The Research that Won the 2020 Nobel Prizes in Physics
This year’s Nobel Prize in Physics has got to be one of the most exciting of my lifetime. The 2020 Nobel Prizes spotlighted a physics concept that has fascinated the minds of both scientists and non-scientists alike: Black Holes. The prize was shared by three winners: Sir Roger Penrose, Dr Reinhard Genzel, and Dr Andrea Ghez. Ghez is the fourth woman to ever receive the Nobel Prize in Physics now joining the ranks of winners Marie Curie (1930), Maria Mayer (1963), and Donna Strickland (2018). 
Sir Roger Penrose shared half the Prize for his 1965 proof that black holes are indeed a prediction within General Relativity, something which Einstein himself thought was physically impossible. Penrose transformed our understanding of black holes with his discovery of Closed Trapped Surfaces which are 2D topological surfaces that can describe the inner region of a black hole, just beyond the event horizon. This topological surface is a solution that is defined within General Relativity and describes a surface on which light is stationary relative to the black hole. The apparent horizon below the event horizon is a region in which all rays orthogonal to the surface (going outward from within the black hole) converge in the future (Fig 1), which is the opposite of the property of orthogonal light rays diverging in the future from a spherical surface in Euclidean space. In 1965 Penrose used Einstein’s General Relativity to show that during the collapse of a star, once the trapped surface forms, the collapse cannot be stopped by any force and the star becomes an infinitely dense singularity inside the surface, creating a black hole. 
Before this idea of CTSs we believed that black holes simply had two regions: the ‘event horizon’ and ‘beyond the event horizon’ at which all physics ceases to exist. The idea of an apparent horizon below the event horizon has also led to the development of several prominent theories for a possible solution to the Black Hole Information Paradox, one of which was proposed by Penrose’s longtime friend the late Stephen Hawking.  With all of this, there is no doubt that through the lens of General Relativity, Penrose’s analysis of black holes and supermassive black holes has greatly expanded our horizons of the inner workings of our Universe.
Figure 1: Image Source: Joshi P.S. (2014) Spacetime Singularities. In: Ashtekar A., Petkov V. (eds) Springer Handbook of Spacetime. Springer Handbooks. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-41992-8_20
Genzel and Ghez shared ¼ each of the rest of the Prize for their work in the early 1990’s on the movement of stars near the centre of the Milky Way from which they independently discovered a supermassive “compact” object at the centre of our galaxy. Teams led by Genzel (Very Large Telescope) and Ghez (Keck Observatory) traced the motion of Sagittarius A (Fig 2) from a period between 1992 and 2018.
Figure 2: Image Source: The Royal Sweedish Academy of Sciences (2020)
They discovered that the motion of stars in this region is being influenced by the presence of a supermassive dense object, calculated to be 4 million times the mass of our Sun. Genzel’s team were able to trace the relativistic precession of Sagittarius A at the VLT, which The Royal Sweedish Academy of Sciences considers “a truly remarkable experimental achievement addressing fundamental physics.”  This prize highlights both researcher’s efforts to incorporate increasingly precise instrumentation and techniques in adaptive optics to overcome obstacles (such as interstellar gas clouds across the Milky Way) over a period of almost four decades to accurately map the motion of a group of stars that are 26,000 lightyears away from Earth. Their observations of these stars offer the most convincing evidence we have for the existence of a supermassive black hole residing at the centre of the Milky Way.
“The discoveries of this year’s Laureates have broken new ground in the study of compact and supermassive objects ... These exotic objects still pose many questions that beg for answers ... questions about their inner structure, but also questions about how to test our theory of gravity under the extreme conditions in the immediate vicinity of a black hole” - David Haviland, Chair of the Nobel Committee for Physics. 
Cover Image: Todd, I., 2020. Trio Win Nobel Prize In Physics 2020 For Work On Black Holes. [online] skyatnightmagazine. Available at: https://www.skyatnightmagazine.com/news/nobel-prize-physics-2020-work-black-holes/
 The Nobel Committee for Physics, 2020., All Nobel Prizes in Physics. [online] The Nobel Prize. Available at: https://www.nobelprize.org/prizes/lists/all-nobel-prizes-in-physics/
 Penrose, R., 1965., Gravitational Collapse and Space-Time Singularities. Phys. Rev. Lett. 14, 57. DOI: https://doi.org/10.1103/PhysRevLett.14.57. [online] American Physical Society. Available at: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.14.57
 Aron, J., 2014., Stephen Hawking's new theory offers black hole escape. [online] NewScientist. Available at https://www.newscientist.com/article/dn24937-stephen-hawkings-new-theory-offers-black-hole-escape/
 The Nobel Committee for Physics, 2020., Theoretical Foundation for Black Holes and the Supermassive Compact Object at the Galactic Centre. p. 14. [online] The Nobel Prize. Available at: https://www.nobelprize.org/prizes/physics/2020/press-release/
 The Nobel Committee for Physics, 2020., The Nobel Prize in Physics 2020. [online] The Nobel Prize. Available at: https://www.nobelprize.org/prizes/physics/2020/press-release/