An alien is empirically testable by both real-life observation and study, as they would be composed of molecular structures and are animated in a way that fulfills basic needs such as energy absorption, replenishment, reproduction and waste expulsion. I don't mention water explicitly, because I'm sure on dry worlds, life can live without water because it might be composed of naturally forming "nano-machines" via extreme chemical processes, working together like a hive to form a single being. In theory it would only needs certain frequencies of electromagnetic radiation to stimulate them to function, along with maintenance by absorbing chemicals, elements, etc on occasion to replenish the hive.
But to address another subject; the whole speed of light limit is a flaw of human thinking as in reality in my theory, the density of the Higgs Field is what determines the speed of light limit. Photons travel fastest in the Higgs Field because they are massless. However the minimal energy resistance by the Higgs Field of photons passing through it, results in a throttling down of speed to what is currently observed. In my theory if one were to utilize a great amount of electron-positron collisions to generate enough W and Z boson at one point of space (to absorb the Higgs Field), it will reduce the Higgs Field density to zero, rendering that spatial location void of anything temporarily (in a bubble or tube fashion), therefore having no resistance on anything passing through this location. This would result in faster-than-light travel within that location (almost like teleportation, or a portal effect), but quite unrefined. Of course utilizing an immense gravitational field can do the same thing, as having too many Gravitons being generated at once, would cause a void state (as the Higgs Field is always absorbed to generate Gravitons), but then you have complications with a singularity to deal with and to be frank, requires a force similar to a black hole. The principle in layman's term of this is to simply tear a hole in the fabric of space-time literally, and then from that point, all sorts of applications of spatial/dimensional travel, time distortions, spatial distortions, energy farming, etc could be achieved.
All speculation of course, and to advise any physicists here, I base all this on my own model version of physics I personally believe in. I'm not the type to always take someone else's word for it, and like to formulate my own understanding on things instead.
How this is relevant to the topic? Well for the tl;dr (too long; didn't read) people, I'm basically saying this to those using the 'speed of light' argument, don't think with such a closed mind regarding on how aliens would travel, as I'm sure faster-than-light travel is quite possible. Human science and technology is still in it's infancy regarding advanced physics in my opinion. Humans haven't even been beyond this solar system yet, to be able to rightfully make fixed conclusions on anything just yet.
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"Have a nice apocalypse" - Southland Tales