Suppose we have a current-carrying wire and an electric charge which is moving parallel to the wire with the same speed and direction as the electrons in the wire. The wire produces a magnetic field which acts upon the charge through a force. From the charge's reference frame, though, no magnetic field can be said to exist, since the electrons in the wire are at rest with respect to it. The force on the charge is thereofre justified through the following reasoning: the motion of the positive nucleus in the wire with respect to the carge causes a Lorentz contraction which increases the positive volume charge density in the wire, leaving the negative volume charge density unchanged, since the electrons are not moving in this particular reference frame. This unbalance gives rise to an electrical force which acts on the wire. The question: what if my current does not consist of electrons in a wire, but of an electron beam in vacuum? Since there's no positive charges in any reference frame, how can I explain, from the charge's point of view the existence of the electric field?