Intelligent Extraterrestrials either do not exist, or they are all far, far away; and, if they do exist, they have never visited Earth because interstellar travel, let alone intergalactic travel, is not possible.
The Drake Equation
Frank Drake, in 1961, published his now famous equation for determining the 'likely' number of intelligent extraterrestrial societies, IE, alien civilizations, in the Universe:
(From 'Drake Equation – Wikipedia':)
The Drake equation states that:
- N = the number of civilizations in our galaxy with which communication might be possible;
- R* = the average rate of star formation per year in our galaxy
- fp = the fraction of those stars that have planets
- ne = the average number of planets that can potentially support life per star that has planets
- fℓ = the fraction of the above that actually go on to develop life at some point
- fi = the fraction of the above that actually go on to develop intelligent life
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L = the length of time for which such civilizations release detectable signals into space.
The fundamental problem with the Drake equation is that anyone can plug in any set of assumptions they wish in order to come up with their preconceived number! For a couple of Drake's factors, a reasonable estimate might be made, particularly as time passes and there are more and more scientific data. This category might include R* and fp ...and then the problems begin.
First, let's consider an analogy: say some alien somehow had data on precipitation on Earth for precisely one afternoon at precisely one location, a backyard somewhere. Now suppose this alien proceeds to estimate precipitation patterns for the entire planet. Right. Well, that is about where we are for estimating all the other factors in Drake's equation. We're trying to come up with estimates that are applicable throughout the Universe, based on precisely one data point ...our experience on Earth.
Still, let's see what happens if we come up with estimates for the remaining factors in the Drake Equation:
- Ne -- Number of potentially inhabitable planets per star? It might be one in ten (Earth), or it might be one in a million.
- Fℓ -- Fraction that develop life? In might be one out of one (Earth), or it might be one in a million.
- Fi – Fraction that develop intelligent life? It might be one out of one (Earth), or it might be one in a million.
- Fc – Fraction that develop high technology? It might be one out of one (Earth), or it might be one in a million.
- L – Length of time that the high-tech civilization lasts? It might be 100 years (Earth), or it might be a million years. Or a billion years.
- Once we have visited, and gathered data on, a hundred (a thousand?) other potentially habitable planets, we might be then able to use the Drake Equation to make a rough estimate. But as long as we have one data point, the equation is useless. In fact, it is worse than useless because it appears to be a useful scientific-based equation.
- But maybe we can come up with another estimate of the number of nearby alien civilizations by considering the evidence we have for even one such civilization.
EVIDENCE THAT ALIENS HAVE VISITED EARTH
There is absolutely no evidence of an Earth visit, even though, by many estimates, such beings should have existed somewhere in our galactic neighbourhood for the past couple billion years. Many intelligent-life-elsewhere enthusiasts believe that the evolution of life, and intelligence, and high technolgy is common, almost inevitable ...in which case hundreds, or even thousands, of intelligent species should have existed in our neighbourhood during many periods of time over the past couple billion years. Furthermore, some of these species would have been far more intelligent, and far more scientifically- and technologically developed than we are. And some of these societies would have had millions of years to develop their sciences & technologies.
During that time, many inhabited planets and star/planet systems would have been faced with certain extinction. The beings on some planets would have found out that an asteroid or comet too big, or too close to divert was going to collide with their planet, annihilating them. Other beings would have experienced the consequences of their star entering its red-giant phase, which would have meant that any beings on its planet would have faced certain extinction.
Surely these threats to their very existence would have caused these civilizations to use every scrap of intelligence and technology to escape their doomed planets or planetary systems. Had they been able to develop inter-stellar travel, surely in their search for other inhabitable planets, at least one intelligent species from somewhere in our neighbourhood would have discovered Earth! After all, it would have been child's play for them to use their incredibly advanced technologies to scan their neighbourhood to discover nearby inhabitable planets within, perhaps, thousands of light years. Remember, it would have been a life-or-death struggle for them when it became evident to them that their own planet was doomed, so they would have poured all their resources into finding 'a new home' for their species.
With our technology--that is only tens of years developed!--we are already on the verge of being able to discover habitable planets within, say, 100 light years, by studying the spectrum of planetary atmospheres, looking for tell-tale emissions that identify the presence of oxygen, water, and organic molecules. We are also already on the verge of being able to detect powerful radio, TV, microwave, and laser signals, as well as energy bursts from something like atom or hydrogen bombs. Such signals emanating from Earth--and therefore indicating the presence of intelligent (?) life here--have already travelled 100 light years (radio), or 80 light years (TV), or 65 light years (A-bomb), or 60 light years (H-bomb). So highly intelligent societies out there some where should already be quite aware of our existence, just from our technological activities! Just imagine how long they would have known that Earth existed, and was inhabitable, through the use of their million-year-old technologies!
So why have we not heard from them? Any advanced civilization within, say, 1000 light years of Earth should have been keeping an eye (and an ear) on us for millions of years, ever since they determined that Earth was habitable. Surely their monitoring systems should have raised a flag by now, particularly if they are within 100 light years, and most particularly if they are within 50 light years of us. So why haven't they sent a tightly focused beacon, welcoming us to the neighbourhood?
And why have not doomed aliens visited Earth long, long ago--in the process of broad surveys, to confirm habitability (for them)? And if many, many intelligent species exist elsewhere, why has Earth not been added to their data bases of confirmed, inhabitable planets? And why have we not found any high-high-tech stuff that survey parties (or pioneers!) might have left behind? I know the Earth is a big place, but surely at some time somebody, somewhere, would have found at least one item—even a scrap of something--that could be identified with certainty as being of extraterrestrial origin.
I think it's because, however dire their circumstances were, no alien civilization, however long-lived, and however advanced in technology, has been able to make the trip. Either that, or there are no alien civilizations out there; or, at least none within thousands of light years.
- Intelligent life evolves only incredibly rarely, such that the nearest such society is thousands, or millions, of light years away; or,
- Interstellar travel has not been developed even by beings with the highest conceivable intelligence, and the highest-conceivably-developed technologies, with the greatest-conceivable need, that is, survival of the species when faced with certain planetary doom; or,
- Both of the above.
My guess: Both of the above.