1.4.2 The field does NOT propagate instantaneously

The Coulomb field is a fairly familiar concept (see below), valid for a static charge, or when the charge moves with constant velocity. The feature of an instantaneous propagation is implicitly contained in this field.

Two alternative ways of modelling the field are shown in the following two examples.We shall use the description of the field in terms of field lines, as shall be explained in detail below.

In the first case, it may be observed how the field accompanies the charge with a radial shape all across the space, even after the collision has taken place. This means that any point in space "knows" instantaneously any velocity change of the charge. Actually, this information travels at the speed of light in vacuum. Only after a finite time (given by the distance to the charge divided by the speed of light) a point located at a given distance of the charge will "know" that the velocity of the charge has changed. This can be correctly appreciated in the second example. the information propagates as a pulse (kink in the field lines) that travels at the speed of light away from the charge. This is the basis of the radiation phenomenon, which takes place whenever a charge is accelerated.