Caltech Theoretical Astrophysics Professor Receives Dirac Medal

Saul Teukolsky (PhD ’74), the Robinson Professor of Theoretical Astrophysics at Caltech, has been awarded the 2021 Dirac Medal and Prize of the International Center for Theoretical Physics for his contributions in predicting the properties of gravitational waves that emerge from the collisions of black holes. He is receiving the prize jointly with Alessandra Buonanno of the Max Planck Institute for Gravitational Physics in Germany; Thibault Damour of the Institute of Advanced Scientific Studies in France; and Frans Pretorius of Princeton University in New Jersey. Together, the researchers’ contributions provided the theoretical underpinnings for LIGO (Laser Interferometer Gravitational-wave Observatory) to make the first direct detection of gravitational waves in 2015.

“I’m very honored to receive this award,” says Teukolsky. “But quite frankly, this project would have been impossible without the great graduate students and postdocs who have contributed so much.”

“Saul’s contributions to the fields of gravitational-wave physics and astrophysics have been immense and well deserving of the Dirac medal,” says Alan Weinstein, professor of physics at Caltech and a member of the LIGO team. “His work over the decades has dramatically shaped and informed our understanding of compact stars, such as white dwarfs, neutron stars, and black holes; provided precise waveform templates that enable LIGO’s detection of binary black hole mergers; allowed us to probe the structure of black holes through black hole spectroscopy and perform precision tests of general relativity in the strong-field regime; and much more.”

The medal is granted by the International Center for Theoretical Physics in Italy in honor of the late Nobel laureate Paul Dirac, an English physicist who made fundamental contributions to quantum physics.

According to the award citation, the four researchers are being honored for “establishing the predicted properties of gravitational waves in the curvature of spacetime produced when stars or black holes spiral together and merge. This achievement was essential for the LIGO detection of gravitational waves from these energetic astronomical events. The consistency of theory and observation is an impressive check of the accuracy of the general theory of relativity.”

In the early 2000s, Teukolsky who is also a professor at Cornell University, created a group for simulating the collisions of black holes using Albert Einstein’s general theory of relativity, and he has led it ever since. This group, a collaboration of Caltech and Cornell called SXS (Simulating eXtreme Spacetimes), was established with the help of Teukolsky’s former graduate school adviser Kip Thorne (BS ’62), the Richard P. Feynman Professor of Theoretical Physics, Emeritus.

Pretorius was a Caltech SXS postdoctoral scholar when he made a major breakthrough in simulations of colliding black holes, for which he shares the Dirac medal. Buonanno, also a former Caltech postdoctoral scholar, and Damour share the medal for building a formalism that interfaces between the simulations and LIGO’s data analysis.

“We use the biggest computers we can get time on to do these calculations,” said Teukolsky. “But it was never clear to me that nature would provide big black holes merging often enough to be seen. And, of course, we needed our experimental colleagues to build an incredible machine to enable the detections to take place.”

By 2015, SXS had figured out how to simulate the black hole mergers and predict waveforms for collisions that vary in black hole mass, spin rates, and spin directions. LIGO researchers used the waveforms to pinpoint the signatures of merging black holes in their data and extracted from each observed signal the black holes’ masses and spins.

“The accuracy of the SXS team’s predicted gravitational waveforms and the speed and robustness of their computations are remarkable,” says Thorne. “They have not only been crucial for LIGO, they have also led to major new insights into how spacetime behaves when highly warped and disturbed, like the surface of the ocean in a storm.”

Teukolsky received his bachelor’s degree from the University of the Witwatersrand, South Africa, in 1970 and his PhD from Caltech in 1974. He is a co-author of the widely used textbooks Numerical Recipes: The Art of Scientific Computing and Black Holes, White Dwarfs and Neutron Stars: The Physics of Compact Objects. His honors include membership in the American Academy of Arts and Sciences and the National Academy of Sciences. In 2021, he received the American Physical Society’s Einstein Prize.