Quantum field theory is built with causality in mind. However, the way in which this plays out at the level of particle dynamics is not so clear. By considering the role of classical sources, we obtain manifestly-causal transition amplitudes, making comparison with standard S-matrix theory. Subsequently, we prove that these amplitudes are consistent with perturbative unitarity and that they reduce to the corresponding S-matrix amplitudes for positive-energy plane-wave scattering to all orders in perturbation theory.

After highlighting the relevance of techniques borrowed from frameworks of quantum field theory at finite temperature and density, we note the significance of negative-frequency modes in ensuring manifest causality. Drawing upon this observation, we construct a nominal departure from standard quantum field theory that permits states of negative energy, identifying a number of intriguing features. These include possible implications for the cosmological constant and naturalness problems. Subsequently, we highlight issues with perturbative unitarity and Bloch-Nordsieck cancellation, commenting on the potential relevance of resonance phenomena to the breakdown of naive perturbation theory. Nevertheless, as a non-trivial example, we recover the Peskin-Takeuchi parametrization of the electroweak oblique corrections in agreement with the standard result.