Videos taken with multiple government forward-looking infrared systems (FLIRs). This video compilation shows a comparison of normal objects seen in the air and the UFO seen in Jacksonville, Florida on 12-8-2016. In the beginning of the UFO video, I am centering it in to the reticle.
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One of the key things about tracking UAP seems to be narrowing down the spectrums at which they can be observed. Infrared seems to be more frequently reported for visual detection and there are specific radio frequencies they're detected at. Even a presentation by AARO highlighted signatures that they're often detected at.
If it's a cloaked device I expect to bump into it if I attempt to occupy the same space as it.
If it's a device that's out of phase, I will pass through it without bumping into it.
If it's not a device at all, but is a natural organism that's always been just outside our umwelt (subjective surroundings, subjective world), then that's something completely different again.
All of the above, and maybe more, can conceivably look the same to our infrared and visual sensors.
Obviously something is going on, and it's interesting, and we need to investigate further and exercise the highest level of discernment we can muster at this time.
Definitely a good question and one there doesn't seem to be a solid answer too. Also, some of the suggested speeds of UAP should be creating shock waves, whereas they're typically silent and have no discernible impact on their surroundings. It's as if they're not truly occupying the space they're in. Suggestions I've seen are that they're moving outside our known/perceived dimensions or using a means of warping gravity and essentially displacing their surroundings.
I think similarly. I also have a feeling that many (but probably not all) exotic craft are more like our earth skateboards, so we won't find any engines on them. I feel in many cases the occupants move these things around with their minds. The craft just forms a protective barrier, and maybe enhances the mind's function, or something like that.
In any case we need to be careful with our assumptions. Of course we want to find the engine as the first priority, but what if they don't have and don't need engines? Who knows.