**Casimir Effect**: Two uncharged metal plates in a vacuum attract each other slightly because quantum fluctuations create more virtual photons *outside* the plates than between them, generating a tiny pressure. Experimentally verified in the 1940s and refined ever since—it's like the vacuum squeezing the plates together.
**Lamb Shift**: In atomic spectra, electron energy levels are nudged by virtual photons interacting with the electron, shifting spectral lines by a minuscule amount. This was a key confirmation of quantum electrodynamics (QED) in 1947.
**Particle-Antiparticle Pairs**: Fluctuations can produce electron-positron pairs that annihilate almost instantly, contributing to vacuum energy.
