The rationalist community is skeptical of UFOs.

Here's Eliezer Yudkowsky betting 150:1 odds against UFOs being aliens:

UFOs aren't aliens. You can approximately leave that possibility out of your thinking. I've studied some of what one needs to study, to know that a bit more surely. There's vast room above human intellect and human technology, and what that implies is this: if there are hidden aliens, they're successfully hiding. Like, actually successfully, without the whole UFOs thing.

Scott Alexandar agrees:

Eliezer bets $150,000 to $1,000 against UFOs being aliens, and gives the same argument I would - it’s unlikely that any civilization advanced enough to travel through space would still be primitive enough to use macroscopic, biologically-piloted craft that sometimes crash.

Robin Hanson on the prior probability of UFOs:

I estimate a chance of at least 10% for each of the following events, given the prior events:

1. Earth was seeded by panspermia in its nursery
2. A sibling star gave rise to a long-lived advanced civ long before now
3. That civ prevents itself from expanding, tries to prevent siblings from expanding, and long ago traveled to here to wait to enforce this preference,
4. They induce or allow UFO-style encounters while they wait here.

Four factors of 10% gives a minimum prior chance of 10^-4, but as most of the probability weight should above these minimums, I estimate the total chance to be at least 10^-4

There are 95-180 billion habitable planets in the Milky Way alone, and hundreds of billions of galaxies in the universe. The scientific consensus is that there should be extraterrestrial life, and even intelligent life seems probable. There is no longer contention about whether aliens exist, but whether they have visited here.

Even if we assume there are no ways past the speed limit of light, an advanced civilization could easily have identified Earth as a planet that could harbor life and have sent drones at some point in its 4B years of formation. That is more than enough time to travel across galaxies at even fractions of the speed of light, and is likely what we ourselves would do in the not-too-far future.

To believe that UFOs are real is not irrational, it should be the a priori belief of any Bayesian that other intelligences would already be aware of our existence and have had billions of years to travel here if they wanted to. Our current model of cosmology suggests that there should be so many countless other civilizations out there that we should see some trace of them or even direct communications from some by now. The silence of any kind of signal from anywhere in the universe is the mystery, and it's infamously known as the Fermi Paradox.

Maybe there hasn't been silence at all though, we just haven't been paying attention. It would elegantly resolve the paradox with the answer we expected all along. There is an extensive historical record of data and testimonies from intelligence officers, military pilots, and commercial pilots spanning decades and describing the same phenomenon with radar, infrared, and optical sensors. There are mass sightings with dozens to hundreds of witnesses and well documented events where multiple sensors corroborated UAP with the same phenomenal flight characteristics that witnesses described. There are accounts like these and others that can be found all over the world and go back as far as recorded history with uncanny similarities. Here are a few notable examples:

• The USS Nimitz Tic Tac (2004) - The Nimitz Tic Tac was witnessed visually by at least four pilots each with over a decade of experience. It was captured on optical camera, infrared camera, and observed by at least two operators on radar. All sources of data corroborate the same story of an aircraft with unprecedented flight characteristics. Insiders have leaked that intelligence agencies have a classified analysis of the Nimitz radar data that confirms multiple objects moving from 80,000 ft to sea level in an instant.

  Skeptics will say Mick West has debunked this incident, but Mick only evaluates the publicly available FLIR footage and gives no weight to the witness accounts, the radar data, or the classified analysis since he either doesn't trust or doesn't have access to the source. This is fine if your goal is to have the most rigorous standards, but it rejects the majority of the evidence in the case.

• The Michigan UFO Flap (1994) - Over 300 witnesses in 42 counties of western Michigan reported UFOs to radio hosts and 911 operators. Several police officers investigated and witnessed the same objects described in the reports. A radar operator for the National Weather Service also observed the UFOs and was able to rule out weather phenomenon and could corroborate the locations other witnesses had given. (There's an episode about this event in Netflix's Unsolved Mysteries series).

• The NORAD National Air Defense UFO Alert (1957)- Right after NORAD was first established, multiple UFOs were tracked coming off the east coast towards Nebraska with a course flying over several sensitive military bases. It was captured by multiple radar installations as it flew over New England, President Eisenhower and nearly every intelligence agency were alerted. From Christopher Mellon's post: "The UFOs were radar-tracked moving at varying speeds from about 800 to about 19,000 mph, and at varying altitudes from 50,000 to 135,000+ ft altitude."

There are many more examples just like these, a good source with dozens of equally credible reports can be found in Leslie Kean's book: UFOs: Generals, Pilots, and Government Officials Go on the Record.

Skeptics like to brush witness accounts off as misperceptions: just uneducated people mistaking mundane objects for the extraordinary. The sheer number of diverse, corroborating accounts alone should discredit this idea. To satisfy Occam's Razor of finding the simplest explanation for the UFO phenomenon, you would have to theorize (like Carl Jung) that an unknown mental pathology has infected thousands of unrelated people, who are decades apart, to share the same delusion of flying metallic saucers. We would have to believe that flying saucers are somehow embedded into the collective human psyche, but of course there's no evidence of this or anything like it in psychology.

Why Do UFOs Crash?

There are enduring mysteries though. Why do UFOs crash? Why do they perfectly avoid leaving any physical evidence while also flying brightly lit, conspicuous vehicles throughout the skies? Why have they not made direct contact and what interest would an advanced intelligence have in our relatively barbaric civilization in the first place?

These are valid questions, but they don't retract from the ET hypothesis. Eliezer and Scott Alexander often write about their fears of AI takeover and how difficult it is for us to imagine the motivations and behaviors of a superintelligent AI. Our AI would at least be designed and trained on data from human experiences though, we should expect an alien superintelligence to be even more inscrutable. What would be the behavior of superintelligent probes from a civilization of ant-like beings from Sirius with millions of years of technological advancement ahead of ours? We can't even predict how our own society will look in one hundred years. We should expect that any communication or actions from a truly alien civilization would be exceptionally strange, like bees trying to communicate with chimps through performing intricate dances on leaves.

It is expected that any encounter with a non-human intelligence might be beyond our imaginations, and there are many possibilities that could explain the strange behavior we see in UFOs. But this doesn’t discount from the hypothesis that we have already been visited by aliens. I would urge the rationalist community to take a deeper look at the case for the ET hypothesis and consider the entirety of the decades of evidence before drawing conclusions.