Public talk

Thursday, September 11, 2025, at 6:30 p.m.

Auditorium Grenoble Alpes Métropole – 1pl. André Malraux – Grenoble
Free admission, subject to availability
Lecture in English

In 2019, the Event Horizon Telescope (EHT) issued the first-ever image of a black hole, M87*.  Though of solar system size, M87* is so distant that it is tiny on the sky demanding an Earth-sized telescope to see the donut of billion-degree gas surrounding it.

Could we peer deeper into that donut?  If so, it would open to inquiry an astonishing phenomenon: the photon ring, a thin ring of pure light in orbit around the black hole. Formed by light lensed and thrown into near-horizon orbit by gravity, this ring carries everything there is to say about the black hole, mass and spin.

The Black Hole Explorer (BHEX) is a proposed space radio-telescope joined with terrestrial radio telescopes (including some that are part of the EHT). Together the space and terrestrial instruments will form the most powerful telescope in the history of astronomy.

For astronomers: Understanding the photon ring offers a measure of black hole spin, the engine powering huge astrophysical jets that clear space and shape galaxies.

 For physicists: Since photons follow straight lines of curved space, the ring promises insight into the geometry of spacetime bordering the black hole horizon, an edge of our universe.

BHEX launches experimental photon ring science.

Peter Galison is the Joseph Pellegrino University Professor at Harvard University. He directs the Black Hole Initiative at Harvard, and he is the Science Teams Lead for BHEX. Peter’s work explores the interaction between the principal subcultures of physics—experimentation, instrumentation, and theory—and the embedding of physics in the wider world. More recently, as a member of the Event Horizon Telescope Collaboration, he shared in the 2020 Breakthrough Prize in Fundamental Physics for the capture of the first image of a black hole. His other books include How Experiments End; Einstein’s Clocks, Poincaré’s Maps; and, with Lorraine Daston, Objectivity. Peter’s latest feature film is on Netflix: Black Holes | The Edge of All We Know.