Studying the UFO phenomenon, a number of ufologists have come to the conclusion that these objects are not related to aliens from outer space, but represent a purely terrestrial phenomenon that people have observed throughout the history of their civilization. One such hypothesis is that UFOs are real time machines, and it is on them that our distant descendants regularly visit the past for tourist and research purposes …
Gigantic distances even to the nearest stars, where there may be planets with intelligent inhabitants - this is the main argument of skeptics who believe that UFOs are not alien spaceships at all (people often observe them). After all, interstellar travel, even at the speed of light, will take tens and hundreds of years.
Once Carl Sagan, an outstanding American astronomer who did not particularly trust theories about the extraterrestrial origin of UFOs, calculated that a civilization that wants to visit its brothers in mind once a year in Space should send approx. 10 thousand ships … If the Earth is a place of regular and planned expeditions, then we must admit a certain uniqueness of our green planet in the vast Universe, which contradicts the thesis about the multiplicity of inhabited worlds.