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The Gravity Wave Hunter

An astrophysicist sits down with Nautilus to discuss cosmology and culture.

The celestial object came flying across the nighttime sky quickly, moving in an unusual pattern. Chiara Mingarelli and her friend…By Michael Segal

The celestial object came flying across the nighttime sky quickly, moving in an unusual pattern. Chiara Mingarelli and her friend saw it from a field in rural Ontario, their regular nighttime observation post. The interpretation was clear: Aliens. Her next step was also clear, which was to sprint home and explain to her parents her newly precarious position. She’d been caught staring at an alien spaceship.

It’s childhood experiences like this one that laid the foundation for a lifetime love of astronomy. Today Mingarelli is still looking up into the sky, but much, much farther away. Her specialty is hunting for gravity waves, particularly those in the nanohertz frequency range (LIGO’s recent detection was of waves at a trillion times higher frequency). In her capacity as a Fellow at Caltech, which co-led the LIGO effort, she’s well positioned to guide us through the excitement and implications of that recent experimental breakthrough.

But, through her experience with science outreach, working with young women, and being a woman in a male-dominated field, she also sheds light on the evolving culture of her field.

Nautilus sat down with Mingarelli on the Caltech campus.

The video interview plays at the top of the screen.

View Video Interview


Interview Transcript

What was your reaction to the LIGO detection of gravitational waves?

Oh my gosh! I was absolutely delighted. You know, it was almost like it was too good to be true, because we always say five to 10 years, right? We always say that we’re going to detect something in five to 10 years, and to have that be over and to just say, “oh we’ve finally detected something,” is amazing! And it’s a great feeling. I think that it’s going to open a whole new branch of astronomy that just looks at gravitational waves. To be someone who’s been in the field for five or six years already and working on a different experiment, you kind of feel like a pioneer in your own experiment, and that maybe in 10 years when we detect gravitational waves with pulsar timing arrays that we’ll have the same kind of party and fanfare. I think that the reaction, especially from the public, has been really overwhelming; people are so interested in this result and so interested in this new way of thinking about space and time and gravity and these ripples. So I was ecstatic! I couldn’t believe it.

How could you not want to tell all of your friends that you’ve just found evidence that supports Einstein’s theory of gravity?

The fact that these black holes had a mass that was about 30 times the mass of the sun was really surprising. Since day one, the people who do the expectation rates for the LIGO experiment always said that we would detect a neutron star-neutron star coalescence first, because we expect there to be more of these and therefore we should see a binary neutron star first. The fact that we saw a binary black hole first really blew my mind! And then the fact that they were so massive again was just like, phew! It was nuts! It’s one of the wonderful things about this kind of discovery, that it’s never really what you expect it to be. Reality is even cooler than what you could’ve thought of, and I hope that the same thing happens for pulsar timing arrays.

How did other Caltech scientists react to the LIGO detection?

I can tell you about Alan Weinstein’s reaction, and he’s a professor here at Caltech who works on the LIGO experiment. He said when they got the phone calls they were all incredulous because they couldn’t believe that it was real. They’ve been looking for gravitational waves for decades. He said at first he thought that it was a blind injection, that someone had put in a signal and they didn’t know about it and so they thought that they were going to have to go through this whole rigmarole again, to find out that at the end of the day it was a hardware injection. Then they thought that maybe it was double blind because no one seemed to know what was going on. Whoever did the injection didn’t tell anyone, and this is going to be a big secret, and then eventually it’s not going to be a real signal. But then everyone swore that they hadn’t done any injections, and so they were starting to think, “oh my gosh, maybe this is real!” And then Alan thought maybe it was a triple blind experiment, and that just means it’s a malicious hacker who somehow managed to erase all of their steps and get the perfect gravitational wave signal in the mirror, and then will announce that they’ve somehow engineered this in a few months, and embarrass the collaboration. But he also claims that a binary black hole merger is much more likely than someone with that level of computer hacking power who is interested in hacking LIGO.

How did the LIGO results leak early?

I think that there were a lot of different ways that information was leaked. One of the ways was [that] apparently, someone had printed a copy of the draft of the paper and had left it on the printer and people went by and just took pictures of it with their phone and then emailed their friends! So if stuff like that happened, you know? Discussion groups, sometimes people think that they’re closed but they’re public, and you can send an email to a list and you think that it’s safe, but then there are people who are not part of the collaboration who can be on it.  There [are] a lot of different ways that accidents happen and these leaks happen. Also, it’s just so exciting! How could you not want to tell all of your friends that you’ve just found evidence that supports Einstein’s theory of gravity; it’s taken 100 years to detect!

I think that this is the number one thing, that the NSF, I saw one estimate that claims that they funded over $1 billion into LIGO. This is a really expensive experiment, and you have this one goal of detecting gravitational waves. If you actually do that after decades of trying, of course people are going to call and say like, oh my gosh, guess what just happened? So you know, you can ask people to not tell anyone, but at the end of the day I think that people who do this kind of work are really excited about their work and it’s really hard for them to not call their mom or their spouses and tell them that this discovery of fundamental importance has just happened, and this is most likely going to win a Nobel Prize, and you’ve been a part of that work. Of course you want to tell everyone, right? So I think it was really difficult for people to be silent about it. So yes, of course there are a few leaks.

Gravitational waves also stretch and compress time. Why don’t we hear about that aspect of the LIGO detection?

The first thing to think about is that the theory of general relativity is a theory of spacetime. So in the theory of spacetime, space and time are this one quantity. We need to set up our reference frame to describe space and time, and therefore we have to pick a coordinate system. Once we set this up we have a lot of choices as to how we describe how these different coordinate systems move, and we usually pick a coordinate system that is really easy and simple, and in this coordinate system, there’s no time component—but it doesn’t mean that you couldn’t have one; it just means that we pick one where it doesn’t exist but you can make a rotation and end up having a time component. The reason this is important is because if we choose a description that has time dilation in it, it’s really difficult to explain and not maybe very intuitive. To choose a really clear theory we choose a reference frame where gravitational waves are transverse—and by that I mean, if it’s coming out of my forehead and the gravitational wave is coming this way, then it stretches and squashes spacetime this way, right? So it’s coming out like this and it stretches and squashes like this, or also like this. These are the two different plus and cross polarizations of gravitational waves. So there is—or you could think of there being—a time dilation, and in that sense you could describe it mathematically that way, but it’s not intuitive.

So picking the proper reference frame to make this description—which isn’t easy by the way, a lot of people got this wrong—and making the gravitational waves transverse in this reference frame and then having the masses being freely falling in this reference frame and then acting upon the gravitational wave effect, and looking at this spatial distortion is an intuitive way of thinking about it. But you could also think of the laser as picking up a time dilation effect, which is caused by these mirrors being stretched and squashed. So in that sense, you can think of the experiment as also being a kind of time dilation effect where the signal is delayed by a certain amount of time. Or you can think about the experiment as it being stretched and squashed in space and having to move, and then picking up that signal when the lights recombined.

Is there a constant background of gravitational waves?

Gravitational waves interact very weakly with matter and therefore, we’re actually bathed in a background of gravitational waves right now. If we want to be specific and give an example, let’s consider the background of gravitational waves: supermassive black hole binary mergers. So every single galaxy that has merged with another galaxy has had the central supermassive black hole merge with the other supermassive black hole to make a new black hole. This has been happening for, well, billions of years. So what happens is that not only can you have these single sources of gravitational waves from individual systems, but you have a whole background of gravitational waves, like a choppy sea, right? So you have gravitational waves coming from over here and from over here and then they interfere and you get this kind of choppy sea pattern of gravitational waves.

My love affair with science began when I was really young; just thinking about the stars and thinking about the universe.

Now, we have a very good idea of what this kind of signal would look like, especially in a pulsar timing array. We know what the frequency dependence looks like in our signal, and we have rough estimates as to what the amplitude of that kind of background would be. Those estimates come from different galaxy merger scenarios. So the more galaxies that merge, the higher your background is going to be because you had more galaxy mergers with more supermassive black holes. It also depends on the masses of the central supermassive black holes. So you’ll have a stronger background if you have fatter black holes. Now, what we can do, even with no detection of the gravitational wave background, is that we can set upper limits, and all that means is that if the background were at this level it would’ve detected it, and therefore because we haven’t detected it, we can rule out all these theories that claim that we have 10 billion solar mass black holes in the centers of each galaxy, because it’s obvious that we don’t—because if we did, they would’ve made a signal that we could’ve detected. But we haven’t detected it and therefore this theory is no good. So we can keep on ruling out theories until we actually make a detection. And therefore, upper limits can also tell us interesting things about the universe from what we don’t see.

Did the early universe have a gravitational wave background?

Actually, there were quantum fluctuations in the gravitational field in the early universe, which make primordial gravitational waves, and that’s also something that’s really interesting. What happened is that these quantum fluctuations, these fluctuations of the gravitational field, made primordial gravitational waves, which were then blown up during the inflationary period. Now we can look for these primordial gravitational waves, which are kind of analogous to the cosmic microwave background radiation. So this is one experiment where we can use limits from LIGO, we can use limits from cosmic microwave background experiments, and limits from pulsar timing array experiments to put constraints on different theories of inflation, because some of them predict that the science of these gravitational waves would be larger and thus we’d be able to see them today. But we don’t see them today, so again we can put a kind of limit on that and say, well these theories of inflation or these other theories are no good because it predicted a number that we’ve now seen, but we haven’t seen the signal, and therefore we can rule out this theory or say that our evidence does not support this theory. So primordial gravitational waves are really interesting and it’s also an active area of research that I’m participating in right now.

What frequencies can gravitational waves have?

The gravitational wave spectrum is immense and we expect there to be gravitational waves at many different frequencies—at very low frequencies, 10-16, which is tens of a millionth of a billionth of a hertz, if that makes any sense to anyone, going all the way up to LIGO sources, which are at around a kilohertz. The kilohertz band is the interesting band, really kind of tens of hertz to a kilohertz, because this range would be audible if you could hear it, which is why people make these tripping noises when they’re describing the LIGO signal, because it goes, “wooooooop!”; and that’s because if time goes this way and frequency goes this way, you go up in frequency as you go out in time; therefore you get this distinctive chirp signal.

The gravitational waves that I work on are in the nanohertz frequency band, so that’s a billionth of a hertz. These are lower than infrasound and they don’t chirp because they’re not at the end of their lifetime. So this chirp sound comes in during the very last stages of merger, as in you get this huge increase in frequency. But if you’re at the nanohertz level, this is when you have, for example, two supermassive black holes, which are the centers of galaxies, and when the galaxies merge, their supermassive black holes merge. We hear these with gravitational waves when they’re in the nanohertz frequency regime. It wouldn’t have that chirping sound; it would be more of a steady hum.

So there’s a difference in what part of the signal that we see in the nanohertz frequency part of the gravitational wave spectrum, and what LIGO sees. So LIGO would see sources that are a few times the mass of the sun, maybe 10 times or more the mass of the sun, and also neutron stars or other compact objects that are inspiraling at the ends of their lives, whereas the nanohertz gravitational wave experiments see much more massive objects, like supermassive black holes, which are a million to a billion times the mass of the sun in their very early inspiral phase; so you don’t hear that final chirp in the nanohertz frequency band, but you do with LIGO.

Would it be fair for just a few individuals to win the Nobel Prize for the detection of gravitational waves?

The Nobel Committee has this rule that the prizes should be awarded to individuals who make significant contributions to their field. Now, I think that maybe this will have to be revised eventually because we have huge experiments, like there’s the Large Hadron Collider, and the discovery of the Higgs boson, and the fact that the LHC or even CERN, none of those experiments got a Nobel Prize and maybe they should have. It’s also strange that the Nobel Prize for Peace went to the European Union, which is clearly not an individual, and so maybe they can take that precedent and apply it to physics. But these are all my opinions and speculations; I don’t have any insider knowledge on the thoughts of the Nobel Committee. But I think that these projects, if they don’t win a Nobel Prize, should also be awarded some kind of status. There should be some sort of special recognition by the Committee, either funding in perpetuity or something of equivalent importance, because it took over 1,000 scientists to detect those gravitational waves with LIGO. Even I worked on it when I was a graduate student. Anyone who studies gravitational waves will eventually come across LIGO and be a member of the LIGO Scientific Collaboration. So it’s difficult to know how to reward the experiments and the thousands of people who have input into the experiments, over the few people who had the vision early on to really push for these kinds of experiments.

How did your interest in science start?

When I was a kid I really liked to play outside, especially around dusk. And when you’d see the first stars come out, I thought it was just really, really beautiful. I really love star-gazing. Even though I’m from Ottawa, I’m kind of from the outskirts of Ottawa. There’s a small town called Rockland, Ontario, and that’s really where I was raised; they have really dark skies.

When I moved there, the population was just under 10,000. And when our first Subway restaurant opened, we thought that that put us on the map! We finally got a Tim Hortons [and] we were overjoyed at the prospect! So it was a really small town and there wasn’t a lot of light pollution, and at night I would look up at the stars. I always wondered what was out there, and if I could make a contribution to our knowledge of what was out there. Who was I to pretend like I could do such a thing? But I was always really interested and I think that that was really my strength, that I never gave up. Once I found out what black holes were and that you could actually make money studying black holes—and that could be a career path—I really thought, “Well, why would you do anything else? Like this is obviously the thing that we should all be doing! How come everyone is not doing this?”

So yes, my love affair with science, I guess, began when I was really young and just thinking about the stars and thinking about the universe. My father, who’s a mathematician, asked me what I wanted to be, and I said, well I really like astronomy, but I don’t want to just look at the stars. I’d rather do something with a bit more math in it. I like challenging myself. I said, is there such a thing as astrophysics? And I think he almost cried! He was so excited! He was like, “yes, yes, there is! Oh, my daughter is going to be an astrophysicist!” I just have always had this fascination with black holes, and I still do to this day.

Why is the public so fascinated by black holes?

It’s this interesting duality of them being really simple and really complicated. So they’re really simple. You can describe them by their mass and their spin, right? But what’s past the event horizon? What does the singularity actually look like? Is it actually this point of infinite curvature? It is some sort of quark soup? What does it look like? We have some ideas of how light behaves as it approaches the singularity, but actually knowing what it looks like and all of the stuff that’s going on inside the event horizon ... I mean it must be fireworks all the time as matter is approaching the singularity and being shifted and torn apart and emitting X-rays and gamma-rays and all sorts of ionizing radiation that would kill us immediately. But what does all of this look like?

I try to imagine it as like a water fountain, having light coming out and then falling back in on itself as it approaches the event horizon. But there could also be light kind of traveling, in all different sorts of weird and wonderful orbits, and the singularity at the center. If the black hole is spinning, this should actually be a ring singularity instead of just a point. There have been some theories about how we could maybe access different realities, parallel universes, if we were to cross these ring singularities. But this is very speculative, because of course, if you’ve looked at the math behind this, it’s really a copy of the universe that happens after the singularity, but that doesn’t mean that it’s a copy of a different universe or access to a different universe. And also the spacetime around the singularity becomes really unstable. So this might just be a mathematical fluke, but it could be something else.

And that’s what’s so fascinating. All of these different theories of gravity or of black holes or five-dimensional—all of these things have such an ability to tickle your curiosity and to think of amazing things that you don’t think of when you’re going to the grocery store or if you’re typing some manuscript on your computer. A lot of this stuff really requires a lot of imagination, and that’s what’s so fun about it. Again, black holes, they look like they’re possibly the simplest objects we could ever describe because all you can see is the event horizon, right? And if you’re lucky, there might be a companion that is donating mass to the system, and then you can see X-rays that are coming off of an accretion disk around the black hole because X-rays are kind of light—you can see these with space-based detectors. I mean they’re just so cool, right? And I think that a lot of people may share that fascination, that they are these seemingly simple objects that are really complicated.

Did the movie Interstellar portray black holes correctly?

That movie has some of the best scenes of black holes. I mean, Gargantua’s visualization was absolutely stunning. That actually is real. The way that they visualized Gargantua was by taking Einstein’s field equations and actually solving them on supercomputers in such a way that us scientists would have a really difficult time doing. That’s because we don’t have enough money to buy all these supercomputer hours, but this movie did! And so they could run all of these simulations and generated almost a petabyte of data, which we then put together to make that beautiful visualization of Gargantua. But Kip told us that if it didn’t look nice, that it would’ve just been thrown out and completely discarded. This beautiful piece of science that we couldn’t afford to do on our own, if it didn’t look good it’s out the window! So I’m really glad that it looked nice, and it also gave us that really interesting visualization where you have the secondary ring, right? So you have a halo around the black hole and then you have the accretion disk; but actually the halo is just the lensed effect of the accretion disk behind it. This is something that we’ve never seen before, and it makes sense if you think about it, but beforehand no one had ever really thought that that’s what it would look like because we’re all used to looking at these things in two dimensions. We draw this flat fabric of spacetime and we draw this little well and sometimes we make this two-dimensional by shifting it and you can see that it’s this big kind of dip. But imagining this in three dimensions or seeing it and seeing what the effect on light is, is stunning. And that movie is really responsible for that effect, which is beautiful.

What has been your experience as a woman working in a male-dominated field?

It’s a tough question to answer because there’s no right or wrong answer, or an answer that will make me popular! The truth is that it’s really difficult and it’s difficult for a lot of different reasons. It’s difficult because people have preconceived notions of what a scientist should look like and people have implicit gender biases, especially when it comes to scientists. You can take one of the Harvard implicit bias tests and find out, like I did, that you take male scientists more seriously. Even I do! And I’m a female scientist! It’s this bias that everyone has. And acknowledging that that exists is one way of getting rid of it; it’s to remind yourself that you do have this bias and you should always try to keep that in the back of your mind when you’re evaluating someone else’s scientific research. Am I really looking at this objectively? Are things tainting this? How can I try to mitigate this kind of effect?

It’s been shown that science, when we have a lot of diversity, is just so much better.

It’s also difficult in a field, even in all of academia, which is essentially based on a letter-writing campaign. You do work for your Ph.D. supervisor and your Ph.D. supervisor writes you a letter of reference. If your Ph.D. supervisor has bullied you or harassed you in any way, you can’t ask them for a letter because chances are you don’t like them very much and they don’t like you, after all is said and done, especially if you’ve reported them; so then what do you do? You don’t have anyone who has your back and you don’t have anyone who can write you letters to go to the next rung on the academic ladder. That kind of power is almost absolute in the world of academia. So some people just put up with it. Some people just put up with male colleagues leering at them at conference dinners or people getting drunk and putting their hand on your bum, or sometimes a lot more awful things happen. You can have professors, again in these situations, putting their hands up your skirt, and there’s not a lot that you can do. You can complain, but chances are they’re just going to get a slap on the wrist. It makes you wonder if anyone takes your science seriously at all, because you’re a woman, and because you think that people want to talk to you about your science, but sometimes they just want to talk to you to get you drunk or to make out with you or to have sex with you.

So this is one thing that I’ve found the most difficult in my career—is to try to have really amazing scientific conversations with colleagues without having at the back of my mind that they want something else, because all these things have happened to me before where there [have] been ulterior motives. So it’s a really difficult thing to be a minority in the field, and at least I have one advantage—that I’m white! If I were a person of color, chances are I wouldn’t be here at all, because female people of color are some of the most disadvantaged in science. I try to keep my biases in check and I try to encourage other women and support them and also try to encourage people of color and underrepresented minorities to join science. It’s been shown that science, when we have a lot of diversity, is just so much better. People bring in different ideas, you’re more productive, you have better science. But the hard reality is that people like what they know, and they like having clubs of people who are all familiar to each other and who are all friends. That’s really difficult to change, and I think change will come really slowly, unfortunately.

There’s been a lot of press in the past few months about Geoff Marcy at UC Berkeley, or Christian Ott who is here, whose office is down the hall, and these are extreme cases. But one aspect, which isn’t always appreciated, is that these kind of people usually have bullying issues. This is all a power play. Whether you’re a woman or whether you’re someone who won’t stand up to them, they know that because they hold your academic future in your hand, they can essentially do whatever they want. They have this prized position as being a tenured faculty member, which essentially means you can never be fired. So you’re really in a difficult position to try to do anything to report these people. Being a woman puts you in an especially disadvantaged position and it’s something that I’ve tried to deal with myself and I’ve tried to support other women dealing with it. I think it’s important to talk about it in an open and honest way, like I’m doing right now, which is really difficult because there are always naysayers who will say, oh you know, she was drunk and she wanted it, or she was ... Anyhow, there’s a lot of people who blame the victims in this case. But in my experience with all of the colleagues and the graduate students who I’ve supported in different instances of this kind of harassment, it’s mostly the victims who blame themselves for whatever reason, and it’s to try to convince them that, no, they did not deserve to be harassed. Like you can wear whatever you want and people should still respect you as a scientist. So I hope that things get better and I hope that with these really high profile cases, which have been in the media, people will come to realize that this is a real problem and it’s a real issue and people like myself and my colleagues deal with it every single day. I really hope that there’s light at the end of that tunnel and that that comes really soon.

What have you learned from talking to young women interested in science?

It depends on if they’re at a girls’ school or if they’re at a mixed school. The girls who are at all-girls’ schools, in my experience, it’s never occurred to them that they can’t be scientists or leaders. They all seem to be happy asking questions, they all show interest. Obviously not everyone is interested, but you have the right proportion of them who are really interested, some who are mildly interested, and some who just don’t care—and that’s normal. However, I’ve noticed that in mixed schools if we’re doing a demonstration, the boys always come to the front and the girls fall to the back and are really shy to raise their hand and ask a question, whereas the boys feel like it’s they’re entitled to be there; it’s their job to be at the front, to show that they’re asking awesome questions, even if their questions are ridiculous. They still want to be seen as being the leaders of the pack and being at the front and asking questions and participating. So there’s this really interesting separation when you have just girls who are working together and if you have mixed classes.

Hopefully in the future that division will go away and you won’t need to educate girls separately so that these biases become apparent from a very young age, because when they’re really young then there is no difference. When they’re really young, I have like 4-year-olds coming up to some Lycra universe demonstration where you have a piece of Lycra and you put a heavy ball in the middle of this, it stretches out the fabric of spacetime, and everyone wants to play with the golf balls that go in orbit around the sun; and girls and boys both do this. But just a little bit older and they already start to become shyer, they are embarrassed to ask questions, they don’t want to show off, they feel easily embarrassed. Boys don’t seem to suffer the same thing. So I hope that in the future one of the ways that we can measure this improvement is by having young children always being at the same level of interest. Of course not everyone is going to be interested in black holes the way that I was. But maybe they’ll find something else which is interesting to them and they’ll be the version of me for geology or for language studies or for whatever it is that they want to pursue.

What would you be if you weren’t a scientist?

Oh my God, what are you doing to me? Okay, well if I weren’t a scientist I would probably be a linguist. I speak four languages, three of them fluently, and I never pursued that as a career because it was always really easy and I thought that that was somehow cheating. I was like, well if I do this and it’s easy, then what if I really challenged myself, what could I do if I really challenged myself? So I did my masters degree in Italian, in Italy, and spent some time in Spain and also learned Spanish, and I grew up in Ottawa where I went to French school. So English, French, and Italian are very good, and Spanish is okay, and you can usually get away with putting some Italian words into your Spanish in order to make yourself understood. But linguistics has a lot of the same kind of logical rules and structures that you have in science, especially when you go back to Latin. So I find that kind of stuff really interesting, and so I think that I could apply my same kind of curiosity and love of analysis of things and looking for sources to something like linguistics.

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