Except for a few brief periods, Einstein was uninterested in analysing the nature of the spacetime singularities that appeared in solutions to his gravitational field equations for general relativity. The existence of such monstrosities reinforced his conviction that general relativity was an incomplete theory which would be superseded by a singularity-free unified field theory. Nevertheless, on a number of occasions between 1916 and the end of his life, Einstein was forced to confront singularities. His reactions show a strange asymmetry: he (...) tended to be more disturbed by merely apparent singularities and less disturbed by real singularities. Einstein had strong a priori ideas about what results a correct physical theory should deliver. In the process of searching through theoretical possibilities, he tended to push aside technical problems and jump over essential difficulties. Sometimes this method of working produced brilliant new ideas—such as the Einstein–Rosen bridge—and sometimes it lead him to miss important implications of his theory of gravity—such as gravitational collapse. (shrink)

Except for a few brief periods, Einstein was uninterested in analysing the nature of the spacetime singularities that appeared in solutions to his gravitational field equations for general relativity. The existence of such monstrosities reinforced his conviction that general relativity was an incomplete theory which would be superseded by a singularity-free unified field theory. Nevertheless, on a number of occasions between 1916 and the end of his life, Einstein was forced to confront singularities. His reactions show a strange asymmetry: he (...) tended to be more disturbed by merely apparent singularities and less disturbed by real singularities. Einstein had strong a priori ideas about what results a correct physical theory should deliver. In the process of searching through theoretical possibilities, he tended to push aside technical problems and jump over essential difficulties. Sometimes this method of working produced brilliant new ideas—such as the Einstein–Rosen bridge—and sometimes it lead him to miss important implications of his theory of gravity—such as gravitational collapse. (shrink)

The ArgumentThe question of the possible existence of black holes is closely related to the question of the action of gravitation on the propagation of light. It has been raised recurrently from the when that Newton referred to a possible bending of light in hisOpticks. And it relies on apparently simple questions: Is light subject to gravitation? What is the effect of a gravitational field on the propagation of light? Could a particle of light emitted by a star be retained (...) by its gravitational field?From the end of the 1960s, the black hole idea has had a very important place in the relativistic literature, not to speak of the popularization of the theory. It turned out to be not only an important concept but also a tool that permitted us to understand general relativity better, indeed a tool that contributed greatly to changing the interpretation of Einstein's theory of gravitation. Here too I want to use this concept of the black hole to assist our understanding of the history of general relativity: the black hole is a fundamental milestone in the evolution of general relativity. (shrink)

In 1786, Robert Blair, an unknown astronomer from Edinburgh, wrote a paper that would remain unpublished. In his manuscript, Blair gives a systematic treatment of the Newtonian kinematics of light, taking into account in the absolute space of Newton the motion of the light source, that of the observer, and the velocity of the corpuscles of light. Two years before, in the context of Newton's corpuscular theory of light, John Michell had pointed out that the velocity of light could be (...) measured with the help of refraction experiments. Blair went a step further and inferred the existence of what we now call the Doppler effect: a variation of refraction due to a relative motion of the source and the observer. Blair's proposal is at the roots of Arago's well‐known 1806–10 experiments on the velocity of light. In the context of the undulatory theory of light, Blair proposed an experiment to determine the absolute motion of the Earth, laying the bases for the famous experiment performed by Albert Michelson 100 years later. In fact, this manuscript contains the very questions of light relativity, the roots of spectroscopy, and addresses the very problems that would be hotly debated in the nineteenth century, only to be solved by Einstein in 1905. (shrink)

During fifty years Einstein’s theory of gravitation was limited to a backward neo-newtonian interpretation. Nevertheless it was sufficient to as­sure its predominance in regard to Newton’s theory. In the late sixties the studies on the structure of spacetime around a star as well as that concerning the cosmological field were essential for general relativity to get a coherent interpretation. What were the difficulties which explain such a delay to re­consider its interpretation? What were the elements, the concepts that will permit (...) the experts the renewal of the sixties? Cosmology, cosmologists are at the very centre of this revolution. (shrink)

During fifty years Einstein’s theory of gravitation was limited to a backward neo-newtonian interpretation. Nevertheless it was sufficient to as­sure its predominance in regard to Newton’s theory. In the late sixties the studies on the structure of spacetime around a star as well as that concerning the cosmological field were essential for general relativity to get a coherent interpretation. What were the difficulties which explain such a delay to re­consider its interpretation? What were the elements, the concepts that will permit (...) the experts the renewal of the sixties? Cosmology, cosmologists are at the very centre of this revolution. (shrink)