To the editors:

I’d like to offer the following clarification regarding a point mentioned in Steven Wheeler’s essay: “This tiny delay [7 milliseconds] corresponds to the time for a gravitational wave, which moves at the speed of light, to travel the distance between the two detectors: 3,002 kilometers.” For a gravitational wave traveling at the speed of light to propagate directly across a distance of 3,000 km would require 10 milliseconds rather than 7. The shorter delay indicates that the direction of the gravitational wave was neither in the alignment of the two detectors, nor in a celestial plane orthogonal to the half-way distance between the two detectors—for which the time delay would have been zero. Since a pair of detectors cannot be directional (at least three are needed for this purpose), the location of the event was rather imprecise. However, the seven millisecond delay enabled the scientists to localize the source to an elongated area of the sky, in a plane roughly orthogonal to the line joining the two detectors. This point is elaborated in the penultimate section of the essay, but, I fear, a little too late for an uninformed reader.

I was surprised to learn of so much reluctance on the part of some American astronomers to accept that gravitational wave physics was indeed astronomy. This was not at all the case in France. I arrived at the Paris-Meudon Observatory in 1976 as a young researcher in the Groupe d’Astrophysique Relativiste. I can recall that a gravitational wave bar detector, of the type designed by Joseph Weber, was under construction at the time. The project was directed by Silvano Bonazzola, also the director of our group, with Brandon Carter. Several French astronomers, not only theoretical physicists, were also involved (the great John Wheeler paid us a visit around the same time). It was soon realized, of course, that Weber’s claims of a positive detection were erroneous and that bar detectors could not achieve the required sensitivity. Several relativistic astrophysicists from our group, such as Thibaut Damour and Luc Blanchet, subsequently made the analytic calculation of the gravitational signal that would be produced by the coalescence of black holes or neutron stars. They predicted exactly what was found by LIGO, and shared the 2016 Gruber Prize for Cosmology. Efforts to detect gravitational waves have thus always been considered, at least in France, an integral part of relativistic astrophysics, and, more broadly, astronomy.

Jean-Pierre Luminet