FAQ Group: GW-Science Q&A

No. Both ground motion and time variations in familiar Newtonian gravity from spurious mass motions on the Earth prevent observations below about 1 Hz on the ground. It is necessary to make measurements in space in order to observe many of the important astrophysical sources throughout the Universe.

Gravitational waves are ripples in the fabric of space-time generated by some of the most powerful astrophysical events – such as collisions of black holes and exploding stars. Gravitational waves travel at the speed of light through the universe. They allow us to explore the dark side of the universe.

Since gravitational waves are the stretching of spacetime itself, they have the interesting property that the measured displacement between two reference objects scales with the original separation between those objects. In other words, if there is more spacetime to stretch, the total stretch is larger. LISA’s arms are roughly a million times longer than LIGO’s, which means that a gravitational wave of the same amplitude will produce displacements that are roughly a million times larger in LISA. The total displacement is still small, on the order of picometers (one picometer = one trillionth of a meter) but is well within the range of modern metrology techniques. From the metrology perspective, the LISA measurement challenge is “easier” than that of LIGO, which is important given that it has to be robust enough for spaceflight as well as be able to be operated from far away.

The gravitational waves that LISA is designed to observe have typical timescales of hours. So long as the distance between the satellites is smoothly changing over these time scales, the gravitational waves can be observed as an additional modulation on top of this smooth change. Each satellite is in an independent Keplerian orbit around the Sun with the plane of the triangle inclined at 60 degrees to the plane of the ecliptic. Over the course of the mission, the nominal 2.5 million kilometer distance between each satellite will vary by hundreds of thousands of kilometers. LISA will be able to measure the absolute distance between the satellites to a few centimeters and will measure hour-scale fluctuations at the level of several picometers (1pm = 1 trillionth of a meter), the level required to detect gravitational waves.

At any one moment, LISA will be sensing gravitational waves from millions of individual sources. The vast majority of these will be binary systems of compact objects in the Milky Way, but signals will also be received from extragalactic sources such as the mergers of massive black holes. Each of these signals has a distinct waveform that depends on the astrophysical properties of the source (masses, spins, orientations, positions, etc.). Thanks to extensive work in theory and modeling, we have very good templates for these sources which we can compare to the LISA data and extract individual signals using a technique known as matched filtering. The entire LISA data set is processed as a hierarchical global fit, where individual sources are added and subtracted to improve the overall fit. The most significant sources are easily identified and characterized. As the signal strength decreases, a point is eventually reached where no additional sources can be confidently extracted. Simulations with mock LISA data suggest that tens of thousands of individual signals will be identified in the full LISA data set with the remaining Milky Way binaries producing an unresolved, but still detected, foreground of gravitational waves in the lower part of the LISA sensitivity band. The LISA community is continuing to conduct mock data challenges of increasing sophistication to hone the data analysis techniques that will be used to solve this problem.

The Gravitational Universe is a new window in astronomy. Powerful sources of gravitational waves are being used to probe a universe that cannot be explored by other means. Significant advances in astronomy have been made by looking at the Universe using electromagnetic radiation as a probe. But with gravitational waves, we can also study the dark universe, analogous to listening for objects that do not produce light. LISA will enable us to explore the dark universe through gravitational waves.

The gravitational waves that LISA will discover include ultra-compact binaries in our Galaxy, supermassive black hole mergers, and extreme mass ratio inspirals, among other possibilities. LISA will be the first to explore gravitational waves in the frequency range of 0.1 milliHertz to 0.1 Hertz.

Since gravitational waves are the stretching of spacetime itself, they have the interesting property that the measured displacement between two reference objects scales with the original separation between those objects. In other words, if there is more spacetime to stretch, the total stretch is larger. LISA’s arms are roughly a million times longer than LIGO’s, which means that a gravitational wave of the same amplitude will produce displacements that are roughly a million times larger in LISA. The total displacement is still small, on the order of picometers (one picometer = one trillionth of a meter) but is well within the range of modern metrology techniques. From the metrology perspective, the LISA measurement challenge is “easier” than that of LIGO, which is important given that it has to be robust enough for spaceflight as well as be able to be operated from far away.