Eliezer Yudkowsky on Why UFOs are not Aliens

I agree with this completely:

I've bet $150,000 to $1,000 against past UFO sightings being revealed within 5 years to have a worldview-shattering origin.

My model of the world confidently says no to alien UFOs: Their technology would not be such that, having arrived here across interstellar distances and then remained hidden, they'd need to fly around in large visible vehicles. It is definitely the case, given the physics we already know, that the aliens can do whatever surveillance they want using far tinier devices; eg, covalent-bond-strong, micron-sized robots, like bacteria but not with proteins held together by static cling.

Superintelligence is possible - it is just flat wrong that a human is as smart as any physical system can get - and you'd expect something crossing interstellar distances to be long since superintelligent. If they wanted to stay hidden, they'd stay hidden successfully. If aliens wanted to help Earth and not hide, humans would not be dying of cancer. If aliens didn't want to help, nor to hide, the aliens would have harvested the Solar System for matter and energy.

I have enormously wide uncertainty about the distribution of true alien psychologies, or the spacefaring agencies that grow out of them. But it's uncertainty over a metric where--when we look back down at Earth and what those psychologies would mean to us--the supervast majority of probable alien intellects, would not come here across interstellar distances, quietly and hiding on arrival, and then occasionally fly around in giant visible vehicles.

I have enormously wide uncertainty over the possible range of alien technologies. But I can use current knowledge of physics and chemistry, and the advance analyses that others have done of what technological possibilities those imply, to put a lower bound under alien technology that's comfortably above "needs to use giant flying vehicles for travel or surveillance".

UFOs aren't aliens. You can approximately leave that possibility out of your thinking.

He doesn't even mention that current UFO theories require that these alleged interstellar spacecraft keep crashing.

Just because something is unexplained doesn't mean that the actual explanation is your sci-fi fantasy.