Casimir effect
The Casimir effect is a tiny but real force that pulls two uncharged, parallel metal plates toward each other when they are placed extremely close together in a vacuum. It is one of the most direct demonstrations that the quantum vacuum is not empty but seething with field fluctuations. Hendrik Casimir predicted it in 1948, and increasingly precise experiments from the late twentieth century onward have confirmed it.
The standard explanation runs like this: the space between the plates can only host electromagnetic field modes that fit neatly between them, while outside the plates there is no such restriction. With fewer allowed modes inside than outside, the fluctuating vacuum presses inward harder than outward, and the plates are squeezed together. The force is feeble and grows rapidly as the gap shrinks, becoming significant only at separations of well under a micrometre.
The Casimir effect matters far beyond its own curiosity. It is a clean, quantitative check that vacuum fluctuations have measurable mechanical consequences, and it has become a practical concern in the design of tiny machines, where surfaces only nanometres apart can stick together because of it. It is a vivid reminder that the lowest-energy state of the fields is alive enough to push on the everyday world.
Fewer field modes fit between the plates than outside, so the vacuum presses them together.
While usually explained via vacuum zero-point energy, the Casimir force can equally be derived from ordinary forces between the atoms in the plates. So it confirms quantum field theory's predictions but does not, by itself, prove the vacuum energy is 'really there' in any naive sense.