And that's the rub: not only do we not know all the physics yet, but even once we have a working Grand Unified Theory, there will likely be a great deal more physics to learn.

Personally, I believe UFO's exist (though the vast majority of sightings are bunk). After all, even after allowing for all the ways that life as we know it cannot develop, there's still many millions of planets just in our galaxy that may harbor life.

The sightings of UFOs that seem most credible indicate actions and movements that are flatly impossible according to all known physics...so there's somebody out there who knows things we haven't learned yet.

Moreover, if any of those UFOs have life on board, then either that life traveled many, many of our years to get here...or they used FTL.

So that's the hill I'll defend: if there are UFO's, FTL travel is almost certainly a reality.