Investigating Black Holes - The 2020 Nobel Prize Winners

 To earn my place at the University of Leeds I have been asked to complete an assignment on the science and importance of a Nobel Prize discovery. It was daunting at first trying to attempt this piece of work (the three page project brief was enough to make me eyes water) and yet I soon enjoyed picking out my articles, reading them then turning my shorthand attempts into this essay. Happy reading!

Black holes have captured the public’s imagination and inspired a wave of Sci-fi science: from Interstellar to Doctor Who, the black hole is deemed as key to human time travel through the warping of space time. Despite seeming fantastical, the 2020 Physics Nobel Prize winners Reinhard Genzel and Andrea Ghez could bring us a step closer to this improbable reality and provides further evidence for Einstein’s Theory of General Relativity. As a science geek myself, I’m fascinated by the future of space and having the opportunity to further delve into its discoveries was not one to be missed.

The 2020 Nobel Prize for Physics was jointly awarded to Genzel and Ghez ‘for the discovery of a supermassive compact object at the centre of our galaxy’. Our fascination with black holes started, however, in the 18^th century with Pierre-Simon Laplace and John Michell. Using Newtonian physics, Laplace derived 1/2mv² – (GMm)/r = 0 where the test particle’s total rest energy equals 0 which forms r < (2GM)/c² when light is unable to reach infinity. Michell commented “from the motions of these revolving bodies infer the existence of the central ones”. Guided by these principles, Genzel and Ghez proved Laplace’s equation and found what could only be a black hole in the Milky Way. 

Figure 1: Dusk at the Very Large Telescope operated by the European Southern Observatory on Cerro Paranal.  

Genzel, from the Max Plank Institute started work in the early 1990s and was soon followed by Ghez from the University of California. Previously, quasar Q503C273 (a large radiation source) had been detected from Sagittarius A* at the centre of our galaxy by Maarten Schmidt in 1963 and became the focus of Genzel and Ghez’s research. Genzel used the telescopes NTT and VLT (which has the world’s largest monolithic mirrors) in Chile while Ghez used Keck Observatory, Hawaii with its 36 hexagonal segments – each segment controlled separately.

However, in order to create accurate observations, atmospheric air bubbles and gas clouds of varying densities and temperatures must be overcome. The variation of density and temperature results in blurring and reduces the telescope’s diffraction limit as they cause distortion on the light waves. The teams used Speckle Imaging techniques by taking many short images with a sensitive detector and aligning them, hence reducing the blur. Active and Adaptive Optics were also used by measuring the light’s wavelength, calculating the amount of shift and therefore the level distortion for which a correction can be applied by adjusting the telescope’s mirrors. At the Keck Observatory, this correction could be applied individually to each mirror segment – improving the overall image.

In using Adaptive Optics, Genzel and Ghez were able to determine radial velocity as well as projected velocity of individual stars which allowed them to track the celestial bodies’ orbits. From their observations, it was noticed that stars within a radius of one light month didn’t have orderly orbits but stars outside this radius had uniform orbits. Stars closer to the centre also had faster velocities hence following Kepler’s Law and alluding to the presence of a high mass object at Sagittarius A*’s centre.

Figure 2: a) Orbit of SO2. b) Graph showing velocity of SO2 during orbit.

Star SO2 displayed a disorderly orbit and with its short 16-year orbit, it became the focus of teams’ observations. Einstein’s Theory of General Relativity states gravity is the curvature due to objects with mass in space-time and so the rate of change of an object’s orbit is different to that predicted by Newtonian physics. Though already proven in weaker gravitational fields by Mercury and the Sun, SO2 proved Einstein’s Theory held in stronger gravitational fields – providing further evidence for the Theory of General Relativity. This was achieved by the researchers monitoring the Doppler shifts in the returning light’s emission spectrum to infer SO2’s velocity which was used to calculate the radius of its orbit. Throughout the 16-year observation, the radius fluctuated significantly to produce an elliptical orbit. Furthermore, SO2’s movement produced a gravitational redshift caused by the wavelength of light being elongated by the black hole’s strong gravitational field. This is predicted by the Theory of General Relativity.

SO2’s radial velocity was also used to calculate the mass of the central body as suggested by Laplace and Michell which was found to equal to 4 million solar masses. All this evidence further supports the possibility of a black hole at the centre of our galaxy.

Figure 3: η Car’s u−v coverages

The interferometric techniques developed by Genzel and Ghez have been further utilised by Roberto Abuter and his team in tracing the inner structure of wind-wind interaction of η Car. η Car is a stellar system with at least two stars but any other structural details of this system had remained undiscovered. By using spectro-interferometric observations with the K-band (radio wavelengths with frequencies between 18–27 GHz) beam combiner GRAVITY at VLTI, He I emissions (producing Helium ion spectrums) could be tracked therefore showing the cloud movement. The results were consistent with previous hydrodynamical models and can be used to predict the formation of η Car. This research was highlighted in the Scientific Background for the 2020 Nobel Physics prize and I used the author’s name to search for the research on the University of Leeds’ library catalogue. 

References:

·        Press Release: The Nobel Prize in Physics Press release: The Nobel Prize in Physics 2020

Published: 06/10/2020

Date Accessed: 26/05/2021

 

·        Popular science background: Black holes and the Milky Way’s darkest secret The Nobel Prize in Physics 2020: Popular science background

Published: 06/10/2020

Date Accessed: 26/05/2021

·        Scientific Background: Theoretical foundation for black holes and the supermassive compact object at the galactic centre Scientific Background on the Nobel Prize in Physics 2020

Published: 06/10/2020

Date Accessed: 26/05/2021

 

·        Imaging Black Holes Imaging black holes: Physics Today: Vol 71, No 4 (scitation.org)

Published: 01/04/2018

Date Accessed: 29/05/2021

 

·        Nobel Prize in Physics honors the discovery of a supermassive compact object at the heart of the Milky Way Nobel Prize in Physics honors the discovery of a supermassive compact object at the heart of the Milky Way: Physics Today: Vol 73, No 12 (scitation.org)

Published: 01/12/2020

Date Accessed: 29/05/2021

 

·        Compact Objects and Black Holes; 2020 Nobel Prize in Physics Compact Objects and Black Holes | SpringerLink

Published: 30/12/2020

Date Accessed: 07/06/2021

 

·        Milky Way’s black hole provides long-sought test of Einstein’s general relativity Milky Way’s black hole provides long-sought test of Einstein’s general relativity (nature.com)

Published: 26/07/2018

Date Accessed: 28/06/2021

 

Research:

·        GRAVITY chromatic imaging of η Car’s core GRAVITY chromatic imaging of η Car’s core - Milliarcsecond resolution imaging of the wind-wind collision zone (Brγ, He I) | Astronomy & Astrophysics (A&A) (openathens.net)

Published: 23/10/2018

Date Accessed: 11/06/2021

 

·        K-Band - Handbook of Terahertz Technology for Imaging, Sensing and Communications (6.2 Motivation for terahertz wireless communications)  K-Band - an overview | ScienceDirect Topics

Published: 2013

Date Accessed: 21/06/2021

 

Images:

·        Figure 1: Dusk at the Very Large Telescope operated by the European Southern Observatory on Cerro Paranal (photo) Bridgeman Education (openathens.net)

Date Accessed: 17/06/2021

 

·        Figure 2: Nobel Prize in Physics honors the discovery of a supermassive compact object at the heart of the Milky Way. Figure 2. The star S2 follows an elliptical orbit around Sagittarius A*. Nobel Prize in Physics honors the discovery of a supermassive compact object at the heart of the Milky Way: Physics Today: Vol 73, No 12 (scitation.org)

Date Accessed: 17/06/2021

 

·        Figure 3: η Car’s u−v coverages obtained during the GRAVITY runs in February 2016 (top panel) and May-June 2017 (bottom panel). GRAVITY chromatic imaging of η Car’s core - Milliarcsecond resolution imaging of the wind-wind collision zone (Brγ, He I) | Astronomy & Astrophysics (A&A) (openathens.net)

Date Accessed: 17/06/2021