One of Einstein's most far-reaching theories describes the existence of gravitational waves. These are moving ripples in space-time caused by enormous cosmic ruptures such as exploding stars. While still theoretical, many scientists believe that, with the right tools, these space-bending waves can be measured and studied, offering us new insights into the nature of the universe. In this month's feature "Waiting for Gravity", author Trudy E. Bell inspects the efforts of the engineers who, in this hope, have built such exotic and expensive tools: Laser Interferometer Gravitational Wave Observatories, or LIGOs.
Bell, a former Spectrum senior editor, explains that, due to the mysterious composition of gravitational waves, it will take quite a sophisticated instrument to detect them. The basis of the misions of the LIGO observatories in Hanford, Wash., and Livingston, La., however, is fundamental interferometry—create a device that measures interference with the behavior of a known physical constant. In the LIGO operation, twin sets of lasers and mirrors are placed at 90 degree angles and fire coordinated pulses at thousands of times a second to complete a circuit over a specified time. If the time is the same for each circuit in the 'L'-shaped layout, then nothing has interfered with the instrument. But if the two circuits deviate, then something is up.
The problem for the scientists and engineers is that all sorts of things could be up for instruments as sensitive as a LIGO: seismic activity, passing trains, even a routine garbage pickup. To counter this, the LIGO designers have put in place a number of sophisticated passive and active isolation systems to make their interferometers as closely tuned as possible to "listen" just for the "sound"—thought to be about 50 to 2000 hertz—of the passing ripple in the fabric of space-time. And they could be listening for quite a while, because it's no sure thing these costly observatories will detect enigmatic gravitational waves anytime soon. So, there are plans in the works to upgrade to an advanced version of the system.
"If we find events and uncover a new phenomenon, that would be very exciting," Michael E. Zucker, the chief of the Louisiana observatory, told Bell. "But if we see nothing—especially after advanced LIGO—that would be very curious and would carry serious implications for our understanding of general relativity and what's out there in the universe."
It's a big place to search for just the right wave. Let's hope they can catch it.