Pais-Uhlenbeck Oscillator and Negative Energies

Understand

We review the occurrence of negative energies in Pais-Uhlenbeck oscillator.

• We point out that in the absence of interactions negative energies are not problematic, neither in the classical nor in the quantized theory.
• However, in the presence of interactions that couple positive and negative energy degrees of freedom the system is unstable, unless the potential is bounded from bellow and above.
• We review some approaches in the literature that attempt to avoid the problem of negative energies in the Pais-Uhlenbeck oscillator.