An exciting time for gravitational wave astronomy

Abstract

This year marks the centenary of Einstein’s general relativity (GR), a theory whose intrinsic mathematical beauty is only matched by its robustness. Over the past 100 years, GR has stood the challenges of numerous weak and strong field tests. One remaining crown jewel is the direct detection of gravitational waves (GW), or ripples of spacetime curvature. Aside from testing GR, modified gravity theories, and a host of related fundamental physics predictions/conjectures such as cosmic censorship and nohair theorems, such detections would also provide invaluable information about the structure and history of our universe and celestial objects within. By accident or design, significant strides in experimental capabilities are being achieved around this centenary, and right now appears to be an opportune time to offer a practitioner’s perspective on the past, present and future of GW astronomy. The viewpoint expressed in this short piece is necessarily biased and limited by the author’s own experiences, and we apologize for any omissions and refer readers to the recent suite of review papers [1–5] for more detailed discussions. Theoretical predictions for the existence of GW were made soon after the invention of GR, by Einstein himself and collaborators. Although the analytical computations are relatively straightforward (but subtleties exist, such as whether the wave is physical or merely a coordinate effect, a question only resolved much later), the experimental detection is not. Gravity being so remarkably weak prevents us from being able to artificially generate GWs with sufficient amplitudes that they are detectable with our current technology, so we have to look to the cosmos for viable sources, and black holes and neutron stars stand out as obvious candidates. In particular, the collision and coalescence of two black holes can inject such massive amounts of energy into GWs that the total electromagnetic luminosity of galaxies are easily eclipsed. Unfortunately (or fortunately for life on Earth), such extremely violent astronomical events do not occur frequently in our vicinity, so the GW would likely have to travel hundreds of mega parsecs to reach us, by which stage they would have weakened significantly and their induced relative distance changes (strain) are expected to be at the order of 10 . This means GW detection is an incredible technological challenge, where the overcoming of every hurdle leads to significant new innovations, and even the opening up of entirely new fields of research (e.g., quantum optomechanics). The first serious ground-based experiments designed to detect GW were done by Weber in the early 1960s, who used aluminum resonant bars to sense small vibrations caused by GW. This design was subsequently improved upon between the years of 1972 and 2000, through the use of cryogenic techniques, making it possible to detect sources within our Milky Way galaxy. A network of such detectors was built around the world, but the source event rate is too low for them to make a plausible detection. During this period, however, progresses have also been made elsewhere. Hulse and Taylor discovered a binary pulsar system in 1975 and, with colleagues, tracked it for 20 years, demonstrating the predicted orbital decay due to the back-reaction of GWs. This highlighted a promising class of sources—the coalescence of binary neutron stars, F. Zhang Gravitational Wave and Cosmology Laboratory, Department of Astronomy, Beijing Normal University, Beijing 100875, China