GPS Mapping of the Spacetime Around a Black Hole

Pointings by the Robert C. Byrd Green Bank Telescope in the Galactic Center region around the supermassive black hole, Sagittarius A*, with an overlaid total intensity mosaic of the region at a frequency of 1.28 gigahertz from the MeerKAT radio observatory. The central pointing, labeled as A00, indicates the Galactic center location at l = 0°, b = 0°. (Image credit: K. Perez et al. 2026)

The ideal way to map the spacetime around a black hole is by a Global Positioning System (GPS) with a collection of precise clocks travelling through this spacetime. This was my insight in 2003, when I approached the postdoctoral fellow Eric Pfahl and suggested that we write a paper about the feasibility of such a mapping as a test of Albert Einstein’s theory of General Relativity.

My idea was to use a population of millisecond pulsars as an array of natural GPS around the supermassive black hole, Sagittarius A*, at the center of the Milky-Way galaxy. We observe massive stars forming out of dense gas in that region and know that these stars are short-lived.

The observed star cluster around this 4.3-million-solar-masses black hole should have left neutron star remnants. A neutron star contains about 1.4 solar masses within a diameter of order 24 kilometers, comparable to the length of Manhattan Island. This compact remnant is a relic of progenitor stars more massive than 8 solar masses and resembles a giant atomic nucleus, made of neutrons at nuclear density. Neutron stars can spin up to a limiting rate without breaking-up, corresponding to a period of order half a millisecond (=0.0005 seconds). Their stable rotation makes them excellent clocks with precision of a part in a trillion, comparable to common atomic clocks. The ticking rate of these precise clocks can be detected on the sky, as some neutron stars produce a stable radio beam that is misaligned with the spin axis and sweeps the sky periodically like a lighthouse. They are called radio pulsars.

An artist’s illustration of a pulsar, where a radio beam which is misaligned with the spin axis of a neutron star, sweeps the sky periodically like a lighthouse. (Image credit: Mark Garlick/Science Photo Library)

The resulting paper, titled “Probing the Spacetime Around Sagittarius A* With Radio Pulsars”, was published in 2004 and can be found here. It argued that the brightest radio pulsars near Sagittarius A* may be detectable with current telescopes in periodicity searches at radio frequencies near 10 gigahertz, where the effects of scattering by interstellar electrons are modest. Long-term timing observations of such a pulsar would clearly reveal its motion around the black hole and potentially show the effects of Einstein’s relativistic gravity. We discussed how pulsar timing can be used to study the spacetime around the central black hole.

This proposal was published 22 years ago. On February 10, 2026, a new paper led by Karen Perez (available here) reported the discovery of a millisecond pulsar candidate following the deepest radio search in the Galactic center region, conducted by the Robert C. Byrd Green Bank Telescope. The survey operated in the radio frequency band of 8–12 gigahertz, using data from the Breakthrough Listen initiative. Following a comprehensive

periodicity search targeting radio pulsars, the survey was sensitive enough to detect the most luminous pulsars expected in the Galactic center. Among 5282 sources, the researchers identified an interesting pulsar candidate with a 8.19 millisecond period, persistent in time and frequency, and flagged it for follow-up verification.

The scarcity of detected radio pulsars near Sagittarius A* reinforces the concern expressed in my original paper that strong scattering by the dense interstellar gas in the Galactic center obscures pulsar signals. Moreover, the extreme orbital acceleration very close to the central black hole could distort the periodic signals and make their detection challenging.

Scientific progress sometimes occurs at a glacial pace. I am happy to have laid a brick 22 years ago in the construction project that young scientists like Karen Perez are pursuing now. In a recent conversation, I was asked which historic period I would rather live in, given a choice. My response was: “in the future.” When asked “why?,” I explained that I am optimistic that the future will be better than the past thanks to scientific advances.

The collapse of stars at the end of their life leads to compact remnants, such as white dwarfs, neutron stars or black holes. This newly processed image from the NASA/ESA Hubble Space Telescope shows the Egg Nebula, a structure of gas and dust created as a Sun-like star approaches the end of its life. This Egg will eventually hatch, revealing a white dwarf at its center. (Image credit: HST/NASA/ESA)

ABOUT THE AUTHOR

(Image Credit: Chris Michel, National Academy of Sciences, 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

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