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The Casimir effect is a force associated with the change in electromagnetic field fluctuations produced by material boundaries. For ideal perfectly conducting parallel plates separated by $a$ at zero temperature, the attractive pressure isReal measurements require finite-conductivity, temperature and geometric corrections. A classic experiment used a torsion-pendulum electromechanical system to measure the force between nearby conducting surfaces. The experimental comparison concerns the force difference produced by a specified geometry; it does not make an arbitrary absolute vacuum-energy offset observable.^[ref]

Zero-point energy is the ground-state energy above the classical minimum, remaining even at zero temperature. For a harmonic oscillator it is $\hslash \omega /2$; simultaneous vanishing position and momentum fluctuations is forbidden by the uncertainty principle.

A concrete test is the Casimir interaction: changing conducting boundaries changes electromagnetic ground-state modes and produces a measurable force. Ideal parallel plates have pressureA torsion-pendulum experiment measured the attraction of closely spaced conducting surfaces, testing this boundary-dependent interaction.^[ref] Such measurements test changes in ground-state energy, not an arbitrary absolute additive vacuum energy; real-material and thermal corrections must be included when comparing precision data.

An experimental atomic physicist who measured the Casimir force between conducting surfaces in 1997. His electromechanical apparatus used a torsion pendulum to detect the weak attraction over separations of roughly 0.6–6 micrometres. The experiment tested a consequence of changing the allowed electromagnetic field fluctuations by changing boundaries, connecting quantum electrodynamics to a directly measured mechanical force.^[ref]