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Concept

quantum vacuum

In quantum field theory, the vacuum state is not empty but filled with fluctuating fields, each carrying a zero-point energy in every mode. Virtual particle pairs continuously form and annihilate; electromagnetic modes cannot be silent. This active vacuum underlies the Casimir effect, the Lamb shift, and spontaneous photon emission by excited atoms. It is also implicated in the cosmological constant problem: the unsolved question of why empty space is so much less energetic than quantum field theory naively predicts.

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