This post was inspired by the recent selection of LISA as ESA’s third large class mission, L3. This is fantastic news for gravitational astrophysics, and means that construction of the LISA mission can now commence.
LIGO and ground based detectors
With the recent news of another detection of gravitational waves (GWs) by LIGO, it seems that GW astronomy is starting to take its place alongside the classical electromagnetic observations that have been used since the dawn of astronomy. This is especially exciting since the same theory that predicts GWs - general relativity - is a fundamental theory of space and time, and in many ways is much deeper than classical electromagnetism. These gravitational wave observations may then help us to probe fundamental physics in very extreme regimes.
Now, GWs, like all waves, have an associated frequency. The radiation detected by LIGO has a frequency of ~ 100 Hz. This is the typical frequency range of extreme events like the merger of two black holes of similar mass (20 - 30 solar masses). However there are also other events in the universe that produce GWs at lower frequencies. Being a ground based detector, the lowest frequency LIGO can reach is limited by noise to ~ 10 Hz. These noise sources are primarily due to tectonic/seismic activity and general thermal noise. LIGO employs both passive and active methods to correct for this noise; passive methods prevent vibrations reaching mirrors and active methods measure vibrations. Nevertheless the remaining noise ultimately limits the frequencies that can be measured to greater than 10 Hz. An upcoming Japanese GW detector KAGRA looks to overcome these noise sources by being underground and being very cold, but is still likely to be limited to frequencies greater than ~1 Hz.
LISA and space based detectors
So, we need a detector that is capable of examining these very low frequencies. To do this we need longer baselines, allowing for the detection of longer wavelengths, and also a way to avoid all the noise on Earth. The obvious solution is to go to space, and this is what is being proposed with the LISA mission
The concept of LISA is entirely analogous to that of LIGO. It is simply a very large interferometer in space. It will be composed of 3 spacecraft in a V-formation. At the apex of the V sits the ‘mother’ craft, whilst 2 ‘daughter’ craft rest at the ends of the arms. Within each of these craft is housed a test mass. These test masses follow a certain path around the Sun. In the absence of any non-gravitational forces (e.g. drag, dust, radiation), we say they are in ‘free-fall’ and their trajectory is specified by general relativity. Consequently, for an appropriate initial configuration the distances between these masses remains unchanged and forms a baseline with which gravitational waves can be detected.
In order to ensure that these masses are indeed free-falling (i.e. only gravitational forces are acting), the craft housing the mass acts as a drag-free casing. This works by continually measuring the distance between the shell of the craft and the test mass and correcting for any change in this distance. This correction is applied via micro-newtonian thrusters on the spacecraft, which adjust the craft’s position by forces equivalent to the weight of a fly.
Measuring the distances between the craft using interferometry is more difficult than the standard method used in an interferometer on Earth. On Earth, interferometers work by the direct reflection of light. With LISA, this is not possible since the distances involved are so much greater; the typical LISA baseline will be ~1 million km. Over such distances, the power from a laser beam would be attenuated by a factor of ~, equivalent to the detection of one photon every 3 days. To remedy this, the lasers on LISA act as transponders; the mother spacecraft sends out a laser signal to the two daughter craft. This incoming laser is phase locked and a beam is returned. By comparing any changes in the laser beam phase, the distances can be measured.
Science with LISA
Science goals with LISA are broad, covering astrophysics, cosmology and fundamental physics. Of particular personal interest is the use of LISA for gaining a greater understanding of fundamental physics. This would be an entire blog post in itself, but LISA will be able to perform precision tests of general relativity in the strong field regime by observing gravitational radiation emitted as some small object (e.g. a pulsar or stellar mass black hole) orbits a much larger supermassive black hole. Such systems are called Extreme Mass Ratio Inspirals (EMRIs). For more information on using EMRIs for precision tests of GR see Amaro-Seoane et al., 2012.