One of the lesser known facts about me is that I’m one of the few world experts on anti-gravity. That’s because 20 years ago I was convinced that repulsive gravity could explain some of the puzzling observations astrophysicists have made which they normally attribute to dark matter and dark energy. In today’s video I’ll tell you why that didn’t work, what I learned from that, and also why anti-matter doesn’t fall up…
But if there is so little anti-matter around us and it lasts only for such short amounts of time, how do we know it falls down and not up? We know this because both matter and anti-matter particles hold together the quarks that make up neutrons and protons.
Inside a neutron and proton there aren’t just three quarks. There’s really a soup of particles that holds the quarks together, and some of the particles in the soup are anti-particles. Why don’t those anti-particles annihilate? They do. They are created and annihilate all the time. We therefore call them “virtual particles.” But they still make a substantial contribution to the gravitational mass of neutrons and protons. That means, crazy as it sounds, the masses of anti-particles make a contribution to the total mass of everything around us. So, if anti-matter had a negative gravitational mass, the equivalence principle would be violated. It isn’t. This is why we know anti-matter doesn’t anti-gravitate.Sabine Hossenfelder, “Does Anti-Gravity Explain Dark Energy?” at BackRe(Action) (November 29, 2021)
Much more at the link.
She learned, by the way, that she ought to listen to her own advice from time to time. She incorporated some of that in her book, Lost in Math: How Beauty Leads Physics Astray.