Latest / Daniel and Kelly’s Extraordinary Universe / How to use the whole galaxy to hear huge gravitational waves

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How to use the whole galaxy to hear huge gravitational waves · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, what's the biggest science experiment ever built?
• 0:12
  Speaker 2: Oh? Well, the large Hadron Collider is pretty big. It's
• 0:15
  Speaker 2: thirty three kilometers around.
• 0:17
  Speaker 1: That is pretty big, I guess for human scale, but
• 0:20
  Speaker 1: compared to the universe. I mean the universe is huge, right.
• 0:22
  Speaker 2: That's true. And I guess astrophysicists use the whole universe
• 0:26
  Speaker 2: as an experiment when they like watch black holes collide.
• 0:29
  Speaker 1: M the whole universe is an experiment. What are the results?
• 0:33
  Speaker 2: They're pretty universal?
• 0:34
  Speaker 1: Are they positive or negative?
• 0:36
  Speaker 2: We have only one data point so far, so I
• 0:37
  Speaker 2: think it would be presumptuous to make any conclusions.
• 0:39
  Speaker 1: That's a hypothesis for the universe. It's awesome or not awesome,
• 0:43
  Speaker 1: it's bunkers. I guess you use data from the whole universe.
• 0:46
  Speaker 1: But usually you only detect things with a small telescope, right,
• 0:50
  Speaker 1: or a small Parkle collider.
• 0:52
  Speaker 3: Yeah.
• 0:52
  Speaker 2: The kind of eyeballs we build are usually small, even
• 0:55
  Speaker 2: when we are observing really large things. But there are
• 0:58
  Speaker 2: some really clever approach, which is that use like the
• 1:01
  Speaker 2: whole galaxy as a detector.
• 1:03
  Speaker 1: What is that experiment called?
• 1:05
  Speaker 2: Because it's so gargantua and of course they use the
• 1:07
  Speaker 2: word nano in the title of the experiment.
• 1:10
  Speaker 1: What I guess it's nana compared to the size of
• 1:13
  Speaker 1: the universe.
• 1:13
  Speaker 2: I can't defend astronomers when it comes to naming their experiments.
• 1:16
  Speaker 1: But you can defend particle physicists. You didn't defend anyone
• 1:22
  Speaker 1: in science or how they name things.
• 1:24
  Speaker 2: I'm gonna plead no contest, The.
• 1:25
  Speaker 1: Experiment says no. Hi. I'm Jorhem Mac, cartoonist and the
• 1:43
  Speaker 1: author of Oliver's Great Big Universe. Hi.
• 1:46
  Speaker 2: I'm Daniel. I'm a particle physicist and a professor at
• 1:49
  Speaker 2: UC Irvine, and I think a universe is pretty well named.
• 1:53
  Speaker 1: Apparently not because there's something called the multiverse. So I
• 1:57
  Speaker 1: think if you have to put something in front of
• 1:59
  Speaker 1: it later, it wasn't well named.
• 2:01
  Speaker 2: Oh well, like Superman is not well named because he's
• 2:04
  Speaker 2: a superversion of a man.
• 2:05
  Speaker 1: Well, he's special, but if you have to sort of
• 2:07
  Speaker 1: redefine what that is like, if you had to later
• 2:10
  Speaker 1: name him ultra Superman, then maybe he didn't name him
• 2:13
  Speaker 1: well the first.
• 2:14
  Speaker 2: Time, multi Superman.
• 2:16
  Speaker 1: Maybe there's a universe where Superman was well named, or
• 2:21
  Speaker 1: physicists named their experiments in easy to understand ways.
• 2:24
  Speaker 2: That would be like a superphysicist.
• 2:26
  Speaker 1: But anyways, welcome to our podcast Daniel and Jorge Explain
• 2:29
  Speaker 1: the Universe, a production of iHeartRadio.
• 2:31
  Speaker 2: In which we use our definitely not super brains to
• 2:34
  Speaker 2: try to understand the super mysteries of the entire universe.
• 2:38
  Speaker 2: We hope that everything out there in the universe can
• 2:40
  Speaker 2: be described by simple mathematical recipes that make sense to
• 2:45
  Speaker 2: our mammalian brains, and we do our best to apply
• 2:48
  Speaker 2: those rules to the whole universe to see where they work,
• 2:51
  Speaker 2: where they break down, and where we can explain all
• 2:54
  Speaker 2: of them to you.
• 2:55
  Speaker 1: Yeah, because it's amazing that our tiny little brains can
• 2:58
  Speaker 1: understand universe, that we can look out into the cosmos
• 3:01
  Speaker 1: to get data and figure out how things work. Fortunately,
• 3:04
  Speaker 1: the universe likes to reveal itself sometimes.
• 3:08
  Speaker 2: Sometimes we have to be clever and build really interesting
• 3:11
  Speaker 2: little apparatuses to like smash particles together or balance balls
• 3:16
  Speaker 2: against each other in order to extract some information from
• 3:18
  Speaker 2: the universe. We like concoct special setups that we hope
• 3:22
  Speaker 2: will reveal deep truths about the universe. But not everybody
• 3:26
  Speaker 2: gets to do that. Not everybody gets to build their
• 3:28
  Speaker 2: own experiments. Some people have to go out there and
• 3:30
  Speaker 2: find the experiments happening in nature.
• 3:33
  Speaker 1: Yeah, because science is a continuing story and there are
• 3:37
  Speaker 1: new things being learned every day and new ways to
• 3:39
  Speaker 1: look at the universe being discovered every day. And so
• 3:42
  Speaker 1: this week there was a very special news about a
• 3:45
  Speaker 1: new experiment that just revealed the data it's found.
• 3:49
  Speaker 2: That's right after analyzing fifteen years worth of observational data.
• 3:54
  Speaker 2: The Nanograph experiment has just made a dramatic announcement about
• 3:58
  Speaker 2: their fantastic new discscovery.
• 4:00
  Speaker 1: You might have seen it on the news. It's sort
• 4:02
  Speaker 1: of a big deal for this community of astrophysicists.
• 4:05
  Speaker 2: It's literally sending waves through the cosmological community.
• 4:09
  Speaker 1: But hopefully the results aren't the wavy or shaky.
• 4:12
  Speaker 2: It's a bit of a treacherous territory because, as we
• 4:14
  Speaker 2: all know, claims of the discovery of gravitational waves have
• 4:17
  Speaker 2: either been verified and led to Nobel prices or have
• 4:20
  Speaker 2: been debunked and led to some quite red faces in
• 4:24
  Speaker 2: the cosmological community.
• 4:25
  Speaker 1: And so this week there was a big announcement by
• 4:28
  Speaker 1: an experience called Nanograph. Now, Daniel, is that an acronym
• 4:32
  Speaker 1: or do they just like the word nano?
• 4:34
  Speaker 2: You know, I think The answer to that is yes
• 4:36
  Speaker 2: on both counts. It is an acronym. It stands for
• 4:40
  Speaker 2: North American Nanohertz Observatory for Gravitational Radiation. So they both
• 4:47
  Speaker 2: like the word NANO because it's in their title and
• 4:50
  Speaker 2: it's an acronym.
• 4:51
  Speaker 1: Wait what, it's a recursive title, like the word nano
• 4:54
  Speaker 1: is in the acronym NANO exactly.
• 4:57
  Speaker 2: And because NANOGrav is one of these really clever device
• 5:00
  Speaker 2: that uses the entire galaxy as a detector for gravitational waves,
• 5:05
  Speaker 2: you might think that they would choose something which describes
• 5:07
  Speaker 2: the scale the grandeur of that experiment, But instead they've
• 5:11
  Speaker 2: chosen NANO, which reflects the very short frequency of these
• 5:15
  Speaker 2: gravitational waves.
• 5:16
  Speaker 3: Mmm.
• 5:17
  Speaker 1: Yeah, I saw Did you said the word nano hurts?
• 5:20
  Speaker 1: I guess that's the frequency of gravitational waves that they
• 5:23
  Speaker 1: have detected.
• 5:24
  Speaker 2: That's right. The huge scale of the galaxy allows them
• 5:27
  Speaker 2: to measure really long wavelength, low frequency gravitational waves that
• 5:32
  Speaker 2: other gravitational wave detectors Ligo and virgo and all of
• 5:35
  Speaker 2: those could never see, giving us a whole new window
• 5:38
  Speaker 2: into what's going on in the universe.
• 5:40
  Speaker 1: So today on the podcast, we'll be tackling the question,
• 5:48
  Speaker 1: how did the nanograph experiment use the galaxy as a detector? Now,
• 5:55
  Speaker 1: did they ask the galaxy's pervision before they did this?
• 5:57
  Speaker 2: We've been over this before. The universe has no right
• 5:59
  Speaker 2: to privacy inherently, man, we can ask whatever question we want.
• 6:03
  Speaker 1: Is it in the fine print? Or are we going
• 6:04
  Speaker 1: to get sued later?
• 6:06
  Speaker 2: I checked with our legal team. They're fine with it.
• 6:08
  Speaker 1: We have a legal team.
• 6:09
  Speaker 2: I'm the legal team.
• 6:10
  Speaker 1: Yeah. I think that means we don't have a legal team.
• 6:16
  Speaker 1: That's I check. You're not a lawyer, that is correct.
• 6:19
  Speaker 2: Do not take any legal advice for me.
• 6:23
  Speaker 1: So this is one of those large physics collaborations. And
• 6:26
  Speaker 1: I guess nano sound small because we're used to associating
• 6:30
  Speaker 1: the word nano with distances, right, like nanometer as being
• 6:33
  Speaker 1: super tiny, or the scale of atoms and things like that.
• 6:36
  Speaker 1: But here it sort of refers to frequency, which actually
• 6:39
  Speaker 1: is sort of like the inverse kind of our intuition
• 6:42
  Speaker 1: a lot of times, right, And so nano here actually
• 6:45
  Speaker 1: means big.
• 6:46
  Speaker 2: Yeah, that's right. You have to think about waves wiggling, right,
• 6:48
  Speaker 2: and waves that wiggle at really high speed, things like
• 6:51
  Speaker 2: gravitational waves which move at the speed of light have
• 6:55
  Speaker 2: a connection between their frequency and their wavelength, just the
• 6:58
  Speaker 2: way light does. High frequency light has very short wavelengths.
• 7:03
  Speaker 2: Very low frequency light like radio waves, has longer wavelengths.
• 7:07
  Speaker 2: And the nanograph experiment is looking for huge gravitational waves,
• 7:11
  Speaker 2: gravitational waves which dwarf the size even of our solar system,
• 7:16
  Speaker 2: and so they have to look for very very low frequency,
• 7:19
  Speaker 2: very tiny frequencies nano hurts, which means like a frequency
• 7:22
  Speaker 2: of one times ten of the minus nine.
• 7:25
  Speaker 1: Yeah, because I guess when you hear the word like
• 7:27
  Speaker 1: giger hurts or megaherts, it's a super high frequency that
• 7:30
  Speaker 1: happens really fast and very short wavelengths. But if you
• 7:33
  Speaker 1: hear nanoherds, that means that it's like a super low frequency,
• 7:38
  Speaker 1: like it takes years and years for a wave to
• 7:42
  Speaker 1: go by.
• 7:42
  Speaker 2: Maybe that's right. One nanohurtz means one wiggle every thirty years. Whoa.
• 7:48
  Speaker 1: And so they recently announced a big result after a
• 7:51
  Speaker 1: long time that they've been at this using their technology
• 7:55
  Speaker 1: to detect gravitation waves, and this week they made the announcement, right,
• 7:57
  Speaker 1: which made the news.
• 7:59
  Speaker 2: That's right, And they've been hinting at this for several
• 8:01
  Speaker 2: weeks that they have something very big to share, and
• 8:04
  Speaker 2: people have known that they were going to have enough
• 8:06
  Speaker 2: data to say something interesting for a little while. We
• 8:09
  Speaker 2: covered them on the podcast a few years ago when
• 8:11
  Speaker 2: they had very preliminary data. They didn't have yet conclusive results,
• 8:14
  Speaker 2: but we were very excited to hear what they were
• 8:17
  Speaker 2: going to have to say in a few years. And
• 8:18
  Speaker 2: that day is today.
• 8:20
  Speaker 1: So let's dig into what their result was and how
• 8:23
  Speaker 1: this experiment worked and how they use the whole galaxy
• 8:25
  Speaker 1: basically as a detector.
• 8:27
  Speaker 2: So what they're looking for are gravitational waves, which you
• 8:31
  Speaker 2: have to remember are not a wave like a wave
• 8:33
  Speaker 2: in water, that they have a lot of similar mathematical properties.
• 8:37
  Speaker 2: Gravitational waves are waves in space itself. General relativity tells
• 8:42
  Speaker 2: us that gravity is the curvature of space, which really
• 8:45
  Speaker 2: just means you're changing their relative distances between points in space,
• 8:50
  Speaker 2: and so gravitational waves are like ripples through space where
• 8:54
  Speaker 2: things get closer together and further apart, closer together and
• 8:56
  Speaker 2: further apart, like space itself is oscillating.
• 9:00
  Speaker 1: Yeah, because we know space isn't just like the emptiness
• 9:02
  Speaker 1: of the universe. It's actually sort of like a thing, right,
• 9:05
  Speaker 1: like you can bend it and squeeze it and curve it. Right.
• 9:08
  Speaker 2: Yeah, that's exactly right. And those features of space are
• 9:11
  Speaker 2: what give rise to our sense of gravity. If space
• 9:14
  Speaker 2: was totally flat and smooth, there had no curvature to it,
• 9:17
  Speaker 2: then things would just moved through it in straight lines
• 9:19
  Speaker 2: that look straight to us. But because space has this
• 9:22
  Speaker 2: sort of invisible curvature to it, things in free fall
• 9:25
  Speaker 2: tend to follow the curvature of space, following those curves
• 9:29
  Speaker 2: and bends and wiggles, which to us look like something
• 9:32
  Speaker 2: is pushing on it to make it change its direction.
• 9:35
  Speaker 1: That's kind of the Einstein view of the universe, right,
• 9:38
  Speaker 1: that gravity is not a force that pulls on you,
• 9:40
  Speaker 1: but it actually kind of bends space time around you
• 9:43
  Speaker 1: to make you move in certain ways.
• 9:45
  Speaker 2: That's right. That's general relativity in fifteen.
• 9:47
  Speaker 1: Seconds done in nanotime, nanorelativity nanoscience podcast. But yeah, it's
• 9:55
  Speaker 1: kind of this idea that space kind of bends, right,
• 9:57
  Speaker 1: and it can wiggle, especially when you move masses through it.
• 10:01
  Speaker 2: Exactly, And we know that Einstein improved on Newton's idea
• 10:05
  Speaker 2: of gravity. Newton had the idea that gravity was a force,
• 10:08
  Speaker 2: and Einstein replaces it with this concept of space being bent.
• 10:12
  Speaker 2: But there's another important consequence of Einstein's update to Newton's
• 10:16
  Speaker 2: idea which gives us these waves, which is that gravitational
• 10:20
  Speaker 2: information is not instant. According to Newton, if you had
• 10:23
  Speaker 2: deleted the Sun from the universe, you would instantly feel
• 10:26
  Speaker 2: across space and time the absence of the Sun's gravity.
• 10:30
  Speaker 2: But Einstein tells us that if you delete the Sun,
• 10:33
  Speaker 2: that information takes time to propagate. Space doesn't like snap
• 10:37
  Speaker 2: back instantaneously. Everywhere in the universe, there's propagation of information,
• 10:42
  Speaker 2: and you can think of gravitational waves sort of as
• 10:44
  Speaker 2: the propagation of information, the same way that if you
• 10:47
  Speaker 2: wiggle an electron, you make wiggles in the electromagnetic field,
• 10:50
  Speaker 2: which we can think of as photons. If you wiggle
• 10:54
  Speaker 2: the Sun, or if you move any massive body, you
• 10:56
  Speaker 2: make wiggles in space, which we can think of as
• 10:59
  Speaker 2: gravitravitational waves.
• 11:01
  Speaker 1: Yeah, it's interesting you call it the propagation of information.
• 11:05
  Speaker 1: It's sort of I guess like if you stand in
• 11:07
  Speaker 1: the middle of the field and you scream, it's going
• 11:09
  Speaker 1: to take a while for that scream to get to places.
• 11:13
  Speaker 2: Man, what a dark example. Why are you standing in
• 11:15
  Speaker 2: the middle of fields and screaming. Is this like some
• 11:17
  Speaker 2: new performance art.
• 11:18
  Speaker 1: Yeah, I'm screaming with joy. Yeah at the fact that
• 11:23
  Speaker 1: nanograph has discovered that's something interesting this week. Yeah.
• 11:27
  Speaker 2: Maybe what we've actually discovered is aliens all across the
• 11:29
  Speaker 2: universe screaming in their fields. Yes, but you're exactly right.
• 11:33
  Speaker 2: Information always takes time to propagate, and that includes the
• 11:37
  Speaker 2: curvature of space time. So anything that wiggles, anything that's
• 11:40
  Speaker 2: accelerating in the universe is making gravitational waves. Now, gravity
• 11:44
  Speaker 2: is a really weak force, so when you accelerate your
• 11:47
  Speaker 2: arm up and down, you are technically making gravitational waves,
• 11:50
  Speaker 2: but they're super duper weak. So in order to see
• 11:53
  Speaker 2: gravitational waves, we tend to need really really high masses
• 11:57
  Speaker 2: undergoing huge accelerations, which is why the target foreseeing gravitational
• 12:02
  Speaker 2: waves are typically things like black holes swinging around each
• 12:06
  Speaker 2: other super duper fast the moments before they merge.
• 12:10
  Speaker 1: It's interesting, just going back you describe it as how
• 12:14
  Speaker 1: information about it propagates. I guess it's sort of like
• 12:16
  Speaker 1: if the sun suddenly moved a meter to the right,
• 12:19
  Speaker 1: it would take some time for us, and the gravity
• 12:22
  Speaker 1: we feel from the Sun to feel that shift in
• 12:25
  Speaker 1: the sun, right, and then if the sun went back
• 12:27
  Speaker 1: to its original position, it would take a little bit
• 12:29
  Speaker 1: of time for us to feel that it went back
• 12:31
  Speaker 1: to its original position. And so that kind of going
• 12:33
  Speaker 1: back and forth is kind of what you can call
• 12:36
  Speaker 1: gravitational wave.
• 12:36
  Speaker 2: Right.
• 12:37
  Speaker 1: It's like the effect of the Sun through gravity. How
• 12:40
  Speaker 1: the effect of the Sun propagates mm hmm.
• 12:42
  Speaker 2: If you're holding a string and your friend a mile
• 12:44
  Speaker 2: away is holding a string and they wiggle their end
• 12:46
  Speaker 2: of the string, You're not going to feel your end
• 12:48
  Speaker 2: wiggle instantly. It's going to take a while for that
• 12:50
  Speaker 2: wiggle to travel down the string. If you wiggle the Sun,
• 12:53
  Speaker 2: it changes the curvature of space and it takes a
• 12:55
  Speaker 2: while for that wiggle to get to Earth. Those are
• 12:57
  Speaker 2: gravitational waves.
• 12:59
  Speaker 1: So I guess you could describe it two ways. It's
• 13:01
  Speaker 1: like it's the Sun bending the space around it, or
• 13:04
  Speaker 1: it's also you can think of it as the effect
• 13:06
  Speaker 1: of the Sun through gravity being kind of changing over time.
• 13:10
  Speaker 2: Yeah, I think those are both accurate. I tend to
• 13:12
  Speaker 2: think of it the first way. That's the Einsteinian way.
• 13:14
  Speaker 2: It's changing the curvature of space time around it gravity
• 13:17
  Speaker 2: is an effect of that curvature.
• 13:19
  Speaker 1: So a gravitational waves sort of made the news maybe
• 13:22
  Speaker 1: like five I think, I want to say five maybe
• 13:25
  Speaker 1: years ago. That's kind of when they entered the popular culture,
• 13:28
  Speaker 1: because that's when we started to be able to listen
• 13:30
  Speaker 1: to them, right through an experiment called Ligo.
• 13:32
  Speaker 2: That's right. We had strong hints that gravitational waves were
• 13:35
  Speaker 2: a real thing decades before that, when we saw neutron
• 13:39
  Speaker 2: stars orbiting each other and they're orbit decayed and exactly
• 13:42
  Speaker 2: the way you'd expect if they were losing energy to
• 13:44
  Speaker 2: gravitational radiation. But the first direct proof was observations by
• 13:49
  Speaker 2: Ligo and Virgo in twenty sixteen of black hole mergers
• 13:53
  Speaker 2: giving off these gravitational radiations using this incredible bravura technique
• 13:58
  Speaker 2: of these lasers underground between mirrors kilometers apart, looking for
• 14:03
  Speaker 2: variations in the distance between these mirrors of one part
• 14:06
  Speaker 2: in ten to the twenty. So that was really a
• 14:09
  Speaker 2: very spectacular confirmation that gravitational waves are a real thing
• 14:13
  Speaker 2: in our universe.
• 14:14
  Speaker 1: Yeah, we know they exist, and they're pretty awesome because
• 14:16
  Speaker 1: they tell us a lot about these huge events going
• 14:19
  Speaker 1: out there in the cosmos. But Apparently, these things only
• 14:22
  Speaker 1: tell us about a very maybe narrow window of things
• 14:25
  Speaker 1: that can happen with gravitational waves out there in the
• 14:27
  Speaker 1: universe exactly.
• 14:28
  Speaker 2: Logo and Virgo are sensitive to gravitational waves of a
• 14:32
  Speaker 2: certain frequency basically because of their size, Lego and Virgo
• 14:36
  Speaker 2: can only really see gravitational waves that are about the
• 14:39
  Speaker 2: size of their detector. The gravitational wave was much much
• 14:43
  Speaker 2: bigger than it wouldn't have an observable effect on their detector.
• 14:47
  Speaker 2: It would like wiggle too slowly. So Logo and Virgo
• 14:50
  Speaker 2: are built to be able to see stellar mass black
• 14:53
  Speaker 2: holes like black holes of ten or twenty or thirty
• 14:57
  Speaker 2: or forty times the mass of our Sun emerging. They've
• 15:00
  Speaker 2: seen dozens and dozens of those. It's very, very exciting.
• 15:03
  Speaker 2: But they can't see things like super massive black hole mergers,
• 15:07
  Speaker 2: which are predicted to happen when galaxies collide. And they
• 15:10
  Speaker 2: also can't see like echoes from the Big Bang itself,
• 15:13
  Speaker 2: which would leave gravitational waves with huge wavelengths interesting.
• 15:17
  Speaker 1: All right, Well, let's dig into these other kinds of
• 15:19
  Speaker 1: gravitational wave events that can happen out there in the
• 15:22
  Speaker 1: universe and how the NANOGrav experiment has apparently maybe seen
• 15:26
  Speaker 1: some of those, So let's dig into that be first,
• 15:28
  Speaker 1: let's take a quick break. All right, we're talking about
• 15:44
  Speaker 1: the recently announced results of the NANOGrav experiment that have
• 15:49
  Speaker 1: to do with gravitational waves, big gravitation waves.
• 15:51
  Speaker 2: That's right, and not to take anything away from Lego
• 15:54
  Speaker 2: and Virgo, it's really impressive that they saw anything at all.
• 15:57
  Speaker 2: I mean, I remember having the opportunity to join those
• 16:00
  Speaker 2: bariments when I was picking graduate schools, and I thought,
• 16:03
  Speaker 2: those guys are never going to make that work. And hey,
• 16:06
  Speaker 2: that's not the first Nobel Prize that I've missed out on.
• 16:09
  Speaker 1: You've had several happened, we all.
• 16:11
  Speaker 2: I mean, there's this thing in research where you have
• 16:13
  Speaker 2: to sort of make a gamble, like is this going
• 16:16
  Speaker 2: to be a fruitful thing to do? And part of
• 16:17
  Speaker 2: it depends on your smarts and your hard work, and
• 16:20
  Speaker 2: part of it just depends on the universe. Is there
• 16:23
  Speaker 2: something there to discover? You build this new kind of
• 16:26
  Speaker 2: telescope and use it to look out into the world.
• 16:28
  Speaker 2: Is there something there for you to see? Lego and Virgo,
• 16:31
  Speaker 2: We're definitely lucky because there are a lot more black
• 16:33
  Speaker 2: holes out there than we thought.
• 16:35
  Speaker 1: Wow, aren't you sort of assuming that if you had
• 16:38
  Speaker 1: joined that collaboration, they still would have discovered gravitational waves?
• 16:42
  Speaker 1: I mean, aren't assuming something there?
• 16:45
  Speaker 2: Yeah, you're right, I might have poisoned the entire process,
• 16:47
  Speaker 2: or maybe it would have happened much faster. Who knows.
• 16:50
  Speaker 1: We'll never know or never.
• 16:53
  Speaker 2: No, you never really get to open those other doors
• 16:55
  Speaker 2: and know what your other lives might have been.
• 16:58
  Speaker 1: You need like a superhero movie to explore the multiverse.
• 17:02
  Speaker 2: But we all do make these choices as sciences to
• 17:04
  Speaker 2: say I'm going to devote my life to this one
• 17:06
  Speaker 2: particular way to maybe learn something about the universe.
• 17:09
  Speaker 1: Cool. Well, we're talking about there recently announced results from
• 17:12
  Speaker 1: the NANOGrav experiment, which is a big collaboration of many
• 17:15
  Speaker 1: different places, right, not just in North America. I mean
• 17:18
  Speaker 1: the name is North American Nanohertz Observatory, but actually they
• 17:23
  Speaker 1: use I think places from all over the world.
• 17:25
  Speaker 2: Well, NANOGrav uses Arecibo and the Green Bank Telescope in
• 17:28
  Speaker 2: West Virginia and the very large Array in New Mexico.
• 17:32
  Speaker 2: But you're right, there are definitely pulsar observatories all over
• 17:34
  Speaker 2: the world. In Australia and in Europe and in China,
• 17:38
  Speaker 2: and there are other competing collaborations. So NANOGrav represents like
• 17:41
  Speaker 2: the North American slice of the world.
• 17:44
  Speaker 1: And so we were talking about earlier how NANOGrav looks
• 17:47
  Speaker 1: for gravitation waves in different wavelengths of gravitation waves than
• 17:52
  Speaker 1: Ligo and virgo.
• 17:53
  Speaker 2: That's right, NANOGrav is looking for something much much bigger
• 17:56
  Speaker 2: than Lego and Virgo is looking for, and something much
• 17:59
  Speaker 2: bigger than like and Virgo can even see. Something important
• 18:02
  Speaker 2: to understand is like when black holes merge, the ones
• 18:05
  Speaker 2: that Lego and Virgo do see, they're emitting gravitational waves
• 18:08
  Speaker 2: during the entire merger, but Lego and Virgo can only
• 18:11
  Speaker 2: see at the very end because it speeds up and
• 18:13
  Speaker 2: gets shorter wavelengths, and that's what Lego and Virgo can see.
• 18:17
  Speaker 2: So they're only seeing like the last little bit of
• 18:19
  Speaker 2: that merger when it's in their little window.
• 18:22
  Speaker 1: Now, is that because of the sort of frequency limitation
• 18:25
  Speaker 1: or is that because when those black holes merge, that's
• 18:29
  Speaker 1: when the gravitation waves get really big, Right when they're
• 18:32
  Speaker 1: closing in on each other and circling each other really fast,
• 18:35
  Speaker 1: that's when there's a lot of acceleration by those masses,
• 18:37
  Speaker 1: and that's when maybe we get waves that we can
• 18:40
  Speaker 1: detect here on Earth.
• 18:41
  Speaker 2: Yeah, it's both factors. The amplitude is too small during
• 18:45
  Speaker 2: the earlier part of the merger, and the frequency is wrong.
• 18:48
  Speaker 2: Loco and Virgo are sensitive across a certain frequency range,
• 18:52
  Speaker 2: and it's limited by a couple of things. One is
• 18:54
  Speaker 2: just the size of their detector. Your few detector is
• 18:57
  Speaker 2: only a few kilometers long. You can't detect changes over
• 18:59
  Speaker 2: light years. There's like no variation across your detector.
• 19:03
  Speaker 1: Wouldn't be easier though, Like if you you know, like
• 19:06
  Speaker 1: a slower wave, wouldn't your detector be able to catch
• 19:08
  Speaker 1: those better if it had the same amplitude.
• 19:11
  Speaker 2: Well, think about it in terms of the distance, Like,
• 19:12
  Speaker 2: it's much harder to measure the curvature of the Earth
• 19:16
  Speaker 2: if the Earth is much much bigger than your ruler. Right,
• 19:18
  Speaker 2: like to us, the Earth looks almost flat. You can't
• 19:21
  Speaker 2: even really detect the curvature, and so detecting small changes
• 19:24
  Speaker 2: in the curvature is really hard. But if you're like
• 19:26
  Speaker 2: the little prints and you're on a planet where like
• 19:28
  Speaker 2: the planet is basically the size of your ruler, it's
• 19:30
  Speaker 2: much easier to measure the curvature and the changes in
• 19:33
  Speaker 2: the curvature. So here and now we're like sitting on
• 19:35
  Speaker 2: a huge wave. If the wave is like the size
• 19:37
  Speaker 2: of the galaxy, there's no way that your little two
• 19:40
  Speaker 2: kilometer ruler is going to be able to measure any
• 19:43
  Speaker 2: change in that.
• 19:44
  Speaker 1: Wave unless the change is really big, right, like the
• 19:47
  Speaker 1: amplitude of those low frequency waves. We would be able
• 19:50
  Speaker 1: to detect them, but maybe we don't get those here.
• 19:53
  Speaker 2: Yeah, if it's big enough, then you can detect it
• 19:55
  Speaker 2: in any way. But there's another factor, which is noise suppression.
• 19:58
  Speaker 2: These things have to be able toinguish real gravitational waves
• 20:02
  Speaker 2: from other kinds of wiggles. And because Ligo and Virgo
• 20:05
  Speaker 2: are on Earth, there's seismic noise, and that seismic noise
• 20:09
  Speaker 2: tends to be lower frequency, and so it sort of
• 20:11
  Speaker 2: creates a wall that Ligo and Virgo just can't see beyond.
• 20:15
  Speaker 1: Okay, So then the nanograph is a different experiment from LGO,
• 20:18
  Speaker 1: and it use is a totally different technology and method. Right.
• 20:22
  Speaker 1: They didn't even sort of have to build really a detector.
• 20:26
  Speaker 1: They just sort of used existing radio antennas that we have, right, Yeah,
• 20:30
  Speaker 1: they both.
• 20:30
  Speaker 2: Use existing sources of radio waves, meaning pulsars scattered through
• 20:34
  Speaker 2: the galaxy and existing telescopes that can see those radio waves.
• 20:39
  Speaker 2: So it's really just like a clever combination of stuff
• 20:42
  Speaker 2: that's already out there. It's really one of my favorite
• 20:44
  Speaker 2: examples of like just ingenuity and physics.
• 20:46
  Speaker 1: Yeah, it's pretty cool. And so the idea is that
• 20:48
  Speaker 1: they're using pulsars to detect kind of how the whole
• 20:54
  Speaker 1: galaxy reacts to a gravitational wave, right.
• 20:56
  Speaker 2: Yeah, that's exactly right. They want to see really big
• 20:59
  Speaker 2: gravity waves on the scale of the galaxy, and so
• 21:02
  Speaker 2: they want to see the galaxy itself sort of change shape.
• 21:06
  Speaker 2: Like what Lego and Virgo do is they have this
• 21:08
  Speaker 2: big l they construct with mirrors, and they see a
• 21:10
  Speaker 2: chain shape. They see one side get shorter, another side
• 21:13
  Speaker 2: get longer, the whole thing oscillates. So NANOGrav and the
• 21:16
  Speaker 2: other pulsar timing arrays want to do the same thing
• 21:19
  Speaker 2: for the whole galaxy, but they can't like go out
• 21:21
  Speaker 2: there and you know, build lasers and mirrors that are
• 21:24
  Speaker 2: on the scale of the galaxy. So they're just watching
• 21:27
  Speaker 2: the pulsars and they're using very precise timing measurements from
• 21:30
  Speaker 2: these pulsars to measure how the galaxy itself is squishing.
• 21:34
  Speaker 1: Well, I mean, come on, they could build space lasers
• 21:37
  Speaker 1: if they wanted to.
• 21:39
  Speaker 2: Who doesn't want to write? I mean, when I go
• 21:41
  Speaker 2: out in a field and scream, I'm screaming, let's build
• 21:44
  Speaker 2: space lasers.
• 21:45
  Speaker 1: Yeah, let's do it.
• 21:49
  Speaker 2: But this is really awesome because it takes advantage of
• 21:51
  Speaker 2: the universe as sort of a natural set of clocks.
• 21:54
  Speaker 2: These pulsars are really cool, fascinating end point of stars
• 21:59
  Speaker 2: and your stars out there that are burning bright and
• 22:01
  Speaker 2: turning their fuel into light, and then eventually they collapse
• 22:04
  Speaker 2: with like a type two supernova and they leave behind
• 22:07
  Speaker 2: this core, this very very dense object, a neutron star,
• 22:12
  Speaker 2: which is something that has like a ten kilometer radius
• 22:15
  Speaker 2: but weighs as much as our sun.
• 22:18
  Speaker 1: Yeah. I think we've had one or a couple of
• 22:21
  Speaker 1: episodes about pulsars and neutron stars. They're not burning like
• 22:24
  Speaker 1: regular stars, but they are giving off a lot of
• 22:27
  Speaker 1: light right because they are still really hot.
• 22:30
  Speaker 2: They are definitely still hot because they're super dense, but
• 22:32
  Speaker 2: there's no fusion going on inside of them, so they
• 22:35
  Speaker 2: don't glow in the typical wavelengths. Sometimes you can see
• 22:38
  Speaker 2: X rays from cracks on their surface. But what we're
• 22:40
  Speaker 2: interested in this case is the beam that they emit
• 22:43
  Speaker 2: along their magnetic north and south poles. They generate some
• 22:46
  Speaker 2: radiation from like motion of charged particles on the surface
• 22:49
  Speaker 2: on the crust of the neutron star, and they have
• 22:52
  Speaker 2: this very strong magnetic field which slurfs those charged particles
• 22:56
  Speaker 2: in that radiation sort of up the north pole and
• 22:59
  Speaker 2: down the south pole creates these massive beams of radiation
• 23:03
  Speaker 2: along the north and south pole of the pulsar.
• 23:06
  Speaker 1: Now this happens with every neutron star, or only some
• 23:09
  Speaker 1: neutron stars have this beam of radiation going out of
• 23:13
  Speaker 1: its poles.
• 23:14
  Speaker 2: Not every neutron star is a pulsar. And we do
• 23:17
  Speaker 2: not understand very well both the source of the radiation.
• 23:20
  Speaker 2: You think maybe it comes from a combination the rotation
• 23:22
  Speaker 2: of this object and the charged particles on the surface,
• 23:25
  Speaker 2: And we don't really understand very well the source of
• 23:28
  Speaker 2: incredibly strong magnetic fields from neutron stars. But not every
• 23:32
  Speaker 2: neutron star spins this way and has these magnetic fields
• 23:36
  Speaker 2: and is a pulsar.
• 23:37
  Speaker 1: Or can we see it right? Because I think the
• 23:39
  Speaker 1: idea is that you have this neutron star that's a
• 23:42
  Speaker 1: magnetic field. It's shooting out basically like radiation beams out
• 23:46
  Speaker 1: of its poles, and it's also spinning. The whole thing
• 23:49
  Speaker 1: is spinning sort of like a lighthouse, right.
• 23:51
  Speaker 2: And the magnetic north pole is not aligned with the spin,
• 23:55
  Speaker 2: so the magnetic north pole and the beam sweeps across
• 23:58
  Speaker 2: the universe.
• 23:59
  Speaker 1: Yeah. I guess it's sort of like when you spin
• 24:01
  Speaker 1: a top on the floor and it starts to slow down,
• 24:04
  Speaker 1: it starts to kind of spin in a kind of
• 24:06
  Speaker 1: wiggly fashion, right, And so if it has like a
• 24:09
  Speaker 1: flashlight at the top of the top, then that flashlight
• 24:12
  Speaker 1: is going to kind of sweep around, sort of like
• 24:14
  Speaker 1: a lighthouse.
• 24:15
  Speaker 2: Yeah. Well, these guys aren't wiggling, like they're spinning very
• 24:17
  Speaker 2: very regularly. But the flashlight is sort of offset from
• 24:20
  Speaker 2: the axis of spin, so it's sort of like you're
• 24:22
  Speaker 2: spinning around. If you held the flashlight straight up, it
• 24:25
  Speaker 2: would always point in the same direction. But if you
• 24:27
  Speaker 2: held the flashlight at an angle from the axis where
• 24:30
  Speaker 2: you're spinning, then it's going to sweep around the room
• 24:32
  Speaker 2: and light up different corners. So that's what's happening with
• 24:35
  Speaker 2: a pulsar is the angle of radiation is different from
• 24:38
  Speaker 2: the angle of the spin of the star.
• 24:41
  Speaker 1: But I guess what would make the magnetic axis be
• 24:43
  Speaker 1: different than the spin axis.
• 24:45
  Speaker 2: Yeah, I wish I knew. It's the same on Earth though, right.
• 24:47
  Speaker 2: The Earth's magnetic north pole is not the same as
• 24:49
  Speaker 2: our spin axis north pole.
• 24:51
  Speaker 1: Right, And that's because the stuff inside of Earth is
• 24:53
  Speaker 1: spinning in a kind of weird way, right.
• 24:55
  Speaker 2: Yeah, And again not something that we understand very well.
• 24:58
  Speaker 2: In the Earth's magnetic field even fl every once in
• 25:01
  Speaker 2: a while, as does the suns, So that's a whole
• 25:04
  Speaker 2: murky era of research we don't understand very well. But
• 25:07
  Speaker 2: something that is amazing, and it makes the physics that
• 25:09
  Speaker 2: nanograb is doing possible, is that the whole thing is
• 25:12
  Speaker 2: super duper regular. It doesn't wiggle a lot. When this
• 25:15
  Speaker 2: thing spins, it spins at a very specific rate. And
• 25:18
  Speaker 2: when a pulse reaches Earth, it reaches Earth very very regularly.
• 25:21
  Speaker 2: Like the time between the pulses is extraordinarily predictable.
• 25:26
  Speaker 1: I guess what makes it so predictable. I guess just
• 25:29
  Speaker 1: because it's an object in space spinning, and so therefore
• 25:33
  Speaker 1: it's pretty regular.
• 25:34
  Speaker 2: It's very dense, it's very high energy. I guess it's
• 25:36
  Speaker 2: just not interfered with a lot. You know, in principle,
• 25:39
  Speaker 2: everything is predictable, but some things are more complicated because
• 25:41
  Speaker 2: they are chaotic. They're like multi object systems. But here
• 25:45
  Speaker 2: you have an intense source of radiation and an object
• 25:48
  Speaker 2: that's sort of isolated. It's the left over core of
• 25:50
  Speaker 2: this star.
• 25:51
  Speaker 1: Okay, so that's a pulsar, which is a kind of
• 25:53
  Speaker 1: neutron star that has its neetic axis, ask you from
• 25:57
  Speaker 1: its spinning axes, and so therefore we can sort of
• 26:00
  Speaker 1: we sort of get hit by its radiation beams sometimes
• 26:04
  Speaker 1: or regularly like a clock. These are kind of spread
• 26:06
  Speaker 1: all over the galaxy.
• 26:07
  Speaker 2: Right, Yeah, that's exactly right, and they vary in their frequency.
• 26:10
  Speaker 2: Some of them spin super duper fast. These are called
• 26:13
  Speaker 2: like millisecond pulsars, which means that it's spinning so fast
• 26:17
  Speaker 2: that the time between pulses is in the order of milliseconds,
• 26:20
  Speaker 2: which means the whole neutron star spins around thousands of
• 26:23
  Speaker 2: times per second. It's wild, it's really incredible, and by
• 26:26
  Speaker 2: looking at the pattern of the timing, we can extract
• 26:29
  Speaker 2: a lot of information about what's going on near that
• 26:33
  Speaker 2: neutron star.
• 26:34
  Speaker 1: So you're saying the frequency sort of depends on the
• 26:36
  Speaker 1: kind of the physics of that star, what's going on
• 26:39
  Speaker 1: inside of it exactly.
• 26:40
  Speaker 2: And you can watch one of these things for a
• 26:42
  Speaker 2: long time and learn what its frequency is, and then
• 26:45
  Speaker 2: if that frequency changes, if you notice like, oh wait,
• 26:47
  Speaker 2: there was a longer time between these last two blips,
• 26:51
  Speaker 2: that tells you something about what's going on between you
• 26:54
  Speaker 2: and that neutron star. If, for example, a huge gravitational
• 26:58
  Speaker 2: wave wiggle through the gallery, see, it would make some
• 27:01
  Speaker 2: of these neutron stars further away from us and other
• 27:03
  Speaker 2: neutron stars closer, and so it would vary the timing
• 27:07
  Speaker 2: pattern of those neutron star pulse.
• 27:10
  Speaker 1: Arrivals because it's making the star further and pushing it
• 27:15
  Speaker 1: away from us and then towards us. Or because it's
• 27:18
  Speaker 1: sort of affecting like the path that the light has
• 27:22
  Speaker 1: to travel as it gets here.
• 27:24
  Speaker 2: What's the difference between those two.
• 27:26
  Speaker 1: Like you could maybe have a gravitation wave happen in
• 27:28
  Speaker 1: the middle and it might not move necessarily the pulsar,
• 27:33
  Speaker 1: but it might affect the photons that are on the way.
• 27:35
  Speaker 2: Well, this is really similar to the way we think
• 27:37
  Speaker 2: about the expansion of space. You know, how the universe
• 27:40
  Speaker 2: itself is expanding. Gravitational waves have that same effect. They
• 27:43
  Speaker 2: expand or contract space and in the same way. You
• 27:46
  Speaker 2: can think about in two different ways, like the literal
• 27:49
  Speaker 2: distance between us and the object is increasing, or that
• 27:53
  Speaker 2: it's expanding the photons as they're moving between here and there. Fundamentally,
• 27:58
  Speaker 2: those are the same picture of mathemat to me, the
• 28:00
  Speaker 2: most intuitive way to think about it is that it's
• 28:02
  Speaker 2: literally increasing the distance between us and the pulsar, and
• 28:05
  Speaker 2: so it takes longer for those photons to arrive.
• 28:08
  Speaker 1: That's a pretty cool idea. I guess you're sort of
• 28:10
  Speaker 1: listening to the blips coming from this pulsar, like it's
• 28:13
  Speaker 1: going beep beep, beep, beep, beep, beep beep, and if
• 28:16
  Speaker 1: the frequency changes, like it only gets really fast and
• 28:19
  Speaker 1: then really slow, then you know that something happened maybe
• 28:23
  Speaker 1: to the distance between here and that pulsar, and that
• 28:26
  Speaker 1: change in the distance could be a rotation wave.
• 28:28
  Speaker 2: Exactly, And for an individual pulsar, lots of things could
• 28:31
  Speaker 2: do that, Like we actually have discovered planets orbiting pulsars
• 28:35
  Speaker 2: by how that planet has tugged on the pulsar and
• 28:38
  Speaker 2: changed that series of blips. But what we're looking for
• 28:40
  Speaker 2: here are correlations among many, many pulsars. So they're looking
• 28:44
  Speaker 2: through the sky for lots and lots of these examples,
• 28:46
  Speaker 2: and they want to see an overall effect where a
• 28:48
  Speaker 2: bunch of pulsars are squeezed towards us and a bunch
• 28:51
  Speaker 2: are squeezed away from us. And there's very specific predictions
• 28:55
  Speaker 2: made by two physicists Hellings and Downs, that predicts a
• 28:58
  Speaker 2: very particular kind of sh shift in the pattern of
• 29:01
  Speaker 2: pulsars that would come from gravitational waves, and that's what
• 29:04
  Speaker 2: NANOGrav and the other arrays are looking for.
• 29:07
  Speaker 1: MM So, I think you're saying, like, if you look
• 29:09
  Speaker 1: out to a whole bunch of pulsars out there in
• 29:11
  Speaker 1: the night sky and you see sort of a ripple
• 29:14
  Speaker 1: go through all of these different frequencies of light that
• 29:17
  Speaker 1: you're getting from these pulsars, then you know that maybe
• 29:19
  Speaker 1: a gravitational wave kind of spread out there in space exactly.
• 29:23
  Speaker 2: And these could be gravitational waves from like the mergers
• 29:26
  Speaker 2: of supermassive black holes, things that could have been washing
• 29:30
  Speaker 2: over us basically our entire existence. But we could not
• 29:33
  Speaker 2: detect that even ligo and virgo could not see. So
• 29:36
  Speaker 2: this is like opening a new kind of eyeball to
• 29:39
  Speaker 2: a new kind of frequency of gravitational wave nobody's seen before,
• 29:42
  Speaker 2: from a different kind of source that nobody's heard from before.
• 29:46
  Speaker 1: That's pretty cool. And so I guess how many pulsars
• 29:49
  Speaker 1: did NANOGrav look at for this latest set of results.
• 29:52
  Speaker 2: So nanograph has been looking at sixty eight pulsars and
• 29:55
  Speaker 2: studying them for fifteen years. So that's a good amount
• 29:59
  Speaker 2: of data. As you say, they're using existing facilities, but
• 30:02
  Speaker 2: they still have to like occupy time on those facilities.
• 30:05
  Speaker 2: They're not general purpose telescopes that listen to the whole sky.
• 30:08
  Speaker 2: Got to like point the Green Bank telescope at the
• 30:11
  Speaker 2: right part of the sky to listen to a pulsar.
• 30:13
  Speaker 2: So it has taken a significant amount of our sort
• 30:15
  Speaker 2: of astronomical resources that we could have spent listening to
• 30:19
  Speaker 2: other stuff.
• 30:20
  Speaker 1: Oh you mean, like we don't listen to all sixty
• 30:22
  Speaker 1: eight pulsars at the same time. We have to kind
• 30:24
  Speaker 1: of go through them one by one mm hmm, yeah, exactly.
• 30:27
  Speaker 1: And so you're pointing all your telescopes to the first pulsar,
• 30:30
  Speaker 1: measuring its frequency, going to the next one, going to
• 30:33
  Speaker 1: the next one, sixty eight of those, and then you,
• 30:35
  Speaker 1: I guess you cycle back around and you start with
• 30:38
  Speaker 1: the first one again, and you do that for a
• 30:39
  Speaker 1: long time.
• 30:40
  Speaker 2: Yeah, you need lots of hours, and they observe each
• 30:42
  Speaker 2: one monthly, so they know roughly the pattern of these
• 30:45
  Speaker 2: things and they sort of cycle through them. And these
• 30:48
  Speaker 2: pulsars are very faint, so you can't always hear them
• 30:50
  Speaker 2: very well, So you need lots of hours to sort
• 30:52
  Speaker 2: of like light these up in NANOGrav Even though it's
• 30:55
  Speaker 2: trying to use the whole galaxy as an observatory, it's
• 30:57
  Speaker 2: only still really sensitive to the ones within a few
• 31:00
  Speaker 2: thousand light years of Earth because the limitations just of
• 31:04
  Speaker 2: like hearing these things, they're very faint sources.
• 31:08
  Speaker 1: And I think you have to do it for a
• 31:10
  Speaker 1: long time, like you said, because these waves are so slow, right,
• 31:14
  Speaker 1: like you said, they have a period of maybe thirty years, fifteen.
• 31:18
  Speaker 2: Years, thirty years, yeah, exactly, And so these are very
• 31:21
  Speaker 2: slow moving things, and so to see slow moving things,
• 31:24
  Speaker 2: you need data over a large period of time. It's
• 31:27
  Speaker 2: hard to measure slow effects over a very short time period.
• 31:30
  Speaker 2: The longer your lever arm in time, the better you're
• 31:33
  Speaker 2: able to see effects that are very slow.
• 31:36
  Speaker 1: And so specifically things that would give gravitational ways that
• 31:41
  Speaker 1: are so big and so slow thirty year period. Those
• 31:43
  Speaker 1: are very special events in the universe, right like, they
• 31:46
  Speaker 1: don't just happen all the time, or maybe they do.
• 31:48
  Speaker 1: I guess that's part of the question.
• 31:50
  Speaker 2: They don't just happen all the time, but they are
• 31:52
  Speaker 2: very slow events. So when you have like two galaxies
• 31:55
  Speaker 2: merging and then they each have like a super massive
• 31:58
  Speaker 2: black hole that's like millions to up to ten billion
• 32:01
  Speaker 2: solar masses, they don't merge instantaneously. They dance around each
• 32:05
  Speaker 2: other for like you know, sometimes twenty five million years
• 32:08
  Speaker 2: before they actually coalesce, which means these things are generating
• 32:12
  Speaker 2: gravitational waves for twenty five million years. And so while
• 32:15
  Speaker 2: galaxy mergers aren't happening all the time nearby, their radiation
• 32:20
  Speaker 2: does last for a long time.
• 32:21
  Speaker 1: And so just like LAGOA, I guess we're looking for
• 32:24
  Speaker 1: the moment where these black holes are right before they
• 32:28
  Speaker 1: smush together, right like, because that's when they spin around
• 32:31
  Speaker 1: each other super duper fast and cause big waves and
• 32:33
  Speaker 1: the gravitational fabric of space and time.
• 32:36
  Speaker 2: Yeah, but NANOGrav is sentenced to them well before they
• 32:38
  Speaker 2: actually hit each other, so we can listen to almost
• 32:41
  Speaker 2: any part of that and the current results from NANOGrav
• 32:43
  Speaker 2: they do see these galaxy size gravitational waves, but they
• 32:47
  Speaker 2: can't pinpoint individual collisions. It's sort of an incoherent superposition
• 32:51
  Speaker 2: of mergers all over the nearby part of the universe.
• 32:56
  Speaker 1: I imagine it's hard because these polsers are different distances
• 32:59
  Speaker 1: from us, right, So, like some of them are maybe
• 33:01
  Speaker 1: one hundred thousand or I don't know, fifty thousand light
• 33:05
  Speaker 1: years from us, and so like the data you get
• 33:07
  Speaker 1: from them, they come from kind of different times in
• 33:09
  Speaker 1: the universe, don't they.
• 33:10
  Speaker 2: Yeah, that's right. All the pulsars that we're looking at
• 33:13
  Speaker 2: are a few thousand light years from Earth. But they
• 33:15
  Speaker 2: definitely take this into account. But again, the events we're
• 33:17
  Speaker 2: looking at are very very slow.
• 33:19
  Speaker 1: Moving, meaning like maybe there's our two super massive black
• 33:23
  Speaker 1: holes merging somewhere in the universe, but they've been doing
• 33:25
  Speaker 1: it for you know, thousands and thousands of years, and
• 33:28
  Speaker 1: so if you detect all of your pulsars kind of
• 33:31
  Speaker 1: wiggling at the same frequency, maybe they come from the
• 33:34
  Speaker 1: same event exactly.
• 33:36
  Speaker 2: But again, nanograph can't pinpoint individual events, not yet. At
• 33:40
  Speaker 2: least what they see so far is totally consistent with
• 33:44
  Speaker 2: a lot of gravitational waves adding up from lots of
• 33:47
  Speaker 2: supermassive black holes merging.
• 33:49
  Speaker 1: Cool. Well, Daniel, you got to interview one of the
• 33:51
  Speaker 1: scientists that works on this nanograph collaboration, Professor Kiaramingarelli, and
• 33:56
  Speaker 1: so you have an interview with Earth.
• 33:58
  Speaker 2: That's right. I had a lot of fun chatting with Cool.
• 34:00
  Speaker 1: So when we come back, Daniel, we'll interview Professor Kara Mingarelli,
• 34:03
  Speaker 1: who's part of the nanograph collaboration, and she'll talk about
• 34:06
  Speaker 1: some of the results and what that means for her
• 34:09
  Speaker 1: and for the scientists that worked on it, and what
• 34:11
  Speaker 1: that means for our knowledge of the universe. But first,
• 34:13
  Speaker 1: let's take another quick break. All right, we're talking about
• 34:28
  Speaker 1: the nanograph results unveiled this week, which are big news
• 34:32
  Speaker 1: because I guess it, let's us see another part of
• 34:35
  Speaker 1: the universe we couldn't see before, right.
• 34:37
  Speaker 2: That's right. It let us listen to things going on
• 34:39
  Speaker 2: out there that we couldn't hear before, specifically the mergers
• 34:43
  Speaker 2: of super massive black holes, which are already things we
• 34:46
  Speaker 2: don't understand very well.
• 34:48
  Speaker 1: Yeah, I mean I think they're tied even to like
• 34:50
  Speaker 1: the mystery of how galaxies form, right, because we sort
• 34:53
  Speaker 1: of are not sure about that, right.
• 34:55
  Speaker 2: We definitely don't understand how galaxies form and how they
• 34:58
  Speaker 2: get these super massive black holes and why there are
• 35:01
  Speaker 2: so many of them. The black holes at the hearts
• 35:03
  Speaker 2: of galaxies are bigger than anybody expects, and they're everywhere,
• 35:06
  Speaker 2: and they formed earlier in the universe. Then we understand.
• 35:10
  Speaker 1: Cool. And so you talked to one of those scientists
• 35:12
  Speaker 1: that worked on this. How excited was she?
• 35:13
  Speaker 2: She was very excited. She had met her entire career
• 35:16
  Speaker 2: since she was a graduate student on this project, and
• 35:19
  Speaker 2: now she's getting the payoff.
• 35:21
  Speaker 1: Awesome. Well, here is Daniel's interview with Professor Kiera Mingarelli
• 35:26
  Speaker 1: from You and the NANOGrav experiment.
• 35:36
  Speaker 2: All right, so then it's my great pleasure to welcome
• 35:39
  Speaker 2: to our program professor Kiera Mongarelli, professor at Yale who
• 35:43
  Speaker 2: works on NANOGrav. Kierra, thank you very much for joining
• 35:47
  Speaker 2: us today.
• 35:47
  Speaker 3: Thank you so much. I'm so happy to be here.
• 35:49
  Speaker 2: So tell me first, what is the basic idea of NANOGrav?
• 35:54
  Speaker 2: How does keeping track of pulsars help you spot gravitational waves.
• 35:58
  Speaker 3: So NANOGrav uses the galactic population of millisecond pulsars to
• 36:04
  Speaker 3: look for gravitational waves. We can do this because millisecond
• 36:08
  Speaker 3: pulsars are nature's best clocks. So they spin around about
• 36:13
  Speaker 3: one hundred times a second. Their masses are maybe one
• 36:16
  Speaker 3: and a half times the mass of the Sun, and
• 36:19
  Speaker 3: they would fit inside Manhattan. So if you can imagine
• 36:22
  Speaker 3: taking the Sun, shrinking it to the size of Manhattan
• 36:25
  Speaker 3: and putting it in a blunder, that is a millisecond pulsar.
• 36:30
  Speaker 3: And there are ticks when they arrive at the Earth.
• 36:32
  Speaker 3: So every time they spin around, they send a flash
• 36:35
  Speaker 3: of radio waves to the Earth. And those flashes are
• 36:39
  Speaker 3: so precise that we can time them to hundreds of
• 36:42
  Speaker 3: nanoseconds over a decade, so they are very very stable clocks.
• 36:47
  Speaker 3: That point is really important because gravitational waves change the
• 36:51
  Speaker 3: distances between objects, and so as a gravitational wave transits
• 36:57
  Speaker 3: the galaxy, it will change the distance between the Earth
• 37:00
  Speaker 3: and the pulsar. And so now those super stable radio
• 37:04
  Speaker 3: flashes arrive early, and then they arrive late as the
• 37:08
  Speaker 3: gravitational wave transits through the galaxy, so a little bit
• 37:11
  Speaker 3: early and then a little bit late, and then a
• 37:13
  Speaker 3: little bit early again. And so what we look for
• 37:17
  Speaker 3: are those changes and the arrival times of the pulses
• 37:20
  Speaker 3: from these ultra stable pulsars. That's how we can turn
• 37:23
  Speaker 3: the whole galaxy into a gravitational wave detector.
• 37:26
  Speaker 2: Awesome, Well, it sounds great in principle, but I'm sure
• 37:29
  Speaker 2: in practice there are a lot of things to get right,
• 37:32
  Speaker 2: what kind of like technical and data analysis challenges that
• 37:35
  Speaker 2: you have to overcome to make this crazy idea work.
• 37:37
  Speaker 3: So it's taken a lot of people a lot of
• 37:40
  Speaker 3: time to overcome all those challenges. In fact, we have
• 37:43
  Speaker 3: some of the best people in the world working on
• 37:45
  Speaker 3: the data analysis and the noise models, because you're right,
• 37:50
  Speaker 3: there's a lot of noise involved here. We can't walk
• 37:53
  Speaker 3: over to our pulsars and turn them on and off
• 37:55
  Speaker 3: again because we think something funny happened. Right. One of
• 37:58
  Speaker 3: the big challenges that we have is the interstellar medium,
• 38:01
  Speaker 3: so the gas and dust between us and the pulsars.
• 38:05
  Speaker 3: There are some maps of that that they're all very approximate,
• 38:10
  Speaker 3: and this affects our signals because the radio wation the
• 38:14
  Speaker 3: pulses are dispersed by the interstellar medium, and that happens
• 38:18
  Speaker 3: at different frequencies radio frequencies, and so we have to
• 38:21
  Speaker 3: take this signal that's then been spread out by the
• 38:24
  Speaker 3: interstellar medium and d disperse it to make it a
• 38:29
  Speaker 3: very clean signal again, and so that can add some
• 38:33
  Speaker 3: noise to what we're looking for. And unfortunately that noise
• 38:38
  Speaker 3: looks very similar to the gravitational wave signal that we're
• 38:42
  Speaker 3: looking for. So we have to be very very careful
• 38:46
  Speaker 3: when we're making these noise models for the pulsars that
• 38:50
  Speaker 3: we're not going one way or the other. That is
• 38:52
  Speaker 3: to say that we're not taking a gravitational wave signal
• 38:56
  Speaker 3: and thinking that it's just noise in our pulsar because
• 38:59
  Speaker 3: they look so sim and therefore taking the signal away
• 39:03
  Speaker 3: from any possible detection. And at the same time, we
• 39:06
  Speaker 3: need to make sure that our signal models are good
• 39:08
  Speaker 3: enough that we don't mistake some noise for a possible signal.
• 39:14
  Speaker 3: And so this right now is really really important. Getting
• 39:18
  Speaker 3: those individual noise models correct for the pulsars is really
• 39:23
  Speaker 3: really important.
• 39:24
  Speaker 2: So how do you distinguish between them? How do you
• 39:26
  Speaker 2: know you're getting the noise right?
• 39:28
  Speaker 3: So we can distinguish between the signal and the noise
• 39:32
  Speaker 3: in a few ways. Number One, the gravitational wave background
• 39:37
  Speaker 3: has some characteristic amplitude, and that's a function of the
• 39:41
  Speaker 3: astrophysics of the source that's creating the signal. So if,
• 39:45
  Speaker 3: for example, it's super massive black holes, and we'll probably
• 39:49
  Speaker 3: get to that in a second, but if, for example,
• 39:51
  Speaker 3: it's that that's creating the signal, then the amplitude of
• 39:55
  Speaker 3: the signal is based on the astrophysics that governs their mergers.
• 39:58
  Speaker 3: So of are the black holes, what gravitational wave frequency
• 40:03
  Speaker 3: are they emitting at, how far away are they How
• 40:06
  Speaker 3: many super massive black holes are there per unit rendshift?
• 40:10
  Speaker 3: So what's the density of black holes in the universe.
• 40:14
  Speaker 3: And we know that that amplitude has a predicted and
• 40:19
  Speaker 3: characteristic way of varying as a function of frequency. So
• 40:23
  Speaker 3: we have lots of different frequency detector bins in our experiment,
• 40:29
  Speaker 3: and we know exactly how much signal should be in
• 40:31
  Speaker 3: each one of those bins from our simulations, and we
• 40:33
  Speaker 3: can measure what's there and the way that the signal
• 40:37
  Speaker 3: is distributed in those bins can be characteristic of, for example,
• 40:42
  Speaker 3: super massive black hole binaries generating the background. So that's
• 40:45
  Speaker 3: one part of the puzzle. The second part is something
• 40:50
  Speaker 3: called the Hellings and Downs curve. And so, back when
• 40:53
  Speaker 3: people were thinking in the eighties about how do we
• 40:57
  Speaker 3: actually go about detecting a gravitational wave back, the zero
• 41:01
  Speaker 3: thworter thing to do, and I think a lot of
• 41:03
  Speaker 3: us that have done undergraduate degrees in STEM fields have done,
• 41:06
  Speaker 3: is a cross correlation analysis, where you say, all right,
• 41:09
  Speaker 3: there's one signal in my data, So I'm going to
• 41:12
  Speaker 3: take all of these different pieces of data and like
• 41:15
  Speaker 3: bash them together and see what signal is present in
• 41:18
  Speaker 3: all of my different pieces of data. And so that's
• 41:22
  Speaker 3: what we do with pulsar timing. We take all of
• 41:24
  Speaker 3: these different timestamps from pulsars and cross correlate them, and
• 41:27
  Speaker 3: we look for the gravitational wave background signal. What's really
• 41:30
  Speaker 3: interesting is that not only do you get this amplitude
• 41:33
  Speaker 3: of the gravitational wave background that's correlated, but there's an
• 41:37
  Speaker 3: additional geometric piece predicted by general relativity, and that additional
• 41:42
  Speaker 3: cross correlation term varies as a function of angle between
• 41:46
  Speaker 3: the pulsars, and so if pulsars are close together, they
• 41:50
  Speaker 3: have a stronger correlation, a stronger response to the gravitational
• 41:54
  Speaker 3: way background. If they're a little bit further away, it
• 41:56
  Speaker 3: gets weaker. But then interestingly, because gravitational have this quadrupolar shape,
• 42:02
  Speaker 3: which is like a cosine sort of shape, the signal
• 42:06
  Speaker 3: increases again right as they get further apart. And so
• 42:11
  Speaker 3: to create a signal that looks like that, that kind
• 42:14
  Speaker 3: of cosine style shape in addition to an amplitude, which
• 42:18
  Speaker 3: is predicted by all of our simulations, that's impossible to fake.
• 42:23
  Speaker 3: And that's what we've seen now in the nanograph data.
• 42:26
  Speaker 3: So previously we'd seen evidence for this amplitude and it
• 42:30
  Speaker 3: was so loud we where were scratching our heads for
• 42:34
  Speaker 3: a hot second being like can this be real or
• 42:37
  Speaker 3: is this just noise? Right? And we had to wait
• 42:39
  Speaker 3: until we had this distinctive Hellings and Downs curve, until
• 42:43
  Speaker 3: we had evidence for that as well, because nothing can
• 42:46
  Speaker 3: fake the Hellings in Downs curve the noise. You know,
• 42:50
  Speaker 3: there could be unknown unknowns that just happened to be
• 42:53
  Speaker 3: generating some sort of correlated noise and all of the
• 42:57
  Speaker 3: pulsar signals we don't know, but we do know for
• 42:59
  Speaker 3: sure that this distinctive quadrupolar shape that's the result of
• 43:05
  Speaker 3: this sellings and downce curve can't be faked. So having
• 43:07
  Speaker 3: those two things at the same time makes us very
• 43:10
  Speaker 3: confident that what we're seeing now is evidence for the
• 43:12
  Speaker 3: gravitational wave background.
• 43:14
  Speaker 2: Do you feel like you have to be extra careful
• 43:16
  Speaker 2: about these claims in the wake of the BICEP two debacle.
• 43:20
  Speaker 3: Extraordinary claims require extraordinary evidence, And I feel like as
• 43:24
  Speaker 3: a collaboration, we've done a really great job at being
• 43:27
  Speaker 3: very conservative. When we first found the hint that what
• 43:31
  Speaker 3: we were seeing was a gravitational wave background back in
• 43:33
  Speaker 3: twenty twenty, we were very careful to say that this
• 43:37
  Speaker 3: could be the first part of the signal, but we're
• 43:40
  Speaker 3: certainly not saying that we've seen the whole signal, and
• 43:43
  Speaker 3: this is certainly not any kind of evidence for the
• 43:46
  Speaker 3: gravitationalive background, but heck, it's sure interesting, and we're going
• 43:50
  Speaker 3: to keep timing the data and we'll let you know
• 43:52
  Speaker 3: when something cool happens. So another part of, you know,
• 43:57
  Speaker 3: why we're so confident that this is evidence for the
• 43:59
  Speaker 3: gravitation background is that not only have we seen this signal,
• 44:05
  Speaker 3: but so have the Europeans. So in Europe they have
• 44:08
  Speaker 3: their own pulsar timing array and in Australia they have
• 44:11
  Speaker 3: a pulsar timing array, and in India they have a
• 44:14
  Speaker 3: pulsar timing array, and so far we've all been seeing
• 44:19
  Speaker 3: consistent signals the O. There's different levels of evidence for
• 44:23
  Speaker 3: the signal depending on the data set. But these data
• 44:26
  Speaker 3: sets are all different, they have different systematics, they're taken
• 44:31
  Speaker 3: with different telescopes. We use some similar analysis tools that
• 44:35
  Speaker 3: we're now even having independent analysis tools for the pulsars,
• 44:40
  Speaker 3: and so this makes us all very confident that what
• 44:44
  Speaker 3: we're seeing has to be real.
• 44:46
  Speaker 2: So why did you choose this as your particular slice
• 44:49
  Speaker 2: of physics to pursue? Doesn't it seem quite a risky
• 44:53
  Speaker 2: bet for a young researcher.
• 44:54
  Speaker 3: Oh yes, when I was even younger, I had a
• 44:57
  Speaker 3: lot of professors tell me why I was wasting my
• 45:00
  Speaker 3: I'm looking for low frequency gravitational waves. And even you know,
• 45:04
  Speaker 3: I've been doing this work since twenty ten, that's when
• 45:08
  Speaker 3: I started working on pulsar timing rays and graduate school,
• 45:11
  Speaker 3: and you know, even then working on LIGO was risky.
• 45:14
  Speaker 3: I in fact started my life as a ligone and
• 45:18
  Speaker 3: did a lot of work in helping to develop something
• 45:22
  Speaker 3: called LOO, and that was terrible for me and my
• 45:28
  Speaker 3: professor at the University of Birmingham, my supervisor, Alberto Vecchio,
• 45:33
  Speaker 3: was like, well, maybe you want to do something with
• 45:35
  Speaker 3: pulsar timing arrays and I was like, oh, I don't know,
• 45:38
  Speaker 3: let's think about it. So the idea of a pulsar
• 45:41
  Speaker 3: timing array, of having nature create a gravitational wave detector
• 45:46
  Speaker 3: for you, if you're just clever enough to use it,
• 45:49
  Speaker 3: really blew my mind. It was such a beautiful idea
• 45:54
  Speaker 3: that I thought, you know, this is really the experiment
• 45:56
  Speaker 3: for me. And I also felt like at the time
• 46:00
  Speaker 3: that it was risky to join a collaboration that had
• 46:03
  Speaker 3: a lot of the theoretical predictions already in place, and
• 46:07
  Speaker 3: at the time pulsear timing arrays didn't have some of
• 46:12
  Speaker 3: the fundamental papers that have now been written that I've
• 46:16
  Speaker 3: helped to write, and so I felt like it was
• 46:19
  Speaker 3: maybe less of a risk or I don't know, it
• 46:21
  Speaker 3: depends on how you look at it. It was risky
• 46:23
  Speaker 3: for sure to go into a field when it was
• 46:26
  Speaker 3: very exotic, even compared to ligo, which at the time
• 46:30
  Speaker 3: was also exotic, right, So it was writing down the equations,
• 46:35
  Speaker 3: trusting that the math was right and then just kind
• 46:38
  Speaker 3: of looking at nature hoping that the merger rates would comply,
• 46:41
  Speaker 3: because there's really nothing you can do, Like, even if
• 46:44
  Speaker 3: you have the perfect instrument, if super massive black holes
• 46:47
  Speaker 3: never merge, you're not going to find a signal. Right,
• 46:50
  Speaker 3: So you're right. It was risky, and I had several
• 46:53
  Speaker 3: people tell me that I should absolutely not be doing this,
• 46:56
  Speaker 3: that it should definitely not be my photos. But I
• 47:00
  Speaker 3: mean it was good advice at the time, right that
• 47:02
  Speaker 3: if you're a stake in your career on something that
• 47:05
  Speaker 3: isn't a proven technology yet, it's a big risk.
• 47:09
  Speaker 2: I love the idea that we build a new kind
• 47:11
  Speaker 2: of eyeball and we use it to look on to
• 47:13
  Speaker 2: the universe, but we don't know what we're going to see.
• 47:16
  Speaker 2: And in the case of Ligo and Virgo, they were
• 47:18
  Speaker 2: lucky because there's so many more gravitational waves than people
• 47:22
  Speaker 2: initially expected. What's the situation here. Are we surprised at
• 47:26
  Speaker 2: the sort of amplitude of these gravitational waves? Is it
• 47:29
  Speaker 2: more or less than we expected?
• 47:31
  Speaker 3: I'm very surprised at the amplitude of the gravitational wave background.
• 47:35
  Speaker 3: It is firmly twice as loud as I thought it
• 47:39
  Speaker 3: would be from my own models. So that's very surprising
• 47:43
  Speaker 3: to me and delightful. I couldn't be happier. Thank you
• 47:48
  Speaker 3: universe exactly, Thank your universe for loving super massive black
• 47:52
  Speaker 3: holes as much as you love stellar mass black holes.
• 47:55
  Speaker 3: But that's assuming that all of this signal is coming
• 47:57
  Speaker 3: from these merging super massive black holes. So I guess
• 48:02
  Speaker 3: one thing that I haven't said yet is that the
• 48:04
  Speaker 3: signal that we found evidence for is this gravitational wave background,
• 48:10
  Speaker 3: and that comes from the cosmic merger history of super
• 48:13
  Speaker 3: massive black holes. So it's not one signal that we're
• 48:16
  Speaker 3: looking for. It's this aggregate signal. It's the incoherent superposition
• 48:22
  Speaker 3: of all of the super massive black hole mergers that
• 48:25
  Speaker 3: have ever happened. So it's the total opposite of ligo
• 48:29
  Speaker 3: in so many ways. It's very low frequency, so frequencies
• 48:33
  Speaker 3: of nanohurts, and if you're not used to thinking about nanohurts,
• 48:37
  Speaker 3: one nanohertz is like an orbital period of thirty years,
• 48:41
  Speaker 3: So these are very slowly orbiting super massive black holes,
• 48:45
  Speaker 3: and by super massive, I mean a billion times the
• 48:48
  Speaker 3: mass of the Sun. Our signals are also very long lived,
• 48:51
  Speaker 3: so one of the reasons that there is this buildup
• 48:54
  Speaker 3: of gravitational wave signals at very low frequencies is because
• 48:58
  Speaker 3: the black holes evolve soly, their mergers are so slow.
• 49:01
  Speaker 3: They take about twenty five million years to merge while
• 49:05
  Speaker 3: emitting gravitational waves, and so our signals are rare in
• 49:12
  Speaker 3: terms of rates. One super massive black hole binary system
• 49:16
  Speaker 3: takes a whole galaxy merger to happen, right, and so
• 49:20
  Speaker 3: that's not happening all the time everywhere you look. But
• 49:23
  Speaker 3: the merger is so slow, and the signals are so powerful.
• 49:27
  Speaker 3: They are a million times stronger than what you can
• 49:29
  Speaker 3: see in LEGO. But when they build up, they create
• 49:31
  Speaker 3: this wopping loud signal that we can look for with
• 49:35
  Speaker 3: pulse our timing arrays.
• 49:36
  Speaker 2: So you say there're one million times more powerful than LEGO,
• 49:39
  Speaker 2: that's like at their source or to that factor in
• 49:41
  Speaker 2: also their relative distance, because they're further away also than
• 49:44
  Speaker 2: what LEGO can see.
• 49:45
  Speaker 3: Right, So I mean those are slightly different questions. So
• 49:49
  Speaker 3: when we talk about gravitational waves, our currency, the measurement
• 49:54
  Speaker 3: that we use is the strain and so that's how
• 49:56
  Speaker 3: much are the gravitational waves changing space time that you
• 50:01
  Speaker 3: can think about that is a distance over distance or
• 50:05
  Speaker 3: time over time, because we have space time, so you
• 50:07
  Speaker 3: can pick one. So for Ligo, they can look for
• 50:10
  Speaker 3: signals that have a strain that has a value of
• 50:13
  Speaker 3: ten to the minus twenty one. But what does that mean.
• 50:15
  Speaker 3: That's something like that's the fraction of a size of
• 50:17
  Speaker 3: a proton over the length of their detector, and I
• 50:20
  Speaker 3: think that's the tagline that was very successful for them.
• 50:24
  Speaker 3: For us, this is more like one meter per light year.
• 50:28
  Speaker 3: But what's a light year. I don't have a physical
• 50:31
  Speaker 3: intuition for how long a light year is, and Americans
• 50:34
  Speaker 3: are not good with meters, so I think that change
• 50:37
  Speaker 3: in time over time is more intuitive. And so for us,
• 50:42
  Speaker 3: the strain that we're looking for is ten to the
• 50:43
  Speaker 3: minus fifteen. So that's one part in a million billion
• 50:47
  Speaker 3: and that's about one hundred nanoseconds over a decade. So
• 50:51
  Speaker 3: that's how much the gravitational waves are changing the space
• 50:55
  Speaker 3: time fabric around us. And while one part in a
• 50:59
  Speaker 3: million bills is incredibly small, it's still a million times
• 51:03
  Speaker 3: louder than what Ligo is able to detect.
• 51:07
  Speaker 2: I see, yeah, that's the relevant comparison. Very cool, So
• 51:10
  Speaker 2: you say that we can see this overall background hum
• 51:13
  Speaker 2: and that we attribute it to supermassive black holes. How
• 51:16
  Speaker 2: do we know it's not also coming from like primordial
• 51:19
  Speaker 2: gravitational waves from before the CMB, etc.
• 51:23
  Speaker 3: That's a fantastic question. The answer is that we are
• 51:27
  Speaker 3: not sure. In fact, there's a whole paper about, you know,
• 51:31
  Speaker 3: exotic physics that we can now constrain with this amplitude
• 51:36
  Speaker 3: of the gravitational wave background that we found. So one
• 51:39
  Speaker 3: of the things that we'll do in the future is
• 51:42
  Speaker 3: to try to characterize the gravitational wave background and to
• 51:46
  Speaker 3: you know, see how does the amplitude vary as a
• 51:49
  Speaker 3: function of frequency. Right now, all of the sources that
• 51:53
  Speaker 3: we know of very in similar ways, right and the
• 51:57
  Speaker 3: error bars will include all of your standard model right. So,
• 52:01
  Speaker 3: right now we have as our key targets. Are the
• 52:04
  Speaker 3: prime suspects for sourcing this gravitation oive background are super
• 52:09
  Speaker 3: massive black holes. First and foremost. You know only the
• 52:13
  Speaker 3: cosmic merger history of super massive black holes of cosmological
• 52:17
  Speaker 3: or primordial gravitational wave background, and that is due to
• 52:22
  Speaker 3: quantum fluctuations in the early universe that were then blown
• 52:25
  Speaker 3: up to the size of the whole universe, and then
• 52:27
  Speaker 3: cosmic strings, which are these defects in the fabric of
• 52:31
  Speaker 3: space time. There's matter density spaghetti in the universe that
• 52:35
  Speaker 3: are vanishingly small that can also create gravitational waves. Now,
• 52:40
  Speaker 3: those three sources might all be contributing to this signal
• 52:44
  Speaker 3: and might help to explain why it's so loud, but
• 52:48
  Speaker 3: it's not necessary to include them. But at the same time,
• 52:52
  Speaker 3: right now, all of the predictions for how the amplitude
• 52:56
  Speaker 3: of the signal varies as a function of frequency, you know,
• 52:59
  Speaker 3: they're all about minus one. They all have like this
• 53:01
  Speaker 3: almost minus one slope, and that means that we won't
• 53:06
  Speaker 3: be able to know for another five years or so
• 53:08
  Speaker 3: as to exactly what's happening and what's sourcing this background,
• 53:12
  Speaker 3: or at least we can say what is the primary
• 53:15
  Speaker 3: source of this background. And that's not even to get
• 53:19
  Speaker 3: into the fact that it's still possible that there might
• 53:22
  Speaker 3: be some noise that's leaking through the pulsars that's masquerading
• 53:25
  Speaker 3: as some background amplitude when it's really noise. We know
• 53:30
  Speaker 3: for sure that there is a background because we can
• 53:32
  Speaker 3: see the hellings and downs curve and nothing else can
• 53:35
  Speaker 3: fake that, so we know that it's not all noise.
• 53:38
  Speaker 3: But we now need to figure out what's sourcing this
• 53:41
  Speaker 3: signal and create, you know, better custom noise models for
• 53:47
  Speaker 3: our pulsars to make sure that we completely the best
• 53:50
  Speaker 3: of our ability understand how the astrophysics of the signals
• 53:54
  Speaker 3: propagating from the pulsar to the Earth will affect things.
• 53:59
  Speaker 2: So then what are the prospects for nanograb you talked
• 54:01
  Speaker 2: about the next five years. Is that gathering more pulsars
• 54:04
  Speaker 2: or more analysis of the data, or combining with the
• 54:08
  Speaker 2: other pulsar timing arrays on Earth.
• 54:10
  Speaker 3: It's all of those things. It's all of those things.
• 54:14
  Speaker 3: So the signal to noise that we measure with our
• 54:17
  Speaker 3: experiment increases as the number of pulsars that we add
• 54:21
  Speaker 3: and as the square root of the time. So it's
• 54:23
  Speaker 3: really important to add more pulsars. One way of adding
• 54:26
  Speaker 3: these new pulsars is to go out and search for
• 54:30
  Speaker 3: more pulsars. That's one thing that you can do, and
• 54:32
  Speaker 3: that's one thing we should absolutely be doing. The more
• 54:35
  Speaker 3: immediate thing that we can do is to collaborate with
• 54:37
  Speaker 3: our colleagues and Europe and Australia and India and China
• 54:41
  Speaker 3: and South Africa and share our data and make these
• 54:45
  Speaker 3: huge mega combined data sets that will give us immediately
• 54:49
  Speaker 3: access to the southern hemisphere to all of those pulsars.
• 54:53
  Speaker 3: And when we combine our data streams from all the
• 54:56
  Speaker 3: individual pulsars, we'll get denser data sets, and that can
• 54:59
  Speaker 3: make us much much more sensitive to individual inspiraling super
• 55:04
  Speaker 3: massive black hole binaries. So now that we have evidence
• 55:07
  Speaker 3: for the background, which, to be honest, is a foreground.
• 55:10
  Speaker 3: It's what we were looking for. It's not a nuisance
• 55:14
  Speaker 3: that was the thing. Now it will be a background.
• 55:17
  Speaker 3: Now it'll be a source of noise that we're trying
• 55:19
  Speaker 3: to get rid of. And so what's underneath right, Well,
• 55:22
  Speaker 3: there'll be some anisotropy in the gravitational wave background, similar
• 55:26
  Speaker 3: to the cosmic microwave background. We'll be able to make
• 55:29
  Speaker 3: maps of the gravitational wave universe. And what I'm really
• 55:34
  Speaker 3: excited for is when we find some sort of gravitational
• 55:37
  Speaker 3: wave hotspot on one of those maps and there's no galaxy,
• 55:42
  Speaker 3: right Like, what happens if there's gravitational waves that are
• 55:45
  Speaker 3: coming from a place where there's no known galaxies, That's
• 55:48
  Speaker 3: when I think things just start to get really interesting.
• 55:52
  Speaker 3: So stuff like that could be right around the corner
• 55:55
  Speaker 3: and that's super exciting. We can also do tests of
• 55:58
  Speaker 3: general relativity. Relativity predict two gravitational wave polarizations, so just
• 56:04
  Speaker 3: like light, you know, we have plus and cross polarizations
• 56:07
  Speaker 3: with gravitational waves, but extensions to general relativity predict even
• 56:12
  Speaker 3: more polarizations, like for example, there could be a breathing mode.
• 56:16
  Speaker 3: So instead of gravitational waves stretching and squashing the fabric
• 56:21
  Speaker 3: of space time like a plus or a cross, it
• 56:24
  Speaker 3: breathe so all of the space time, you know, goes
• 56:28
  Speaker 3: out like you're taking a big breath, and then collapses
• 56:30
  Speaker 3: in on itself like a balloon, getting bigger and then
• 56:33
  Speaker 3: getting smaller again.
• 56:35
  Speaker 2: Great, well, this is very exciting. Congratulations and thanks very
• 56:38
  Speaker 2: much for taking some time to chat with me today.
• 56:41
  Speaker 3: Thank you very much. I just want to really emphasize
• 56:45
  Speaker 3: that NANOGrav is an experiment that's been taking data for
• 56:49
  Speaker 3: fifteen years and it's taken a team of one hundred
• 56:52
  Speaker 3: astrophysicists to get this result. So it's a huge, huge experiment,
• 56:59
  Speaker 3: take in a lot of time and a lot of money,
• 57:02
  Speaker 3: and it's just part of this global effort to detect
• 57:04
  Speaker 3: gravitational way. So I really want to give a shout
• 57:06
  Speaker 3: out to my colleagues all over the world. I started
• 57:09
  Speaker 3: my career in Europe, right so I was part of
• 57:12
  Speaker 3: the European Pulsar Timing Array for many years. And I've
• 57:15
  Speaker 3: written papers with the Parks pulsar Timing Array in Australia,
• 57:19
  Speaker 3: and you know, of course many papers with NANOGrav here
• 57:22
  Speaker 3: in the US and in Canada. So I just want
• 57:25
  Speaker 3: to mention that this is a huge experiment that's taken
• 57:29
  Speaker 3: decades to get to where it's at right now.
• 57:31
  Speaker 2: Absolutely, and I love when scientists from all over the
• 57:33
  Speaker 2: world can come together to make a project bigger than
• 57:37
  Speaker 2: anyone scientist. Absolutely. Well, congratulations again and thanks for joining us.
• 57:42
  Speaker 3: Thank you so much.
• 57:43
  Speaker 1: Awesome, pretty exciting it must be. And I when an
• 57:46
  Speaker 1: experiment your work done for so long, it pays off
• 57:48
  Speaker 1: and potentially changes how we see the universe.
• 57:51
  Speaker 2: Yeah. I love the bit where she says that people
• 57:53
  Speaker 2: warned her not to work on this experiment because it
• 57:55
  Speaker 2: was such a long shot.
• 58:01
  Speaker 1: Do you wish you had maybe listened to that advice
• 58:03
  Speaker 1: or somebody had giving you that advice.
• 58:06
  Speaker 2: No, I'm pretty happy with where I ended up, but
• 58:07
  Speaker 2: I'm just glad that somebody's out there swinging for the
• 58:10
  Speaker 2: fences and trying to discover crazy things about the universe.
• 58:14
  Speaker 2: One of my favorite things about this discovery is that
• 58:17
  Speaker 2: it was a surprise, you know, that they're seeing super
• 58:19
  Speaker 2: massive black hole gravitational waves at like twice the amplitude
• 58:23
  Speaker 2: that they expected. The universe is louder in super massive
• 58:27
  Speaker 2: black hole gravitational waves than we thought.
• 58:29
  Speaker 1: Yeah, sort of the same thing happened with Ligo, right, Like,
• 58:32
  Speaker 1: there are more of these events, or at least we
• 58:34
  Speaker 1: could listen to more of these events than we thought
• 58:36
  Speaker 1: was possible or actually happening.
• 58:39
  Speaker 2: Yeah, we have an episode coming up about how surprising
• 58:41
  Speaker 2: it was that Lego saw so many black hole mergers
• 58:44
  Speaker 2: when they did. And in this case also we were
• 58:47
  Speaker 2: sort of lucky the universe was louder to our new
• 58:49
  Speaker 2: kind of ear than we even dared. Hope.
• 58:53
  Speaker 1: The universe is out in the field screaming with joy and.
• 58:56
  Speaker 2: Gravity and finally we're able to listen.
• 58:59
  Speaker 1: All right, welcome gras to the scientists at NANOGrav and congrats,
• 59:03
  Speaker 1: I guess to all of us. Right, it's humanity opening
• 59:06
  Speaker 1: up a new eyeball into.
• 59:07
  Speaker 2: The universe, that's right, and hearing new kinds of physics
• 59:10
  Speaker 2: going on out there. Hopefully in the future, as NANOGrav
• 59:13
  Speaker 2: and their international partners stitch together their data into a
• 59:16
  Speaker 2: massive data set and collect more pulsar as we can
• 59:19
  Speaker 2: learn even more things about gravitational waves from the early
• 59:22
  Speaker 2: universe and maybe even figure out how this whole crazy
• 59:26
  Speaker 2: universe came to.
• 59:27
  Speaker 1: Be and why it's in the middle of the field
• 59:29
  Speaker 1: screwing for joy or not, it might be a different experiment.
• 59:32
  Speaker 1: All right. Well, we hope you enjoyed that. Thanks for
• 59:34
  Speaker 1: joining us, See you next time.
• 59:44
  Speaker 2: Thanks for listening, and remember that Daniel and Jorge Explain
• 59:47
  Speaker 2: the Universe is a production of iHeartRadio. For more podcasts
• 59:51
  Speaker 2: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
• 59:56
  Speaker 2: you listen to your favorite shows.