# Causality and gravity

@inproceedings{Donoghue2021CausalityAG, title={Causality and gravity}, author={J. Donoghue and G. Menezes}, year={2021} }

In our classical world, causality is taken to mean that there is no effect before its cause. Technically, we therefore use retarded propagators, which vanish for propagation backwards-in-time, to describe classical physics. Quantum physics is a bit different. Using Feynman boundary conditions, propagators do have a backwards-in-time propagation for so-called negative energy modes. Indeed this is required [1] in order to obtain a more general definition of causality that field operators commute… Expand

#### One Citation

The Ostrogradsky instability can be overcome by quantum physics

- Physics
- 2021

In 1850, Ostrogradsky analysed Lagrangians which contained higher time derivatives and showed that such theories are classically unstable [1, 2]. Classical instability need not imply quantum… Expand

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