In all matter there are continuous jostlings of positive and negative charges; at every point in a material body or in a vacuum, transient electric and magnetic fields arise spontaneously. These fluctuations in charge and in field occur not only because of thermal agitation but also because of inescapable quantum-mechanical uncertainties in the positions and momenta of particles and in the strengths of electromagnetic fields. The momentary positions and electric currents of moving charges act on, and react to, other charges and their fields. It is the collective coordinated interactions of moving electric charges and currents and fields, averaged over time, that create the van der Waals or “charge-fluctuation” force.

It turns out that such charge-fluctuation or “electrodynamic” forces are far more powerful within and between condensed phases—liquids and solids—than they are in gases. In fact, these forces frequently create condensed phases out of gases. The electric fields that billow out from moving charges act on many other atoms or molecules at the same time. The particles in a dilute gas are so sparsely distributed that we can safely compute the total interaction energy as the sum of interactions between the molecules considered two at a time. Rather than think in terms of a gas of pairwise particles, the modern and practical way to look at van der Waals forces is in terms of the electromagnetic properties of fully formed condensed materials. These properties can be determined from the electromagnetic absorption spectrum, i.e., from the response to externally applied electric fields.