Catching gravity, rolling by
gravity: sits a massive laboratory, its two long arms stretching off into the distance. On windy days, tumbleweeds roll by, piling up against the arms’ concrete housing and creating headaches for the lab’s maintenance workers. Inside, though, there is a buzz of activity as scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) prepare for the most exciting period in the facility’s 14-year history. Later this month, they will begin observations with an upgraded machine, new instruments and a corresponding sense that this time, when they go on a gravitational-wave hunt, they’re going to catch a big one.
Gravitational waves were predicted by Albert Einstein in 1916 as a consequence of the field equations of his general theory of relativity. These 10 coupled nonlinear equations formulate the universe as the dynamic interplay between mass–energy and space–time. massive bodies move around, they cause the fabric of space–time to warp, generating ripples that propagate outwards at the speed of light. These ripples are known as gravitational waves, but they are not the familiar sinusoids found in electromagnetism. Instead, they stretch space in one direction perpendicular to the line of travel, while simultaneously compressing it in the other – a bit like lips puckering up and down for a kiss.
No prediction made by Einstein’s equations has ever been proved wrong, and in the 1970s observations of a binary pulsar – a rapidly rotating neutron star in orbit around another neutron star – strongly suggested that gravitational waves do indeed exist (see “Pulsar detectives” below). However, nobody has ever detected such waves directly, despite decades of trying.
LIGO was built to change that. From 2002 until 2010, laser beams travelled from the lab’s hub down its two long, perpendicular arms, where they reflected off huge, hanging masses and were recombined back near their origin. The idea was that a passing gravitational wave would cause the masses to move enough for the length of the arms to change and produce a detectable phase shift in the interference pattern of the recombined laser beams. On a handful of occasions during LIGO’s first period of experimental operations, researchers thought they had spotted such a shift – only for the supposed signal to be revealed as noise or, in one case, a deliberate fake generated by researchers within the collaboration, as a test of their internal data-checking procedures.
Now, however, the LIGO facility near Hanford, Washington – along with its twin in Livingston, Louisiana – is entering a new era. In March contractors completed a $221m upgrade of the dual facilities that improved their ability to detect the feeble waves of gravity by a factor of 10. Thanks to this upgrade – known as Advanced LIGO, or aLIGO – researchers should be able to detect gravitational waves that originate anywhere within a sphere of about 420 million light-years in radius, centred on the Earth. That is still only a small fraction of the total universe, but it’s a thousand-fold increase (by volume) on what was possible before the upgrade. As the upgraded system kicks into high gear and achieves its designed sensitivity in 2016 or 2017, the scientists at LIGO are quietly confident that they will see something real.
Only an attometre
The recently completed aLIGO upgrades all have the same goal: reducing noise. Noise poses challenges for many physics experiments, of course, but for the dual LIGO machines and their interferometric kin, the problem is particularly acute. Although gravitational waves come from some of the most massive and energetic systems in the universe (such as a pair of black holes or neutron stars orbiting one another) their amplitudes are exceedingly small by the time they reach Earth. In fact, a passing gravitational wave is expected to change the length of LIGO’s 4 km-long arms by only a few attometres (10–18 m) – around 1000 times less than the diameter of a proton (see “How LIGO works” below).