Cal Professor Reinhard Genzel awarded 2020 Nobel Prize in Physics for Black Hole work

Genzel shares a half of the prize with UCLA's Andrea Ghez for experimental proofs that a supermassive blackhole is at the center of the Milky Way

The Nobel Laureate parking on campus will get just a bit busier after today.

While there may not be any Cal sports results to cover yet this fall, I am happy to write my first post in months about the latest (108th, if you are counting at home) Nobel Prize laureate to be affiliated with the University of California, Berkeley.

Emeritus Professor of Physics Reinhard Genzel has been awarded the 2020 Nobel Prize of Physics for his experimental work related to the Black Hole, the epitome of suckiness so perfectly fitting for 2020. Genzel shares half of the prize with Andrea Ghez (a faculty at Cal’s sister campus in LA, who is also just the 4th woman to get the Physics Nobel) with the other half of the prize going to famed theorist/mathematician Sir Roger Penrose of Oxford University.

Genzel is the 34th Cal affiliated Nobel Physics Prize winner and the 49th official Berkeley’s Nobel Laureate (including alumni and faculty); counting only current faculty prizes, Genzel is the 24th Nobel Prize (this appears to be the preferred way to count by the University). He is the 8th faculty member to win the Nobel Physics Prize while active at Cal, joining Saul Perlmutter (Nobel Prize 2011) and George Smoot (Nobel Prize 2006) as Nobel Laureates on the current Cal Physics faculty. Genzel has been a Cal faculty member since 1981, from being an Associate Professor of Physics and Associate Research Astronomer at Space Sciences Laboratory (1981–1985) to become a Full Professor of Physics (1985–1986 and again 1998-).

In addition to his role on the Cal Physics faculty, since 1986 Reinhard Genzel also holds the position of the Director of the Infrared and Submillimeter Group at Max Planck Institute for Extraterrestrial Physics in his native Germany, where he got his bachelor (University of Freiburg) and Ph.D. (University of Bonn) degrees.

Specifically, Genzel and Ghez are awarded the Nobel Prize “for the discovery of a supermassive compact object at the centre of our galaxy”. That “supermassive compact object” is more commonly known as a black hole. A black hole is an object that has such a strong gravitational field (because it is so massive) that it sucks in all matter and radiation, so it appears to be “black” (this is not entirely true since there are relativistic jets of ionized particle shot out of black holes, the mechanism that causes this is still mostly unknown).

From the official Berkeley News release, Genzel’s ties with Berkeley dated back to the late 60s when he was a postdoctoral fellow working with the late Nobel Laureate Charles Townes. Famous for being one of the inventors of laser (technically the maser), Townes was also renowned for his subsequent career shift into astrophysics (and leveraging his Nobel Prize and fame for funding). Genzel and Townes did the first work back then that hinted at the presence of a supermassive black hole at the center of the Milky Way; this was a result that Genzel greatly and steadily improved upon over the years to earn him this great honor today.

Sagittarius A* is the compact object at the center of the Milky Way, believed to be 4 million times the mass of the sun. Genzel and his team have developed both ground- and space-based ways to conduct high precision measurements of the star movement in that region. The supermassive black hole at the Galactic Center was more or less confirmed by 2002. In particular, both Genzel’s group at Max Planck Institute for Extraterrestrial Physics, using the European Southern Observatory, and Ghez’s group at UCLA both independently studied the movement of the star S0-2. Located quite close to the black hole (1400 Schwarzschild radii from Sgr A*), Genzel’s group confirmed gravitational redshift to the greatest precision yet in 2018 with their measurements of the highly elliptical 16-year orbit of S0-2, at the end of 26 years of careful observation.

Animation of stellar orbits around the Galactic Center can be viewed below:

You may be more familiar with the regular redshift for light from velocity. If an object is moving away from you, its observed frequency by you is shifted toward the red (longer wavelength). Conversely, an object is shifted toward the blue if it is moving closer to you. Consequently, if you observe a blue bumper sticker in the car in front of you, you may be driving too fast (if that sticker appears to be red when at rest). Of course, if there is a Stanford fan driving a red car in front of you, you can observe that car turning to blue by just driving toward it at the speed of light.*

Like movement, a strong gravitational field can also impact the observed frequency and stretch its wavelength toward that of red (boo!), essentially from the relativistic effect of time slowing down near a black hole. In their paper in 2018, the international team led by Genzel used several technological upgrades of their instruments to “robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for [S0-2] of z = Δλ λ ≈ 200 km s−1/c”. The movement of S0-2 has clearly violated the standard Newtonian dynamics and was further proof for Einstein’s theory of general relativity. This was the first time that general relativity has been confirmed by something orbiting a supermassive black hole.

You can watch a public lecture given by Genzel in 2016 about his work below.

Genzel shares the 2020 Nobel Prize with theorist Roger Penrose (1/2 of the prize), and fellow experimentalist Andrea Ghez (1/4 of the prize).

The official citation awards this Nobel Prize to Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity”. Penrose and Stephen Hawking shared the 1988 Wolf Prize for their Penrose-Hawking Singularity theorems. Known for a long list of things in mathematics, Roger Penrose and his father Lionel, also invented the “Penrose Stairs”, the always ascending or descending closed staircase depending on if you follow it clockwise or counterclockwise, that is featured in the several famous drawings by M.C. Escher. There is an interesting BBC documentary from 2015 about Roger Penrose’s personal ties to the artist M.C. Escher.

It is important to reiterate that Ghez is merely the 4th woman to be awarded the Nobel Prize in Physics after Marie Curie (1903), Maria Goeppert Mayer (1963), and Donna Strickland (2018). Ghez leads a competing experiment to Genzel in studying the center of the Milky Way. Ghez’s group used the Keck telescope in Hawaii to image the Galactic Center also in the infrared.

Congratulations to all three brand new Nobel Prize Laureates, especially the Golden Bear in Reinhold Genzel! Stay tuned to Write for California to see if have any more Golden Bear Nobel Laureates later this week.

*That is obviously an obligatory (forced) joke (AKA the hallmark of the CGB/W4C Nobel Prize coverage), please drive responsibly and do not get into a car accident.