At the first annual conference of Harvard’s Black Hole Initiative, a philosopher concluded his talk by stating that “conversations with some prominent theoretical physicists led me to conclude that if the physics community agrees on a research program for over a decade, then it must be correct.” I realized that his conclusion must have been inspired by a scientific culture in which authority sets the tone. My personal experience has taught me otherwise.
By the year 2000, astronomers had identified a correlation between the mass of black holes at the centers of galaxies and the luminosity of the spheroid, or bulge, of stars that surrounds them. It appeared to me that any such correlation is probably due to the black hole saturating at a mass where its powerful energy output expels the gas reservoir that feeds it, similarly to a baby throwing food off the table when it becomes too energetic.
The self-inflicted limit on a black hole growth must therefore depend on the depth of the surrounding gravitational potential well, which could be gauged through a measurement of the velocity dispersion of stars. At a conference in Leiden, in the Netherlands, I proposed plotting the correlation between black hole masses and the velocity dispersion of their host spheroid of stars. This proposal was immediately rejected as uninteresting or impractical by experts at the conference.
Upon returning to Harvard, I attended two lectures by candidates in an assistant professor search of our department, Laura Ferrarese and Karl Gebhardt, who both presented the standard correlation with spheroid luminosity in their job talks. In my subsequent meetings with each of them separately, I suggest that they plot the correlation with velocity dispersion instead.
Two months later I received a note from each of them informing me that the correlation with velocity dispersion is tight and that they along with their research groups were about to submit an exciting paper on the subject. This would become the hottest result in this field for over a decade and the two teams fought fiercely among themselves for the credit of being first to derive the well-established correlation between black hole mass and velocity dispersion.
The second counterexample was triggered two years later, during my sabbatical at Princeton when I realized that imaging the motion of a “light bulb” just outside the Innermost stable circular orbit (ISCO) in the highly curved spacetime of a black hole could establish a new test of Einstein’s theory of gravity. I conjectured that such a light bulb could take the form of a “hot spot” in an accretion disk, heated through reconnection of magnetic field lines that cross each other, similar to the flares occurring on the surface of the sun.
When I suggested this simpleminded idea to “experts” at Princeton, they dismissed the notion of a hot spot as unrealistic given the turbulent dynamics of gas near a black hole. They argued that any “hot spot” will quickly dissipate through turbulence or be sheared away by the differentially rotating gas.
Based on my earlier experience with the black hole correlation idea, I decided not to give up, and upon my return to Harvard, I suggested this idea as a research project to a brilliant postdoctoral fellow, Avery Broderick, who had just arrived to our newly established Institute for Theory and Computation. In the subsequent few years, Avery and I wrote a series of papers on the observable consequences (light curves, time-dependent images and polarization maps) of a simple-minded “hot spot” moving around the largest black hole on the sky, Sgr A*. This black hole resides at the center of the Milky Way galaxy and weighs 4 million Suns.
We also made predictions for the second largest black hole on the sky, M87, which weighs 1,000 to 2,000 times more but resides at a distance that is 2,000 times farther. The giant black hole in M87, located at the center of the Virgo cluster of galaxies, launches a tightly collimated jet out to a distance of thousands of light years.
This month a team of astronomers at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, led by Reinhard Genzel and spearheaded by Frank Eisenhauer, announced the observational discovery of hot spots moving in a circle on the sky for three flares near the ISCO of Sgr A*. Their exciting observational data was obtained with the GRAVITY instrument on the Very Large Telescope Interferometer in Chile.
As Galileo reasoned after looking through his telescope, “in the sciences, the authority of a thousand is not worth as much as the humble reasoning of a single individual.” To which I would add the footnote that sometimes Mother Nature is kinder to innovative ideas than people are.