Quantum fluctuations give rise to van der Waals and Casimir forces between electrically neutral conducting objects. For free space, the electromagnetic field can be identically zero in the classical picture. In quantum electrodynamics, on the other hand, electromagnetic fields can never be exactly zero in free space because they are continuously fluctuating. The energy of a photon mode is given by (n+1/2)ħω, where ħ is the Planck’s constant/2π, ω is the frequency of the mode and n is the number of photons. In other words, there is a finite zero-point energy (1/2)ħω even when no real photons are present. For a cavity formed by two perfectly conducting plates, the electromagnetic field must satisfy the boundary conditions. The zero-point energy density for the electromagnetic fluctuations between the plates is smaller than that in free space. As a result, there is a net attractive force between the plates.