Dr Siebren Kuiper

Fundamentally different from and complementary to other astrophysical ‘messengers’ such as photons, neutrinos or cosmic rays, GWs provide unique information about the most energetic astrophysical processes in the Universe by carrying information about the dynamics of massive objects such as black holes and neutron stars moving at relativistic speeds. As predicted by general relativity (GR), GWs are transverse (oscillating perpendicular to the direction of propagation), travel at the speed of light and possess two polarizations.

GWs physically manifest themselves as time-dependent strains, h, in spacetime, or, more precisely, h = δL/L, where L is the distance between two reference points in space and δL is the induced displacement over the baseline L. GR predicts that the induced strain is perpendicular to the axis of GW propagation and is quadrupolar, that is, a wave travelling along the z-axis stretches (then compresses) the path along the x-axis while shrinking (then stretching) the y-axis (for one polarization; in the orthogonal polarization, the elongation/compression occurs along axes rotated 45\xb0 relative to the x-axis and y-axis). GW detectors rely on a measurement of the variations in the light travel time between separated reference points — or ‘test masses’ — caused by a passing GW. The test masses are configured such that each is in near-perfect free fall (and, as such, approximate a local inertial frame) and are separated over very long baselines. The light travel times between pairs of test masses are monitored and read out via a detector such that any changes in the spacetime curvature caused by passing GWs induce modulations in these light travel times.