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Gravitational astronomy: listening to the whispers of the universe It was about 10 am, Greenwich time, on September 14th 2015 when scientists detected something which had never been detected before: the “sound” of two colliding black holes which, after orbiting around each other for billion years, approached closer and closer until colliding, giving rise to a single huge black hole. A tiny fraction of second before the collision, they sent a signal towards the rest of the universe, their “swansong”, a vibration which propagated in space and time at the speed of light, getting to the Earth 1.4 billion years later. A gravitational wave. A hundred years after the publication of the General Relativity, the Gravitational Waves detection confirms once again the validity of Einstein’s Theory, and paves the way for a completely new research field. Space-time ripples poorly interacting with matter and giving rise to extremely tiny effects, Gravitational Waves need very sophisticated instruments to be detected. In this paper we will retrace the history of this discovery, starting from the breath of revolution of General Relativity which carries along the theorization of Gravitational Wave. We will then go through the analysis of the effect that Gravitational Waves induce on the matter, and we will describe the basic principles of the instrument which detected them, to finally get to the first discovery which opens the way to a completely new era for astronomy.

This paper aims to discuss two realist conceptions about causation in the light of the general theory of relativity (GTR). I first consider the conserved quantity of causation, which explicitly relies on the energy conservation principle. Such principle is however problematic within GTR, mainly because of the dynamical nature of the spacetime structure itself. I then turn to the causal theory of properties, according to which (fundamental physical) properties are such that insofar as they are certain qualities, they are powers to produce certain effects. In order to be compatible with GTR, such theory has to assume non-trivial global conditions on the spacetime structure; such assumptions seem to deprive the „singularist‟ non-Humean feature of this theory of causation. The question of the possible causal nature of spacetime (metrical) properties is addressed in the conclusion.

This paper addresses the extent to which both Julian Barbour‘s Machian formulation of general relativity and his interpretation of canonical quantum gravity can be called timeless. We differentiate two types of timelessness in Barbour‘s (1994a, 1994b and 1999c). We argue that Barbour‘s metaphysical contention that ours is a timeless world is crucially lacking an account of the essential features of time—an account of what features our world would need to have if it were to count as being one in which there is time. We attempt to provide such an account through considerations of both the representation of time in physical theory and in orthodox metaphysical analyses. We subsequently argue that Barbour‘s claim of timelessness is dubious with respect to his Machian formulation of general relativity but warranted with respect to his interpretation of canonical quantum gravity. We conclude by discussing the extent to which we should be concerned by the implications of Barbour‘s view.

A draft of a possible comparison between the use made of mathematics in classical field theories and in quantum mechanics is presented. Hilbert’s space formalism, although not only elegant and powerful but intuitive as well, does not give us a spatio-temporal representation of physical events. The picture of the electromagnetic field as an entity which is real in itself – i.e., as a wave without support – fostered by the emergence of special relativity can be seen as the first step, favored by many physicists and philosophers, of a gradual “escape” from intuition into a purely mathematical representation of the external world. After the introduction, in recent theoretical physics, of fiber bundle formalism the classical notion of field acquires a new spatio-temporal intuitiveness. This intuitiveness is clearly foreshadowed in the Kantian and Meinongian analysis of the notion of magnitude. At the end of the paper we show that, contrary to what happens in quantum mechanics, mathematics plays a truly explicative role in general relativity, without any loss of spatio-temporal intuitiveness.

The debate on the ontological status of spacetime in General Relativity has historically seen two principal philosophical contenders: substantivalism, roughly the view that holds that spacetime exists apart from the material contents of the universe, and relationism, the doctrine that spacetime does not exist, i.e., it is a mere abstract web of spatiotemporal relations among bodies. This dispute, however, has rarely been fought on a cosmological battlefield. In this paper an attempt in this direction is made. The question at issue is the following: is there any feature of our universe that requires or is best explained in terms of a substantival space? I claim that there is indeed: the expansion of the universe, perhaps the most important phenomenon in cosmology, can play such a role.

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We reexpress in tetrad form the background dependent family of superpotentials for the first-order Lagrangians which generate Einstein’s field equations, found in an earlier paper. Using a result on the pull-back properties of the Poincar´eCartan forms for second-order Lagrangian field theories, we show that these superpotentials are the superpotentials corresponding to a Lorentz invariant family of first-order tetrad Lagrangians.