Covid has renewed my interest in fundamental physics. But I notice that my knowledge has rusted a little bit over the years. So please bear with me.
Since unification of gravitation and quantum field theory is still an open topic, I became stuck with the following problem I found interesting:
According to the equivalence principle, inertial and gravitational mass are proportional. But, since this principle has evolved from macroscopic observations, I am not so sure whether it excludes negative gravitational masses (i.e. proportionality factor including a minus sign). More specifically, does any physical law (not just gravitation) exclude the possibility that gravitational mass and charge always have the same sign? To my knowledge, positivity of mass is always just silently introduced, without justification.
If there was no such constraint, electrons could be the sources of a negative gravitational potential (or a corresponding metric). However, since other electrons had the same signed mass, electrons would still attract each other. Similar would hold true for protons with a positive gravitational potential, but still attracting each other. The only difference, as far as I could see, would be a repulsive gravitational force between electrons and protons, which would be weak by a factor of the electron/proton mass ratio. Neutrons, if considered a compound of protons and electrons united by the process of electron capture would present a reduced gravitational potential just like a hydrogen atom would.
Astronomical objects then would be sources of a gravitational field that is reduced by ~1/2000th for the mass of the electrons contributing a negative fraction to the overall gravitational field. But, since the ratio of electrons to protons is always the same, the reduction would be everywhere the same, and hence, could be already absorbed into the gravitational constant / the proportionality of inertial and gravitational mass.
Last but not least, anti-matter would then repel ordinary matter due to the opposite charges of their baryons (I have seen a paper https://arxiv.org/abs/1103.4937, that derives such a repulsion from assuming CPT invariance for GRT, but I guess that is a different story). There would be probably (I haven't checked) be anti-Schwarzschild solutions of the Einstein equations, and anti-Blackholes would also repel ordinary Blackholes.
Ist there any fallacy in my thinking? Or is there any experiment that can rule out the above? I guess not, since measuring the gravitational effect on electrons fails for reasons of accuracy/weakness of gravity vs. electromagnetism. But it sure would make a difference on the theory level, wouldn't it.
Edit: I am a bit unsure about my original (speculative) conclusion, that electrons would still attract each other, in case of their mass being negative; from the point of view of newtonian mechanics we could probably argue that a mass sign would enter quadratically into the gravitational force of like charges, resulting in an overall attractive gravitation between electrons (inertial mass must always remain positive if Lorentz' force law shall not be compromised) and a repulsive gravitation between electrons and protons; but from the point of view of GRT, the weak equivalence principle would dictate that gravitation is just geodesic motion in the external field (in this case of an electron), regardless of what kind of other charged particle falls into this field; so if electrons had negative gravitational mass, their metric and the corresponding Levi-Civita connection would be somehow inverted, but it would act the same on other electrons and protons; meaning if this metric causes repulsion, it would repel protons as well as other electrons; but then again, the question remains whether the weak equivalence principle (which has been experimentally confirmed only for makroscopic matter) could also be confirmed for elementary particles...