The early history of gravitational wave detection in Italy: from the first resonant bars to the beginning of the Virgo collaboration Adele La Rana - TERA Foundation and Sapienza Università di Roma - [email protected] Leopoldo Milano - Università di Napoli “Federico II” and INFN, Sezione di Napoli - [email protected] Abstract: At the very beginning of the 1970s, two experimental activities for gravitational wave (GW) detection were being carried out in Italy: one in Frascati, at the European Space Research Institute (ESRIN), performed by Karl Maischberger and Donato Bramanti; the second one set up by the group of Edoardo Amaldi and Guido Pizzella at the University of Rome. This second activity pioneered the field of cryogenic resonant bar detectors, which dominated Italian research on GWs until the 1990s, when the French- Italian project for a giant interferometric detector came into life: Virgo, led by Alain Brillet and Adalberto Giazotto. The present paper gives a brief overview of the early history of GW experiments in Italy and is the result of a historiographical research based on archival resources, scientific papers and interviews of some of the protagonists, among which is one of the authors. Keywords: General Relativity, gravitational waves, detector, Virgo. 1. Introduction The age of GW detection begins with the experimental work of the American physicist Joseph Weber, at the end of the 1950s. When Weber proposed for the first time the idea of an apparatus capable of vibrating at the passage of gravitational radiation and measuring the amplitude of the vibration, the very existence of GWs was a still debated theoretical question. Were GWs purely mathematical entities – an artifact related to the choice of the coordinate system in the field equations of General Relativity – or did they possess a physical reality and were they thus measurable? Weber participated into the theoretical debate – which was then taking place with unprecedented liveliness, as part of the new growing interest in General Relativity lightened in the 1950s – publishing in 1957 a paper with John Wheeler (Weber, Wheeler 1957). The authors supported the hypothesis of a true physical meaning of these undulatory solutions of Einstein’s field equations. Furthermore, beyond the uncertainty about their existence, the extreme weakness predicted for their interaction 186 Adele La Rana, Leopoldo Milano with matter made it seem impossible to accomplish an experiment for measuring the effect of their passage. In spite of these discouraging premises, in 1959 Weber presented at the Royaumont Conference, near Paris, a work about the detectability of GWs, proposing the first experimental ideas for a GW detector (Weber 1962). His 1960 paper (Weber 1960) constituted the start of the experimental search for GWs. Weber had the merit and the audacity of bringing the discussion about the existence of gravitational radiation from the theoretical to the empirical ground. His effort has to be considered inside the wider context of the Renaissance of General Relativity, as historians call the new attitude of the scientific community towards Einstein’s theory which developed between the 1950s and 1960s, bringing it back from the field of mathematics to physics.1 In the following years, Weber made many experimental attempts and built several prototypes of his resonant bars at the University of Maryland, gradually improving the facilities and the data analysis techniques. Finally, in the late 1960s he believed he had detected gravitational waves and announced the discovery in a paper (Weber 1969). He had set up five resonant bars at Maryland University and one at the Argonne National Laboratory, 1000 km away: he observed several coincidences in the data collected by all the detectors working at the same time, as if the six facilities had been crossed by the same signal coming from the sky. The probability that all of these coincidences were accidental is incredibly small. Experiments imply that electromagnetic and seismic effects can be ruled out with a high level of confidence. These data are consistent with the conclusion that the detectors are being excited by gravitational radiation. These words appear in the abstract of the paper. Several experimental activities started all around the world in order to verify Weber’s results, but none of these experiments – in most cases more sensitive than the original ones – could ever confirm the optimistic conclusions traced by Weber. Nevertheless, the scientific effort aimed at detecting GWs had begun. Among the first activities were the two born in Italy: in Frascati, at the freshly founded European Space Research Institute (ESRIN), and, briefly later, at the Sapienza University of Rome. 2. The European Space Research Institute and the search for GWs ESRIN was born in the 1960s as a facility of the European Space Research Organization (ESRO): a laboratory for studying physical and chemical phenomena in space, gathering experts from specialized fields of theoretical and experimental physics, chemistry and plasma physics. The cornerstone ceremony of ESRIN was celebrated on September 27, 1968, with an inauguration speech by the Director General of ESRO, the theoretical physicist Hermann Bondi, one of the protagonists of the debate on the 1 For more details about the Renaissance of General Relativity, see the talk by R. Lalli at the XXXVI SISFA Congress (Naples, October 4-7, 2016). Atti del XXXVI Convegno annuale SISFA – Napoli 2016 187 existence and measurability of GWs during the Renaissance of General Relativity. It was not by chance that one of the first activities at the new laboratory was the construction of a Weber-type detector, initiated already in 1969. The enterprise was undertaken by the German electronics engineer Karl Maischberger and the Italian physicist Donato Bramanti, assisted by the German technician D. De Loie, expressly assigned by ESRIN to the GW experiment. It is interesting to note that the beginning of this experimental research was stimulated by the theoretical physicist Bruno Bertotti, a former student of Erwin Schrödinger, who had worked at Princeton University besides John Wheeler and Robert Dicke, giving relevant contributions in the field of General Relativity.2 In 1972 Maischberger and Bramanti published a first paper reporting about their activity, in which they acknowledged “the valuable suggestions from Prof. J. Weber” and thanked “Prof. B. Bertotti for initiating this work at ESRIN” (Bramanti, Maischberger 1972). Bramanti and Maischberger’s resonant detector was a room-temperature aluminum cylinder, as Weber’s, 153 cm long, with a diameter of 70 cm and weighing 1580 kg, suspended by a steel wire in a vacuum chamber at 5 x 10-4 torr. An explicit optimism transpires from the paper: We have decided to repeat Weber’s experiment not only to check his results, but because so far, it seems to be the most sensitive device for detecting gravitational waves. Furthermore, it was planned to install a coincidence network with Weber’s detectors at the University of Maryland and the “Weber copy at the Max-Planck Institut in Munich”, where Heinz Billing and Walter Winkler had started in 1971 a Weber-like experiment.3 As Winkler recounted to the authors of this paper, he met Maischberger presumably in 1971 at a conference, where the ESRIN scientist gave a speech about the Frascati detector. Winkler was not aware of the GW activity in Frascati before, nor did he know Maischberger. Afterwards he contacted Maischberger and the two decided to establish a collaboration, which gave birth to the Munich-Frascati experiment. In the meantime, a British computer scientist, Donald Parkinson, had joined the ESRIN team and collaborated to the data analysis of the detector in Frascati (Bramanti et al. 1973). Between 1971 and 1973, ESRO was changing the destination of ESRIN, gradually transforming the space physics laboratory in a canter for Space Documentation and for the development of the Information Retrieval Service database. The GW experiment at ESRIN continued until 1975, thanks to the financial support received from the Max Planck Institute in Munich, which took over the activity. Between July 1973 and January 1975, the detectors in Munich and Frascati worked simultaneously for 350 days. It was the most sensitive room temperature Weber type experiment accomplished so far. As J.L. 2 Bertotti was in Princeton in the years 1958-1961. Among his publications of that period, we remember the fundamental work on the so-called Bertotti-Robinson solution of Einstein’s equations (Bertotti 1959). 3 In Munich the experimental work on bar detectors started on January 1st, 1971, when the young physicist Walter Winkler was hired to work on this topic under Billing’s supervision. By 1976 Billing’s gravitational team in Munich was composed by Walter Winkler, Albrecht Rudiger, Lise Schnupp, Roland Schilling, Peter Kafka. (Source: email-interview released by Walter Winkler to Adele La Rana, 15/9/2016). 188 Adele La Rana, Leopoldo Milano Levine remembers in his historical review about the early years of GW detectors, at the time of room temperature bars the Munich-Frascati experiment unquestionably provided the most stringent test of all for the detection of gravity waves. However, it did not confirm Weber’s results (Levine 2004).4 Nor did the experiment in triple coincidence set up among the Munich-Frascati detectors and the resonant bar at the Observatory of Meudon (Paris), built by the group of Silvano Bonazzola (Bonazzola et al. 1973). In 1975 the Frascati detector was dismantled and later rebuilt in Garching, where Billing’s team had moved in the meantime; it was the definitive end of ESRIN’s brief GW activity. Maischberger was integrated into Billing’s group, which had turned its interests to interferometric detectors around 1974.5 The first paper (Gertsenshtein, Pustovoit 1962) proposing an interferometric method to detect gravitational radiation was published in 1962 by the Russians M.E.