The Casimir force
It was the Dutch theoretical physicist Hendrik Casimir (1909-2000) who first realized that when two mirrors face each other in a vacuum, fluctuations in the vacuum exert "radiation pressure" on them.
On average the external pressure (red arrows) is greater than the internal pressure (green arrows). Both mirrors are mutually attracted to each other by what is termed the Casimir force. The force F ~ A/d4, where A is the area of the mirrors and d is the distance between them.