Anti-gravity, mass, inertia. FTL is actually just the side benefit of being able to manipulate these.

Assuming "alienness" implies FTL, but don't get distracted from the immediate here and now evidence that physics is reaching a crux in its path. Either we know most of the things about reality and nature's laws already, or we are reaching a crossroad where we know almost nothing and have to start anew with different methods, materials and questions at a level that is orders of magnitude more powerful than we are accustomed to.

UAPs imply the latter, and that we have been throwing a bunch of money into a blind canyon instead of building a bridge to the other side.

This doesn't mean we haven't already built it (black projects) and then cloaked it to prevent self-destruction and to keep the present economic system afloat on oil, mining, power grids, automobiles, etc.

UAPs present a different economic universe than we have the conscious ability to manage. The science might be better left alone until we dissolve our petty differences as peoples, tribes and nations. A single terrorist with anti-gravity technology could destroy the planet in several ways. The energy sources implied by UAPs would obsolete our power grids and petroleum sectors.

We haven't yet developed socially to have people live without competing over useless fanaticisms, so I don't see why there's such a rush to empower humans to travel to the stars.