How does the Casimir effect show that empty space is not truly empty?

The Casimir effect was predicted in 1948 by Dutch physicist Hendrik Casimir. When two uncharged conducting plates are placed extremely close together, the quantum electromagnetic field between them is restricted to fewer allowed modes than outside, creating a tiny net force pushing the plates together. This does not mean empty space contains ordinary matter, but it does show that quantum vacuum has physical effects. The force becomes noticeable only at very small separations, typically on the order of micrometers or less. It is one of the clearest demonstrations that vacuum in quantum physics is more subtle than classical emptiness.