Experimental physicist key to the detection of gravitational waves.
2017 Nobel Prize in Physics
Comment & Review
The detection of gravitational waves is the culmination of many decades of persistent theoretical, observational and engineering work. While heralded as surprising, that the first detected wavescame from binary black holes was indeed theoretically expected.
A momentous signal from space has confirmed decades of theorizing on black holes — and launched a new era of gravitational-wave astronomy.
The announcement confirming the discovery of gravitational waves created sensational media interest. But educational outreach and communication must remain high on the agenda if the general public is to understand such a landmark result.
Gravitational waves are predicted by general relativity, but their direct observation from astronomical sources hinges on large improvements in detection sensitivity. The authors review how squeezed light and other quantum optical concepts are being applied in the development of next generation interferometric detectors.
The Laser Interferometer Gravitational Wave Observatory in the USA is searching for gravitational-wave emissions from cataclysmic astrophysical events. The task has required the construction of the world's largest and most sensitive optical strain sensor.
Observables & Implications
The general theory of relativity predicts that all accelerating objects produce gravitational waves — analogous to electromagnetic waves — that should be detectable for instance in the case of extremely massive objects such as black holes undergoing acceleration. The existence of such waves has been inferred indirectly, but an important goal in physics is their direct observation, a feat that would both validate Einstein's theory and lead to new areas of cosmology. Now early results from LIGO (the Laser Interferometer Gravitational-Wave Observatory), one of the handful of detectors searching for gravity waves, have provided a starting point for further gravity hunts by deriving an upper limit for the stochastic background of gravitational waves of cosmological origin. The data rule out models of early Universe evolution with a relatively large equation-of-state parameter, as well as cosmic (super)string models with relatively small string tension that are favoured in some string theory models.
Advanced LIGO has detected gravitational waves from two binary black hole mergers, plus a merger candidate. Here the authors use the COMPAS code to show that all three events can be explained by a single evolutionary channel via a common envelope phase, and characterize the progenitor metallicity and masses.
One of the best-measured parameters from the gravitational wave chirps caused by merging binary black holes is the effective spin of the binary—a combination of the spins of the individual black holes. If the black holes came from a pre-existing binary star system, then the expectation is that the spins will be aligned. On the other hand, if the binary black hole systems were formed through dynamical interactions, the spins will be randomly aligned. William Farr et al. examine the spin parameters for the four mergers reported so far and find at 2.4σ significance that the spins were not aligned. Only ten more merger events will be needed to raise this to 5σ if most of the spins are not aligned.
The first gravitational-wave source from the isolated evolution of two stars in the 40–100 solar mass range
Krzysztof Belczynski et al. present numerical simulations of the formation of binary black holes that provide a framework for interpreting the recent detection of the first gravitational-wave source (known as GW150914) — a merger of two massive black holes. Their models imply that these events take place in an environment where the metallicity is less than 10 per cent of that of the Sun, and that the progenitors are stars with initial masses of 40–100 solar masses that interact through mass transfer and a common-envelope phase. The calculations predict detections of about a thousand black-hole mergers per year once gravitational-wave observatories reach full sensitivity.
Squeezed states of light have been experimentally demonstrated to improve the performance of the Laser Interferometer Gravitational-wave Observatory (LIGO) in astrophysically relevant frequency regions. This enhanced performance may help to reach the sensitivity required for detecting gravitational waves.
Researchers demonstrate a laser interferometer that achieves simultaneous nonclassical readout of two conjugated observables. Because their system uses steady-state entanglement, it does not require any conditioning or post-selection. By distinguishing between scientific and parasitic signals, its sensitivity exceeds the standard quantum limit by about 6 dB.
Quantum metrology employs the properties of quantum states to further enhance the accuracy of some of the most precise measurement schemes to date. Here, a method for estimating the upper bounds to achievable precision in quantum-enhanced metrology protocols in the presence of decoherence is presented.
‘Squeezed light’ enables quantum noise in one aspect of light to be reduced by increasing the noise, or more accurately the quantum uncertainty, of a complementary aspect. This has now been used to push the detectors at the heart of the GEO600 gravitational wave observatory to unprecedented levels of sensitivity.
Substantial improvements, through the use of squeezed light, in the sensitivity of a prototype gravitational-wave detector built with quasi-free suspended optics represents the next step in moving such devices out of the lab and into orbit.
On astronomical scales, gravity is the engine of the Universe. The launch of LISA Pathfinder this year to prepare the technology to detect gravitational waves will help us 'listen' to the whole Universe.
Hubble Space Telescope observations of the location of the short-duration γ-ray burst SGRBH 130603B, which was detected by the Burst Alert Telescope on NASA's Swift satellite on 3 June 2013, provide support for the favoured model for the origin of such bursts — the merger of two compact stellar objects. Nial Tanvir et al. imaged the position at optical and near-infrared wavelengths at about 9 and 30 days after the bursts and observe signs of a faint, fast transient or 'kilonova'. The simplest interpretation of the data is that the burst was a compact object merger. The authors suggest that such mergers are probably sites of significant production of heavy elements through r-process nucleosynthesis.
The first detection of electromagnetic emission from a gravitational wave source bridges the gap between one of the most energetic phenomena in the Universe and their dark, difficult to detect progenitors.