NASA's LISA Array: Using Interferometry to Measure Gravitational Waves

Posted by: Mike Pallante on January 22, 2011 at 2:22PM
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NASA Announces LISA

Earlier this year NASA announced plans to implement the Laser Interferometer Space Antenna, or LISA mission, as early as 2016. LISA, the first space project dedicated to direct observation of gravitational waves, consists of three space craft launched into solar orbit. The three space craft, arranged in a triangle, will use a science called laser interferometry to detect distortions in space-time created by gravitational waves.

Interferometry and Wave Physics, A Crash Course

In wave physics, “interference” refers to the combination of two similar waves into a single new waveform. Interferometry measures the interference patterns. Waves are composed of peaks and valleys (think of waves in a bath tub moving up and down when you dip your toe in the water) and the frequency of a wave is the number of peaks and valleys (or cycles) in a given time frame (usually seconds) and measured in Hertz (cycles per second). The phase of a wave is the particular point in the undulation of a wave at any given time. If two waves with the same frequency are reaching their peak (or crest) in unison they are described as in phase. If two waves of similar frequency and similar phase are combined, their peaks combine resulting in a greater amplitude or overall maximum crest. The new combined wave is the sum of the two initial waves. If two identical waves are reaching their crest at different times then they are out of phase. If two out of phase waves combine, the result is a reduction in amplitude- the maximum crest of one wave combining with the deepest trough of the other. Think of adding -10 and 10, the sum is zero! An interferometer is a system which combines, or interferes, waves. Most interferometers use electromagnet waves, or light.

A Michaelson type Interferometer splits an incoming laser beam in two identical beams, usually with a partially reflective mirror, sending each beam along a unique path. The beams are reconstituted and an optic reader compares the combined beam to the original source - a phase difference between the two reconstituted beams results in a difference in intensity and indicates a difference in beam paths. If one beam traveled farther than the other, the beams fall out of sync.

How Phase Differences Reveal Gravitational Waves

Interferometry measures information about combined discrete wave sources; however, by a happy accident interferometry may reveal much about gravitational waves as demonstrated by the Laser Interferometry Gravitational-Wave Observatory, or LIGO, a ground based observation station. LIGO operates as a Michaelson Interferometer with paths, or “arms”, over 4.2 Km long. A beam fires into a central point and splits at 90 degree angles in an L shape returning them to an optic reader after recombination. The beams are tuned to return exactly out of phase- canceling each other out completely so the reader observes 0 intensity. As the LIGO sites are dug into the ground the arm lengths remain fixed and only a major geological event like an earthquake could change the path of the laser resulting in a phase change. An earthquake- or a gravitational wave.

Einstein defined gravity as a curvature of space-time (space-time: space and time wrapped up together, moving and changing in unison). Super massive objects in space, like planets or stars, wrap space-time around them causing gravity. These massive objects move throughout space causing ripples in the fabric of space-time which propagate, or move, like waves through the universe at the speed of light, losing intensity but never stopping. A distortion, or ripple, of space-time passing through the arm of a LIGO laser interferometer changes the space through which the laser passes and consequently the distance of the beam's journey. Gravitational waves generated by the motion of massive objects light years away would diminish significantly by the time they reached Earth but still register on an interferometer which can observe even the most minute change in the path of its lasers.

Building Massive Interferometers in Space: The LISA Array

Because of limitations due to available space and the curvature of earth (a laser beam can only follow a straight path, so after a while the beam is shooting off into space) ground based Michaelson type interferometers have an upper limit of effectiveness. NASA's LISA mission works on the same theory as the LIGO interferometer, however, free of terrestrial limitations, LISA plans arm lengths over 5 million kilometers. While more accurate and capable of observing a larger range, space-based interferometers face obstacles, too. For example, the laser intensity diminishes significantly after a 5 million kilometer journey. Where a mirror would reflect a beam in a normal interferometer, LISA's optics read the phase of the wave and generate an identical copy from a separate laser for the second leg of the path. Unlike LIGO, LISA employs three arms for more coverage and accuracy. However, while LIGO's arms are rooted in the ground, NASA had to find a way to fix the arms of the LISA array in space. Floating in orbit, external disturbances such as solar wind and debris would cause the arms to simply float away from each other. However, if the lasers are in absolute free fall (without any external forces) the three LISA space craft would remain in fixed relation indefinitely.

The three LISA craft are essentially space bubbles which surround what NASA calls proof masses - 46 mm gold-platinum alloy cubical masses. The “Space Bubbles” house the proof masses, shielding them from disturbances, achieving free fall. Highly accurate internal censors read the position of the craft relative to the proof mass. If the “bubble” floats out of equilibrium with the proof mass then Newton Thrusters (highly accurate micro thrusters) adjust the position of the space craft to compensate. The laser arms, fired from the LISA space craft, work from a position relative to the proof masses.
The three space craft of the LISA mission form an accurate laser interferometer in space capable of reading gravitational distortions passing through the 5 million kilometer arms in a range from .03 millihertz to 100 millihertz. If LISA achieves direct observations of gravitational waves then not only do we confirm a prediction of Einstein's General Relativity, but change the way we observe the universe. The gravity produced by heavenly bodies in motion could reveal much about the inner workings of the distant universe.