First there were optical telescopes. Then radio wave detectors and X-ray observatories gave researchers new "eyes" with which to view the cosmos. The next big window to the universe may be through gravitational waves.
Through a project called LIGO (Laser Interferometer Gravitational-wave Observatory) -- a joint effort of the California Institute of Technology, MIT and a group of about 20 other institutions represented in the LIGO Scientific Collaboration supported by the National Science Foundation (NSF) -- scientists expect to "see" the universe in a fundamentally new way.
Or with a whole new wave. In 1915, Einstein showed that accelerating masses, such as those occurring in an exploding star, create gravitational fields that radiate from their sources much like ripples spreading from a stone dropped in a lake. These fields, which warp the shape of space and time and travel at the speed of light, are called gravitational waves. They are difficult to detect because they interact so weakly with matter.
By the time they reach us, gravity waves, roaming through space since the beginning of the universe with little to dampen them, are minuscule disturbances in the space-time fabric.
Unlike electromagnetic waves such as radio waves or X-rays, gravity waves are not scattered as they travel. Scientists hope that this means they will open a unique window into the innermost and densest regions of space and provide information about violent events in the regions where they originated.
"What we're trying to do is actually explore nature in a way that is different," said Rainer Weiss, professor of physics at MIT and LIGO's integration scientist. "That's what's so exciting."
AMBITIOUS UNDERTAKING
To date, researchers have indirect but strong evidence that gravitational waves actually exist. This came from radio observations of a binary stellar system consisting of a pair of compact stars made by 1993 physics Nobelists Russell A. Hulse and Joseph H. Taylor Jr. at the University of Massachusetts and Princeton University.
The system exhibits an energy loss best attributed to the radiation of gravitational waves. To detect gravity waves directly, researchers need an extremely sensitive instrument: one capable of a task tantamount to identifying a single, particular grain of sand on all of Cape Cod's beaches.
The unprecedented sensitivity required to directly detect gravitational waves has made the development of the detectors a first-rank technical challenge of such difficulty that it is sometimes met with skepticism.
To make matters worse, once it was realized that gaining the requisite sensitivity would involve large baseline systems and thereby large costs and all the trappings of big science, new challenges became evident. The scientists in this field had to convince their colleagues that not only would the technology development succeed but also that the eventual scientific returns were worth the costs. Even with consistent support by the NSF, it took several decades to develop the scientific case. LIGO represents years of effort on the parts of Professor Weiss and many other scientists.
The decades of persistence paid off. Late last year, a first-of-its-kind instrument to search for gravitational waves was inaugurated in Livingston, LA. A twin facility exists in Hanford, WA. The $300 million LIGO project is the largest single enterprise ever funded by the NSF.
The detection concept is to measure the time it takes light to travel through the space distorted by the gravitational wave using precision interferometry. At each facility, laser light is injected from a central building into orthogonal tubes 2.5 miles long. Mirrors at the ends of the tubes reflect the light back to the central building.
A gravitational wave, should one happen along, will increase the light travel time in one arm while decreasing it in the other. This time difference is converted into a change in intensity and measured on photodetectors.
Among the cataclysmic cosmic events that should produce detectable gravity waves are compact stars (neutron stars or black holes) spiraling into companion compact stars, the collapse following a supernova, the characteristic oscillations of space-time when a black hole is formed or perturbed, the motion of nuclear fluid on the surface of a neutron star, or the waves produced by the Big Bang itself.
In addition to attempting to answer what Professor Weiss calls "sharpshooter questions," for which there is a specific prediction, LIGO researchers and their instrument also will be open to "buckshot questions," which are more broad-based. "I think it's glorious to go looking, not knowing what you'll find," he said.
THE SEARCH IS ON
The search for gravity waves is only around 40 years old and involves a handful of scientists around the world. Professor Weiss's interest was sparked early on, shortly after "acoustic bar" gravitational detectors were first proposed. By 1970, he had already come up with the concept of an interferometer-type detector.
In recently renovated space in Building NW17, there are huge metal tanks resembling a large distillery that will be used to develop the next generation of detector. The tanks and their connecting tubes contain "a few optics, a lot of light and very little air," according to David Shoemaker, senior research scientist for LIGO at MIT.
Because the effect of a passing gravitational wave is so small, causing only motions of the order of 10-19 (0.0000000000000000001) meters on the mirrors, every aspect of the instrument must perform at the limits of fundamental physics: the mirrors must show only Brownian motion, and the interferometric sensing must be limited by quantum effects.
In the future detectors presently under study, even Heisenberg's uncertainty principle comes into play.
PLANNING AHEAD
The detectors, designed and constructed by Caltech, MIT and members of the LIGO scientific collaboration, are expected to record their first observations in 2002. Researchers already are preparing for the future with a more sensitive LIGO II, which is planned to be more sensitive by roughly a factor of 10.
Because the volume of space "seen" by the detector grows with the cube of the sensitivity, if only one event were detected during the first go-round, 1,000 events would be found with LIGO II.
In addition to US-based efforts, there is another interferometer planned in Australia and smaller units in Germany and Japan. In Italy, a detector called VIRGO is expected to come on line in 2002.
The ground-based detectors would be complemented by a space-based antenna planned for a decade down the road. LISA, the space-based project, a joint effort by the European Space Agency and NASA, would search for gravitational waves at much lower frequencies than the ground-based observatories. LISA would involve laser ranging between satellites three million miles apart in solar orbit.
Researchers expect to get dramatic results from LISA, as well as before LISA is up. "There is a reasonable chance that the currently planned ground-based interferometers will detect gravitational waves from astrophysical sources," Professor Weiss said. "Current estimates of source strengths and rates would predict a high probability of detection with the sensitivity improvements associated with LIGO II. Using the current knowledge of astrophysical phenomena, the LISA project is expected to make high signal-to-noise detections of massive black collisions throughout the universe.
"There is a strong chance," he added, "that gravitational wave detections will become a standard branch of observational astrophysics in the next two decades."
A version of this article appeared in MIT Tech Talk on March 29, 2000.
