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Register a new userQuantum effective potential from discarded degrees of freedom
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Luke M. Butcher
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I obtain the quantum correction $Delta V_mathrm{eff}= (hbar^2/8m) [(1- 4xi frac{d+1}{d})(mathcal{S})^2 + 2(1-4xi)mathcal{S}]$ 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 $-langleDelta V_mathrm{eff}rangle$ that pushes the expectation value $langle xrangle$ 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 $mathcal{S}(x)$.
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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 Synges 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 Schrodinger equation; in particular, it allows us to consider discarded variables with a history-dependent information capacity $mathcal{S}=mathcal{S}(x,int^t f(x(t))mathrm{d} t)$. History dependence does not alter the formula for $Delta V_mathrm{eff}$.
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