Definition
The Casimir effect is the small attractive force that arises between two parallel, uncharged, electrically conducting plates placed close together in vacuum. The plates restrict which electromagnetic field modes can exist in the gap between them, lowering the vacuum energy there relative to outside and producing a net inward pressure.
It was predicted by Hendrik Casimir in 1948 and measured to good precision starting with Lamoreaux in 1997.
Why it matters
How it works
In quantum field theory, every field — including the electromagnetic field — has a zero-point energy: even in its lowest state, it cannot be perfectly still, because the uncertainty principle forbids simultaneously vanishing field and field momentum. The vacuum is therefore awash in fluctuating virtual photons across every wavelength.
Place two parallel conducting plates a small distance d apart. Inside the gap, the conducting boundaries force the electromagnetic field to vanish on the plates, which means only standing-wave modes with wavelengths fitting in the cavity are allowed. Outside the plates, all wavelengths exist. There are fewer modes inside than outside, so the vacuum energy inside is lower.
Nature pushes systems toward lower energy. The plates feel a force pulling them together — the Casimir force — proportional to ℏc/d⁴ per unit area. For metal plates one micrometer apart, this is a tiny but measurable pressure of about 1.3 millipascals.
Experimentally, Casimir's prediction has been confirmed in setups using a sphere-plate geometry (easier to align than two flat plates), torsion balances, and atomic-force microscopy. The effect now constrains designs for very small mechanical devices, where Casimir forces can cause moving parts to stick.