“Right now, your body is being stretched and squeezed.” So Harry Ward from the University of Glasgow tells me as I try to get my head round the head-scratching concept of gravitational waves. “Typical gravitational waves that are passing through you at the moment are probably stretching your feet and head apart maybe a billion trillionth of a metre,” he says.
Gravity, according to Ward, is the master of the Universe. “It’s what controls the Universe on the grandest scales. Gravitational waves are ripples in the fabric of space-time.”
These ripples permeate out from violent events like colliding black holes or exploding stars. These waves were predicted almost 100 years ago by Albert Einstein's general theory of relativity, but physicists deem gravity to be a weak force, and the predicted effects of gravitational waves from distant events are almost unimaginably small. So small, that no one has yet detected them in experiments. That is what Ward’s team, and its international partners, is trying to do. The trouble is that it may take some time.
In his narrow, windowless lab crammed with equipment, Ward points at what he hopes will help reveal one of the universe’s biggest secrets. Pride of place among shelves stacked with computers and oscilloscopes, and a bench strewn with electronic components, is a laptop-sized square of smoked glass covered in rectangular glass mirrors, standing upright like dominoes.
“This,” exclaims Ward proudly, “is an interferometer – an assembly of mirrors used to direct laser beams.” It is the prototype for an instrument that will use lasers to measure – to a thousandth of one millionth of a millimetre – the position of two free-floating 46mm gold-platinum cubes inside a spacecraft. The real instrument has just been shipped off to Germany to be integrated into Laser Interferometer Space Antenna (Lisa) Pathfinder, the first stage of a Europe-led effort to measure gravitational waves.
However, this ambition is so challenging that the European Space Agency’s (ESA) Lisa Pathfinder mission will not be attempting to measure gravitational waves, but instead proving the technology works for a spacecraft that might. This brings to mind the Deep Thought computer in the Hitchhiker’s Guide to the Galaxy, which produces the answer to life, the universe and everything, but has to design another computer to calculate what the question actually is.
What all the physicists around the world working on Lisa Pathfinder really want to build is a mission, known as eLisa, consisting of three spacecraft, flying in space some million or so kilometres apart. Each would contain a free-floating cube, or test mass, the theory being that any relative movement between these masses would be caused by gravitational waves.
For that to work, the masses have to be completely isolated from all other external forces. This includes movement of the spacecraft, stray electrical forces and locally fluctuating gravitational forces – caused by planets and the like. They cannot be allowed to expand or contract through heat or cold and have to survive the traumas of launch and separation – going from our atmosphere to the vacuum of space. These masses need to be completely free floating, with the spacecraft built around them.
“But,” confesses Ward, “it’s been difficult to convince people we’re so clever!”
So, Lisa Pathfinder is a “stepping stone” mission to prove the technology for this ambitious gravitational wave experiment actually works. The Lisa Pathfinder payload – now being put together in Germany – will contain two of these test masses, only 40cm apart. They will be held in special chambers and the spacecraft’s position controlled with new “micro-thrusters” to within a thousandth of one millionth of a metre. When the payload is fully assembled it will come to the UK to be fitted into the main spacecraft, which is ready and waiting.