Question posed by famous physicist Enrico Fermi, regarding the existence of extraterrestrial life. He asked his colleagues:

If there are so many advanced civilizations out there, where is everybody?

The implication of this paradox is that we are alone in this universe. However, the question whether there is life in space is inherently more complex than this, and thought must be given to the massive distances involved, development of space travel technology, and other factors.

Astronomer Frank Drake penned an equation that approximates the probability of existence of technologically advanced life in space and that we will find out about it anytime soon.

N = R* * fp * ne * fl * fi * fc * L

N = Number of intelligent civilizations in space
R* = Number of stars
fp = Fraction of stars with planets
ne = Probability of number of planets with habitable environments
fl = Fraction of planets where life has originated
fi = Chance of life evolving into a condition of intelligence
fc = Percentage of life that has technology to send out signals into space
L = The "longetivity" factor, from 0 to 1, gauging the lifespan of the average intelligent civilization

Due to the number of unknowns in this equation that we will probably not find out within our lifetimes, it is still a mere conjecture on the subject. Estimates on the number of intelligent civilizations in the universe range from over the millions to zero.

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Simply put:

If there are other intelligent beings in the universe, then why aren't they here?

Though the scales of distance in the universe are large, the lengths of time necessary for traversing it is not as large as the time used during evolution. And because the amount of time from where civilization is developed to the point where interstellar travel and von Neumann probes seem reachable are significantly smaller, it seems likely that when a civilization reaches this point they will quickly spread around the universe.

Thus, if there are some out there, then why haven't they spread to here, or at least close enough to be detected? In some ways, it implies the lack of these other civilizations.

There have been various attempts to explain this apparent problem. Some suggest that we've been set aside, like a preserve, to achieve technology on our own. Others suggest that they transcend much before they really spread out into space. Or that they're actually here, but hidden on the nanoscale, or in some other manner.

Also see: Fermi's Paradox, Drake Equation, the great filter

Please Note: Yes, there are assumptions here, and possible explanations, but this is a short descriptive writeup, not a long paper to discuss it in detail. A search on the web for the Fermi Paradox will turn up many articles about it which deal with many of the things I have left out.

If there are other intelligent beings in the universe, then why aren't they here?

First of all, I have to question the use of the word "other". Are you suggesting that there are intelligent beings on Earth? I assume you're referring to humans.

That having been said, okay, pretend there are other "intelligent" beings out there, same intelligence as humans. Why haven't they come here? Why haven't we gone there? We're the same intelligence as them, right?

The correct question would be "if there are much more intelligent beings in the universe, then why aren't they here?" The answer is because their first contact with us was an intercepted transmission of the Jerry Springer Show.

Intelligence, as we think of it, is a very narrow spectrum. Extraterrestrials would think completely differently from us (I am Spock had some insights from a SETI scientist on this). Math is not a social construct in that 1+1=2 &c, but life on other planets would think this in a completely different manner, completely undecypherable by humans.

While we're at it, what we consider life is a very narrow spectrum. In the Star Trek episode 'Devil in the Dark' they discovered Horta, which is a silicon based lifeform. Bones, of course, instantly argues with that analyses "That's impossible!" After all, we've never seen a lifeform like that, so it can't exist.

Or maybe they're hugely advanced but have never been enticed by that big void filled with little glowing white things. Maybe they're just interested in neurobiology, genetics, and philosophy. and mathematics.

Define intelligent. Define life.

Ohh... But look at all those assumptions you've gone and made!

1. Extraterrestrials are not now living among us.

2. The population density of Intelligent life is high enough that it is practical to commute.

3. Aliens can survive away from the planet's surface. (Low G isn't healthy for everyone).

4. Aliens are not xenophobic pacifists.

5. They find us interestering (But they're intelligent! Exactly...)

6. They reproduce fast enough to populate the stars.

7. They can see us. (Meat?!)

8. There is a spoon.

9. They would come at the right time and/or stay long enough to intersect with human civilization.

10. The life cycle of an intelligent species is long enough to leave a star system. (We're almost there... Cut it out with those nukes!)

If they were here, would we know?

If we were to detect, say, a century from now (to be pessimistic about the advance of our technology), another civilization which we were interested in visiting, I suspect that we would have many technologies at our disposal for doing so undetected. Nanotechnology is the most obvious.

Given this, I find it impossible to believe that any visitor to our civilization would be so clumsy as to attempt and fail to conceal themselves, as so often happens in popular accounts of extraterrestrials.

It is an unpleasant thought, but the possibility that we are being watched without our knowledge does not even require invocation of Clarke's Third Law any more. Of course, the probability of such surveillance is another matter, but set that aside for a moment.

A better argument against extraterrestrial intelligent life is that we haven't heard them yet...the "Why is the universe silent?" question. As the SETI folks argue, it makes more sense for us to listen. Nothing on the radio yet, though. Perhaps we should be theorizing about how our technology may soon advance to the point where radio waves will no longer escape our planet.

I had never heard of Fermi having any particular expertise in astronomy, so always wondered why he should have thought of this. But it has just occurred to me: he's there in Chicago in 1942 or whenever, he's about to make the world's very first atomic pile, they've worked out with pencil and paper that it's not going to blow up Illinois and set the atmosphere on fire, but nevertheless he's wondering to himself... "I wonder if this is why advanced civilizations don't last long enough to travel in space."

Enrico Fermi's paradox is swallowed easily as soon as you realize alien existence does not have to involve alien intelligence making actual contact or even detectable alien presence. Nevermind the fact that every radio pioneer before Marconi claimed they received intelligent transmissions: every paranoid schizophrenic knows those are stunts pulled by British MI who had that technology for centuries. Francis Bacon wasn't just making up Rosicrucian telepathy claims in the New Atlantis - he was making technological statements.

( And of course Egyptians knew electricity and the Assassins are the link in the chain that connected British Freemasons with electrical secrets through the Knights Templar. But of course! You clearly must have been the victim of communist indoctrination at school. Next thing I know you'll be telling me human civilization is less than 10,000 years old. )

As TekkonKinkreet and Saige put it, the paradox actually concerns advanced civilizations capabilities of piercing the veil of long distances searching for signals of life: electromagnetic, water, air, organic chemicals and so on. And they could easily do that undetected supposing they can either travel FTL or image FTL.

General Wesc raises an important point: really advanced civilizations would also search for the stuff of other kinds of intelligent life that we aren't aware of. And maybe there's even a way to know beforehand where it will eventually develop, you know, through time manipulation? That introduces yet another philosophical question in the trekkie tradition: if their models show possibility of negative entropy molecular machines self-assembling, do they meddle with it? Game it a little, perhaps? It's a far more interesting question than the moot point of the original paradox, due to the invisibility question.

Maybe they miss the heroic transmissions darkened by fiber optics. They could search and search and hardly find more intelligent television. I do hope they are helping our helpers through coincidence control.

As Karl Marx would have put it after reading Charles Fort and being visited by Men In Black: We are the cattle of our future selves. Oh yeah, that already happened and that old weasel is Robert Anton Wilson. Therefore, you will find intelligent life - Level 2 Civilizations - inside wrongly understood "black holes", "binary star systems" and the like. I doubt such advanced characters would waste their time giving you anal probes.

Definitely the worst part of this overhyped "paradox" is that it leaves room for deep black shady government syndicates deciding to stage a world government coup after major disaster strikes. You know what I'm talking about, the Watchmen scenario meets Rael and all of a sudden the peaceful wise beings from outer space show up with their UFOs everywhere to help bad bad stupid mankind. You can bet my guerrilla comrades will be testing if Riconosciuto's a-neutronic bomb can down UFOs. I bet it can.

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