In a nutshell: could the "real", physical UFOs - if there are any - be von Neumann probes? Probes that are sent throughout our galaxy to protect their masters against hostile civilizations?
Let me explain.
First, von Neumann probe means a self-replicating robotic spacecraft in case you didn't know.
Then back to my idea. There might be or have been several technological civilizations in our galaxy - some estimate there are 10000 at any given moment.
Some of those civilizations have possibly survived long enough to reach a technological level sufficient to build a von Neumann probe.
When they have realized a concept of von Neumann probe, they have simultaneously realized that if only one civilization builds a von Neumann probe and equips it with weapons, that civilization will rule the galaxy as it likes. That is simply because the power of von Neumann probe army increases exponentially - something that stationary defense system hardly can match, no matter how sophisticated it is.
Let's take an imaginary example civilization that is relatively peaceful. After realizing things said above, our example civilization has two options: to just wait and see if hostile von Neumann probes appear some day, or to build what I believe is only effective defense system against exponentially growing enemy - another von Neumann probe. The one that is programmed to spread all over the galaxy and "stamp out" aggressive and hostile von Neumann probe infections before they run out of control.
It is not self-evident that they will just wait and see. After all, what proof we (they) have that somewhere in our galaxy there are't a creatures like Hitler, Stalin or bin Laden with a capacity to launch an aggressive von Neumann probe to do things it considers to be just and meaningful? And I doubt humans are the most aggressive race out there: after all, we are not even inherited from real predators, like sharks or crocodiles.
So could there be one or more of these defensive von Neumann probe "families" in our solar system, looking for signs of any hostile von Neumann probe? They certainly would have had enough time to arrive. It has been theorized that a self-replicating starship utilizing relatively conventional theoretical methods of interstellar travel resulting to an average cruising speed of 10% of speed of light could spread throughout a galaxy the size of the Milky Way in as little as half a million years.
If they are here what are they doing? Possibly interfering with our life as little as possible but simultaneously looking for signs of another von Neumann probes, either probes built by local civilization (=human race) or probes arrived from elsewhere and started reproduction here.
If they are here and consider human race a possible near-future source of von Neumann probes it might explain why the modern UFO phenomenon started during WWII. During WWII human race developed space-faring capacity (German V-2 rockets) and nuclear power, required for nuclear pulse propulsion. Also, it might be possible the probes noticed our WWII breakthrough in computer science, possibly by listening our crypted and decrypted radio signals. Thus, the probes changed our status from "primitive technological civilization" to "rapidly developing space-faring civilization" and started intensified reconnaissance in order to determine whether we are building our own von Neumann probe.
That's it. The whole "thought experiment" is of course highly speculative, but please tell me what you think about it. Is there any logical fallacies I didn't notice or something that is clearly incompatible with our current understanding of science and technology?
I tried to keep my main idea as short as possible for not to scare readers away. If you still have an energy and interest to read about this topic I will try to further clarify some of my points.
I'm not trying to use my idea to prove the "real" UFOs exist. That would be silly. Neither I'm a UFO believer: the evidence so far doesn't convince me were have been visited by artificial extraterrestial crafts, even though some cases are very interesting. But my point is that maybe scientific community as a whole shouldn't judge the alleged UFO phenomena outright ridiculous and impossible. If there isn't any obvious impossibilities in my thought experiment it could be one possible starting point to explain the most curious cases and aspects of the UFO phenomena. At least my idea would more or less explain how UFO's could travel such a long distances and why they would bother. Also it would explain the statistical improbability that several UFOs would visit the Earth during a short period. And it would explain why they seem to be just buzzing around, apparently without any sensible purpose. All these questions have been repeatedly used as a proof why there couldn't be alien spaceships near the Earth at this very moment. Plus: it could explain the sudden start of UFO sightings at 40's.
I would like to add there is also a possibility that "police probes" would be sophisticated enough monitor us effectively without ever revealing themselves. In that case we could forget the UFO phenomena connection.
I assume that if my idea will generate some discussion the most popular objection will be something like "but advanced civilization can't be hostile, otherwise it would have nuked itself away, so there's no need for any kind of 'police probes'". I can agree to that to some extent. Sure, if a civilization is aggressive it is reasonable to believe it is in greater danger to destroy itself before developing technology needed for advanced self-replication and interstellar probes. But can we rule out the possibility there could be a sentient, intelligent race that is simultaneously both highly social and highly aggressive. They would be always ready to sacrifice themselves for the cause of their tribe, never violent towards one of their own but always hostile towards others. In fact, their feelings towards other creatures than members of their tribe could be as warm as what we feel towards spiders. So, this race would be like an advanced version of anthill. Or the Japanese in WWII.
It might be rather unpleasant emotionally if we could really prove there are von Neumann probes in our solar system. Of course there is ever-present fear towards something new - it's not pleasant to change a way we perceive the world. And many people would be nervous about motives of "intruders". Also, we wouldn't be the crown jewels of the nature anymore, not even in our own domain. It also might prove the modern scientific community could be fooled for decades (if there is an UFO sightings connection). And as an ultimate insult, they wouldn't even seem to bother to come to "shake hands" with us - almost like they were treating us like a children. However, I believe people would soon get used with the idea and there would be many positive psychological effects as well. (Addition 31. May -09: I might have been too optimistic about consequences, please read this and this post as well)
As far as I know this is the first time anyone proposes that there is an interstellar self-replicator arms race going on and UFO phenomena are a side product of it - correct me if I'm wrong. The idea of interstellar self-replicator arms race, or "police probe" scenario isn't entirely novel: as far as I know, it was introduced first time last year (2008) by futurologist Anders Sandberg on this posting.
After reading Mr Sandberg's posting I felt rather depressed, since it felt like all that was original in my idea was took away from me before I had a chance to publish it. But then I understood something crucial was missing: it seems Mr Sandberg have never tried to connect his insight to the UFO phenomena. So as I felt there might be still something original and valuable in my ideas, I rushed a half-ready website to the Internet. And there it still stands:
There are lots of misspelled and missing words and sentences, sometimes even paragraphs. The looks of the site is hideous and there are cheap ads placed everywhere by the hosting service (I'd like to point out I don’t earn a single penny from those ads, everything goes to host). However, the text should be somewhat readable and understandable. I should have fixed it all but after initial upload never found time or energy.
So, I'm very happy if someone made it all the way here. I would like to very much hear your comments. But one wish if you may: even though this is very controversial subject let's try to keep the discussion boringly calm, shall we?
Insults, arrogant remarks and ad hominem arguments can't prove any idea or hypothesis wrong. My opinion is that they usually just indicate one is afraid their world view is somehow threatened. But I'm glad to notice the tone of discussions on this forum seems to be mostly polite and civilized.