The announcement from the researchers behind Laser Interferometer Gravitational-Wave Observatory (LIGO) that they had directly observed gravitational waves. It’s an amazing achievement, one made all the pertinent by the fact that it was made 100 years after Einstein predicted it with his theory general relativity. It was the last remaining piece of the theory which had yet to be observed and with LIGO’s results it’s finally complete. However this is far from being the end of research into gravitational waves and there are some incredibly ambitious missions planned with one already on its way.
LISA Pathfinder, pictured above, was launched on December 3rd, 2015. Inside the craft are two small test masses which are sitting on opposite ends of the craft, 40cm apart. It arrived at its destination, a special place called the Sun-Earth Lagrange point 1 (chosen due to the fact that the gravity of the sun and earth cancel each other out) on the 22nd of January 2016. After it has been commissioned it will set those two mass free, allowing them to experience near perfect free fall. It will then attempt to measure the distance between both of them using the same kind of laser interferometry that the LIGO detector used here on earth. It will also test various systems to account for other forces that are acting both on the craft and the test masses as well as providing insight into the longevity such systems will have in space. It’s essentially a smaller version of LIGO in space, one that will be critical for further planned missions.
As its name implies LISA Pathfinder is the trailblazer for another, much more ambitious craft that’s scheduled to be launched in 2034. LISA Pathfinder should be able to provide evidence that the systems work as intended although I wasn’t able to find any official source that said it will definitely provide direct observations of gravity waves itself. Indeed LIGO has been running since 2002 and was unable to detect anything until the recent upgrade was completed in September 2015. However the data provided by those observations helped in determining what level of detection was required and its likely that LISA Pathfinder will provide the same assurance for its successor craft, eLISA.
Comparatively LISA Pathfinder and eLISA are not even in the same ballpark. Where LISA Pathfinder has 2 small masses separated by 40cm eLISA will have 3 distinct craft, each carrying a 2KG weight and separated by 1 million kilometres. The principals behind them are the same however as they will precisely measure the distance between each other using laser interferometry. eLISA will be able to detect gravity waves at a much lower frequency than its ground based peers, allowing us to see a much wider range of events that generate them. For comparison LIGO can only detect frequencies about 10 orders of magnitude higher than what eLISA will be able to, a significant improvement in sensitivity.
Suffice to say it’s an incredibly exciting time for researchers in the world of general relativity. With the foundations of the theory backed up with observational data there’s now a whole world of new physics for them to explore. Soon there will be troves of data for them to pour through, much of which will be used to design the eLISA craft. Whilst it’s going to be some time before we see eLISA launching into space we at least know that when it does it will be able to provide us incredible insight into our universe.