Quantum effective potential from discarded degrees of freedom

Abstract: I obtain the quantum correction $\Delta V_\mathrm{eff}= (\hbar^2/8m) [(1- 4\xi \frac{d+1}{d})(\mathcal{S}')^2 + 2(1-4\xi)\mathcal{S}"]$ that appears in the effective potential whenever a compact $d$-dimensional subspace (of volume $\propto \exp[\mathcal{S}(x)]$) is discarded from the configuration space of a nonrelativistic particle of mass $m$ and curvature coupling parameter $\xi$. This correction gives rise to a force $-\langle\Delta V_\mathrm{eff}'\rangle$ that pushes the expectation value $\langle x\rangle$ off its classical trajectory. Because $\Delta V_\mathrm{eff}$ does not depend on the details of the discarded subspace, these results constitute a generic model of the quantum effect of discarded variables with maximum entropy/information capacity $\mathcal{S}(x)$.