# Are fundamental forces always attractive/repulsive, i.e. parallel to the separation?

If magnetic monopoles existed it would not be the case - the forces on an electron and a magnetic monopole passing by each other would be at right angles to the vector connecting the two particles!

But is there some reason that real forces should always be in the direction of the particles they connect (that is, attractive or repulsive)?

Edited to add: I guess I should limit the discussion to forces mediated by virtual particles, otherwise an oscillating (or just accelerating) charged particle can produce a wave which acts on another charged particle in a direction orthogonal to the wave's propagation.

• Force is a classical concept which is meaningless with relation to particle interactions. One can't attach a force gauge to an electron, neither can one measure an electron's acceleration close to another strongly interacting particle. The action of a classical magnetic monopole on a charge particle can be analyzed, but such a thing does not exist in the real world, except in a fairly poor approximation as the end of a long magnetized rod. Dec 20 '14 at 1:32
• @CuriousOne, you can always obtain classical behaviour in the limit. Consider the question to be about "an electrically charged large object" passing by "a magnetically charged large object". Dec 20 '14 at 1:37
• So you're asking for examples of when Newtons third law doesn't apply? :) Dec 20 '14 at 1:56
• You mean a potential like discussed here? arxiv.org/abs/physics/0701232 Dec 20 '14 at 2:23
• Theory can not tell you if something is possible or not. At most it can tell you if something fits into an existing theoretical framework. Do magnetic monopoles fit into a classical framework? Sure. You can plug ANY potential into classical physics and turn the cranks. Do elementary magnetic monopoles fit into the standard model of particle physics? Absolutely, or we wouldn't be searching for them. Dec 20 '14 at 2:56