Last month Sir Roger Penrose was announced as the joint winner of the 2020 Nobel Prize in Physics. Sir Roger Penrose previously held a visiting Professorship at the Centre for Research in String Theory (CRST) at Queen Mary University of London. In this blog, Professor David Berman and Professor Malcolm Perry from CRST discuss some of Penrose’s revolutionary ideas and how they have and continue to shape the field of particle physics.
In 1915 Einstein's theory of general relativity first unified the concepts of space and time with the phenomenon of gravitation. Initially it had considerable success, helping to explain an observed drift in Mercury’s orbit around the Sun and predicting the deflection of starlight passing close to the Sun, which was famously first observed by Eddington during a total eclipse. But once these initial achievements became firmly established, the study of general relativity remained relatively untouched. There were two reasons main reasons for this; firstly it was thought to be experimentally inaccessible and secondly, mathematically it was extremely complicated which made mastering the subject a very intimidating proposition.
However, slowly attitudes began to change. During the 1920s the theory general relativity was successfully applied to model the Universe as a whole, albeit this research was quite controversial. And just before WW2 it was again used in the exploration of gravitational collapse - the end-point to the life of stars - that can result in the formation of black holes. In the late 1950s, once the war had ended it then became an active area of research and it was during this revival that Sir Roger Penrose began to apply modern mathematical ideas to general relativity with spectacular results.
At this time, the research community had started to show interest in three main issues that revolutionised our picture of space, time and gravitation; gravitational radiation, the gravitational collapse of matter to form black holes and lastly, the modelling of the universe. Sir Roger Penrose became a leader in all three of these areas, making major contributions that completely changed our ideas about the physical world.
Firstly, Penrose used spinors – originally invented in 1919 - to reformulate general relativity, which made many previously horribly complicated computations relatively simple. Using these methods, he developed our understanding of gravitational radiation to what it is today. Subsequently, some of these ideas also allowed us to understand the set of events in spacetime and the relationship between these events, known as the causal structure. Without his contributions, our present understanding of black holes would be impossible. He also discovered the Penrose process, an unexpected method for extracting energy from rotating black holes, which ultimately contributed to our understanding of how extremely bright quasars are powered.
Perhaps his most spectacular and revolutionary invention was that of the first singularity theorem in general relativity. Spacetime singularities, which are thought to be boundaries to spacetime, have been known since Einstein’s theory of general relativity was first developed. Initially singularities were thought to be artefacts of the simplifications made to Einstein’s complicated theory rather than being something physically real. However Penrose proved that if sufficient matter was clustered to form a black hole, a spacetime singularity was inevitable. His first singularity theorem was revolutionary, proving that general relativity is an incomplete theory.
To go with the singularity theorems, he proposed the cosmic censorship hypothesis. The cosmic censorship hypothesis has not been proved and remains a major challenge. The idea is that any singularity associated with a black hole would be hidden from the view of those outside the black hole. You would reach the singularity only if you fell into the black hole. If the cosmic censorship hypothesis is true, then spacetime singularities need not bother us too much as long as we do not fall into a black hole. Later, Penrose worked with the famous Stephen Hawking to generalise these theorems to a much wider range of situations. Perhaps the most far-reaching of these new singularity theorems is the proof that in an expanding universe there must be a singularity in the past, the big bang singularity. These singularity theorems were one of the last nails in the coffin of steady-state cosmology – the idea that although the universe is continually expanding its density remains constant, with matter being created to form new stars and galaxies at the same rate as old ones are being lost.
Penrose’s great contributions to the study of general relativity have not gone unnoticed. He has been knighted, appointed to the Order of Merit, elected to the Royal Society, and awarded several medals from prestigious societies across scientific fields. His work is still studied by students all over the world, and in any course on general relativity, students will learn about Penrose diagrams, his graphical representation of the causal relations of spacetime.
One of his many notable inventions, twistor theory, is the one that connects him to Queen Mary. This theory was first developed in the late 1960s, but took almost 40 years to enter the mainstream of physics research. Since 2004 Penrose's twistors have led to major advances in our understanding of the scattering of fundamental particles and triggered a radical reformulation of the traditional textbook methods. Far from being abstract ideas these scattering amplitudes are measured at CERN's Large Hadron Collider. Whilst calculating them was traditionally done using a diagrammatic method introduced by Richard Feynman, the Centre for Research in String Theory at Queen Mary have taken Penrose's twistor ideas and extended them so that scattering amplitudes can be computed much more easily and efficiently. Physicists at Queen Mary are part of a global programme working in this field and lead the innovative training network Scattering Amplitudes: from Geometry to Experiment (SAGEX).
As the importance of Penrose’s twistor variables become increasingly apparent, the field of particle physics is starting to be rewritten and a new generation of physicists are taking forward his ideas. During his time as a Visiting Professor at Queen Mary, Penrose was a key participant in several scientific conferences for researchers here working on twistors, and showcased his admirable public speaking skills, at an impressive talk on his revolutionary ideas in cosmology in 2010. He was and continues to be an inspiration to all the members of the Centre for Research in String Theory at Queen Mary, and many more all over the world.
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