Casimir effect

The Casimir effect is a physical force arising from the quantum fluctuations of a field, first predicted by Hendrik Casimir in 1948. When two uncharged conducting plates are placed very close together in vacuum, the quantum modes of the electromagnetic field between the plates are restricted relative to those outside, producing a pressure difference that pushes the plates together. The effect was experimentally confirmed in 1997 by Steven Lamoreaux and has since become one of the most direct demonstrations that vacuum energy is physically real, not merely a mathematical artifact.

The Casimir force is intimately connected to the cosmological constant problem: if vacuum fluctuations produce a measurable force between plates, they should also curve spacetime, yet the observed curvature is 120 orders of magnitude smaller than quantum field theory predicts. This tension suggests that our understanding of how vacuum energy couples to gravity is incomplete. The Casimir effect is not a curiosity of boundary conditions; it is a window into the deepest puzzle of quantum gravity — why the vacuum weighs so little.