2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/192703Contrary to common belief, it is shown that theories whose field equations are higher than second order in derivatives need not be stricken with ghosts. In particular, the prototypical fourth-order derivative Pais-Uhlenbeck oscillator model is shown to be free of states of negative energy or negative norm. When correctly formulated (as a $\cP\cT$ symmetric theory), the theory determines its own Hilbert space and associated positive-definite inner product. In this Hilbert space the model is found to be a fully acceptable quantum-mechanical theory that exhibits unitary time evolution.revtex4, 4 pagesHigh Energy Physics - TheoryGeneral Relativity and Quantum CosmologyQuantum Physicstext