The results of gravitational waves from LIGO are still warm off the press, and the race is on for the next generation of gravitational wave detectors.
Gravitational waves are detected by carefully measuring the distances between reflecting mirrors. As a gravitational wave passes through a detector, the distances shift slightly due to the warping of space by the gravitational wave. Of course on Earth there are all sorts of things that can cause the mirrors to shift, from minor tremors to the rumbling of a nearby truck. Most of the “noise” is actually larger than the signal you want to detect. It is like trying to perform a careful experiment where every now and then someone walks by and starts pounding on your lab table.
There are ways to reduce the level of noise, and ways to distinguish a real gravitational signal from other things, but all those background events limit the sensitivity of LIGO. The obvious solution is to simply put the whole thing in space. Floating in space, there aren’t any ground vibrations to bother you. Problem solved!
Except it isn’t quite so simple. On Earth the LIGO detectors are assembled within a rigid structure, which is easy to do when you’re on the ground. We can’t do the same thing in space. For example, LIGO’s mirrors are spaced about 4 kilometers apart. To make a rigid structure in space would require the construction of the largest space-based object by far. Even if we did construct such a monstrosity, gravitational torsion and the heating and cooling of the support structure would create far more noise than even LIGO has.
A better idea is to simply let each mirror float freely in space. That way the system will simply orbit the Earth or Sun in a predictable way, and any short-term deviation will be due to a gravitational wave disturbance. At least that’s how it should work, but we couldn’t be sure until it’s tried. Enter LISA Pathfinder.
Pathfinder was designed as a proof of concept. It is a single spacecraft containing two blocks about a meter apart. Reflecting lasers of the block allows us to measure how far apart they are. The blocks are designed to float freely within the spacecraft, and were enclosed in gas-filled containers to dampen any vibrations once in orbit. The key test of Pathfinder was to determine just how much of an issue background noise would be. What was found is that the noise is much smaller than expected. In fact, at key frequency ranges, the noise limit was primarily due to the thermal noise (brownian motion) from the gas surrounding the blocks. Over time this noise would die down as gas is purposely leaked away from the containers.
This is a great result, and it’s a big step toward a new era of space-based gravitational astronomy.
Paper: M. Armano et al. Sub-Femto-gg Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results. Phys. Rev. Lett. 116, 231101 (2016). doi: 10.1103/PhysRevLett.116.231101