Letter: Focusing on gravity

From Barry Barish, Caltech, director of LIGO, James Hough, University of Glasgow, co-director of GEO600 and member of the LISA International Science Team (LIST), Bernard Schutz, Max Planck Institute for Gravitational Physics, Golm, co-director of GEO600 and member of LIST, Kip Thorne, Caltech, co-founder of LIGO and member of LIST and Rainer Weiss, Massachusetts Institute of Technology, co-founder of LIGO

The goal of detecting gravitational waves is, as Stephen Battersby writes, truly breathtaking (7 September, p 26). But we strongly reject the suggestion implicit in his article that our project, known as LIGO, offers an inefficient route to this goal.

LIGO’s task is daunting. It must monitor the separation of mirrors 4 kilometres apart to an accuracy of 1/1000th the diameter of an atomic nucleus, while protecting the mirrors from all forces except gravitational waves. But the reported claim of one critic that we “could have developed better technology first on a smaller machine” overlooks the fact that all the technologies required for LIGO had been invented and tested by the early 1990s.

The key challenge is to make these technologies robust and work together in a complex gravitational-wave detector 100 times larger than our 1990s test beds. That is why we formulated a staged plan to scale up the equipment.

Step one was to build LIGO’s 4-kilometre-long vacuum systems and install initial detectors. These are not “pushed to the limits of current technology” but use simplified designs and the most robust of our technologies.

Step two, where we are now, is to perfect the interactions of these technologies and look for gravitational waves for several years at a sensitivity where the most optimistic estimates say waves will be seen. For step three, in about 2007, we will upgrade to our complex designs and delicate technologies in detectors sensitive enough for even the most pessimistic estimates.

LIGO is an international collaboration of 440 scientists from 16 countries, and includes the smaller British-German GEO detector. GEO takes part in the initial LIGO wave searches while testing the interactions of LIGO’s advanced technologies. GEO’s mirrors are kept still by hanging them from fused quartz fibres attached to cascaded pendulums. The fibres, developed in the early 1990s, were too delicate for the initial LIGO while the pendulums, developed in the 1980s, were too complex. But both will be used in LIGO’s 2007 upgrade, as will “signal recycling”, described by Battersby as a “new” technology even though it was invented in the 1980s.

LISA, the Laser Interferometer Space Antenna (planned for launch in 2011), will look for gravitational waves with wavelengths 10,000 times as long as those LIGO seeks, so its view of the sky will be radically different. LIGO and LISA are pricey, but extensive and repeated peer reviews have concluded that their pay-offs are worth it. Through the new windows they open onto the dark side of the Universe we will probe curved space and warped time in ways not even imagined a few decades ago.