= Casimir energy
{c}
The <Casimir energy> is the configuration-dependent part of a quantum field's vacuum energy after a specified subtraction or renormalization. Only an energy difference or its derivative is physically meaningful in the idealized calculation; the separate zero-point sums are divergent.
For the electromagnetic field between two infinite perfectly conducting parallel plates at zero temperature and separation $a$, the renormalized energy per area is $E/A=-\pi^2\hbar c/(720a^3)$. Differentiation gives the attractive pressure $-\partial(E/A)/\partial a=-\pi^2\hbar c/(240a^4)$. Real materials, finite temperature and geometry alter this result. The <Casimir effect> therefore tests a response to changed boundaries, not a direct weighing of an unregularized infinite energy.
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