Dr. Ian Jubb
address: Rm. 208, DIAS, 10 Burlington Rd, D04 C932.
PhD Imperial College London (2017),
MPhys University of Manchester (2013)
Our current understanding of the universe rests on two great pillars of theoretical physics. The first is General Relativity, Einstein’s theory of gravity as the curvature of spacetime itself, which describes phenomena such as black holes and gravitational waves. The second is Quantum Field Theory, the theory of matter and its interactions, which can describe processes such as quantum tunnelling and particle-antiparticle pair production. These theories represent two of the most significant scientific achievements of the last century, and yet they are mutually incompatible. A coherent understanding of the most fundamental aspects of our universe requires that quantum physics and gravity be reconciled into a single, as of yet, unknown framework — Quantum Gravity.
The long-standing problem of Quantum Gravity is the central motivation behind my research, and I am currently investigating it from three main directions:
Measurements in Quantum Field Theory
Quantum Field Theory has been experimentally verified to extremely high accuracy. Nevertheless, there are still outstanding issues within the theory, unaddressed by any experimental tests. For instance, there are many observables that the current theory says should be measurable, but, at the same time, we can also show that if we were to measure them we could send signals faster than light! If we are to reconcile quantum theory with relativity, and Einstein’s theory of gravity, then such superluminal signals should be avoided. The framework of Quantum Field Theory, therefore, needs to be modified in some way, either by excising these superluminal signalling observables, or by updating our description of measurement within the theory. Regardless of the particular resolution, a consistent description of measurement in the relativistic setting of Quantum Field Theory will likely have important implications for any complete theory of Quantum Gravity.
Spacetime Topology Change
Predictions in quantum theory can be derived by summing over all the possible trajectories that the particles can follow. In direct analogy, physicists have tried to quantise gravity by attempting to sum over the different ways that spacetime can curve. One unanswered question regarding this sum is whether we should sum over spacetimes that “split” and/or “re-attach”, that is, spacetimes that undergo a topology change. A dramatic example of this would be the universe splitting in two. As radical as this sounds, similar processes may occur at the quantum level. There are multiple types of topology changes that can occur, and to properly address the question of whether a particular change can happen at the quantum level we must include the
effects of matter via Quantum Field Theory. The only topology change that has been properly studied in this way is the 2D trousers spacetime, in which a single circle splits into two circles. In this case, the matter appears to prevent the trousers topology change from occurring. There are hints that all the different topology changes in 3D and 4D are also prohibited in the same way, except one. Remarkably, this single (possibly) viable topology change occurs in black hole pair-production — a process that intimately ties together quantum theory and gravity. Evidence for or against the viability of this topology change, and other spacetime topology changes, will undoubtedly be beneficial in the search for Quantum Gravity.
Causal Set Theory
Causal Set Theory is an approach to Quantum Gravity in which the continuous spacetime of General Relativity is substituted for a discrete network — a causal set — in which the vertices are the ‘atoms’ of spacetime, and the links between vertices are
causal relations. This approach is motivated by the fact that almost all of spacetime geometry, and hence gravity, can be encoded in causal relations. Additionally, the discrete nature of the theory provides a physical reason as to why black hole entropy should be finite. The manifestly causal nature of Causal Set Theory has also inspired the development of novel methods in Quantum Field Theory, e.g. the Sorkin-Johnston formalism. Further study of Quantum Field Theory and black holes within Causal Set Theory will undoubtedly improve our understanding of how to embed quantum theory within a relativistic framework.