Who are the aliens?
'Who are the overlords of the UFO?' asked an infamous and much mocked 1970s television documentary. But the question is a valid one. If there are UFOs someone or something must make them. And since these UFOs exhibit flight characteristics beyond what we have developed here on Earth, whoever built them must have more advanced science than us.
In the 1950s 'alien contactees' always reported that visitors came from Venus or Mars. We now know that both of those planets are uninhabitable for lifeforms with Earth biology. Since they appear to be completely lifeless, except for the possibility of microbes beneath the Martian surface, we can assume that there are no alternative biologies which can live on them either.
The term 'aliens' is is in common use nowadays and we must be clear that by 'aliens' we mean the entities who appear to emerge from UFOs and are glimpsed by those who observe the UFOs, usually when the craft is landed on the ground. There are also people who have waved to entities in UFOs and sometimes the entities even waved back.
We cannot be sure that entities are actually biological at all. Since visitors would by definition be more advanced than we humans, and our own robotics is advancing fast, then the makers of UFOs might be so advanced that they can build entities which look and behave like living beings. These machines might be so advanced that they are semi-biological, and able to reason and make decisions. They may be made to look familiar to us so as not to frighten us. We will use the term entities so as to make this clear.
These entities are usually humanoid with a head, two legs, two arms and two eyes. Of course that could be convergent evolution - sharks, plesiosaurs and dolphins all have similarities in their configuration because of the laws of fluid dynamics. So evolution on a cosmic level could make all beings similar. To advance and thrive advanced beings would surely need hands to make and build things, eyes to see things and legs to walk about.
There are basically 3 theories about who or what the entities could be and we will now explore them.
Extra-terrestrial entities
The E.T. theory has been the dominant theory in public opinion and as far as is known in government circles too. It is the obvious theory and one that goes back to when primitive 19th century telescopes first saw 'canals' on Mars. The popular imagination then began to theorise about Martian beings and their civilisation. Space probes soon destroyed the Mars canals theory, but the space programmes of the late twentieth century also opened our eyes to the possibilities of space travel, and that if there are other beings out there then they might travel too for the same reasons as us.
A problem with the E.T. theory is the vast distances of space. The nearest star to Earth is Alpha Centauri at 4.367 light years away. That is 25,670,000,000 miles. Or 25.67 trillion miles in shorthand. NASA has said that its typical space craft travelling at 17,500 mph or 28,000 kmph would take more than 148,000 years to get there. That journey is not possible for human beings, period. E.T.s would of course have found a way round this.
There are people who claim to have had access to government UFO study programmes. They claim that governments know that the E.T. visitors have technology which can turn off gravity, enabling travel faster than light. Einstein and physicists after him famously proved that matter cannot ordinarily travel faster than light at 186,000 miles per second. But if gravity can somehow be disabled a new physics of space travel is possible.
Time travelling entities
This theory suggests that the entities are time travellers. Perhaps they are even human beings from the future. The grey spindly slight bodies often described by witnesses could be our future emaciated selves? Our bodies smaller and thinner and our heads larger? We do not have sufficient data to be able to assess this theory further.
Extra-dimensional entities
When the Soviet government asked its scientists to try and explain UFOs the scientists theorised that the craft and their crews might be visiting us from other dimensions. Some Soviet scientists thought this a more likely explanation that the E.T. theory because of the vast distances in space between stars and solar systems.