Nano Meets Astro: A Dialogue with MacArthur Recipients Michal Lipson and Nergis Mavalvala

The two explored some of their shared interests in a teleconference with the Kavli Foundation on Dec. 6, 2010. They had never talked to one another before, and the conversation was as much a personal introduction as a scientific dialogue. Is a collaboration in the works? Time will tell, but the dialogue suggested a number of potential areas in which they might work together, from the detection of black hole horizons to the management of light forces. “If you look at the overlap between the kinds of things we do, at least in concept it’s actually huge,” Mavalvala said. Lipson responded: “We absolutely must sit down” and meet in person.

Michal Lipson is an associate professor in the School of the Electrical and Computer Engineering at Cornell University and is a member of the Kavli Institute at Cornell (KIC). She has emerged as a leader, despite relatively modest resources, in designing optical and hybrid opto-electronic devices with silicon-based fabrication methods. Lipson holds numerous patents on novel micron-size photonic structures for light manipulation, and she has pioneered several of the critical building blocks for silicon photonics including the GHz silicon modulators. In addition to the 2010 MacArthur Fellowship, Lipson's honors and awards include OSA Fellow, IBM Faculty Award, and NSF Early Career Award.

Nergis Mavalvala is a professor of Physics at MIT and is on the faculty of the MIT Kavli Institute for Astrophysics and Space Research. She has been involved with the Laser Interferometric Gravitational Wave Observatory (LIGO) since her early years in graduate school at MIT and her primary research has been in developing instruments for interferometric gravitational-wave detectors. Her recent research focuses on minimizing, if not circumventing, the barriers imposed by quantum physics on the precision of standard optical interferometers. Her honors and awards, along with the MacArthur Fellowship, include the 2007 Edgerton award for faculty achievement at MIT, the 2006 John de Laeter medal of the Australian Institute of Physics, and a 2005 Sloan Foundation Fellowship.

Michal Lipson: I was born in Israel, and I grew up in Brazil. And then I went back to Israel to do my undergrad and PhD in physics, in the Technion. Then I did my post-doc at MIT in materials science for two and a half years. And then I came to Cornell in 2001 to the ECE Department.

Nergis Mavalvala: I was born in Pakistan – that’s actually where I grew up, in Karachi – and I came to the United States as a teenager to attend college. I did my undergraduate degree at Wellesley College and I was a graduate student at MIT. I was a post-doc at Caltech, and then I joined the Physics faculty at MIT in 2002. Michal, you must have been at MIT exactly the years I was at Caltech.

Michal Lipson: Exactly, yes

Michal Lipson: I was interested because I did my PhD in quantum microstructures – meaning, basically, it was optical spectroscopy of micro-cavities that confined the quantum wave as well as optical wave. That was very basic physics. And when I did my post-doc, I moved to an area that was also micro-cavities but these only confined the optical wave, not the quantum. And at the time, Lionel Kimerling, who was my post-doc [advisor], was very interested in the field of silicon photonics. And when I came, I used my background in light confinement for this area. That was the basis for starting the group here [at Cornell] – doing strong light confinement for silicon photonics, for applications in optics for microelectronics.

Nergis Mavalvala: My story is not quite as directed as Michal’s. I came onto gravitational waves quite by accident. I’ve been doing gravitational waves since I was in graduate school, [but] I started graduate school working in cosmic microwave background, which is another area of astrophysics. The group I was working in was moving to another university, and so I was kind of shopping around and bumped into Rai Weiss, who eventually became my PhD advisor. If you ever meet him, you’ll know why I didn’t have to think any more about this being the right thing. He is very dynamic. It was such a crazy idea – this is the thing I remember most about starting to work on gravitational wave detection – this was in 1991 – and the ideas were really amazing and far out. I thought, OK, you know, I’m crazy, but this is what I’ve got to do. So I was not as directed. And then I did my post-doc also in gravitational waves, but on a very different side of gravitational waves. For my PhD, I had worked on proof-of-principle tabletop experiments that inform us how to operate more sensitive gravitational detectors. When I was a post-doc and also a few years into my faculty appointment, I was working on the long base-line gravitational wave detectors. Along with many others, I was working on making them work and start being [actual] gravitational wave detectors as opposed to tabletop or prototype experiments.

Michal Lipson: Nergis, I should tell you a little bit about what I’m doing now, which I think you’ll find interesting, hopefully.

Nergis Mavalvala: Yes, I’m very interested.

Michal Lipson: I started working with my father [Reuven Opher] – he is a professor in physics working in cosmology – and we are working now on analogies of black holes, basically demonstrating horizons, or black hole effects, in the lab using photonic elements. So basically it’s all based on the analogy between gravitational waves and optical waves.

Nergis Mavalvala: Wow. I have to say that sounds really far out. It sounds like the kind of thing a person could get hooked to.

Michal Lipson: Yeah. … I wouldn’t say this is my main stream of research at all. It’s just one little crazy project. The rest of my projects are more connected to industry in the sense that the industries are very interested to see if there is a solution for the next generation of computers using optics.

Michal Lipson: In this field, there are several papers out there now that – recent Physical Review letters, and a recent Nature paper; though it’s not an area that I would naturally work on, the technology is very much what I do. My father is in cosmology, and I’m in photonics. But this seemed to be a point of [joint] interest – the analogies of black holes using photonics.

Nergis Mavalvala: That’s awesome.

Nergis Mavalvala: I think the overlap is really remarkable. One of the things that Michal works with – and correct me if I’m wrong here, Michal – is light force, and how light force can be used to move nanoscale objects around.

Michal Lipson: Absolutely, yes.

Nergis Mavalvala: I do pretty much exactly that. But my experiments, instead of nanoscale, span from about microscale to kiloscale. Let me just say a little bit about one of the big problems we face in gravitational wave detection. The idea that we are working on is quite simple. You watch the distance between two particles change as a gravitational wave comes by. Now how do you measure that change in distance? You just shoot a laser out at the particle – in our case the particle is a mirror – if you have a very precise clock, you count up how long it took the light beam to go out to the mirror and reflect from it and come back to you, and then that travel time will change as the gravitational wave comes by.

Michal Lipson: OK.

Nergis Mavalvala: So that’s pretty much the principle.

Michal Lipson: Yeah, yeah.

Nergis Mavalvala: The problem of course is the light force. It’s a fundamental problem in quantum measurement, which is [that] your meter, which is the light, is also kicking your particle, which in our case is a kilogram-scale mirror, but because we use huge amounts of laser power, the actual momentum transfer is very significant, and we do disturb the measurements. So I think if you look at the overlap between the kinds of things we do, at least in concept it’s actually huge.

Michal Lipson: Yes, yes. Oh my God, we absolutely need to sit down.

Nergis Mavalvala: And maybe even detect some black hole horizons.

Michal Lipson: Yeah. I think if we talk enough, something will come out of it.… I’m sure there’s a lot of stuff that we’ve developed you could apply to your research, and the other way around.

Nergis Mavalvala: In fact, in my log – this was a little bit of a secondary experiment where we use laboratory-scale tests of effects we expect will be dominant in our long kilometer-scale detectors, but you know we have a cryogenically cooled micro-cantilever which we move around. It’s coupled to an optical cavity, just like you do – you know, confined optical fields. It’s at the nanoscale, and we’re basically studying the quantum back-action of the photon fluctuations on this little cantilever motion.

Michal Lipson: Fantastic.

Nergis Mavalvala: So if nothing else, maybe you’ll help me make good devices.

Michal Lipson: Absolutely.

Michal Lipson: When I started my research here, the main goal was overcoming the fact that silicon, which is the material we make all of our microelectronic devices from, is not an optical material. It doesn’t have at all an electro-optical [effect]. It doesn’t have any of the optical properties that are usually seen in optical materials. Silicon is really an electronic material. And we basically make it an optical material by confining light, by making light stay in one place for a very long time, and increasing the interaction of light with the material. So even just a very, very small electro-optical effect] gets amplified by the fact that light is there, bouncing around many, many times and interacting with the material.

Michal Lipson: Yes. It’s still a major goal. We’ve demonstrated that it might be possible to overcome some of these hurdles, but it is still definitely very much a microelectronic material.

Nergis Mavalvala: Well, definitely. I’m far, far from an expert on materials, but the material that we work with in our micro-cantilevers turns out to be a cousin of silicon. We use aluminum gallium arsenide, and we use it precisely because for our purposes its optical properties are actually quite a bit better than silicon at the wavelengths of light we are using. So I’ll throw that out there. But to come back to your original question -- in gravitational wave detection, if you ask what are the biggest hurdles to be crossed, I think they’re three-fold. One comes, I think, from the theoretical side, which is [that] we actually don’t know in many cases exactly what the gravitational wave will look like from a given source. And it’s partly because we don’t know some of the parameters of the objects we are looking at or looking for. That’s really an astrophysics theory and general relativity theory type of issue to solve. [There is a] very clever cohort of people working on that. I don’t work on that part of the physics myself at all. The second challenge is on the detection side, how do you make the detectors more and more sensitive. There I think one of the problems is the one I described – the fact that you have light that is made up of photons, these discrete particles that have quantum properties, and that the quantum properties of the light – the quantum noise of light -- end up being a limitation for our detectors. The third problem – which is maybe fundamentally harder to solve – is that when you’re trying to make a measurement, when you’re trying to say, “I have this test particle whose motion I’m going to measure as a way of seeing if a gravitational wave comes by” – you have to make sure nothing else moves it. And it turns out we live on a very vibrant planet. Everything else on the planet wants to move our mirror more than the gravitational waves.

Nergis Mavalvala: We have to be a little careful there. Everything we do right now in gravitational wave detectors themselves are very sort of macroscopic-scale things. [But] I can give you one example of a place where nanomaterials and structures can help us. Our mirrors have optical coatings on them just to make them very reflective at the wavelengths of the light that we use, and those optical materials that we use for coatings have limitations. There are people who are thinking about using heterostructure materials as high-reflectivity coatings that may not have some of the mechanical problems that our standard coating materials have. So that’s a place where we look to the materials world to give us some insight on how to do things. Certainly I can imagine that the optical sensing techniques that Michal uses either are similar to ours -- because we have the same issues of making sensitive measurements -- or can teach us something. I don’t know, maybe Michal, you can tell me.

Michal Lipson: I have to think about it a little bit, but I’m sure there might be some overlap there.

Michal Lipson: Not directly, no. But I must say that both my father and my twin sister are astrophysicists, so it was always a goal to try to find a common language between the three of us, my father, my sister and me. So maybe Nergis can help me here. [Laughs]

Michal Lipson: It was one of my strongest decisions to actually go and do something that’s not pure physics, absolutely.

Michal Lipson: It’s pretty hard. Right now, my father is actually visiting me here in Cornell, and he’s sitting next door studying all the [science] papers that suggest analogies, digging a little deeper trying to see exactly what would it mean to demonstrate some kind of, in this case, a black hole. What would the astrophysicists learn from that? That’s the goal – trying to find out what information would we learn by bringing these two fields together.

Michal Lipson: Well, like Nergis said, a lot of the micro-mechanical work where we move little objects using optical forces -- overlap with the gravitational wave community. They are very interested in detection of ultra-small forces or movement. In that respect, yes. I had several discussions with colleagues who are more involved in gravitational waves. Nergis, I’m sure you’re aware of the very strong community at Caltech for example involved in optomechanical [research], people like Oskar Painter and Kerry Vahala...

Nergis Mavalvala: Keith Schwab was at Cornell and now is at Caltech, too.

Michal Lipson: Oh yes, Keith Schwab, absolutely, absolutely. We had several discussions. It didn’t actually lead to anything concrete, but we did bounce a few ideas.

Nergis Mavalvala: I want to add that this concept of bouncing ideas off people in other fields is highly underestimated. This is really how many new ideas come about. Certainly some of my quantum measurement and position measurement work has come from talking to people in the micro-cantilever community. So I’m a big fan of bouncing ideas, even if at first the idea makes no sense to me. I’ll [think] what, what are you talking about? And then, you know, a month or two later, after the wheels start turning, there’s usually an “aha” moment: Oh, that’s what she meant!

Michal Lipson: Absolutely, absolutely. I am definitely coming to visit you the next time I’m in Boston, and actually my sister is moving to Boston University, so I’ll be there and you’ll be the first one I’ll come to visit.

Nergis Mavalvala: Please do.

Michal Lipson: I think a lot of what drives the communities is funding. There’s no question about it. There are several programs that encourage different communities to interact, and I think that’s helpful. But I’d also like to see more interdisciplinary funding opportunities. That definitely will induce a lot of cross-fertilization.

Nergis Mavalvala: I agree. You know, when you keep and open mind and you go to talks and you hear some ideas, you may never -- at least I, with my limited abilities can’t always -- get to the next stage and say, OK, this is how this idea could be worked out in my lab. So I think having more targeted opportunities, like what Michal was saying, symposia that are designed to bring together people from different areas, and funding opportunities and symposia where people are … forced to say, here is what I do, here’s how I do it and here’s how I think it could be useful. That’s really important. And I have to say, my sense is that the funding agencies have become aware of that in recent years. Certainly the word goes around a lot: “cross-disciplinary.”

Michal Lipson: Yeah, I agree. I agree.

Michal Lipson: Thank you very much for bringing us together.