['Just', 'as', 'in', 'the', 'electromagnetic', 'case', ',', 'the', 'bag', 'boundary', 'condition', 'modifies', 'the', 'vacuum', 'fluctuations', 'of', 'the', 'field', ',', 'which', 'results', 'in', 'the', 'appearance', 'of', 'a', 'Casimir', 'force', '\xa0 ', '[', '14–18', ']', '.']