Kate Dooley ’06, an assistant professor of physics and astronomy at the University of Mississippi, is an experimental gravitational wave physicist and member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration, whose recent discovery proved the existence of gravitational waves—originally predicted by Albert Einstein in his theory of general relativity. (The achievement was recently reported in the New York Times and other publications.) Dooley's work focuses on the sensing and control of the mirrors in the interferometer as well as the incorporation of squeezed light. Her work helped to detect the first confirmed existence of gravitational waves—fluctuations or distortions in spacetime—in September when two black holes violently merged a billion light years away.
When did you first become interested in science and gravitational waves?
My particular interest in studying the universe really comes from my curiosity about what is out there in the universe. As a child, I realized how insignificant we are, in some sense, when you consider how big it is. There was a drive to understand the universe, and it really piqued my interest in this particular field.
Entering the field of gravitational wave research actually originated at Vassar. My professor, Eric Myers, taught the modern physics lab. One of our projects was to build a tabletop Michelson interferometer—a basic version of what the LIGO detectors are. The basic design is a laser shot at a mirror that equally splits the light into two paths (50 percent goes straight through the mirror and 50 percent gets reflected at a 90 degree angle). The reflected light then hits two mirrors some distance away and is reflected back and rejoins. We looked at the interference pattern of the light and I remember feeling very frustrated at the alignment of these mirrors and how terrible a job that was. Professor Myers pointed out the LIGO detectors, saying that they also had interferometers and their mirrors are four kilometers away from each other. He said, “Can you imagine having to align the laser beam to something so far away?” That greatly impressed me, the thought of scaling up this small project.
How did you become involved with LIGO?
I originally became involved with LIGO as an undergraduate after my junior year. In the summer, I spent 10 weeks at Caltech as a researcher in a program very similar to URSI, and that was my first experience working on an experimental research project. I realized then that it was the right fit for me.
While earning my Ph.D. from the University of Florida, I spent all but one year at the LIGO Livingston Observatory in Louisiana. After earning my Ph.D., I moved to Germany for a postdoctorate and spent three years there working at the GEO600 gravitational wave detector—part of the LIGO Scientific Collaboration—near the Albert Einstein Institute in Hanover. The GEO600 detector is a smaller version of the LIGO detector. The really great aspect of it is that it’s a beautiful testbed for experimenting with new technologies that we might want to later install in larger observatories.
What is the nature of your work with LIGO?
I focus on improving the angular sensing and control systems. We have a lot of mirrors that all need to be pointing, precisely, in the right direction so that the laser beams will resonate in the cavities at all times.
How are gravitational waves measured?
Gravitational waves distort spacetime in a way that it stretches it in one direction while compressing it in another, and vice versa. So, if a gravitational wave were to pass through you—coming straight at you—you would first get taller and skinnier and then shorter and fatter, and then repeat the cycle back and forth. What we do is measure this stretching and shrinking in spacetime by using a laser and mirrors. We place mirrors four kilometers apart from one another and effectively see how long it takes for the light to travel from one end to the other and back. We know the speed of light is constant, so if this time that we measure for the light to go four kilometers and back changes, then that means the distance between the mirrors has changed. We’re using light as the ruler to detect gravitational waves.
What was the experience of hearing the gravitational waves for the first time?
The way that the detection has captured public attention has been the most exciting part to witness. I hope it will inspire a new generation of scientists.
When we first became aware of the possible detection [of gravitational waves], I was rather dismissive of the news. It wasn’t until I saw the data with my eyes that I was completely blown away. The signal is beautiful; you can see it in the data with your bare eyes. We never expected our first detection would be so loud and clear.
What created the gravitational waves?
The fact that our first detection of gravitational waves came from a binary black hole system is very exciting. It’s not what we expected. Not only did we discover gravitational waves, but we also observed a binary black hole system; no one had seen one before. We theorized that they should exist, but now we know that they do.
The speed at which they were traveling before they crashed together was about half of the speed of light. It’s certainly nothing that humans can picture. This final merging of the two black holes produced an extraordinary amount of energy that was converted into gravitational waves. The two black holes, one weighed 29 times the mass of the sun and the second, 36 times the mass of the sun. If you add that together, you get 65 solar masses. However, the final black hole we observed weighed only 62 solar masses. That means three times, the mass of the sun disappeared during this collision and it disappeared into the form of gravitational waves. If this energy had been light that we can see with our eyes, this explosion would have briefly out-shown a full moon, even though it happened over a billion light years away.
How sensitive is the equipment you work with?
This is the most sensitive measurement device in the world. What we need to measure is changes in the distance between the two mirrors—four kilometers apart—that is less than 1,000th the diameter of a proton. It’s equivalent to measuring from Earth to the nearest star and being able to tell whether that distance changes by the width of a human hair. That’s the sensitivity we’ve achieved with the Advanced LIGO system.
In order to achieve that, we have to control the mirrors and the laser and stabilize everything with computers and through very extensive technology that’s been developed over the past 40 or 50 years.
What will you work on in the future?
I’m building up a lab at the University of Mississippi to build, design, and construct some new technology that could be installed as part of an upgrade in the LIGO detectors years from now.
One particular project we are working on is to build better seismometers, which are important because we use them to measure the ground motion and predict how it will make the mirrors move and we can then correct for that. What we’re interested in is measuring the side-to-side motion of the ground, but seismometers also register when the ground tilts, which is something we don’t want to measure. My graduate students and I are working on a project to create a seismometer that’s nonsensitive to the tilt of the ground. This should ultimately improve the LIGO detectors to stay operational for longer periods of time. They should also help with the low-frequency sensitivity so that the next time we see a binary black hole, for instance, the signal will be even louder and clearer in the data.
What’s the importance of the discovery?
The achievement took the work of over a thousand people, so it’s a very collaborative effort and requires people with varied expertise—from engineers and theorists, to computer scientists, experimental physicists, and data analysts.
What we have to look forward to is the opening of an entirely new field—the field of gravitational wave astronomy has been born. We can now listen to the cosmos, whereas before we were limited to looking at it with our eyes. Gravitational waves are different from light and we can think of the LIGO detectors as ears, rather than eyes, for observing the universe. Who knows what surprises will be out there.
-Binary black hole graphic courtesy of eXtreme Spacetimes Project; top photo of Kate Dooley by Gabriele Vajente; bottom photo of Kate Dooley by Valera Frolov and Gary Traylor.