Sigma Club Seminar by Dr Karim Thébault (University of Bristol)
Title: Time and Knowledge in Quantum Cosmology
Speaker: Dr Karim Thébault (University of Bristol)
Abstract: A fascinating and yet under-explored question in the foundations of physical theory relates to the status of the distinction between constants of nature and constants of motion. A plausible line of argument, dating at least as far back as Poincaré, suggests that the distinction may be a fluid one depending upon our limited epistemic vantage point. Poincaré suggested that there may be circumstances where a constant is initially understood to be ‘essential’ (i.e. a constant of nature) but by knowing more we later understand it to be ‘accidental’ (i.e. a constant of motion). This foundational question takes on a more physical character in the context of quantum theories where constants of motion and constants of nature are treated entirely differently. Whereas superpositions of the first can correspond to pure states, the second are subject to superselection rules.
In the context of the problem of time in quantum gravity this difference in treatment of constants, when considered in the context of the cosmological constant, corresponds to the difference between a timeless Wheeler-DeWitt type equation subject to a big bang singularity and a dynamic Schrödinger equation with, in simple cosmological models at least, a unitary cosmic bounce. This cosmological model, first studied in the context of unimodular cosmology by Unruh and Wald (1989), was demonstrated to be unitary and non-singular in Gryb and Thébault (2018), cf. Gielen and Menéndez-Pidal (2022). Furthermore, recent work by Gielen and Menéndez-Pidal (2025) has shown that the approach can be extended towards black hole singularity resolution. In this talk I will review the formal and conceptual foundations of these ideas with a particular focus on the connection to the novel approach to the problem of time developed in Gryb and Thébault (2024).
This event will take place in person on LSE’s campus. However, those unable to attend in person will have the option of taking part online.
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Further reading
- Gielen, S. and L. Menéndez-Pidal (2022). Unitarity, clock dependence and quantum recollapse in quantum cosmology. Classical and Quantum Gravity 29 (7)
- Gielen, S. and L. Menéndez-Pidal (2025). Black hole singularity resolution in unimodular gravity from unitarity. Physical Review Letters 134(10), 101501.
- Gryb, S., & Thébault, K. P. (2018). Superpositions of the cosmological constant allow for singularity resolution and unitary evolution in quantum cosmology. Physics Letters B, 784, 324-329.
- Gryb, S. and K. P. Y. Thébault (2024). Time Regained : Volume 1 : Symmetry and Evolution in Classical Mechanics, Volume 1. Oxford University Press.
- Unruh, W. G. and R. M. Wald (1989). Time and the interpretation of canonical quantum gravity. Physical Review D 40(8), 2598.
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