Classical and quantum pictures of interactions

Classically, interaction at a distance, e.g. in electromagnetism, is described in terms of a potential or field due to one charged particle acting on another, the field permeating the whole of space around the source charge. A conceptually more appealing idea, perhaps, is that of an exchange interaction, where the object exchanged carries momentum from one charge to the other, the rate of exchange of momentum providing the force. A frequently quoted everyday example is of two ice-skaters travelling side by side, initially moving parallel, who start to diverge as they repel one another by throwing a ball back and forth.

In quantum theory, action at a distance is indeed viewed in terms of an exchange interaction, the exchange being of a specific quantum (a boson) associated with the particular type of interaction. Since the quantum carries momentum and energy, say ΔE, the conservation laws can only be satisfied if the process takes place within a timescale Δt limited by the Uncertainty Principle, i.e. ΔE Δt ≃ ħ. Such transient quanta are said to be virtual.

The quantum concept of an electromagnetic interaction is of the continual exchange (emission and absorption) of virtual photons by the charges, which is no more or less fictitious than the classical concept of an all-pervading field surrounding a source charge. Neither the field nor the virtual quanta are directly observable; it is the force that is the measured quantity.