Today a major press conference was held about the initial results from LISA Pathfinder. LISA Pathfinder is a spacecraft that was launched back in December in order to test key technologies for the LISA mission, a space-based gravitational wave detector.
After launch, LISA Pathfinder travelled 1.5 million kilometres from the Earth, heading towards the Sun. When it was done winging its way across the solar system it began it’s scientific mission, starting on 1st March.
In some respects, LISA will be a little like Advanced LIGO, the ground-based detectors which were the first to directly detect gravitational waves back in September 2015, an event that was eventually announced in February this year, after rigorous testing and analysis of the data. Like Advanced LIGO, LISA will utilise lasers to very precisely determine the position of “test masses.” If a test mass moves in Advanced LIGO (and effects like seismic motion of the Earth can be ruled out) then the cause is a gravitational wave. in Advanced LIGO’s case, the test masses are huge, perfect, fused silica glass mirrors.
In space, seismic motion isn’t really a problem. Even aside from seismic motion, space is a lot quieter than the Earth. Advanced LIGO has to be careful with all sorts of things that might move their mirrors and would look like a gravitational wave; eLISA might have other challenges to overcome, but earthquakes will never be one of them.
Also, eLISA doesn’t use mirrors, at least not in the traditional sense. It uses gold-platinum cubes.
Its these cubes and the sensors that determine their precise position that are being tested in the LISA Pathfinder mission. The cubes are in the most perfect free fall ever produced in space. By keeping them in free fall, any changes in their motion must be due to changes in gravity; gravitational waves.
Over the past two months the LISA Pathfinder team have been carrying out painstaking experiments to find out how accurately they can measure the positions of the cubes. This kind of mission has never flown before and its success is absolutely necessary to guaranteeing that eLISA will fly at some point in the future. Space missions don’t come cheap – if there isn’t a good chance everything will work, they don’t get to launch at all. So understanding these new technologies properly will make all the difference.
Because it’s totally new technology, LISA Pathfinder was absolutely essential. In the full-scale LISA mission, spacecraft could be as much as 5 million kilometres apart, but we’ll need to know the precision to picometre precision – that’s less than a tenth of the size of an atom. Millions of kilometres don’t worry space engineers, but picometres sound ridiculous. There would be no point in building and launching LISA if there’s no evidence we can measure to that precision.
The good news is that the scientists working on LISA Pathfinder absolutely smashed the precision requirements. Pathfinder is not only sensitive enough to prove that it’s possible to measure to the necessary precision, it’s already as good as we’d hope that LISA itself would be. They’ve almost made it look easy.
This is a green light for LISA. There’s now really no good reason why the mission shouldn’t go ahead and that’s really exciting news.
So, given that Advanced LIGO has already detected gravitational waves, where is the additional benefit of detecting them in space? Well, although detection was the first goal, it wasn’t the end point of the research. Rather, the end goal is to do astronomy with gravitational waves. The way that there are telescopes all over the world, working collaboratively to do astronomical research, we would like a global network of gravitational wave detectors.
The advantage of having a space-based detector to compliment the global network is that a space-based detector will be able to “see” gravitational waves that a ground based detector never will. Different kinds of gravitational waves come from different kinds of event. The one that was announced in February arose thanks to the collision of a pair of black holes with a few solar masses. LISA could detect collisions between “galactic scale” black holes, super massive black holes, black holes with masses equivalent to a few million stars. These have much longer signals but would be impossible to detect on Earth.
The field of Gravitational Wave Astronomy is only just getting started. In the next few years and decades we’re going to see some incredible things.