Bright Spot in Orbit Around a Black Hole

During my sabbatical year in 2002/3 at the Institute for Advanced Study in Princeton, I came up with an intriguing “thought experiment” (gedankenexperiment) on how to probe the curved spacetime theory of Albert Einstein, who was faculty there half a century earlier.

Imagine a source of light in orbit around a stationary black hole. Spacetime is strongly distorted near the black hole horizon, which marks the outer boundary of the prison walls from where nothing, not even light, can escape. The prediction for the extremely curved spacetime in that region was derived in 1916 by Karl Schwarzschild, merely a few months after Einstein published his equations of General Relativity. The gedankenexperiment suggests that as the source of light approaches the black hole horizon, we would see it slowing down and shifting its emission to longer wavelengths. Its image would fade away and eventually freeze, like a cowboy riding towards the horizon at the end of an old Western movie.

After thinking about it for a while, it occurred to me that this thought experiment can be turned into a real experiment. To image the motion of the light source, one better use the largest black hole projected on our sky. This happens to be Sagittarius A* (abbreviated as, Sgr A*) at the center of our own Milky Way galaxy. It is located twenty-four thousand light years away and has a mass four-million times larger than the Sun. The Schwarzschild radius of the horizon is a tenth of the Earth-Sun separation.

Now that we identified the black hole, what would be the light source? Unfortunately, it is not possible to use a star, because the strong tide induced by SgrA* would “spaghettify” a star like the Sun into a stream of gas at a distance that is ten times larger than the horizon. But fortunately, SgrA* shines at radio waves and the gas around it is transparent to this emission all the way down to the horizon at wavelengths shorter than a millimeter. This is a lucky consequence of the low accretion rate of gas into SgrA* at the present time, nearly a billion times below the maximum rate it can accept through its horizon. Nature is kind to astronomers. It made the horizon visible to sub-millimeter telescopes. A couple of years before my sabbatical, a paper by Heino Falcke, Fulvio Melia and Eric Agol, noted that the image of SgrA* can be resolved by an array of sub-millimeter telescopes around the globe.

In this backdrop, it occurred to me that a hot spot of enhanced brightness in the gas orbiting SgrA* could be used to map the spacetime near it. I suggested this idea to a few local experts on accretion flows and they all dismissed it as unrealistic because such a spot would disperse quickly owing to its enhanced pressure and, moreover, it would be sheared away by differential rotation around SgrA*. None of these experts was interested in collaborating with me on a theoretical paper proposing a real experiment based on my gedankenexperiment.

When I returned back from my sabbatical, I met a new postdoctoral fellow in our theory group at Harvard, named Avery Broderick. Avery just finished his PhD at Caltech, in which he developed a radiative transfer code for simulating the light emitted from hot gas near a black hole. This was an opportunity I could not miss.

At our first meeting, I proposed to Avery that we explore the appearance of a bright spot moving around a black hole. Thanks to Avery’s brilliant insights and technical skills, we wrote thirty papers on this subject, starting in 2005 with a paper titled: “Imaging bright-spots in the accretion flow near the black hole horizon of Sgr A*,” that included predictions for the variability and polarization of the light emitted by the bright spots. In 2009, we published the first detailed paper suggesting to image the black hole in the giant elliptical galaxy, M87. Our proposal was realized a decade later by the Event Horizon Telescope, a collaboration in which we participated as the initial members.

Whether theoretical calculations are relevant to reality is judged by observations. Gladly, nature was kinder to us than the experts in the field. In 2018, a German team used the GRAVITY instrument on the Very Large Telescope Interferometer to image the motion and measure the polarization of a bright spot in orbit around SgrA* at infrared wavelengths. Last week, a new paper reported polarization analysis of bright spots in orbit around SgrA*, guided by data from sub-millimeter telescopes.

Tradition instructs us to reflect about the past at the beginning of each Jewish new year, which always coincides with the academic year. What are the lessons learned from this experience? First, we should not be deterred by experts. Those who base their prestige on past knowledge are often tied up in routine thoughts and keep taking the beaten path. Intellectual innovation does not necessarily align with authority. Second, we must allow nature to surprise us in realizing circumstances that experts cannot imagine. The final verdict lies with what exists out there, not with the barricades that humans place on exploring it. Gladly, young scientists like Avery Broderick tend to jump over these artificial roadblocks. Their wonder makes the bright spot in my own spacetime trajectory.


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 chairs the advisory board for the Breakthrough Starshot project, and 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.



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Avi Loeb

Avi Loeb


Avi Loeb is the Frank B. Baird Jr Professor of Science and Institute director at Harvard University and is the bestselling author of “Extraterrestrial”.