I'm aware that this Topic is generally considered hypothetical, but at least I'm not talking about something that someone can say, "hey, that's your personal theory". A decent number of respected physicists have published papers on one aspect of the subject, or another. There is a Question, though, that has been nagging me for some time, and because it is about particles, that's why I chose this particular category for this Message Thread. Some background information: No examples of negative mass are known to exist. There is no known theoretical reason why the stuff couldn't exist. If we did have a sample of negative mass, it is expected to exhibit certain distinctive behavior. For example, if a sample of iron had negative mass instead of ordinary mass, and we tried attracting it with a magnet, we expect it would actually be repelled. That's because the equation F=(m)(a), when a negative mass is involved, becomes F=(-m)(-a). Now, regarding some particles: Most ordinary mass-possessing particles also have an electric charge associated with them. The neutron, for example, contains some charged quarks, and those charges happen to balance each other out, leaving the neutron with zero net electric charge. About the only particles that I'm aware of, that might possess mass without any hint of associated electric charge, are the Z boson and the neutrino. For the purpose of the Question I want to ask, I'll choose to use the neutrino in my lead-up. I don't want any unnecessary forces, such as might be associated with interacting electric charges, to interfere with the description that follows. Let us assume that a particle equivalent to the neutrino, except possessing negative mass instead of ordinary mass, might be able to exist. My Question relates to an examination of what might happen if that negative-mass neutrino (-m) collides with an ordinary neutrino (+m). Here is a simple sketch of the event: (+m)(+v)--->*<---(-m)(-v) If we assume the two particles have equal magnitudes of mass, despite their opposite signs, and we assume they are moving in opposite directions at equal speed, then when the collision is done, we might expect the mass to be "cancelled out" or "nullified" (this word courtesy of the late Dr. Robert L. Forward). We also can expect that there will be Zero Kinetic Energy left over (since equal and opposite magnitudes of KE are carried into the interaction). However! It looks like there will be Momentum left over, since we all learn in about the 4th grade of school that two negative numbers, multiplied together, make a positive number (and so both particles bring positive amounts of Momentum to the interaction). What form can that Momentum have, after the nullification event completely dissociates it from both mass and kinetic energy??? That's my Question. For anyone with an Answer, Thanks In Advance!