LIGO’s second time

According to Einstein’s General relativity, the structure of space-time is not fix and invariable but, just like a rubber sheet supporting bodies, it is bent by the mass and energy distributed within itself. While this metaphor may be slightly misleading -explaining gravity with gravity- it allows to grasp  what happens when, for instance, two bodies rotate around each other: the rubber sheet gets creased. Similarly, gravitational waves are creases in the fabric of space-time, propagating at the speed of light.

Einstein was quite skeptical about the possibility of gravitational waves ever being detected. Nevertheless, in 1974, their indirect effects were observed on the binary system PSR B1913+16, made up of a neutron star and a pulsar. This last one is a star spinning quickly around itself, characterized by a highly directional emission at radio frequency. It’s like a space radio beacon: every time the radio beam points in our direction, as a result of the star spinning, we detect the presence of the pulsar. Twenty years’ worth of observation have showed that the orbital period of the pulsar around its mate is decreasing at the exact same rhythm as foreseen by the hypothesis describing the loss of the system’s energy at the emission of gravitational waves.

This result, while important –those who discovered the binary system, Hulse and Taylor, were awarded the Nobel Prize in Physics in 1993- doesn’t quite satisfy physicists: the final proof of the existence of gravitational waves requires direct observation. Gravitational waves act like the tide: when they run through the monitor of the computer you’re reading this article on, they pull it slightly in one dimension, compressing it in the other, without changing its surface; these distortions are the bigger the wider the screen. For this reason gravitational waves detection is based on the measurement of the distortion they induce among the parts of an extended body, or in the distance between to independent bodies.

The first technique is the one allowing the operation of resonating antennas, in which the mechanic deformation that the wave induces on a suspended cylinder –generally in aluminum- is measured. Only two of this sort of detectors are currently in operation, both Italian, at the INFN: AURIGA in Legnaro and NAUTILUS is Frascati. These detectors are only sensitive to high frequency waves, close to a kHz.

The distance measurement is carried out by interferometric detectors: the American couple LIGO – one in Washington state, the other in Louisiana – and the Italian-French Virgo, locate in Cascina (Pisa). In these detectors, the two independent bodies, whose distance is to be measured, are the mirrors of an interferometer. From this interferometer, two light beams are produced from the same spot simultaneously, aiming in two perpendicular directions so that, reflecting on the mirrors, they come back following the initial directions, recombining at the starting spot and generating an interference figure, a succession of dark and light bands, alternated. A gravitational wave passing by would alter the light’s path along the two directions, producing a variation in the interference figure. The interferometers work at higher sensitivity and broader band (from a few Hz to a few kHz), and as such, they tend to garner more attention from physicists.

The problem is that these deformations are incredibly tiny. Even the most powerful source, a supernova exploding inside our galaxy, would produce a variation in the Earth-Moon distance of the order of one atom’s diameter. This forces us into building interferometers whose mirrors are a few kilometers away from the light source and into operating resonating antennas at temperatures close to the absolute zero (at about -273 centigrade), in order to reduce as much as possible the thermal deformations of the cylinder.       

On September 14^th, 2015, as if in celebration of the centenary of the publication of the General Relativity theory, the LIGO interferometers (each 4 km long and 3000 km apart) registered, for the first time, the gravitational waves generated by the fusion of two black holes about 1.3 billion of light years away from Earth. They were two “small” black holes, with solar masses of 36 and 29 (nothing to do with the monster of millions of solar masses at the center of our galaxy), which resulted into one black hole of about 62 solar masses. The 3 missing solar masses (36 + 29 = 65!), were radiated as gravitational waves during the fusion process. The analysis of this event –called GW151409- appeared on the renowned American journal Physical Review Letters on February 11^th , 2016. [1]

On the same journal, on June 15^th, the analysis of a second event was published, GW152612, due to the fusion of two black holes of 14 and 8 solar masses, which resulted into a 21 solar masses black hole. [2] So, in this case, only the energy corresponding to one solar mass was converted into gravitational waves. Since these black holes were lighter, their fusion process lasted longer than the one in September 2015, allowing the LIGO physicists to observe it, estimating the rotation speed of one of the two black mirrors.

It’s the dawn of a new era, the one of gravitational astrophysics. We have developed a new “eye” for gazing at the sky, a new sense is joining the electromagnetic one (light, x-rays, gamma-rays) so far used in astrophysics. New discoveries are just around the corner… we just need to turn on Virgo. (Danilo Babusci)

 

[1] B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Observation of Gravitational Waves from a Binary Black Hole Merger
Phys. Rev. Lett. 116, 061102 (2016) – 11 February 2016    http://arxiv.org/abs/1602.03837

[2] B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence Phys. Rev. Lett. 116, 241103 (2016) – 15 June 2016    http://arxiv.org/abs/1606.04855