The 2020 Nobel Prize in Physics has been awarded to three individuals for their seminal work on black holes. One half of the prize went to University of Oxford Scientist Roger Penrose who showed that black hole formation is a “robust prediction” of Einstein’s general theory of relativity; the other half went jointly to Germany’s Max Planck Institute/UC-Berkeley Scientist Reinhard Genzel and UCLA Scientist Andrea Ghez, for their discovery of a supermassive compact object (a/k/a/ supermassive black hole) at the center of our Milky Way galaxy. These theoretical and observational advances were key components of the now firm foundation for the existence of perhaps the most exotic and important objects in the Universe.

Florida State University Professors of Physics Bill Green and Peter Hoeflich shared their explanation of the prize.

How long have scientists believed that extremely dense or heavy objects might trap light?

Even before Einstein’s theory showed how heavy objects can bend spacetime, John Michell (1783) and Pierre-Simon LaPlace (1796) independently applied Newton’s simpler theory to predict that light would be unable to escape from objects substantially heavier than the sun. This led Michell to speculate that the discovery of such “dark” objects would therefore need to be made indirectly — i.e., by how they influenced the motions of nearby “luminous objects” (e.g., stars) that happened to revolve around them. That is the very approach taken by Genzel and Ghez to discern the mass of the black hole at the Milky Way center.

As for Penrose’s work, the implications of Einstein’s theory (1915) became better understood as time passed. By the mid-1960s, its predictions for black holes were a hot item. It was then realized by Penrose that the mathematical tools needed to understand them needed to be improved. Basically, Penrose developed techniques showing that black holes had an “horizon” around them which marked the distance inside of which nothing, including light, could go fast enough to escape. More important, once inside the horizon, time itself runs in only one direction — toward the center of the black hole. This meant that not only must one exceed the speed of light to escape from inside the horizon (which the theory forbids), but one also could not turn around and try to leave unless time itself reversed — i.e., unless one travelled into the past. So, simply put, once in, any normal stuff has no choice but to fall to the center of the black hole where time and space stop. That location is called a singularity because known physics also comes to a stop there. The key point: theory says black holes should exist.

And how did the Genzel and Ghez teams show that there was a big black hole in the Milky Way?

As for the Milky Way, it was clear that a compact source of radio frequency emissions exists at its center, known as Sgr A*. This source is about 25,000 light years from Earth. The Genzel and Ghez teams monitored stars near Sgr A* for three decades, looking for patterns of motion that, like the solar system, could allow them to determine whether the stars were in orbits around a central massive object, and if so, would allow them to determine its mass. State of the art telescope techniques were used to achieve the time and position values for specific stars. By the late 1990s, Genzel’s team showed that a handful of stars, like our planets, had orbits expected for orbits around a central massive point object. By 2002–/2003, Genzel’s and Ghez’s teams showed that a particular star (S2) orbited Sgr A* in about 16 years, allowing them to establish that the Sgr A* black hole has a mass about 4 million times heavier than our Sun. More recent measurements by others are sufficiently precise to even allow confirmation of subtle orbital effects predicted by Einstein’s theory. For non-rotating black holes, a key measure is the innermost stable circular orbit, which is 3 times farther from the center than the event horizon. [Any matter (people, rocks or stars) that ventures inside this distance will fall in]. Flaring objects corresponding to the region just outside the innermost stable orbit of Sgr A* have now been detected.

So, what do we know about black holes now overall?

In sum, Penrose’s work supported the idea that the theory of general relativity makes black hole formation in our Universe unavoidable. Genzel and Ghez revealed that the behavior of stars in the center of the Milky Way strongly supports the conclusion that they are orbiting a supermassive black hole. And of course, the gravitational waves detected from merging black holes by LIGO (Nobel Prize 2017) and the picture of a black hole at the center of another galaxy 55 million light years from Earth (Event Horizons Telescope (2019)), have removed any further doubt. Current estimates are that the each of the Universe’s ~2 trillion galaxies possess a supermassive black hole at its center that greatly influences the evolution of it stars and other features. Black holes are here to stay. For more information, visit the Nobel Prize website to read the committee’s explanation of the winning work.