UFOs are both amorphous and solid. They appear as "lightforms" as often as they appear to be constructed of metal They can be plasma-like but they can also take form as "nuts and bolts" craft. Some of the aerial "plasma light" phenomena appears to be self-organized and self-directed, even exhibiting some type of intelligence. They can hover, move instantly, morph shape, blink out then reappear elsewhere...or fade into nothingness. Explanations have been proffered that the lights are unknown natural earth or atmospheric events or processes. Maybe they somehow relate to piezoelectricity, ions, earth lights- or unique combinations of these things. Or even still, some feel they may be some sort of unknown aerial life forms (i.e. Trevor Constable's "Space Critters" theory.)
These are interesting concepts that may well explain some UFO sightings... but some of the phenomena represent craft that are clearly constructed of solid material and that are likely controlled or piloted by sentients.
How can we reconcile the phenomena of luminous and amorphous aerials with ships made of actual alloy? We can do this because they are one and the same. All matter is one matter. The four known states do not really define all matter. It is not just four forms. And these states of matter will one day be found to be all of "one thing" -but with potentially infinite combinations of properties, existing in infinite phases. For instance, in just the past half-century we have learned that:
Solids can be ionic solids, covalent solids, molecular solids, metallic solids and amorphous solids (i.e. solids existing between two states such as found in some advanced polymers.) Aerogels (or "frozen smoke") are open cell polymers that are causing some Materials Scientists to reconsider what it even means for matter to be "solid."
Liquids have definite "volume" but they change shape. Viscosity, surface tension, vapor pressure and boiling points can be altered to create fundamental changes in liquids that make them appear distinctly "unliquid." One Roswell crash debris witness said that the material "moved" and "ran like water."
Gases exist without definite form or volume - unless "constrained" into shape, like some UFOs.
Plasma is ionized gas- a cloud of protons with the ability to act as a whole rather than as just a bunch of atoms- "fuzzy" but self-organized. Plasma can be naturally occurring as well as engineered.
Yet a "fifth state" of matter recently postulated is the "Bose Einstein Condensation" which relates to temperature and the collapse of atoms into a single quantum state.
There will be many more "States of Matter" discovered. Of course these other states have always existed- we just have not yet "manipulated" them into our existence. We will continue to find new states and phases of matter and energy for as long as we continue the march of science. And sentients incalculably advanced from us can likely engineer material in ways that we can only begin to conceptualize. It all relates to the continuum of Materials Science. It is future engineering. There really is nothing "strange" about it. Just unfamiliar.
Of course the idea of conventional thrust and propulsion to traverse the cosmos is silly, it is not possible. Traditional "pop can" craft could not possibly allow for crewed interstellar travel. ET must have in some way mastered the ability to manipulate states of matter and energy so that they become interchangable or so that they can co-exist. This is why the craft appear sometimes to be comprised of 'engineered light' and sometimes they appear solid and metal-like. It is only in this way that they could travel the distances required. They can change solid matter into a variety of other states at will. It is also likely that solid matter can change form to accommodate its existence in different dimensions. The interplay of Matter, Energy, and Mind likely manipulates Time in some way. It is necessary for such far travel. The interstellar dynamic requires the interdimensional dynamic. The morphing, fuzzy, phase-changing craft are that way for a reason. The craft's materials of construction change state to suit the environment in which they are required to operate.
In the coming months newly discovered information will be released revealing that the US government conducted some very interesting metallurgical studies- studies whose impetus was without doubt the Roswell debris recovery in 1947!
Official science documents (whose provenance is unquestionable) will confirm that studies were conducted on morphic metals and even on the influence of the mind on Roswellian material. The shape-memory qualities of the Roswell debris speak to the metal's remembrance of a "Malleable Universe." When published, the information will provide stunning insight into the crash event -and even more profoundly- into the true nature of UFOs and the States of Matter.