Here is the Wikipedia article. But basically it is a response to Fermi’s Paradox that says there are advanced ET’s out there, they just purposely choose not to interact with us for ethical reasons.
My question is, is it mainstream? And could it be true?
I know that I have said before, that some scientists are just too critical, when it comes to ET theories. Roswell and Area 51 are likely nonsense. But Ancient Astronaut Theory has some validity, IMHO at least.
Thoughts? 
WEll,
Ancient Astronaut Theory has no validity at all, and
The Zoo Hypothesis, or what one might call the Alien Prime Directive, is theoretically possible, of course. It is rather exceedingly unlikely, like “close enough to zero that it may as well be.”
“Explanations of the Fermi Paradox” is a field in which the term “mainstream” doesn’t have any real meaning. Sure, take any professional in a related field, and most of us will have our own preferred guess as to the explanation. But we also know that that’s all they are: Just guesses. For something to be considered a mainstream view among scientists, it would need to be a scientific view, and we don’t have enough data to form one of those.
To reiterate what @RickJay said, Ancient Astronaut Theory is bogus, racist, and not even slightly scientific. We’ve gone over this in your previous threads and you don’t seem to have learned from them.
The mainstream approach to aliens having landed on Earth is “it cannot absolutely be ruled out but we have zero verifiable evidence for that.”
The mainstream approach to aliens existing elsewhere in the universe is “that is likely but we have zero verifiable evidence for that.”
And the approach to “If there are intelligent aliens in our universe, why haven’t they knocked on our door or sent us a telegram” is “yeesh, folks, remind me how many millennia we’ve had the technology to notice if they had? and do you realize how freaking big our back yard is, seriously?”
If the hypothesis is true, aliens must have self-control far, far, far beyond us humans.
We humans can’t resist poking animals with sticks or throwing objects into zoo enclosures to entertain ourselves out of curiosity. How could alien kids see us on Earth and not hurl a couple of asteroids into London or Tokyo just to see those tiny humans scramble like ants out of an anthill?
Essentially the same question was asked, by the same OP, in the thread Is ‘Zoo Hypothesis’ Plausible? and Fermi’s Paradox and the Prime Directive.
Perhaps others, this was just a quick search.
So, I am going to repeat myself here: based on our knowledge of the universe right now, the zoo hypothesis is physically possible.
However, it requires every individual, of every advanced species, to all agree to one particular philosophy for millions of years. Or, one single dominant species with the power to prevent any other species from making contact. Which is all physically possible too, but when a hypothesis needs a number of quite fantastic suppositions to hold up, it gives us increased reason to doubt the hypothesis.
I mean, no particular theory on extraterrestrials should have adherents at this moment, since we just don’t know, and we should be guided by empirical data. But if we’re going to WAG we should WAG towards hypotheses that need the fewest extra suppositions.
I work closely with astronomers, and from what I can tell it is mostly accepted that there is life on other planets.
I used to not take SETI seriously, but many serious astronomers do.
That said, apart from SETI, I haven’t heard any serious considerations of intelligent extraterrestrial life by my astronomer colleges. I suspect because it isn’t something you can study scientifically.
So, my observations with my astronomer colleges is that the Zoo hypothesis isn’t even thought about, nevertheless mainstream.
Jim_B, throw out your books on alien astronauts and quit reading websites about them. Quit wasting your time with this and find a new hobby. Any theory you read about aliens and their relationships with the Earth are nothing except utter guesses that there’s no evidence either for or against.
If you’re interested in astronomy, read the news stories every few weeks that actually come from astronomers. There is a lot going on about exoplanets. A few decades ago we knew nothing about what the planets of other solar systems were like. Now we discover a few dozen new exoplanets every year. We know about more than four thousand of them at the moment. We learn new things about what kinds of stars and galaxies are possible and how this affects our basic theories of physics. This is actually science.
If aliens are so powerful that they can manipulate what we can learn about the universe, they are too powerful for us to ever learn of their existence. The people who wrote the books about alien astronauts were not doing science. They were just trying to make some money selling books that they wrote. If they couldn’t get a book published, they publicized their theories by whatever methods they could so they could feel important. Nothing they did can be called science.
I’m curious. How is trying to find and confirm the truth to a very high standard “…too critical…”?
It isn’t unless you define it as I believe in aliens on earth and they don’t, therefore they are being too critical of the scanty, unverifiable, contradictory, illusory, falsified, and misread and misunderstood evidence that convinced me.
The revolution that was the scientific method was that a scientist should be their own biggest critic, that they should endeavour to prove themselves wrong.
So, it is not wrong to say that scientists are too critical to entertain a hypothesis with no evidence for it, and boatload of evidence against. But that’s a feature, not a bug.
As to the question of the OP, I would say that among those who believe that there is extraterrestrial life that has extended into interstellar space, the zoo hypothesis is a relatively popular notion. However, I do not thing that there being extraterrestrial life that has extended into interstellar space is mainstream in and of itself.
Is there life on other planets? Probably. Somewhere.
Is that life sufficiently advanced to be able to cross vast interstellar distances within their lifetimes? Highly unlikely.
Even merely being developed enough to communicate with us would still require them to be quite close on a cosmic scale and to know we were here, both of which lower the odds of it happening by quite a lot.
To quote Douglas Adams: “Space is big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to space.” There are galaxies billions of lightyears away. Science fiction may posit wormhole travel and warp drives and other means of crossing the stars in no time at all, but there’s no reason to believe any of those things are actually practicable.
To quote him again, here’s what he had to say about the population of the universe:
Love Douglas Adams, but this entry into the Guide isn’t correct. It is possible that there are only a finite number of populated worlds, but that does not follow from the statement. If half are populated, or a tenth, or a millionth, or even a quadrillionth, then there are still an infinite number of populated worlds.
And six times nine doesn’t equal forty-two either.
Nor do five books form a trilogy.
I always thought there was something fundamentally wrong with the universe.
However, in terms of why we haven’t seen evidence of ETs yet, it is not necessary to postulate FTL travel.
I’m not sure why “developed enough” is the clause in this sentence, but regardless it’s not true.
Radio as we know it tends to weaken over distance, but simply the fact that we can see objects billions of light years away implies that it is theoretically possible to send signals over huge distances. And the fact that a species’ signal from the other end of the galaxy would take 100,000 years to reach us is irrelevant when compared against the age of the galaxy.
It does in base 13.