This year's Nobel Prize in Physics recognizes groundbreaking research on black holes, including the discovery of the colossal one in the center of our Milky Way Galaxy.
Half of the title goes to Roger Penrose, an Oxford University mathematician, for his research on the structure and stability of black holes in the 1960s. Two astronomers share the other half: Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics and Andrea Ghez of the University of California, Los Angeles, respectively. Both led opposing scientific groups since the 1990s, observing stars at the core of the Milky Way and showing that their orbits were distorted by what is known as a supermassive black hole (SMBH).
Over the decades, the idea of a black hole, an object so large that its gravity stops light from leaving, appeared in parts. In 1915, Albert Einstein published his gravity theory, the general theory of relativity. It says that gravity occurs when the fabric of space and time is skewed by mass and energy, allowing the trajectories of freely falling objects to bend like the elliptical orbit of Earth around the Sun.
Just one year later, German physicist Karl Schwarzschild worked out the shape of the pit in space-time that a point mass would generate and proved that it predicts an event horizon. This means the boundary of a sphere around the mass of the point from which light can still exist.
The entire theory that stars burnt out could potentially point to these bizarre voids in space, however, it did not arrive until 1939. This is when physicists J. Robert Oppenheimer and George Volkoff predicted that if a neutron star becomes too massive, it would collapse to an infinitesimal point under its own weight, leaving only its ultra-intense gravitational field behind.
Their research presaged the current understanding of stellar-mass black holes, which form when sufficiently massive stars burn out and their cores collapse.
Oppenheimer and his contemporaries could not prove that an event horizon had to be created by the imploding star. It was possible that the matter would swirl away somewhere or that the gravitational field of the dying star would not hang around. With extreme mathematical rigor, Penrose proved in the 1960s that the creation of a black hole was ultimately unavoidable and that it would be indestructible, expanding as more mass was absorbed.
In short, Penrose demonstrated general relativity, proposing that a black hole is real, stable astrophysical object.
Astronomers have uncovered a wealth of data for black holes following Penrose's advances. They discovered stars circling unseen companions, and when they vanished into putative black holes, they could see hot burning superheated gases. Gravitational-wave detectors, but not the interstellar giants, offered the clincher for black holes. S
Sagittarius A * (Sgr A *) is the one at the center of the Milky Way, weighing millions of solar masses and is only 26,000 light-years distant. But it's very small, in addition to being black: its event horizon will fit inside the orbit of Mercury.
The rival teams of Ghez and Genzel carried out a very basic analysis by pushing observational techniques to their limits: they mapped the development of a single star as it orbited close to Sgr A * and revealed, using basic Newtonian mechanics, that the object they were orbiting had to have a gigantic mass.
Ghez is just the fourth woman to win the Nobel Prize in Physics and the second to win the Nobel Prize in the last three years. At 55, she is a relatively young laureate as well. Penrose, 89, is one of the oldest.
