ESPERIMENTI DI ETHER-DRIFT NEL XX SECOLO - CASI STORICI A CONFRONTO: EFFETTO SAGNAC (FRANCIA, 1913) ED ESPERIMENTI DI MILLER (USA, 1921-26)

One of the most studied topics of the history of Modern Physics is the transition between the 19th century newtonian world-view and the 20th century physics based on the development of relativity theories and quantum mechanics. After Kuhn’s definition of “scientific revolution” and “normal science”, several scholars considered the creation, development and reception of special and general relativity theories a scientific revolution. In particular, this thesis refer to Renn’s writings, which defined “relativity revolution” the process that led to ‘the introduction of fundamentally new concepts of space, time, matter and radiation’ in 1905, and was completed with Einstein’s creation of the theory of general relativity in 1915. One of the main features of the relativity revolution was the explicit dismissal of the luminiferous ether as the real support of the light and/or electromagnetic waves. The conceptual changes linked to the relativistic physical theories (including the refusal of the ether) caused several kinds of controversies within the national scientific communities. Some scientists opposed Einstein’s theories on the basis of epistemological analysis and metaphysical conceptions of reality. This field of study is considered to be part of the historiographical context of the “reception of relativity” and the few scholars who concerned themselves with the analysis of this framework underlined the existence of national differences in the reception of the physical meanings and epistemological features of the relativistic theories. The thesis is a part of this (not developed) historiographical context, and it approaches some specific experimental issues whose features has never been analysed in their complexity. At the beginning of the 20th century (in the period of relativity revolution) two ether-drift experiments were performed, which created a debate on their relevance and meaning within the framework of relativity theories: the discovery of the Sagnac effect (Paris, 1913) and Miller’s repetition of the Michelson-Morley experiment (Mount Wilson, CA, 1921-26). Both Sagnac and Miller interpreted their experiments as a confutation of the principles on which special relativity theory was based. They were not alone in the anti-relativistic interpretation of their findings. A minority of scientists are still claiming that these discoveries were fundamental and that they were discarded (or put aside) without acceptable explanations. Even though the controversy about these events is now at the fringe of the development of physics, these two cases could represent interesting examples of the national differences in the reception of relativity and the indissoluble link between theory and experiments in the interpretation of observational results. Moreover, the use of these experiments by dissident authors created a break between the “standard history” and the “unorthodox history” that needs to be analyzed in detail. The aim of the thesis is to address the historiographical lack on these experiments with a comparative analysis of their history from their immediate reception in the national scientific community in which they were performed until their eventual inclusion in the development of normal science. The choice of a comparative analysis using the spatial unity of nation is justified by the historiographical tradition involved in the studies of the reception of relativity. Apart from the ether context in which these experiments were made, their results have very different scientific meanings. The Sagnac Effect is a fringe shift at the first order in an interferometer set in uniform rotation with respect to the fringe position for the stationary one. The result obtained by Sagnac in 1913 was accepted and the Sagnac effect is used nowadays in aero-spatial navigation and in the synchronization of GPS. Thus the historiographical questions about this experiment are related to the interpretations of the effect within the debate between ether theories and relativity theories. According to the historian of Physics J. Stachel the uniform rotation constitute a “missing link” between special relativity theory and general relativity theory. In this passage the German mathematician T. Kaluza derived the formula of the Sagnac effect in the development of relativity programme in 1910, before the discovery of the effect. In this way, the effect was included in the relativity revolution without any ad hoc hypothesis. The situation was very different in France, where the effect was used by anti-relativistic scientists until P. Langevin published a paper in which he derived the Sagnac formula within the general relativistic framework. Miller’s experiments were a repetition of the Michelson-Morley experiment. They showed a positive result, that is a slight fringe shift periodic with respect to the sidereal time. Miller interpreted his data as a demonstration of the earth’s absolute motion through the ether, which has the value of 10 km/s at the top of Mont Wilson. Contrary to the Sagnac effect, Miller’s results were eventually discarded by the scientific community for several reasons. One of the main reasons was that between 1926 and 1930 some researchers repeated the MM experiment, but no one found the same effect. This lack of confirmation and the non-existence of an ether theory that could compete with special relativity theory made Miller’s result an enigmatic one. Thus it became of little interest to the scientific community, until 1954 when Shankland and three colleagues reanalysed the data and found a possible explanation in a temperature effect. The scientific community regarded this explanation as the conclusion of the matter without any debate, but some contemporary anti-relativists contest Shankland’s methodology. The comparative analysis realized in the thesis was based on different questions: How the national scientific communities reacted to the claims of Sagnac and Miller? In which way these results were included in the development of physical theories in the ‘20s? What was the approach of leading theorists to these experiments? When and how did the MM experiment end? When and why was Miller’s result discarded? When and why was the Sagnac effect included in the relativistic research programme? Which role (if any) did sociological elements play in the US and French reactions to these experiments? How was Langevin’s derivation received by French scientific community? Why did Shankland decide to perform the difficult re-analysis of Miller’s data? How did Shankland’s re-analysis find a possible solution? Other questions could also be asked in relation to philosophical and methodological analyses of this history: Which were the common and the conflicting features amongst the different repetitions of the same type of experiment? Which behaviour could be called rational and which could be called irrational in the history related to these experimental results? The comparative analysis was made on the scientific papers as well as on private documents of the principal actors involved in the events. In particular, the use of private documents has permitted to underline some sociological elements that played a relevant role in the history. The analysis of differences and analogies between the two cases in their national context at the beginning of the 20th century highlight the deep break that there was between the followers of relativity and the physicists of the “old school”. Moreover the thesis highlight some epistemic factors that played an important role in the accumulation of consensus on relativity during the 20th century.