Time Reversal Symmetry in Particle Physics

Abstract

This thesis addresses current debates about the representation of time reversal by dif-
ferent transformations in physics. Up to now, the controversy about how time reversal
should be represented focuses on theoretical criteria, such as the invariance of charge, the
positivity of the Hamiltonian, the reversal of momentum, the invariance of equation of
motions, and invariance of magnetic fields under time reversal transformations. On the
other hand, this thesis adopts an integrated history and philosophy of science approach
to the issue and so considers the history of physical practice that has so far largely been
overlooked both in the philosophy of physics literature and physics textbooks.
In fact, the primary sources discussed here show that experiments (explained in Wu
et al. and Christenson et al.) were the primary factor in determining the appropriate type
of time reversal transformation. In particular, the rejection of the unitary time reversal
operator is linked to experiments involving weak interactions and the preservation of
spin-statistics. Notably Pauli (1936), Racah (1937), Lüders (1954) explicitly embraced
unitary time reversal operation, and it was implicitly adopted by Stückelberg (1941),
Feynman (1949b), and Bell (1955).
Symmetries can be defined in terms of invariance at several levels including solutions,
equations of motion, the Lagrangian, the Hamiltonian, and the Action. The Action be-
came particularly significant for understanding time reversal symmetry as a result of
developments in relativistic quantum field theory. By analysing the symmetry transfor-
mations at the level of Action, which is historically in line with the approach of the
founders of the CPT Theorem, we show that both the unitary time reversal operator
and the anti-unitary operator can plausibly represent time reversal transformation. We
criticize theoretical explanations for the choice of time reversal transformation, as our
historical approach demonstrates that experimentation motivates the choice of correct
transformation.

Date of Award	27 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Karim Thebault (Supervisor) & James A C Ladyman (Supervisor)