arivero said:
I recall that there was an argument from Born expansion showing that exchange of odd spin between equal sign charges generates a repulsive potential, and if the charges are different or the spin is even the potential is attractive.
I wonder, how does it work for non abelian gauge theory charges? Is it still the same?
My intuition is that this is not true at that level of generality.
This flows from three concerns discussed below the "UPDATE" section.
UPDATE:
It occurs to me that by "exchange of odd spin" you mean gauge theories with a carrier boson that has an integer spin 1, 3, 5, . . . I hadn't caught that in the initial read of the quoted language. So, disregard the following three paragraphs.
As a counter-example, suppose that you devised a non-abelian gauge theory with a massless spin-2 carrier boson in which the carrier boson has interactions via that force with other carrier bosons. This is essentially the notional basis of canonical attempts of quantize gravity. In such a theory, the force described by the gauge theory and carried via the carrier boson could be attractive at all times between particles with equal sign and identical charges (e.g. between two particles with identical mass-energy in the quantum gravity context).
Of course, if your hypothetical theory were true, that would be a powerful "no go" theory for lots of kinds of quantum gravity theories. The fact that I've never seen such a "no go" theory articulated in that case also argues against its validity.
At a minimum, I think you need a non-abelian gauge theory with a spin-1 (mod 2) carrier boson for this to be the case (the graviton needs to be spin-2 rather than spin-1 since it is a force that is always attractive.
END UPDATE
1. I'm not convinced that even in that case (i.e. an "odd, integer spin carrier boson"), that it is impossible to imagine a theory in which, for example, identical charges are attractive and opposite charges are repulsive.
@
Vanadium 50 states below that "The spin of the exchanged particle determines whether like-like interactions are attractive or repulsive" and if correct, then this basically overcomes this concern.
2. I am also not convinced that there could not be a case in which "equal sign" is ill defined because there are multiple kinds of charges that never present in isolation (as is the case for the eight color combinations of gluons) -
although perhaps it is better defined than "opposite sign." So, perhaps this concern isn't insurmountable either.
3. I'm also not entirely convinced that it is impossible to have a gauge theory in which charged of the same sign sometimes repel each other and sometimes attract each other.
For example, suppose that you had a gauge theory identical to QED except that photons has a positive or negative electric charge, and that were X-rays or more ultraviolet equal signs were attractive and opposite signs repelled each other, while for photons with lower frequencies the usual attraction/repulsion rules of electromagnetism applied. I don't see why a theoretically possible non-abelian gauge theory couldn't have those properties.
I'm not sure if this is overcome by the definition of "gauge theory" which restricts itself to a narrower class of possible force carrying bosons than my expansive imagination of possibilities in theory space could allow. If one accepts that the carrier boson's spin determines attraction/repulsion then indeed, this shouldn't be possible.
It could be that some or all of three of these propositions can indeed be addressed, but I the reasoning to support these points is not self-evident.