I just returned from Brussels, where I had the privilege of taking part in the 30th Solvay Conference on Physics, devoted to “Gravitational Waves: From Cosmology to Black Holes”. It was an exceptional event, and it was impressive to witness how far the field has come since the first detection was announced, ten years ago.
The Solvay Conferences hold a unique place in the history of physics. The first one, chaired by Hendrik Lorentz in 1911, brought together Einstein, Marie Curie, Planck, Rutherford, Poincaré and others to confront the crisis of classical physics in the face of the quantum. The fifth, in 1927, is remembered for the debates between Einstein and Bohr on the meaning of quantum mechanics — and for what is perhaps the most famous photograph in the history of science. Gravitation was last the theme of a Solvay Conference in 1973, when gravitational waves were still just a prediction, more than half a century after Einstein had first written about them in 1916.
For a century, those waves remained out of reach. Then, on 14 September 2015, the two LIGO detectors recorded the signal of a pair of black holes merging more than a billion light years away. The discovery of GW150914 was announced on 11 February 2016, and what followed was a revolution. In 2017, the merger of two neutron stars, GW170817, was observed both in gravitational waves and across the electromagnetic spectrum, and the Nobel Prize was awarded to Rainer Weiss, Barry Barish and Kip Thorne. Today, the LIGO–Virgo–KAGRA collaboration counts some 390 candidate signals observed up to January 2025, pulsar timing arrays have found evidence for a background of gravitational waves at nanohertz frequencies, and ESA’s LISA mission is preparing to open the millihertz window from space.
The conference was chaired by Roger Blandford (Stanford), Alessandra Buonanno (Max Planck Institute for Gravitational Physics, Potsdam) and David Gross (KITP, Santa Barbara). In the Solvay tradition, it was built around discussion rather than talks.
My role was to put on the table the challenges and opportunities for gravitational-wave probes of dark matter. Gravitational waves open up entirely new ways to search for dark matter: through its effect on the dynamics of black hole binaries, through clouds of ultralight particles around spinning black holes, through primordial black holes, and through the stochastic backgrounds that new physics in the early Universe may have left behind. The opportunities are enormous, but so are the challenges, from the accuracy of waveform models to the difficulty of disentangling new physics from astrophysical effects.
The conference will also be remembered for reasons that had nothing to do with physics. Storm Austen swept across Belgium during the last two days of the meeting, closing the city’s parks and bringing down trees, and was followed within hours by Storm Boelo. Fears of a repeat of the riots and looting that had hit the city centre the week before meant that the windows of our hotel were protected with wooden boards. And, as if that were not enough, a food poisoning outbreak spread among the participants. But hey, we are still alive.
A huge thank you to the Solvay family, to the International Solvay Institutes, and to all the organisers for an unforgettable meeting.