Black Hole Spectroscopy

Minutes

On October 9, 2026, Members of the Society and guests joined the speaker for a reception and dinner at 5:45 PM in the Members’ Dining Room at the Cosmos Club. Thereafter they joined other attendees in the Powell Auditorium for the lecture proceedings. In the Powell Auditorium of the Cosmos Club in Washington, D.C., President Larry Millstein called the lecture portion of the 2,542nd meeting of the Society to order at 8:02 p.m. ET. He began by welcoming attendees, thanking sponsors for their support, announcing new members, and inviting guests to join the society. Scott Mathews then read the minutes of the previous meeting which included the lecture by Julia Adler-Milstein, titled “Digitization of the US Health Care System and Early Insights from the AI Era”. The minutes were approved as read.

President Millstein then introduced the speaker for the evening, Emanuele Berti, of the Johns Hopkins University. His lecture was titled "Black Hole Spectroscopy: Listening to a Black Hole’s Gravitational-Wave Ringing."

The speaker began by sharing his personal journey into gravitational physics, sparked as a teenager by reading Stephen Hawking's A Brief History of Time. He noted that the conceptual origins of black holes predated Einstein's theory of general relativity. He said that as early as 1783 simple Newtonian energy arguments were used to predict that a star with the density of the Sun but a radius 500 times larger would possess a gravitational pull so strong that light could not escape. Berti gave a brief history of the modern formulation of black holes, beginning with Einstein's equations of general relativity, published in November 1915, Karl Schwarzschild finding the first exact, spherically symmetric solution to these equations in December 1915, Chandrasekhar predicting the gravitational collapse of white dwarfs in 1930 (the Chandrasekhar limit), and Oppenheimer and Snyder calculating the collapse of a non-rotating, spherically symmetric star, shrinking below the Schwarzschild radius to form a black hole. Following World War II, John Wheeler and his students (including Kip Thorne) continued the study of gravitational collapse, showing that nuclear reactions could not halt the collapse of massive stars in the 1950s. The speaker said that the 1960s and 1970s marked a "Golden Age" of black hole physics. In 1963, Roy Kerr discovered the mathematical solution for rotating black holes, Penrose and Hawking developed singularity theorems proving collapse is a robust physical process, and Maarten Schmidt discovered quasars. Around 1971, Cygnus X-1 was identified as the first black hole candidate.

The speaker described black holes as states of extreme curvature of space-time, produced by the collapse of matter. He illustrated this idea via Kip Thorne's "parable of the ants", where ants on a bending membrane launch outbound communication balls (photons), which can no longer escape once the curvature reaches a critical value. Berti claimed that the study of black holes is now an integral part of modern physics and cosmology. He said that four Nobel Prizes have been shared amongst eight researchers since 1983.

The speaker then discussed gravitational waves; essentially tidal forces which stretch and compress space-time. He said that in the late 1960s, Ray Weiss proposed using laser interferometers to detect these incredibly minute changes. This led to the construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, and Livingston, Louisiana, alongside Virgo in Italy and KAGRA in Japan. On September 14, 2015, gravitational waves from two merging black holes were directly detected for the first time (GW150914), earning a Nobel Prize for Kip Thorne, Ray Weiss, and Barry Barish. Since then, over 450 binary mergers have been detected. Notably, the high signal-to-noise ratio event GW250114 in January 2025 provided an exceptionally clean signal, allowing astronomers to measure black hole properties with unprecedented precision.

The speaker then turned to black hole spectroscopy: the analysis of the characteristic frequencies of the gravitational waves radiated from a merger or perturbation. Theory indicates that such an event should cause “ringdown”: an exponential decay of a characteristic frequency, analogous to the sound of a struck bell.

The speaker said that theoretical analysis attributed to Kerr showed that a rotating black hole is uniquely characterized by only two properties: its mass and its spin. Consequently, the measurement of the frequency and damping time of a fundamental mode of oscillation allows the estimation of the mass and spin of the system. Measuring a second subdominant mode or "overtone" allows the testing of the consistency of the Kerr metric, thereby performing black hole spectroscopy. Subsequently, it was shown that a superposition of overtones could fit the merger waveform all the way to its peak. While early claims of observing overtones in merger events were controversial due to noisy data, the “louder events” (like GW250114) successfully confirmed the frequency of the first overtone, marking the true dawn of experimental black hole spectroscopy.

Looking ahead, the speaker highlighted the bright future of gravitational wave astronomy. The current ground-based network is undergoing a series of upgrades and a fifth observing run is planned for 2029. In the 2030s, the Laser Interferometer Space Antenna (LISA) will be launched into space to detect supermassive black hole mergers and extreme mass ratio inspirals (EMRIs). Ground-based next-generation detectors, such as the Einstein Telescope in Europe and Cosmic Explorer in the United States, will increase sensitivity tenfold, making the detection of non-linear quasinormal modes a routine occurrence. More ambitious concepts include deploying seismometers on the Moon to use the Moon itself as a giant gravitational wave detector.

Berti ended his talk by discussing the fact that noisy data acquired from gravitational wave measurements can be consistent with deviations from General Relativity, but that future measurements with high signal-to-noise ratios will likely exclude this conclusion.

The lecture was followed by a question and answer session.

A guest asked what tool was used to produce one of the animations presented in the talk. Berti replied that there are several tools that can be used to perform numeric, relativity simulations. He said that the techniques used to create that particular simulation “are not state-of-the-art anymore”. He said that there is an entire field of research called “gravitational self-force” which seeks to improve numeric simulations of relativistic, gravitational interactions.

A member asked about the frequency of black hole collisions and other phenomena that cause gravitational waves, asking “How often do they occur and do we know anything about the distribution?” Berti responded that there are several “channels” which control the way black holes merge. The two most dominant channels are isolated binary formation and dynamical formation. He gave a brief description of each of these processes. He said that while it is difficult to estimate the rate of mergers throughout the entire universe, we are seeing hundreds of black hole mergers per year.

A member discussed super-massive black holes at the center of galaxies merging with other, smaller black holes, asking “Why don’t we see gravitational waves from those events?” Berti responded “Because we don’t have the detectors yet.” He said that the frequencies of such events would be too low to be detected by LIGO, but that future detectors, like LISA, should be able to measure these events.
A guest on the live stream asked if a ground-based system, similar to LIGO but with 15-kilometer arms, would be able to measure the gravitational waves from the merger of intermediate mass black holes. Berti responded “Yes”. He said that one of his Ph.D. students investigated this topic for his dissertation.

After the question and answer period, President Millstein thanked the speaker and presented him with a PSW Rosette, a signed copy of the announcement of the lecture, and a signed copy of Volume 17 of the PSW Bulletin. He then announced speakers of upcoming lectures and made a number of housekeeping announcements. He adjourned the 2,542nd meeting of the Society at 10:13 p.m. Eastern Time.

Temperature in Washington, D.C.: 16.1 °C
Weather: Overcast

Dinner Attendance: 43
Lecture Attendance (In-Person): 83
Livestream Attendance: 30
For a Total of 113 viewers
Views of the video in the first two weeks: 786

Respectfully submitted, Scott Mathews, Recording Secretary