Casimir effecteffect

In 1948, the Dutch physicist Hendrik Casimir, based on quantum electrodynamics, predicted that two perfectly conducting parallel plates in a vacuum would attract each other. The vacuum, it turns out, is not empty: it is a sea of virtual photons continuously popping in and out of existence. Between the plates, only certain wavelengths can exist, like standing waves on a fixed string. The smaller number of allowed modes inside compared to the outside space creates a net pressure that pushes the plates together. The force strongly depends on distance: halve the gap, and the force increases 16-fold. The first precise measurement of the effect was conducted in 1997.

Even in complete emptiness, quantum mechanics says that energy cannot be strictly zero. There is always a faint jitter of electromagnetic fields that appear and vanish, like a fizzing foam. When two conducting plates are placed very close, they restrict the set of possible fluctuations in the gap. Inside the slit, fewer such 'bubbles' fit than outside, creating an imbalance: the pressure from outside pushes the plates together. This microscopic force becomes noticeable only at gaps thinner than a hair.

The Casimir effect is a physical force arising from the modification of zero-point energy of the quantized electromagnetic field between two closely spaced uncharged conducting surfaces. Formally, it is derived by summing the allowed electromagnetic modes between the plates and subtracting the infinite vacuum energy of free space, yielding a finite attractive force per unit area. This effect proves that vacuum fluctuations have measurable consequences and is a pure consequence of quantum field theory.