14th feb 2025

June Gilchrist DAS chair introduced Linus Thummel from The Institute for Astronomy University of Edinburgh, our guest speaker for the evening. The title of his talk is: One Equation to Rule Them All: What can space teach us about fundamental physics?

Linus explained he was deeply honoured to be our valentine date for the evening. He then outlined his background as a theoretical cosmologist working in his final year of his PhD. As a theoretical scientist he explained he uses data collected from telescopes but never actually observes the night sky himself. It has been an objective of many theoretical physicists over the years to try and explain the nature of the universe with a few key equations and therefor come up with a grand unified theory of everything. Perhaps in the future cosmology will be able to provide the answers to this problem if it is solvable. By way of an example, it is useful to look at the complexity of a single cell which has proteins again very complex, these consist of molecules themselves consisting of combinations of elements until we get to the atomic level and sub atomic level. What are the forces at the atomic and sub-atomic level? We have the electromagnetic force which mainly affects electrons and the weak force and strong force which applies on distance scales related to proton and neutron interactions. Many experiments have shown that light which is electromagnetic radiation has a dual nature. It can be considered as a particle or a wave. Quantum field theory has been developed to explain interactions on very small scales. In this theory particles can be modelled as excitations in a field and there are Weak Fields, Strong Fields and Electromagnetic Fields.

Lagrangian mechanics have been adopted as a standard model of physics and overcome some difficulties in Newtonian mechanics. The principal of least action is an important difference in the Lagrangian mechanics compared to Newtonian and this principal is important in quantum mechanics. Quantum Field theory is a useful concept but is not actual reality. The concept of complex systems and emergence can be explained with an analogy; we understand neurons, the signals between neurons and we suppose this leads to consciousness but we do not really understand consciousness. Something has emerged which is not measurable in individual components making up the system.

It is interesting to consider the distances over which the various forces are relevant. The strong and weak forces are working on sub atomic scales, perhaps the electromagnetic force can be considered as relevant up to solar system distances. After a certain distance then gravity is the only relevant force in the universe.

Attention turned to Einstein’s General Relativity and its success in predicting phenomena and useful application in technology. The field equation relates spacetime to matter and energy. Curved space time tells matter how to move. How does matter move? In curved space time straight lines are curves. The rubber sheet analogy is useful where the heavy object distorts the sheet creating a well representing the curvature of space indicating the path matter must follow. General relativity successfully explains the precession of the orbit of Mercury. General relativity certainly explains effects on the scale of our solar system.
GPS clocks orbiting very high above the surface of the earth must take in to account two effects explained by General Relativity. The clock in orbit is running at a different speed to that of a clock on earth. Gravity stretches light and therefor affects the signal beamed down to earth. These effects must be taken in to account for the GPS system to work.

Consider a massive object on a very small scale. This would be a blackhole but could also apply to the Big Bang. For large distance such as intergalactic distance we have gravity waves and these have been successfully detected and one possible mechanism is the collision of binary neutron stars. Einstein rings are also large-scale effects of gravity bending light. This phenomenon is on the intergalactic scale which is an extremely large scale.

Going back to the Big Bang at time almost zero quantum fluctuations caused tiny variations in the density of the emerging universe. As the universe expanded this magnified the effect of the variations. General Relativity is doing very well so far explaining a lot of cosmological observations. The expansion however is accelerating and we need energy to generate this acceleration. This takes us to the concept of dark energy for which there is no explanation of what it actually is. Could this be a new cosmic fluid or a modified gravity working on large scales, a fifth force?

When coming up with new theories to explain dark matter and dark energy we have to make sure they match observations. It is not helpful if new theories cannot explain any existing observations. Existing theories make use of the concept of space-time, mass is coupled to gravity, space-time is coupled to mass but can we improve on this. Theoreticians introduce new dimensions or new fields but have struggled to make headway. String theory makes use of multiple dimensions, possibly as many as twelve. These are considered to be hidden well below the sub-atomic scale and therefore not observable. This theory has been studied for decades but has yet to provide a grand unified theory.

The Euclid space craft has recently begun its mission to measure the redshift of distant galaxies. By doing this it is hoped to explain the history of the expansion of the universe and existence of cosmic structure. It is hoped this will provide explanations for how dark energy is accelerating the expansion of the universe.

The Vera Rubin observatory under construction in Chile will carry out observations of weak gravitational lensing to determine the effects of dark energy on the expansion of the universe. As it is ground based it will run for many years.

In conclusion the speaker admits he thinks it unlikely a grand unified theory will be found any time soon. He was happy to take questions and was able to clarify many points in his presentation. He explained a little bit more about string theory and the hidden multiple dimensions and the fact the strings are invisible so can never be proven to exist. He also clarified that Lovelocks theorem is a mathematical theorem justifying that the only possible equations of motion are Einstein’s Field Equations from GR. That new theories seeking to replace GR will disregard this theorem and going back to previous points any such modifications should really only be carried out if they can predict observations. He also clarified that dark matter has been present in the Big Bang and does provide an explanation for the filamentary structure of the cosmos. This structure is three dimensional and has been built up from many observations over the years. The filamentary nature of the structure is due to dark matter and would not exist otherwise.

June thanked Linus for his great take giving a great overview of theoretical research on cosmology and fundamental physics. Members were able to talk personally to Linus after the talk when refreshments were served.

(Thanks to Phil Rourke for a comprehensive report of this meeting)