10 Years in Gravitational Wave Detection

Peter Saulson, Syracuse University saulson@phy.syr.edu

Looking back ten years in the field of gravitational wave detection is especially handy, since the life span of Matters of Gravity roughly coincides with the life span of LIGO (the Laser Interferometer Gravitational Wave Observatory, in case anyone doesn't know) as an approved project. There is of course a long pre-history to LIGO - the inception of the idea, planning and feasibility studies, technology development, and, not least, lobbying the physics community, the NSF, and Congress to earn approval. Still, life changed for all of us when LIGO was approved.

I vividly remember the moment when I learned that the construction of LIGO had been approved. Abhay Ashtekar, who has always been much better plugged in than I, greeted me in the hallway of the Physics Building at Syracuse University with an outstretched hand and a big grin. As he shared the news, I confess that my own feeling was not the unalloyed happiness that showed on his face. I think I mumbled something like, "Oh, shit, now we have to make it work." It was a time in my life when the technological hurdles facing LIGO seemed especially daunting. Today, when a great many bullets have already been dodged, the fear that I felt seems much less justified.

While I'm confessing my old fears, perhaps I should mention another moment when the enormity (literally) of what we have taken on in LIGO came home to me. It was several years later, during my first visit to the LIGO site at Hanford WA, when I walked through the then-empty experimental hall at the vertex, euphoniously called the LVEA (Laser and Vacuum Equipment Area.) Even more than the 4 km arms, the vastness of this workspace brought home to me the magnitude of what we had persuaded the American taxpayer to support. Now that the Hanford LVEA and the one at its sister site at Livingston LA have been filled with their vacuum chambers and then in turn with the optics for LIGO's interferometers, the scale seems fully appropriate. LIGO is big, but it is big for a good reason - to maximize our chances of detecting gravity waves.

Of course, the detection of gravity waves remains to be accomplished. But much along the way has been done successfully, and with a certain amount of style. The aforementioned remote laboratories have been built, equipped, and staffed. All three of LIGO's interferometers have now been installed. They are in various stages of commissioning; the 2 km interferometer at Hanford (the first of the three to be installed) has "locked" in essentially its full servo configuration, and the 4 km interferometer at Livingston has gone almost as far. The servo engineering that has made this possible is a real tour de force, with gains (and signs!) of feedback switching as the interferometer progresses through a series of states approaching the full Power-Recycled Fabry-Perot Michelson configuration. A few years ago we didn't know how to do this, but now it works.

Now that the interferometers are moving into a state where they function, work is commencing on understanding their performance. It must be said that at present the noise levels are substantially poorer than the design performance of the initial LIGO interferometers. But some of the reasons are well understood, so it is reasonable to hope that what is a huge gap at the present will start to close rapidly. Of course, it is hard to predict how quickly the last order of magnitude will be crossed.

This technical progress could only have come about through progress in the social organization of LIGO. The first proposal for LIGO in 1987 (the one before the successful 1989 proposal) listed 18 members of the team. Now, a scan of the LIGO roster reveals over 180 names of staff at Caltech, MIT, Hanford and Livingston (including only one name of the original 18.) Growth of this magnitude could not have been effectively managed without leadership by people experienced with large projects. This expertise came to LIGO in the person of Barry Barish and of the colleagues he brought with him to LIGO from high energy physics in the middle of the last decade.

The social/scientific structure has grown in another important way as well. LIGO is now organized into two bodies. The staff above constitute the LIGO Laboratory, the group responsible for ensuring that the LIGO interferometers function properly. Direction of LIGO's scientific program lies in a larger body called the LIGO Scientific Collaboration (or LSC), consisting (in a recent count) of 112 scientists, engineers, and technicians from within the Lab, plus an additional 239 members from 27 other groups from across the U.S. and, indeed, the rest of the world. And the LSC continues to grow; at its last meeting in August, two new groups joined.

In addition to the dramatic growth of the number of people working on LIGO, another important change has taken place in the style of work. With the installation of the LIGO interferometers at the Observatories, the focus of work has shifted toward the sites. In addition to the staff that have moved to Washington or Louisiana, this has meant a great deal of travel by experts based at Caltech and MIT. Some non-Lab LSC groups have also been a big presence at the sites. I had a chance to view this process first-hand in 2000 when I spent a sabbatical year at the Livingston Observatory. A substantial chunk of the important work was being done by the group from the University of Florida, who supplied the interferometers' Input Optics. Another important presence was that of the growing group at neighboring LSU, who are vital participants in the commissioning work.

Looking back, I must confess that one of my most vivid impressions from that year at Livingston was yet another epiphany of the magnitude of LIGO's work. Early on, I realized that we needed a set of design documents for reference while we worked on commissioning the interferometer. I searched LIGO's on-line Document Control Center, and by the time I was done I had found a bookshelf full of subsystem descriptions, analyses, and plans. This brought home to me not only the complexity of LIGO as a scientific instrument, but the remarkable intensity and quality of the work it has taken to produce it. Visit a LIGO site and marvel at its size, as I did at first; then pause to admire even more the richness of labor it has taken to turn that site into a gravitational wave detector.

Even as commissioning of the interferometers goes on, work has been progressing on preparing to collect and analyze data to search for gravitational waves. Here is another massive effort, largely invisible to outsiders, that has almost completed the data analysis system that will enable the 24/7 search for the gravitational wave needles in the haystacks of data that LIGO will produce.

Over the past year or two, the interferometers have been exercised in a set of Engineering Runs that practiced collecting data for extended periods, while the software has been tested in a set of Mock Data Challenges that practiced analyzing the data. These parallel efforts will come together in an Upper Limit Run, now scheduled for around New Years. We will run the interferometers for two weeks, collect the data, and analyze that data to the point of being able to make scientific claims about the presence (or, more likely, absence) of gravitational wave signals in it. Throughout 2002, LIGO plans to intersperse interferometer improvement with data-taking periods, until full-time operation at design sensitivity is achieved.

While all this has been going on, LIGO has been preparing increasingly detailed plans for a new set of interferometers to be installed at the sites after the initial LIGO Science Run has been completed. Advanced LIGO will have roughly an order of magnitude better strain sensitivity than the initial interferometers, which ought to be enough to guarantee that known sources (especially binary neutron star inspirals) will be within reach of detection.

Most of this review has focused in a rather parochial fashion on the American effort, but it needs to be stressed that equally impressive work is going on at a number of other places around the world. A Japanese 300-meter interferometer, TAMA, has already started operating, and is showing remarkable performance. The British-German GEO 600-meter interferometer near Hannover is almost completely installed; it features advanced mirror suspensions and optics that will pave the way for parts of Advanced LIGO. The 3-km VIRGO interferometer near Pisa (a joint French-Italian project) is also well along; it is demonstrating advanced seismic isolation systems that will let it probe to lower frequencies than any other instrument yet built. One other admirable development should be stressed - the remarkable extent to which scientific cooperation is being maintained between all of these nominally competing efforts. LIGO and GEO have become especially close, with the GEO team having joined the LSC and a full reciprocal data exchange agreement having recently been concluded.

The past decade has seen another remarkable occurrence on the way to the establishment of the science of gravitational wave detection. An interferometer in space has been a gleam in a few pairs of eyes for almost as long as LIGO has, but now it appears well on the road to becoming a reality. The LISA project garnered strong European support several years ago, and appears to be marching inexorably toward becoming a key part of the NASA program as well. With a projected sensitivity that easily places it in a position to study a range of interesting sources, LISA has also been blessed with a remarkable degree of support from the astronomical community.

With any luck, the next 10 year review will be able to look back on the detection and study of gravitational waves.

Jorge Pullin