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