= Solution
<Zero-point energy> is the <ground-state energy> above the classical minimum, remaining even at zero <temperature>. For a <harmonic oscillator> it is $\hbar\omega/2$; 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 pressure
$$
\boxed{P_C=-\frac{\pi^2\hbar c}{240a^4}.}
$$
A torsion-pendulum experiment measured the attraction of closely spaced conducting surfaces, testing this boundary-dependent interaction.<Demonstration of the Casimir Force in the 0.6 to 6 micrometre Range by S. K. Lamoreaux>[Physical Review Letters 78, 5 (1997), primary abstract: electromechanical torsion-pendulum measurement; later erratum noted in bibliography]{ref} Such measurements test \b[changes in <ground-state energy>], not an arbitrary absolute additive vacuum <energy>; real-material and thermal corrections must be included when comparing precision data.
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