As an undergraduate, Sheila Rowan did a summer project in gravitational-wave detector instrumentation. "I was hooked. It was such a fascinating subject, I could not think of a more interesting thing to do," she says.
Just over 20 years later, Rowan heads the same Institute for Gravitational Research at the University of Glasgow where she earned her bachelor's and PhD degrees in physics.
She and others in her field anticipate that their efforts will soon pay off: The new generation of detectors will likely detect the ripples in spacetime caused by inspiraling neutron binary stars or some other violent cosmic event.
Sheila Rowan. CREDIT: University of Glasgow
The Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) is set to start up this month in the US, Advanced VIRGO will turn on next year in Italy, and interferometers in Japan and India are expected to follow within a few years. (See the story in Physics Today, September 2015, page 20.) "People have been working in this field for a long time. The next few years are going to be particularly exciting," Rowan says.
Physics Today's Toni Feder spoke by phone with Rowan.
PT: How did you get into physics?
ROWAN: Believe it or not, I was one of those people who, from when I was very small, always wanted to do physics.
Like many people, the things I found most interesting were the fundamental questions: When you look up and see the stars, how far does space go? How do we know? How far can you go? And what do you see when you get there? And fundamental particle physics—what is stuff made of? How far can you go in that direction? As soon as I understood that those were questions to ask and answer, that is what I wanted to do.
PT: What hooked you on gravitational-wave physics?
ROWAN: It was the way in which the field works. You can do your work with maybe just a small number of people and on a tabletop in the lab, and develop something that's going to contribute to a much bigger picture that involves understanding how the universe works. The combination of these two things I found very appealing.
And it's a very collaborative field, more so than in many areas. A very stimulating part of the job is working with other people. Nonscientists sometimes have a picture of scientists always working on their own in a very solitary fashion in a lab and wearing a white coat. In some parts of science, that is far from the truth.
PT: How do you describe gravitational waves to lay people?
ROWAN: There are very mathematically precise descriptions that people would use if they are gravitational-wave modelers or theorists talking to one another. But the description for talking to nonscientists, or non-gravitational-wave scientists, is actually not too bad in terms of picturing what gravitational waves are.
Think of the universe with nothing in it, no stars, no planets. Just a flat, empty rubber sheet. Then you come along, and you put some mass in the universe. You put a star or a planet on it. You can imagine that the sheet sags under the weight. What does that physically look like? Well, the sheet curves. If you stuck down a second planet, what would happen? The planets would try and roll together.
If the distribution of the mass suddenly changes or does something violent—say a star explodes—then the curvature it has caused will change, and that change in curvature is like a ripple, spreading across the sheet. That ripple is a gravitational wave. That is a geometrical way to picture what the equations of general relativity describe.
PT: You are a professor at the same university where you earned your degrees. What's that like?
ROWAN: Apart from five years when I was mostly at Stanford [University], I have been here. I had a split position, where I spent 75% of my time in Stanford and 25% of my time in Glasgow. I typically spent three months in the US, and then came back to Scotland for a month, and then went back to Stanford. I was employed as a postdoc in both places. I did that between 1998 and 2003. It was in the middle of the tech boom in the Bay Area, and it was a very interesting place to be. I had a fantastic time. Then a faculty position came up back here in Glasgow, I got a very good offer, and I've been a faculty member ever since.
In a scientific career, it's genuinely beneficial to work somewhere else for a while, because it gives you a completely different perspective on how science can be done.
PT: What were some of the new perspectives you gained at Stanford?
ROWAN: At Stanford lots of students and postdocs were very keen and went out and worked for startups, or started up companies themselves. I managed to resist that pull and stayed in science, but it was fascinating to watch that happen. It's a fascinating environment out there. Stanford is a very special place in terms of the whole spin-off culture. I do find that side of science interesting, but my heart is in my research and in the academic side.
PT: Have the connections with industry that you developed during your years at Stanford remained relevant for you?
ROWAN: Yes. There is a very strong linkage between the kind of research that we do in the gravity field and technologies that are commercially interesting. I still collaborate strongly with people there [in the Bay Area] on the optics and on their coatings. To get the performance we need in the gravity-wave field, we have to talk to vendors about the performance and the details of how they are made, how the coatings are deposited. We push frontiers in a way that is interesting for us, and also help their business and help their product.
PT: How big is the institute you head in Glasgow?
ROWAN: We have 10 faculty members, about 20 postdocs, and about 30 graduate students. The institute has several different research areas. There is the materials side—my own research area. There is also a group working on interferometry, and a group that works on data analysis of gravitational-wave signals. And there is a group that works on space-based gravitational-wave detectors.
PT: Is there any advance that you have made or contributed to that you are particularly proud of?
ROWAN: Yes. The mirror suspensions. They are novel in that they are made using fused silica. The mirror is the point in space that you want to be really very quiet. You want it to sit still and not be disturbed by anything else until a gravitational wave comes along. The mirrors are suspended like a pendulum to try to isolate them and protect them from ground motions.
That was something I worked on as a postdoc. The reason we used these ultrapure glass fibers is that the mirrors are at room temperature, and if something is at room temperature, it has thermal energy, and it's vibrating. How much it vibrates depends on how the thermal energy is dissipated. Ultrapure glass turns out to be a very low-dissipation material, and fused silica in particular dissipates very little.
It was a risky thing to do—to try and hang these mirrors on very fine glass fibers. The mirrors on GEO [in Germany] are about 6 kg. It was very successful, and a scaled-up version is now at Advanced LIGO, where the mirrors are about 40 kg. So far, it's worked well.
Originally the idea to use pure fused silica was from a Russian group. But as a postdoc, working with my PhD adviser Jim Hough, I personally did a lot of hands-on development and demonstration in the lab in making the silica parts work. There are a lot of people in the field, and many people contributed to making it work overall. That whole development is something that I have been particularly proud of being involved in.
PT: What science are you most excited about with the imminent start-up of Advanced LIGO and other new-generation gravitational-wave detectors?
ROWAN: The first detection will be terribly exciting. People have been waiting for this for a long time. I've been in this field for just over 20 years, but there are people in the field that have been working for twice that. No matter what is detected, it will be exceedingly exciting.
We think we have a good idea for some sources as to what signals look like—for, say, two neutron stars spiraling in round one another. It will be terribly exciting to see if what we have predicted is what we see. And once we can do more, once we can start to probe with high sensitivity the details of how the stars merge and behave, there is all sorts of exciting science to do.
Imagine seeing the distortions to the fabric of spacetime caused by two black holes circling round one another and then merging. That is a very exciting prospect.
PT: What are the challenges for developing future, more sensitive detectors?
ROWAN: One of the big challenges in instrument science is how to improve the thermal noise from the optical coatings.
PT: On a different topic, last year Scotland voted to remain part of the UK, but in the lead-up that outcome seemed uncertain. What would it mean for science there if Scotland were to become independent?
ROWAN: In Scotland a significant fraction of research funding comes from the UK research councils, and it was unclear what the mechanism would be for anyone in Scotland to access that kind of funding if we were to become independent. Another area that was unclear was charity funding. Medical charities fund a significant portion of the medical research and have their headquarters in England. So long as we are in the UK, that is not relevant. But if we become a separate country, that is again unclear. Many things were not agreed in advance, understandably, but that left a lot of uncertainty.
The political situation is interesting in the UK. Whilst the referendum was a no-vote, it had significant consequences. It had a very large turnout and engaged a lot of people in Scotland. And when it came to the recent general election in the UK, there was a remarkable situation where I think 56 out of 59 MPs [members of parliament] from Scotland were Scottish National Party candidates. I think it's a reflection that people want Scotland to have a larger voice in the UK government.
It's too early to say what the consequences will be. And as for how it affects science, we'll have to wait and see.
