I am preparing tomorrows lesson about electrostatics and the Coulomb law and wondered the following:
By simple experiments we can show that the electric force can be attractive and repulsive, so we need a charge which can be positive or negative. Also, we can show experimentally that bodies whose charges are of the same type (sign) repel each other, while bodies whose charges are of different type (opposing sign) attract each other.
Naturally, one could ask whether a world is possible where it is the other way around: There is an interaction with a charge, which can be positive and negative, as well, but charges of equal sign attract each other, while charges of opposing sign repel each other.
As far as I understand, if we work with Coulomb's force law, it's a matter of adding an additional minus sign, and we would get a theory which describes exactly such a world. However, if we consider quantum electrodynamics, no matter the sign in front of the coupling constant in the gauge covariant derivative $D_\mu = \partial_\mu\pm ieA_\mu$, the sign in the Coulomb law will be the same (I'd not be able to do the calculation off the cuff, but since the conjugation flips the sign, I'd assume that).
So, is it correct to state that classically there could be a consistent theory of such an interaction (Coulomb force with flipped sign) but in quantum theory, as we understand it today, the only consistent theory with one signed charge is the one that we observe in reality?