Path integrals with discarded degrees of freedom

Luke M. Butcher

Published 2018-07-18Version 1

Whenever variables $\phi=(\phi^1,\phi^2,\ldots)$ are discarded from a system, and the discarded information capacity $\mathcal{S}(x)$ depends on the value of an observable $x$, a quantum correction $\Delta V_\mathrm{eff}(x)$ appears in the effective potential [arXiv:1707.05789]. Here I examine the origins and implications of $\Delta V_\mathrm{eff}$ within the path integral, which I construct using Synge's world function. I show that the $\phi$ variables can be `integrated out' of the path integral, reducing the propagator to a sum of integrals over observable paths $x(t)$ alone. The phase of each path is equal to the semiclassical action (divided by $\hbar$) including the same correction $\Delta V_\mathrm{eff}$ as previously derived. This generalises the prior results beyond the limits of the Schr\"odinger equation; in particular, it allows us to consider discarded variables with a history-dependent information capacity $\mathcal{S}=\mathcal{S}(x(t),\int^t f(x(t'))\mathrm{d} t')$. History dependence does not alter the formula for $\Delta V_\mathrm{eff}$.