. But eventually
that fuel gives out and it can no longer provide
the heat and the radiation to prevent gravity from compacting
it even further. And depending on the mass of the star,
it can end up in various scenarios. It might turn
into a black hole if it's very massive, might turn
into a white dwarf basically a hot lump of stuff
(23:33):
if it's not that massive. In the middle is a
category of object called neutron stars. Here there's enough gravity
to compact ify it to squeeze it down really really dense.
We're talking about a significant fraction of the mass of
the Sun in an area like the size of Los Angeles.
It's incredibly dense, it's incredibly weird matter. Also, it's called
(23:53):
a neutron star because it's been taken and squeezed so
much that the electrons and the protons and he had
ms are squeezed together and turn into neutrons. Usually it
goes the other way. You have a neutron hanging out,
it turns into a proton and an electron. But here
because the pressure that's basically been reversed, and you've got
an object which is mostly neutrons and in some really
(24:15):
weird intense state. In addition, these things are spinning really
really fast because they have all the angular momentum of
the original stuff that made them. But now they're a
really small space. And because angular momentum is conserved, you
can't just get rid of it. It doesn't just disappear.
Then it has to spin faster as it gets smaller,
just like a figure skater pulling in her arms and
(24:36):
you get more compact, you need to higher velocity to
match the smaller radius to have exactly the same angular momentum.
All right, So we have a spinning object, the neutron star,
some fraction of these have also really powerful magnetic fields,
and those magnetic fields operate on the particles on the
surface of the neutron star and can generate beams of energy.
(24:57):
They push the protons and the neutrons and they generate
these massive beams of energy which follow the magnetic fields.
So you have this spinning object with a very powerful
magnetic field, with a beam of energy coming out the
top and the bottom, the magnetic north and the magnetic south.
What happens to the spin of this object is not
aligned with the magnetic axis. What if the beam is
(25:19):
not shooting straight up, so it's always going in the
same direction, but sort of off to the side a
little bit, then what happens is that that beam sweeps
around it points in a different direction. Right. Imagine holding
a flashlight and spinning around. If you're holding it straight up,
the flashlight doesn't change as you spin, But if you're
holding it to the side, then you're gonna be blinding
different people as you spin around. Right. That's what a
(25:41):
pulsar is a very intense beam of light pointing outside ways,
so that as it sweeps around, that beam passes over
different things. And from Earth we see these things when
that beam passes us. So there's a lot of pulsars
out there in the galaxy that we can't see because
their beam never passes us. But the ones where the
beam does sweep over the Earth, we see that as pulses.
(26:02):
We got a pulse every time it sweeps by. The
incredible thing is that they're very very regular. Here you
have an object of incredible mass, trillions of tons of
stuff spinning at very high speeds up to like hundreds
of hurts, right like an incredible amount of stuff, spending
many times per second, and doing it very regularly. It's
not like every point two seconds and then every point
(26:24):
three seconds and every point four seconds. These things are
more precise than some atomic clocks. They're like the most
precise natural clocks out there we have found, and the
universe is filled with them. They are all over the galaxy.
So you might imagine then how we might be able
to use them to measure the distortion of space. If
you are on Earth and you're surrounded by a bunch
(26:45):
of pulsars, and even watching these pulsars for a while
so you know them. You know how long it takes
between pulses for a given pulsar then what you can
do is see if that changes. Think about what happens
as a gravitational wave passes over the Earth. It changes
the distance between us and those pulsars. What that means
is that the pulses would take longer or shorter amounts
(27:09):
of time to arrive here on Earth. So if you
know how often the pulses should be arriving and you
see a deviation, you see your residual from what you expect,
then that means something happened. The distance between you and
that pulsar has changed. And so a while ago people
figured out how to use all of these pulsars, these
precise clocks to calculate what would happen if a gravitational
(27:32):
wave past us, and it wouldn't affect all pulsars the
same way, right, because gravitational waves have this sort of
quadruple effect. They squeeze in one direction at the same
time they're pulling in another direction. So we can't look
at an individual pulsar and say, oh, there was a
gravitation wave. What we need to do is have a
whole network of pulsars, have them all around us in
(27:54):
every direction, so that a gravitational wave has a very
distinct signature, so it looks different from other random weird
blips we might see, or changes in our instrument or
anything else that might affect the timing but isn't due
to gravitational waves. Is a classic trick and experimental physics
is to make the thing you're looking for look unique,
(28:14):
so that when you see it, you know you saw it.
And so there were a couple of folks named Hellings
and Downs, and they did this analysis and they showed
what would happen if a gravitational wave passed over the
Earth and between us and a whole network of pulsars,
And what would happen is a predictable pattern in the
way that the pulses arrive on Earth. You can google
(28:37):
this and check it out if you're interested in learning
more details. But there's a particular signature we would expect
to see in the pulses from pulsars and the timing
of those pulsars arriving here on Earth if a gravitational
wave passed. And remember that we're not targeting fast gravitational
waves the ones that liego can see. Those are things
(28:57):
where it's like a hundred hurts in the frequency. There
is fast ripples in space and time. We're interested in
slow ripples in space and time. We're interested in very
long gravitational waves. Were interested in like the beginnings of
black holes coming together, and not just little itty bitty
black holes like the ones that Lego has seen. We're
interested in super massive black holes right because we think
(29:21):
that those black holes also combined. We talked earlier about
galaxy colliders shooting one galaxy at another. Well, that actually
happens in the universe. I don't know who's controlling it,
or if anybody ever is, but galaxies do merge. We
see evidence for this in lots of galaxies. We can
tell that some galaxies have recently undergone a merger because
they're sort of chaotic, and we can see other galaxies
(29:43):
that have had mergers billions of years ago. We think
that the Milky Way, for example, has remnants of other
galaxies that it's eaten. So if galaxies have supermassive black
holes at their center, then what happens when two galaxies merge?
What happens when one eats another one, which you get
is the merger of super massive black holes. These things
(30:04):
are black holes, not like just a little bit bigger
than our Sun. These things have masses like ten million
or sometimes billions of times the mass of our Sun.
It's staggering. It's hard to even get your mind around. Now,
imagine two of them and they're coming together, and they're
eating each other. They're forming one huge Grandma black hole right. Well,
(30:25):
that is going to admit a lot of gravitational waves,
and in the very beginning, very early part of that,
while the galaxies are still merging, while those black holes
are just beginning their dance, there's going to be very
low frequency, long gravitational waves that take a long time
to propagate, in a long time to measure as they
(30:46):
move through the universe. And so that's what a pulsar
array could be sensitive to. It could see gravitational waves
from the collisions of super massive black holes from the
beginning stages of those collisions while the two galaxies are
still beginning to form together. And we also don't understand
that the size of supermassive black holes. We know that
(31:08):
there's a relationship between galaxies and supermassive black holes that
typically the larger the black hole, the larger the galaxy,
But we don't understand how these supermassive black holes got
so big. We look back in the very early universe
and we see that there are already black holes like
a billion times the mass of the Sun, only a
(31:28):
billion years into the history of the universe, and in
our calculations, that's just not enough time to make that
bigger black holes. The deep mystery how these supermassive black
holes got so supermassive, and so one way to figure
this out is to see them merging. Is to understand
what happens when these two things combine. Is to look
at the early parts and say, oh, okay, this came
(31:50):
from too slightly smaller black holes, or maybe three, or
maybe something else entirely is going on. That's why we
are desperate to listen to these messages and to understand
what's going on with these very low frequency black holes.
So we talked about how to listen through low frequency
black holes by building a system of pulsars all across
the galaxy and watching as the signals from those pulsars
(32:14):
shift in frequency as a gravitational wave passes, and we
talked about what might be generating those gravitational waves. I
want to tell you all about an experiment that claims
to maybe have seen some of these low frequency gravitational
waves by using a pulsar array. But first, let's take
another break. All right, we're back and we are talking
(32:45):
about using the entire galaxy as a gravitational wave detector.
We reminded ourselves that gravitational waves are these ripples in
space and time. Sometimes they are generated when two small
black holes merged become a larger black hole, but they
can also be generated by super massive black holes as
galaxies merge and their central masses do a dance to
(33:08):
find out who's going to be in charge of the
new galaxy, and you can use pulsars to watch these
things happen. Pulsars are very regular clocks that send us
pulses at a very precise intervals, and as a gravitational
wave passes between us and them, shortening or extending the
distance between us and them, it can change the frequency
which those pulses arrive and give us a clue that
(33:29):
gravitational wave may have passed us. And this is not
a brand new idea, which means people have been doing
this for a while now. There's a group called Nano
grab that's been doing it for the last fifteen years.
They have a set of about forty five pulsars that
they've been listening to very regularly. They picked them and
they watch them with radio telescopes, and they observe the
(33:50):
frequency at which these pulses arrive here on Earth. And
after twelve and a half years they think they see
something interesting. They see something which they can't explain. They
see deviations in the patterns of these pulsars, right, And
that's exactly what you would expect to see if there
was a gravitational wave. You would expect that the pulsars
(34:11):
wouldn't be sending you their pulses at the very precise
atomic clock level, calibrated pulses that we're used to, but
that there would be these deviations. Now, nanogravi is not
his own experiment. It's of course using pulsars that are
already out there in the universe, and it uses telescopes
that already exist on Earth. For example, the Green Bank
Observatory that we always talked about in the center of
(34:32):
the radio quiet zone in the United States where you're
not allowed to own a telephone or turn on your microwave,
they used the Aristobo radio telescope before it's unfortunate collapse,
and they use all of these things together to try
to monitor all of these pulsars. Now, in January of
one they release their preliminary results, and what they see
is not consistent with no gravitational waves. Right, it's not
(34:55):
what you would expect if everything was normal. Unfortunately, it's
also not it's consistent with gravitational waves. We talked about
how if there were gravitational waves, you would expect to
see sort of a regular pattern. You would see pulsars
in one direction from Earth looking closer to you, and
pulsars in another direction, looking further because gravitational waves squeeze
(35:16):
space in one direction and lengthen it in another direction.
So that's not what they see. What they see can't
be explained by gravitational waves, but it also can't be
explained by anything we know. And that's exciting, right, because
every time you open up a new kind of eyeball
or build a new kind of ear to listen to
the universe's messages, we hear a surprise. Because sometimes we
(35:38):
go out there trying to answer one question and we
get evidence to answer another one, one we didn't even
know existed. We all remember stories of accidental discoveries. In fact,
pulsars themselves were an accidental discovery. Somebody was out there
looking to study quasars in the distant universe and hear
their radio messages and accidentally discovered pulsars. So it would
(35:59):
be pretty funny if pulsars then in turn gave us
clues about something else in the universe and we didn't
even know to look for. There are several of these
groups doing these studies watching pulsars. It takes a while
because we're talking about very low frequency events. We're talking
about gravitational waves. They could take years, decades, centuries to
(36:19):
propagate across the universe. Not that they're moving slowly, but
that their frequency is very very long, So the information
moves quickly, but the ripples in space themselves are moving
at a very slow speed, just like you can have
light traveling at the speed of light. Having very low
frequency waves like radio, and the kind of things they
can look for are not just super massive black hole collisions,
(36:42):
although that is super fascinating. We're also interested in general,
in what is the gravitational signal out there. We recently
did an episode about the cosmic gravitational background because we
suspect that the universe is filled with these low frequency
gravitational waves. We know that every thing that has mass
and accelerates creates gravitational waves. That means that as the
(37:04):
Earth goes around the Sun, it generates gravitational waves. It
means that every time you run to the store to
get a pint of ice cream, you generate gravitational waves.
And so there should be gravitational waves everywhere. There should
be sort of hard to make out. It's not like
we can pick out individual things unless they are very
dramatic events, like to nearby black holes colliding. But in general,
there should be sort of like a low level hum
(37:27):
of gravitational waves in the universe, some of them from
inspiring supermassive black holes and some of them from neutron
stars being formed or other black holes being created, or
supernovas should be generating gravitational waves. It should be everywhere.
So we should be able to sort of pick up
this low frequency gravitational waves as it sort of slashes
(37:47):
through the universe. And if we see something in those
low frequency gravitational waves that we don't expect, we might
learn something new about the universe. For example, we said
that one way to generate frequency gravitational waves that you
could detect with a galaxy size pulsar array come from
inspiraling black holes. Well, that might be true. What it
(38:10):
might be that the way these black holes merge, these
supermassive black holes merge, is different from what we expect.
There might be something else going on, and that might
help us understand how they get so big and how
galaxies form. Because when black holes pull each other together,
mostly what's going on is the force of gravity that
dominates everything. But black holes have other properties as well. Right,
(38:32):
black holes can spin because when something falls into a
black hole doesn't lose its angular momentum. So if something
falls into a black hole with angular momentum, then the
black hole itself has to spin. Anglo momentum doesn't go away,
same way electric charge doesn't go away. If you have
a black hole which is electrically neutral and you throw
an electron into it, what happens, Well, now you have
(38:53):
a black hole with a charge. So there's a famous
theorem called me no hair theorem that tells us that
black holes can have only those properties mass, spin, and charge,
and any other information about what's going on inside the
black hole is hidden from you, and that's not because
you can't give it a charge. It's because it's sort
of unstable that the process is going on there will
(39:14):
seek to balance it out. If, for example, you throw
a cork into a black hole, well, a cork feels
the strong nuclear force. It's a colored object, where color
is the equivalent of electric charge for the strong nuclear force.
What happens if you throw that into a black hole,
but there's so much energy there that it will pull
other corks out of the vacuum and eventually balance itself out.
(39:35):
That's why most things around us don't have a strong
nuclear charge, because those charges are inherently unstable. So that's
something that's going on with black holes, and black holes
are able to neutralize all those forces except for spin,
charge and mass. But what if there are other forces
out there? We know that there's a lot we don't
know yet about the universe. We know that there are
(39:56):
huge questions that are unanswered. There might be entirely new
forces out there. What if, for example, dark matter feels
a force, not a force that we're familiar with, but
a force that only dark matter can feel with itself.
Imagine if there was like a dark photon out there
that interacted with dark matter particles that had a dark charge.
In that case, it might be that supermassive black holes
(40:19):
have more than just spin and mass and electric charge.
They might also have a dark charge, in which case
that could affect the way that these supermassive black holes
fall into each other. It could have a powerful force
that changes the way they're interacting and how fast they're
falling into each other, and that could change the way
these gravitational waves look. These very low frequency gravitational waves
(40:42):
as they're beginning their dance would look different if there
are different forces in play, because it would change the frequency.
Remember that the frequency of the gravitational wave is determined
by how fast the black holes are moving around each other,
which depends entirely on the forces between them. So we
could use these gravitational waves as a probe to look
for new physics, new beyond the standard model, things that
(41:05):
we do not yet understand. So, while it would be
exciting to see gravitational waves and have them be exactly
what we expect, have them be just the kind of
gravitational waves we expect from neutron stars and supernovas and
inspiring supermassive black holes. It might be even more exciting
if these pulse are arraysed detect gravitational waves that we
don't understand that need new explanations, they need new ideas,
(41:29):
because they are clues that there are things going on
out there in the universe that we don't know about.
And in the end, that's the biggest goal. That's the
reason we listen to the nights guy, That's the reason
we do science because we want to find something new.
We want to gain a broader understanding of what's out
there in the universe. We want to break the cognitive
shackles of being here on Earth and be creatures of
(41:52):
the universe. We want to understand everything that's out there,
and we want to use all of our tools to
find it. Unfortunately, we are trapped here on the Earth
and we can only use the signals that get here.
But at the very least we should pay careful attention
to those signals. So even if they are little hints
from pulsear timings, that's something weird is going on in
the space between us and the pulsears, revealing that something
(42:13):
else weird is going on between super massive black holes
as they dance. These are the kind of clues that
we need to unravel, these subtle little stories that tell
us the deepest secrets of the universe. So write to
your politicians and tell them we should fund more science
because we want to learn more about the universe. But
until then, we will come up with clever and cheaper
(42:33):
ways to listen to those signals from the universe and
hope to unravel those mysteries. Thanks for listening to this
crazy story of ingenuity and creativity in astrophysics. Stay tuned,
and if you have a topic you would like to
hear us talk about, please don't be shy. Or if
you have any question at all about physics or something
you read, I answer all my emails, so right to
(42:55):
us two questions at Daniel and Jorge dot com. Can't
wait to hear from you. Thanks everybody, Yeah, thanks for listening,
and remember that Daniel and Jorge explained. The Universe is
a production of I Heart Radio or more podcast For
my Heart Radio, visit the I Heart Radio app, Apple Podcasts,
(43:18):
or wherever you listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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