Suppressed Antigravity Technology
Our knowledge of anti-gravity drives varies from being sufficient to reproduce, to nothing above gossip. We will take the "cream of the crop" of devices, explain how they work, and compare their mode of operation with known physics. Several of the devices are not similar, but disobey similar laws of physics. This suggests which laws of physics are on shaky ground. The devices are well documented in the public literature, but violate laws of physics that are pretty durable. So we could build them, but would not know why they work. Describe the budget and effort required to bring devices like this out of the literature and into interstellar space travel.