Photons, neutrinos, and gravitational-wave astronomy

Before narrowing down what could be (mathematically and astrophysically) the sources of GWs, let's consider when are GR effects most relevant? A typical quantity to look at the so-called "compactness" \(M/R\) with \(M\) mass of a source and \(R\) its linear dimension. Note that in natural units (\(G=c=1\) typically used to simplify the formalism in GR), this is a dimensionless number. For low values of \(M/R\), General Relativity reduces to Newtonian gravity – as it should being an extension of this theory, and in Newtonian gravity the gravitational field is fixed and any change propagates instantly: there are no gravitational waves. N.B.: Just introducing the postulate that "gravity" has a finite speed of propagation in Newtonian physics, one can build a lot of intuition and quantitative results correct to order of magnitude for GW physics, see Schutz 1984 and LVK Collaboration 2017. For general relativity effects to matter, \(M/R\) needs to be "large": either extremely large masse ... Read full article.