But if we assume that the laws of physics as we know them actually hold true in this regime where we’re asking about them, then no, antigravity is not possible.
Well, if you use Einstein's theories and the viewpoint that the Michelson-Morley experiment ingrained in current science, you;'re right. However, if you look at things from a different perspective; Aether exists and the universe works differently than Einstein's universe. Antigravity could well be a possibility. Aether and grit are different and have different properties.