They argue that the vast distances between stars would make interstellar travel nearly impossible. These skeptics also believe that the many varying descriptions of UFOs and their occupants would imply that many alien groups are visiting the earth, which they consider very unlikely. They also argue that aliens would not be so secretive about their activities and would announce their presence in more obvious ways. Finally, skeptics point out that there is no undeniable evidence, such as a truly authentic photograph or metal from a UFO, that would prove their existence. The second theory states that UFOs are unusual natural phenomena.
Ball lightning is an example of a rare and incompletely understood phenomenon.
Opponents argue that the earthlight theory does not take into account all the data. They do not think that geological stress can create a plasma with the size, shape, and duration of reported UFOs. They also question whether an electromagnetically-induced hallucination could create the consistent type of memories reported by abductees. The third theory proposes that UFOs are the result of psychological or sociological factors. Many scientists, particularly those who are skeptical of the existence of UFOs, argue that all sightings are really misperceptions of natural phenomena or conventional aircraft.