Random Walk LISA will measure ripples in space- time. signals, gravitational-wave detectors such as LISA mea- sure waves from all directions, as an ear does. In this way, detecting gravitational waves is like hearing, and with so many potential sources out there, the trick is to figure out which is the black hole and which is the white dwarf. “It’s like listening to an orchestra and trying to tell which is the cymbal and which is the flute, or which is the first violin and the second violin,” says E. Sterl Phinney (BS ’80), profes- sor of theoretical astrophysics, chair of the LISA Science and Sources Working Group, and the leader of the team that A S PACE - TIME S YMPHONY developed NASA’s Beyond Einstein program. LISA will address a myriad of topics, from the astrophysics of black If you could hear the their own undulations of be capable of detecting more holes to particle physics, to sounds of space and time, space-time. sources. When launched, it fundamental mysteries about the universe would be a noisy Although first predicted by will be the only instrument the birth of the universe and place. When those bizarre, Einstein’s theory of general of its kind in space, a mission the nature of gravity. In Sep- light-bending, space-curving, relativity in 1916, gravita- that will observe the universe tember, the National Research and time-warping objects— tional waves have yet to be de- as never before, listening to Council, which provides black holes, neutron stars, tected. While scientists hope the cosmic cacophony that so science policy advice for the and white dwarfs—meet, ground-based observatories far has been silent to us. government, recommended mingle, and merge, they dis- like the Laser Interferometer Gravitational waves are that LISA be made the flag- turb the fabric of space-time, Gravitational-Wave Observa- vibrations of space-time itself, ship mission of the Beyond sending ripples of gravita- tory (LIGO), run by Caltech and they jiggle everything Einstein program. tional waves across the cos- and MIT, will identify a signal they pass through, such as a Among the more promis- mos. But it’s not just black soon, detection is virtually planet or spacecraft—similarly ing phenomena the spacecraft holes and their brethren that guaranteed by the much-an- to how sound waves jiggle will study is the merging of create these waves. The Big ticipated Laser Interferometer the tiny bones in your ear, supermassive black holes. Bang itself, and maybe even Space Antenna (LISA). LISA allowing you to hear. Unlike These events are some of the more exotic objects called will aim at much lower fre- most telescopes, which point most violent and powerful cosmic superstrings, all make quencies than LIGO, and will in a certain direction to detect in the universe, and likewise 36 ENGINEERING & SCIENCE NO . 4 2 0 0 7 produce some of the strongest LISA should also be able to frequency. How quickly the and are detected, they would gravitational waves. When detect a supermassive black orbital frequency changes tells be a great discovery, he says. two of these behemoths meet, hole eating a relatively tiny scientists how fast the system “It’s something of a long shot, they spiral in toward each one, a few times the mass of is losing energy, which then but it’s a really exciting op- other. According to astrono- our sun. But because the stel- tells them how strong the portunity.” mers, nearly every galaxy has lar-mass black hole is millions gravitational waves are. Just While the science prom- a supermassive black hole at to billions of times smaller than as light looks dimmer with ises to excite and amaze, the its center, and when galaxies the supermassive one, it works greater distance, the strength spacecraft is a remarkable feat collide, the central black holes as what physicists call a “point of detected gravitational waves of engineering in and of itself. often merge—which can hap- test mass.” As the smaller black drops if the source is farther LISA consists of three identi- pen somewhere between once hole circles its giant partner, it away. By comparing the mea- cal spacecraft in a triangular and 300 times per year. follows every curve of space- sured strength of gravitational formation. In order to detect Astronomers are find- time. The gravitational waves waves with the theoretical the frequencies researchers ing that the evolution and betray its path, telling physi- value, researchers can figure want, the triangle has to be formation of galaxies are cists how space-time bends out how far away the system gigantic—five million kilome- inextricably tied to their around the supermassive black is. If the two black holes are ters per side, or the same dis- merger history and to their hole. For the first time, physi- coupled with an electromag- tance you’d cover if you drove central supermassive black cists would find out if black netic source, such as when the to and from Pasadena and holes. But since their black holes behave as they think they black hole eats surrounding New York about 1,120 times. holes are always shrouded in do, Phinney says. gas and dust, LISA will make Each craft holds two identical gas, dust, and stars, scientists Merging supermassive black the most accurate measure- instruments, and each instru- can’t directly observe them. holes could also serve as the ments yet of the universe’s ment encases a shiny, free- Gravitational waves, however, most accurate yardsticks yet expansion. Measuring floating, four-centimeter cube zip through everything at the of the universe. A black hole cosmological expansion means that acts as a test mass. Laser speed of light, and with LISA, binary system, in which two measuring dark energy, the beams that bounce between a researchers would be able to black holes orbit each other, mysterious stuff that makes cube in one craft and a cube make the first direct observa- loses energy as it produces up roughly 70 percent of the in another form the three tions of merging black holes. gravitational waves. The universe. “LISA could revo- sides of the triangle. When a “They will tell us something strength of the waves reflects lutionize dark-energy studies,” gravitational wave zips by, it very fundamental about how how much energy is lost. As Prince says. shifts the distance between the galaxies evolved,” says Tom the system loses energy, the Furthermore, gravitational test masses by a tiny amount. Prince, professor of physics, two black holes spiral closer echoes of the Big Bang give The laser beams also shift, the U.S. mission scientist for together, spinning around astrophysicists a powerful way giving scientists a measure- LISA and cochair of the LISA each other faster and faster, to study the universe during ment of the gravitational International Science Team. increasing the system’s orbital its first second of existence. wave. The shifts in distance Conventional observations, are so small that the instru- by way of electromagnetic ment needs to be accurate to waves—light—only allow re- 10 picometers—smaller than searchers to look back to when any atom. Meanwhile, all this the universe was 300,000 years is trailing Earth by 20 degrees old. Before then, the universe of its orbit around the sun, a was a hot plasma soup, too distance equivalent to 25 mil- thick for light to pass through. lion kilometers. But because gravitational One of the biggest chal- waves can pass through the lenges engineers had to over- primordial soup, LISA may be come was that of designing a able to reveal the universe in its spacecraft that would protect infancy. the test mass and keep it in But wait, that’s not all. its smooth orbit. Given the One of the more exotic gravi- extreme sensitivity of the in- tational-wave sources could strument, normally negligible be vibrating cosmic super- effects such as the force from strings, long, one-dimensional sunlight and the gravitational objects that stretch across the field of the spacecraft itself universe. Waves on those must be accounted for. One strings, which were produced solution was to install micro- during the Big Bang, would thrusters to counteract every At the heart of LISA are the free-floating test masses like this one. Tiny move at the speed of light. inadvertent bump. shifts in distances between the test masses would mean a gravitational They would flop around like In 2010, the LISA Path- a loose garden hose, creating finder mission will test this wave is passing through. The cubes’ polished surfaces reflect lasers between gravitational waves, Phinney delicate ensemble. The the spacecraft to measure the shifts. explains. If these strings exist mission, led by the European 2 0 0 7 ENGINEERING & SCIENCE NO . 4 37 Space Agency and with JPL supplying the thrusters, will test the technology in a true zero-gravity environment. There’s no environment on Earth that’s as quiet as the space environment that LISA will experience, Prince says. So to make sure that research- ers understand how the in- strument works, they have to send a prototype into space. M ARS R OVERS : T HE N EXT G ENERATION The real LISA, a collabo- ration between NASA and ESA, won’t fly until 2018 at the earliest. The greatest hurdle so far, Phinney says, is NASA scientists are seeking last much longer than we closer to the poles. At a con- whether NASA will provide big pieces of an even big- anticipate, as the rovers Spirit ference in late October, the enough funding.