Frascati Physics Series Vol. 66 (2018) Frontier Objects in Astrophysics and Particle Physics May 20-26, 2018 GRAVITY AND GRAVITATIONAL WAVES M. Alessandra Papa Max Planck Institut f. Gravitationsphysik, Callinstrasse 38, 30167 Hannover, Germany U. of Wisconsin, Milwaukee, USA Abstract The first gravitational wave detections have opened a new window on the Uni- verse. Full of excitement and curiosity we are now taking the first baby-steps in this new field. 1 Newton and Einstein The opening talk on gravity at a meeting like this one cannot but begin by men- tioning Newton and his Philosophiae Naturalis Principia Mathematica, where the universal law of gravitation first appeared: two masses M and m at a dis- GMm tance R experience a mutually attractive force equal to R2 . Newton recog- nised that this is the force that is responsible for objects falling to the ground as well as for the moon orbiting around the Earth, and the Earth around the Sun. 13 Newton’s law implies an action taking place instantaneously at all distances between the masses. Newton rejected this criticism in the second edition of the Principia (1713) by arguing his famous “hypotheses non fingo” , i.e. that his law well described certain physical phenomena but was never intended to make any reference to how/why those happen the way that his law predicts. The next paradigm shift came nearly 230 years later, with the General Theory of Relativity. The new theory describes gravity in terms of the geometry of space-time, influenced by the masses in it and at the same time determining their motion. No information can travel faster than the speed of light, the theory postulates, so even the information on changes in the gravitational field brought about by a changing mass distribution have to propagate. In Newto- nian gravity the masses carry around the gravitational field rigidly with them, in every point of space, even very far away. In Einstein’s theory gravity is not an instantaneous condition, it propagates. For this reason gravitational waves are an essential prediction of general relativity. The gravitational field equations, in their covariant form, are deceiptfuly simple: 8πG G = T . (1) µν c4 µν On the left hand side is the gravitational field, described by Gµν which is a func- tion of the space-time metric, and on the right hand side the mass-energy tensor Tµν , which contains the information about the sources of gravitational field. The equations 1 are actually complex, because they are non-linear differential equations in the metric and exact solutions can be found only in particular cases. Furthermore, in order to extract predictions on physical quantities these equations have to be projected in the appropriate observers’ reference frame and coordinate system, and a number of degrees of freedom have to be set with suitable choices of gauge. 2 Gravitational Waves Gravitational waves are solutions of the linearised field equations. They are transverse and described by two polarisations, typically denoted as ”+” and ”x” because of the relative orientations of the deformation patterns, as shown in Fig. 1. Even though Einstein published a paper predicting gravitational waves 14 Figure 1: Deformation patterns induced by the two gravitational wave polari- sations, for waves propagating in the z-direction. a few months after original presentation of the Theory of General Relativity1, in the years, he himself became unconvinced that these were actually a real physical phenomenon. The discussion on this topic lasted over 30 years and in- volved some of the most eminent scientists of those times. In fact, in the 1920s Einstein, Rosen and Eddington, in conversations with Bohr and Schwarzschild, convinced themselves that gravitational waves were not real. In 1936 Einstein and Rosen submitted a paper to Physical Review where they stated that grav- itational waves did not exist. The paper was withdrawn when the editor asked Einstein and Rosen to address the comments of the referee (Robertson) who had raised doubts on the correctness of the main result. About a decade later, thanks to the “intellectual mediation” of Infeld, Einstein reconsidered the issue and concluded that Robertson was indeed right 1). In 1956 Pirani also reached the same conclusion. But it was only in 1957 that the controversy was settled for good. In 1957 a conference took place at Chapel Hill, which turned out to be of pivotal importance for gravitational waves. The occasion was the foundation of a new institute, the Institute of Field Physics , thanks to the donation of Mr. Babson. Mr. Babson was a very wealthy business man, who held unorthodox views about how markets worked. Apparently he was convinced that his success was due to having applied to the markets the concept of “what 1The correct version however followed two years later, in 1918. 15 goes up must come down”, from Newton’s law of gravity, and hence felt that he owed “gravity” his good fortune and was eager to pay back. At the same time, probably due to a family tragedy, the drowning of his sister at a young age, he was obsessed with trying to control gravity and wanted to support research in this direction. For these reasons he had founded the “Gravity Research Foundation” and held an annual research-essay contest with the title “finding ways to control the force of gravity”. In 1953 Bryce DeWitt won the essay competition. In his essay he pointed out that the field was not striving and, in order to make progress, young scientists should be encouraged to devote themselves to the study of gravity. Babson was enthusiastic about the proposal and deiced to create an institute solely devoted to the study of gravity, with DeWitt directing it. During the Chapel Hill conference the inaugural ceremony for such insti- tute was held. In order to attract attention and give credibility to this new endevour, the greatest names in physics at the time were invited. Feynman was among the participants and he presented the conclusive argument sup- porting the physical reality of gravitational waves. The heart of the ”sticky bead argument”, as his argument is commonly referred to, shows how a pass- ing gravitational wave can release energy in an external system, for example by heating it, hence proving that gravitational waves do indeed carry energy 3). A first version on this argument had been previously made by Pirani and a detailed case followed by Bondi and Weber and Wheeler 4). 3 The dawn of gravitational wave experiments The pioneer of gravitational wave detection experiments is Joe Weber who build the first gravitational wave detectors at the University of Maryland in the 1960s (Fig. 3). His detectors were large metal cylinders, whose fundamental longitudinal mode would be excited by the energy deposited by an impinging gravitational wave. The oscillations of the mode were recorded in these first bars by piezoelectric ceramics. The bars were sensitive in a narrow frequency band, of order of a Hz, around the resonant modes of the coupled system bar + transducer, typically a about 900 Hz. Many groups followed in Weber’s steps, in America (New Jersey, Yorktown Heights, Stanford, Louisiana and Rochester in the USA and Regina in Canada), in Europe (Bristol, Glasgow and Reading- Rutherford Lab in the UK, Rome/CERN, Legnaro in Italy, Munich in Germany, 16 Figure 2: Weber and his resonant bar detector (credit: Special Collections and University Archives, University of Maryland Libraries) Meudon in France), in Russia (Moskow), in Japan (Tokyo), China (Guangzhou and Beijing) and in Australia (U. Western Australia). Later versions of Weber’s bar operating in the 90s were significantly more sensitive than the first ones and operated as a network combining the results of their analyses, but they were not able to corroborate any of the detection claims made by Weber 5, 6). However the field was kept alive and virtually all of the bar-detector groups active twenty-five or thirty years ago are now deeply involved in the current gravitational wave detection efforts. Interestingly it was a former PhD student of Weber’s, Robert Forward, who built the first interferometric antenna 7). Failure to detect gravitational waves with bars already in the 1970s in- spired visionary scientists, among whom Brillet and Weiss 9), to pursue alter- native routes. The principle of operation of gravitational wave interferometric detectors is simple: laser light is split in two orthogonal beams and it is re- flected back after having traveled a few kilometers. When it is recombined, the two beams interfere destructively unless a gravitational wave has altered the path traveled by the two beams. The idea dates back to Pirani and was proposed independently by several. 17 Figure 3: Principle of detection for a gravitational wave interferometer. Credit: M. Goldman/M.Gerhardt, SITNBoston, Harvard U. 8). 1974 is an important year because Hulse and Taylor discovered the J1913+16 binary pulsar system (J1913+16-1975). In 1979 their paper demonstrated that the observed spin-down of the orbital period of this system was consistent the loss of energy due to gravitational wave emission 11). In 1975 Rai Weiss and Kip Thorne, both attending a NSA meeting, find themselves sharing a hotel room in Washington. They spend the night talking about prospects for gravitational wave detection, and emerge convinced that the times are ripe for setting up a large interferometric detector. Shortly after, an experimental effort begun at Caltech. In 1989 Caltech and MIT put forward a proposal to the National Sci- ence Foundation for the construction, operation and supporting research and development of a laser gravitational wave observatory 13). The pivotal meeting for the Virgo detector happened in 1985, and it was between Giazotto and Brillet during a Marcel Grossmann meeting in Rome.