Given the position the U.S. government has taken a long with releases concerning observed performance of UAP/UFO maybe they are already here.
A few years back I enjoyed this paper : Estimating Flight Characteristics of Anomalous Unidentified Aerial Vehicles
Several Unidentified Aerial Phenomena (UAP) encountered by military, commercial, and civilian aircraft have been reported to be structured craft that exhibit ‘impossible’ flight characteristics. We consider a handful of well-documented encounters, including the 2004 encounters with the Nimitz Carrier Group off the coast of California, and estimate lower bounds on the accelerations exhibited by the craft during the observed maneuvers. Estimated accelerations range from almost 100g to 1000s of gs with no observed air disturbance, no sonic booms, and no evidence of excessive heat commensurate with even the minimal estimated energies. In accordance with observations, the estimated parameters describing the behavior of these craft are both anomalous and surprising. The extreme estimated flight characteristics reveal that these observations are either fabricated or seriously in error, or that these craft exhibit technology far more advanced than any known craft on Earth. In many cases, the number and quality of witnesses, the variety of roles they played in the encounters, and the equipment used to track and record the craft favor the latter hypothesis that these are indeed technologically advanced craft. The observed flight characteristics of these craft are consistent with the flight characteristics required for interstellar travel, i.e., if these observed accelerations were sustainable in space, then these craft could easily reach relativistic speeds within a matter of minutes to hours and cover interstellar distances in a matter of days to weeks, proper time.
It's important to keep in mind that the paper is estimating the flight characteristics based on anecdotal evidence and whats been said about radar signatures measured during the mentioned encounters. A much more accurate estimation could be made if the radar data was actually released.
The estimate of 100g-1000g is what's particularly interesting to me. The most advanced ICBMs are rated at 100g and typically only experience around 60g during reentry. As such, of the estimates are accurate whatever such craft are they have to be using highly advanced material engineering if they are to be considered conventional.
If such craft are considered to exist then they are either made by Humans or not made by humans. If the estimated flight characteristics are only roughly accurate then either some human organization has highly advanced propulsion and material technologies unknown to everyone (some kind of highly secret military program) or they are not of this world.
I don't think its necessarily unlikely they we may be capable of making such things. The first thing to observe is that there are technological gaps between known and highly produced advance technology. For instance, the United States stealth and propulsion programs are about 20-30 years ahead of all other competitors. That is not to say that China or Russia cannot produce something with the same performance as F22/F35, rather they cannot mass produce such aircraft. Examples of this production issue are su57 which is about 10 years behind in propulsion and stealth but have very low production rates compared to F35 and F22 (during its production). Chines have the J20 and J35, and both have decent delivery times both use stealth and propulsion systems on par to what the US was using during the 80's and early 90's.
Given the existence of technological caps between known craft, one can speculate that perhaps the united states or some other nation with advanced aeronautic engineering capabilities has prototyped something insanely advanced that can't be mass produced. Lets say this craft is 50-100 years ahead of what can be reasonably mass produced. This has happened before with the production of the SR71 back in the 60's. Thirty-two were built, though I am not sure when peek production was, and the stealth technologies utilized for that craft are not all that different from what the U.S. used for the F117 back int he 80's and what the Chinese are using now, which is a 30-50 year gap in technological capabilities.
The above is mostly considering the observed UAPs to be conventional, however according to accounts by pilots a long with scanning radar on their craft the UAPs do not use traditional propulsion. If that's the case and they are human made then someone has cracked some exotic physics/engineering. I won't go on about this but their is a very real history of exotic propulsion research programs in the united states that date back to the early 50's.
