The Secrets of Gravity, by Professor Claudia de Rham

As the year 2025 comes rapidly to a close, the Flamsteed Astronomy Society held its last ‘main talk’ of the year at the National Maritime Museum auditorium.

Attendees gathered for the evening and began with an offering of ‘tea and biscuits’, appropriately presented with colourful Christmas lights, and welcomes from Poly Philippou, Clive Inglis and Richard Summerfield. This was also a chance to catch up with familiar faces and be introduced to new members attending.

Suitably refreshed, we made our way to the main auditorium, took our seats, and proceedings began with our customary ‘slide show’ - a selection of members’ astrophotographs. Both the technical and aesthetic quality of the captures noticeably improves from season to season.

Our host for the evening, Yvonne Jacobs, introduced us to our ‘member’s talk’, given by Likhitha Modalavalasa, a software engineer. Her subject was the role of AI (Artificial Intelligence) in astronomy. Likhitha presented a primer to the different types or ‘layers’ now covered by the generic term ‘AI’ - from its beginnings in the early days of Alan Turing’s thought experiments on the application of rule-based logic to solve problems, through to the development of expert systems, machine learning, deep learning, neural networks, and now generative AI engines and the building of LLMs (large language models). A significant use of AI in astronomy is the processing of gargantuan multi-petabyte datasets now coming online, for example the full-sky surveys being captured by the Vera Rubin telescope. Using AI to pattern-search in both supervised (labelled data, e.g. exoplanet searches) and unsupervised (autonomous categorising, e.g. variable star searches) modes allows ‘anomalies’ hidden in these oceans of data to be identified, potentially leading to a raft of new discoveries and subsequent understanding. More recent work includes generative AI engines being used to create ‘training data’ for other AI models in the search for anomalous galaxy formations.

Likhitha giving her talk on the role of AI in astronomy

Suitably ‘mind boggled’, we then moved on to our main talk for the evening as Yvonne introduced our guest speaker, Professor Claudia de Rham, Professor of Theoretical Physics at Imperial College London.

Professor de Rham’s talk, The Secrets of Gravity, began with a historical perspective on how instrumentation needs to be both accurate and resilient to the harshest of conditions - referencing the Harrison clocks used for navigation, and modern space-based telescopes sent to the outer reaches of the Solar System. In both cases, symmetry and equilibrium are critical properties for engineering and for the fundamental laws of nature. De Rham described how, when building mathematical models of the fundamental forces of nature and incorporating the most subtle variations based on observational data, it is possible to introduce ‘hidden symmetries’ that maintain the structural stability of spacetime. De Rham, also a cosmologist, extended this modelling of symmetry to the whole observable universe through the ubiquitous force of gravity.

Professor Claudia de Rham at the start of her talk to the Flamsteed

Gravity is a fundamental part of everything, and as humans we are not able to feel differences in gravity, as it affects every cell in our bodies equally. Gravity affects everything in exactly the same way throughout the universe. De Rham illustrated this with a video clip of astronaut David Scott, who during the Apollo 15 mission repeated Galileo’s experiment of dropping two objects of different mass (a hammer and a feather) from the same height. They landed on the lunar surface at the same time, in the absence of air resistance. This fundamental symmetry of gravity has been tested to 15 orders of magnitude, and Einstein recognised that gravity is not related to individual objects themselves, but is encoded into the very fabric of space and time through the curvature of spacetime. The more massive or dense an object is, the greater and more acute the curvature of spacetime around it.

De Rham went on to show that gravity shares properties with the other fundamental forces, such as wave propagation and spin characteristics. Gravitational waves are produced at detectable levels only when massively dense objects interact, for example when two black holes collide. As with light, whose discrete nature can be defined in terms of ‘quanta’ or particles, the same is presumed for gravity in the form of gravitons. De Rham continued by explaining that the four fundamental forces of nature - electromagnetic, weak nuclear, strong nuclear, and gravitational - form what are deemed to be the unified forces of nature.

Leading on from Maxwell’s discoveries in electromagnetism, De Rham then discussed how the weak nuclear force exhibits a break in symmetry compared to the strong force, and that this ‘breaking’ arises through the Higgs mechanism. Kibble’s work integrated the Higgs mechanism into the Standard Model while maintaining the laws of nature - so ‘breaking’, but not broken. Because of the nature of quantum reality, De Rham explained how the discovery of the Higgs particle, through particle scattering experiments at the LHC (Large Hadron Collider), relied on the comparison of probabilities across many runs. In theory, the same could be true for particles in a gravitational context. However, at the energy levels required to produce this effect - around 15 orders of magnitude beyond the LHC, at or near the Planck scale - and to create gravitons, Einstein’s general theory of relativity breaks down.

Our little corner of space is safe, as we are some 50 orders of magnitude away from the energy scales at which Einstein’s theory of gravity is no longer sufficient. However, within the cores of black holes at the centres of galaxies, this is a different story, at least in theory.

Professor Claudia de Rham

De Rham described the limits of continuity at the event horizon of a black hole, where the curvature of spacetime approaches the Planck scale and Einstein’s equations break down, leading to a singularity where matter and energy become infinitely dense. As to what happens beyond this point, there are various competing theories, including string theory, causal theory and loop quantum gravity. String theory is currently one of the strongest candidate theories, as it maintains symmetry by relating the extremely small with the extremely large.

De Rham went on to show how, on the largest scales, clusters of galaxies are embedded within filaments of dark matter. Dark matter is observed only through its gravitational interaction with light-emitting objects in its vicinity. She also described how the universe is expanding at an accelerating rate - contrary to thinking just 25 years ago - based on the assumption that all observable matter was confined to these vast filamentary structures and subject to gravitational attraction. We now know that even in the largest cosmic voids there exists a state of quantum vacuum energy, sometimes described as a ‘Higgs bath’, where mass is ascribed to fundamental particles and potentially also applies to gravity. This is treated as a cosmological constant and is deemed to drive the expansion of the universe.

However, the implied rate of expansion produces a discontinuity, in that it would, in theory, exceed the speed of light. Something must therefore be moderating this expansion. Some suggest this is ‘dark energy’, while de Rham proposes that the symmetrical nature of the fundamental forces may not be as symmetrical as previously thought. This is now being revealed through differing probabilistic measurements derived from cosmic microwave background data compared with those from supernovae and stars, leading to different inferred expansion rates. This discrepancy is known as the ‘Hubble tension’.

Claudia explains the 'Hubble Tension"

This raises the possibility of a breaking of symmetry in gravity, analogous to that seen in the weak nuclear force, where beyond a certain distance the force may diminish or effectively vanish. De Rham referenced recent theories suggesting a very subtle internal asymmetry within gravity that could lead to such behaviour. In the 2030s, it may be possible to detect this discrepancy using space-based interferometry missions - satellite versions of the Earth-bound LIGO detectors (Laser Interferometer Gravitational-Wave Observatory). Professor de Rham concluded by speculating that one day we might be able to ‘decode’ gravitational waves and understand how we are connected to objects on the far side of the universe.

Two audience questions followed. The first concerned the apparent violation of the conservation of energy. De Rham explained that conservation of energy is a local principle and does not necessarily apply on the largest cosmological scales. The second question asked whether we have proof of the existence of gravitons. De Rham replied that, given enough time - perhaps millions of years - this may one day be possible.

Claudia answering questions at the end of her lecture, with Yvonne as host

To enthusiastic applause, the talk came to a resounding end.

The evening concluded with Paul May reading a heart-warming Christmas message from our Chairman, Bobby Manoo, who is currently convalescing. He reflected on the wonderful year the Society has had, including numerous social events and significant trips to Herstmonceux Castle and the Royal Observatory Greenwich, as well as overseas visits to La Palma to see professional observatories in operation.

We wrapped up the evening with the opportunity to purchase Claudia de Rham’s book The Beauty of Falling: A Life in Pursuit of Gravity and ask further questions, accompanied by a spread of seasonal mince pies, Christmas cake, and non-alcoholic mulled wine - being careful not to break the laws of expansion by over-indulging.

Pictures from the evening (by Mike Meynell):