Since the universe is thirteen billion years old while human civilization is only a few thousand years old, alien civilizations out there would most likely be millions and perhaps billions of years more advanced than us. Given such a lead, it is quite plausible they could make devices able to display all of the phenomena reported for UFOs. There is nothing in physics that suggests UFOs are not aliens. No, the main argument against UFOs as aliens is that this is an implausible social scenario.
Nobel Laureate in Physics
I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extra-terrestrial intelligence.
Artificial Intelligence Researcher
I don't believe that UFOs are alien visitors, but my skepticism has nothing to do with all the crazy people who believe in UFOs - the existence of wacky cults is not much less expected in the case that aliens do exist, than in the case that they do not. ... I don't believe aliens would (a) travel across interstellar distances AND (b) hide all signs of their presence AND THEN (c) fly gigantic non-nanotechnological aircraft over our military bases with their exterior lights on.
Senior Astronomer at the SETI Institute
It's hardly likely to amaze you, but I'm skeptical. I don't think the evidence laid on the table as proof of extraterrestrial visitation is compelling, and I certainly don't buy the argument that better evidence (in fact, all the really good evidence) has somehow been collected by the governments of the world and stacked up in secret storage lockers.