# Classical and quantum ghosts

@article{Sbis2014ClassicalAQ, title={Classical and quantum ghosts}, author={Fulvio Sbis{\`a}}, journal={European Journal of Physics}, year={2014}, volume={36} }

The aim of these notes is to provide a self-contained review of why it is generically a problem when a solution of a theory possesses ghost fields among the perturbation modes. We define what a ghost field is and we show that its presence is associated with a classical instability whenever the ghost field interacts with standard fields. We then show that the instability is more severe at quantum level, and that perturbative ghosts can exist only in low energy effective theories. However, if we…

## 64 Citations

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