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Hendrik Casimir calculated an attractive interaction between ideal parallel conducting plates from the change in electromagnetic zero-point energy. The boundary conditions alter the allowed field modes; comparing the energy with and without the plates gives a separation-dependent interaction. At zero temperature, the ideal pressure is $-{\pi }^2\hslash c/(240a^4)$ for plate separation $a$. The Casimir effect connects quantum field fluctuations with a mechanical force, while real materials, finite temperatures and geometry require additional physical modeling.^[ref]