2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/49810Using the Dirac constraint method we show that the pure fourth-order Pais-Uhlenbeck oscillator model is free of observable negative norm states. Even though such ghosts do appear when the fourth order theory is coupled to a second order one, the limit in which the second order action is switched off is found to be a highly singular one in which these states move off shell. Given this result, construction of a fully unitary, renormalizable, gravitational theory based on a purely fourth order action in 4 dimensions now appears feasible.RevTeX, 8 pagesHigh Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext