Most discussion these days within ufology centers on the Extraterrestrial Hypothesis (ETH) as the explanation for the phenomenon. Where, they ask, are the UFOs from? "Why, another solar system," is the usual answer.
I agree that this is one possibility. But it is unproved, which means that other possibilities may be just as valid (including, of course, that all UFO sightings really do have terrestrial explanations).
One intriguing possibility, rarely mentioned these days by ufologists, is that UFOs may be time travelers.
In short, the question may be not where they come from, but when.
I can hear the chuckles now... "Kimball has been watching too much Star Trek, or Dr. Who" (it is true that I am a big fan of both shows, in all their varied incarnations). Time travel is silly. Can't be done.
Interestingly, pro-ETH ufologists bristle when those same comments are made about travel through space.
But many physicists will tell you today that time travel as a theory makes as much sense as interstellar travel (just as they'll also tell you that both are beyond our level of knowledge right now).
The late Carl Sagan, an ETH skeptic, did not rule out time travel as an explanation. See www.pbs.org/wgbh/nova/time/sagan.html. He stated "There's the possibility that [time travelers are] here and we do see them, but we call them something else - UFOs or ghosts or hobgoblins or fairies or something like that."
Michio Kaku, while not explicitly linking it with UFOs, has nevertheless written extensively about the possibililities of time travel. For a short introductory article, see www.mkaku.org/articles/phys_time_travel.shtml
Stephen Hawking rejected time travel for years. These days, he's not so sure.
Then there's the philosophical implications behind the concept of time travel, which Joel Hunter of the University of Kentucky has speculated about. See www.iep.utm.edu/t/timetravel.htm.
A couple of other sites worth checking out are:
www.bbc.co.uk/science/space/exploration/timetravel/index.shtml (Neil Johnson, physicist, Oxford University); and
http://science.howstuffworks.com/first-time.htm?referer=time-travel.htm (a good, quick "primer" - time travel film in-joke intended - for non-scientists like me).
The concept of time travel raises as many interesting possibilities about the UFO phenomenon as does the idea of interstellar travel - perhaps even more. For example, for those worried about the future course of humanity, it provides hope. After all, if we make it far enough to master time travel, we should be doing okay otherwise (at the very least, we'll still be here... er, there...).
Even more interesting are the religious implications. Some ETH advocates (like Barry Downing) have posited the theory that the God of the Bible was really extraterrestrial in origin. But what if we are the God of the Bible. One of the central ideas behind God, after all, is that He is eternal - yesterday is today is tomorrow to Him. If we ever do master time travel, we become capable of the same thing. We become, perhaps, our own God - masters of the Eternal Now.
Now, that's probably all a bit much for ufology. But perhaps it shouldn't be.
After all, if we conclude that aliens more advanced than us could figure out how to get from there to here (as ETH proponents like Stan Friedman do), what's to say that, hundreds or thousands of years from now, WE couldn't figure out how to get from then to now?