Reinhard Genzel receives the Nobel Prize for Physics 2020

The Max Planck director is honoured for his observations of the black hole in the galactic center

Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics in Garching, receives the Nobel Prize for Physics 2020 together with Roger Penrose and Andrea Ghez. The Royal Swedish Academy honours the scientists for their black hole research. Using high-precision methods, the group around Genzel also observed bursts of brightness from gas in the immediate vicinity of the black hole and a gravitational redshift caused by this mass monster in the light of a passing star.

Genzel’s work spans both galactic and extragalactic astrophysics. He and his group made pioneering observations to map the motions of stars close to the Galactic centre, leading to firm evidence for the existence of a supermassive black hole at the centre of the Milky Way and to an accurate estimate of its mass.

With the help of adaptive optics to compensate for air turbulence and a method called speckle interferometry, the group succeeded in precisely measuring stellar velocities in the gravitational field of the supposed black hole up to a distance of 0.1 arcseconds. From this, Genzel and the astronomers from the Max Planck Institute for Extraterrestrial Physics determined the mass of the black hole with quite high accuracy to about 4.31 million solar masses.

Subsequent research by Genzel’s group has demonstrated the unusual mass-spectrum and geometry of stars at the centre of the Galaxy and to the discovery of infrared flares that are thought to arise from gas close to the inner accretion disc of the black hole.

Only last year, Reinhard Genzel succeeded for the first time in demonstrating the so-called gravitational redshift on a star. To observe the galactic centre, astronomers use sensitive instruments such as Gravity, Sinfoni and Naco. All of them belong to the European Southern Observatory's (ESO) Very Large Telescope, were built under the direction of the Max Planck Institute for Extraterrestrial Physics, and are scanning the sky in infrared light.

The researchers turned their attention to a star called S2 and followed it on its orbit around the black hole, which it came particularly close to in 2018. Thus, the smallest distance between S2 and the black hole was about 14 billion kilometers on May 19. The star moved at a speed of more than 25 million kilometers per hour - corresponding to almost three percent of the speed of light. It takes about 15 years for a complete orbit.

The scientists compared the position and velocity measurements of Gravity and Sinfoni and those from earlier observations of S2 with the predictions of Newtonian gravitational physics, general relativity and also other theories of gravity. In fact, the new results contradict Newton's predictions, but are in excellent agreement with those of general relativity.

 The measurements clearly showed an effect called gravitational redshift: The light of the star S2 is stretched to longer wavelengths by the very strong gravitational field of the black hole and therefore appears reddish. And this change in wavelength was exactly in line with the prediction of Einstein's general theory of relativity. This was the first time that researchers had observed this deviation from the predictions of the simpler Newtonian theory of gravity in the motion of a star around a supermassive black hole.

 Also in 2018, Reinhard Genzel published a paper on the observation of bursts of brightness in the immediate vicinity of the galactic black hole. The researchers saw three such flares. All of them had the same orbit radii and the same periods of revolution. The movement of these three hot spots in the galactic centre can be explained by a simple orbital model whose radius is three to five times larger than that of the event horizon of the black hole. Genzel's group found that gas is swirling around it at a speed of 30 percent of the speed of light - in line with the theory.

It was only in the spring of this year that a team led by the Max Planck Director succeeded in making another significant discovery. Years of observation of the star S2 showed that its orbit does not remain stationary in space, but moves forward, creating the shape of a rosette. This effect was predicted by Albert Einstein in his general theory of relativity, and explains, for example, the rotation of Mercury's orbit, which has been known for a long time.  

This discovery was also made with the Gravity instrument, which combines the light from all four eight-meter mirrors of ESO's Very Large Telescope. Thanks to this technique called interferometry, Gravity generates the power of a virtual telescope with an effective diameter of 130 metres.

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Reinhard Genzel, born in Bad Homburg in 1952, studied physics at the University of Bonn and received his doctorate from the Max Planck Institute for Radio Astronomy in 1978. He went on to join the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, where he was a Miller Fellow from 1980 to 1982, and became a Professor at the University of California, Berkeley in 1981. In 1986, Genzel was appointed Scientific Member of the Max Planck Society and Director of the Max Planck Institute for Extraterrestrial Physics in Garching near Munich. Since 1999, he has been full professor at the University of California, Berkeley. Reinhard Genzel has received many prizes and awards, including the Gottfried Wilhelm Leibniz Prize of the German Research Foundation in 1990, the Balzan Prize in 2003 for his work on infrared metrology, the Shaw Prize in 2008 and the Crafoord Prize in 2012.