One of the greatest conundrums to face humanity is the question of extraterrestrial life. Many explanations have been posited over the years to explain why we have yet to make first contact, some better than others. Here are seven of the weakest solutions to the vexing problem known as the Fermi Paradox.

For those of you new to Great Silence, I recommend this series of articles I wrote a few years back: "The Fermi Paradox: Back With A Vengeance" parts one, two, and three. This list is helpful, too.

1) Aliens Haven't Had Enough Time to Reach Earth

Actually, they have. This is one of the most common excuses for the Great Silence, but it's also one of the weakest. As a solution, it belies a complete misunderstanding of the paradox's central assertion — that time is not a factor.

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Fermi's paradox is driven by three rather powerful premises: 1) our galaxy has been around for a very, very long time — about 13.2 billion years (or 13,200 million years), 2) intelligent life was capable of emerging when the galaxy was still very young, and thus, 3) aliens have had plenty of time to visit, colonize, and reshape every portion of the Milky Way. Yet it doesn't appear that any civilization has done so.

Tiago Campante/Peter Devine.

As time passes, and as our view of the cosmos comes clearer into focus, the Fermi Paradox as a problem is getting worse and not better. Earlier this year, astronomers came to the realization that terrestrial, Earth-like planets began to emerge as long as 11.2 billion years ago, a mere two billion years after the Milky Way formed. At the same time, we're also learning that the quantity of potentially habitable planets — both in the past and in the present — is exceptionally massive. A recent calculation predicts hundreds of billions of Earth-like planets in our galaxy alone.

2) The Stars Are Too Far Apart

As Douglas Adams once said: "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's, but that's just peanuts to space."

Interstellar (2014).

Yup, space is big, no doubt. But it is this really the reason we haven't seen any signs of extraterrestrial life? For this solution to work, it would mean that all alien civilizations would have to find space too big to bother with, or that interstellar space travel is technologically unfeasible. Neither is likely true. Take a look at us. We've already got an interstellar space traveler in the form of the Voyager probe, and there's no shortage of visions as to how we might embark on such missions.

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And indeed, the first generation of interstellar space explorers will likely come in the form of self-replicating von Neumann probes. This would involve an exponentially expanding bubble of probes that could, in theory, colonize the galaxy in as little as 10 million years. Cosmologically speaking, that's not a lot of time. A single civilization could have colonized the galaxy 1,120 times over the course of the past 11.2 billion years — and that's just one civilization.

Alternately (or in conjunction), there's also the potential to "transmit" streams of uploaded minds at the speed of light from star system to star system. As Charles Stross once said, starships are a myth.

So no, the stars are not too far apart — not when you have billions of years to work with and the power of exponential growth at your disposal.

3) Aliens Are Honoring the Prime Directive

Back in 1972, astronomer John Ball came up with the Zoo Hypothesis, a proposed scenario in which advanced extraterrestrials deliberately resist making contact with humans in favor of watching us from a safe distance. As Ball noted, "they have just set us aside as part of a wilderness area or zoo." Reasons for doing so include the fear of biological and/or sociological cross-contamination, i.e. a "Prime Directive," or simply for studying us for scientific and/or entertainment purposes.

ST:TNG: "Pen Pals." Captain Picard violated the Prime Directive on at least nine occasions.

There are a number of problems with this solution. First, it's completely untestable and even a bit conspiratorial. Second, it's anthropocentric; the Zoo Scenario assumes that all aliens have the same motivations we do in terms of ethics, scientific inquiry, and recreation. It's also arrogant. What could we possibly offer alien intelligences in terms of our science? An extraterrestrial civilization at this stage in its development would very likely be post-biological in nature and in the possession of artificial superintelligence (actually, it probably would be an artificial superintelligence). To them, we'd be as interesting as microbes.

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It's also worth pointing out that the Prime Directive is ethically dubious. It would be incumbent upon a post-Singularity, space-faring civilization to intervene in our affairs to save us from ourselves, eliminate suffering, and uplift us into the broader galactic community.

Lastly, this theory (and many of the other solutions on this list) violates non-exclusivity. While some aliens might put us in a zoo and/or honor the Prime Directive, it cannot speak to all alien civilizations. All it would take is just one to ruin the show.

4) God Created the Universe Just For Us

This religious argument works to resolve the Fermi Paradox by suggesting that human beings were deliberately created by God to be alone. This idea goes back to Aristotle and Thomas Aquinas, who both argued that humans are unique to God's plan.

It's an idea that's still around today, as witnessed by Pat Robertson's recent rant in which he stated that NASA should stop its efforts to explore the cosmos because it contains "nothing but gaseous balls and barren rocks," adding that "This planet [Earth] is where God has got an experiment in what he wants to have accomplished."

But as noted by Carl Sagan, "The universe is a pretty big place. If it's just us, seems like an awful waste of space." Indeed, if God created the Universe just for us, why such elaborate digs? Why would there be trillions upon trillions of stars in the cosmos and not just one? What's more, why create such a seemingly biophilic Universe, one that appears to be cosmologically consistent throughout? Simply put, it's an untestable argument that defies both logic and common sense.

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It is possible, of course that we are alone in the Universe, but if that's the case, there are much better explanations than invoking God.

5) Aliens Were Here, But Then They Left

Some people argue that aliens visited our Solar System in the past and moved on and/or left traces of their existence, such as the Face on Mars, or the pyramids and other archaeological remnants. Of course, the Face on Mars has been debunked as being extraterrestrial in origin, and we have perfectly good explanations for how the pyramids were built. Explanations that attempt to bring aliens into the discuss are often sensationalistic, completely without foundation, and in violation of Occam's Razor.

The Fifth Element (1997).

Others contend that aliens visited Earth long ago, but then moved on without a trace. This is unlikely for several reasons. First, an advancing wave of extraterrestrial intelligence, likely in machine form, would reconstitute practically everything in its wake, converting matter from inert substances into something more useful, such as computronium. Alternatively, they could — and probably would — have uplifted and integrated any life into their civilization. What's more, given the age of the Galaxy and its apparent biophilic constitution, our Solar System could have been visited time and time again by a plethora of different civilizations. Yet according to this proposed solution, not one of them stayed behind or left any kind of discernible trace. Highly unlikely.

6) They're Actually Here Right Now!

So-called UFOlogists would have us believe that aliens are here right now, tormenting us puny humans with their exceedingly primitive flying saucers, strangely anthropocentric physical forms, and a rash of abductions. In a word, no.

The late Carl Sagan said these claims are almost entirely anecdote:

Someone says something happened to them, and people can say anything. The fact that someone says something doesn't mean it's true. Doesn't mean they're lying, but it doesn't mean it's true.

To be taken seriously, you need physical evidence that can be examined at leisure by skeptical scientists: a scraping of the whole ship, and the discovery that it contains isotopic ratios that aren't present on Earth, chemical elements from the so-called island of stability, very heavy elements that don't exist on Earth. Or material of absolutely bizarre properties of many sorts—electrical conductivity or ductility. There are many things like that that would instantly give serious credence to an account.

But there's no scrapings, no interior photographs, no filched page from the captain's log book. All there are are stories. There are instances of disturbed soil, but I can disturb soil with a shovel. There are instances of people claiming to flash lights at UFOs and the UFOs flash back. But, pilots of airplanes can also flash back, especially if they think it would be a good joke to play on the UFO enthusiast. So, that does not constitute good evidence.

Indeed, the rash of crop circles and apparent accounts of flying saucers have not stood the test of time, and are a projection of contemporary notions of technological wonder.

7) We Haven't Been Searching for Aliens Long Enough

It's true that we only started the search for ETIs in earnest with the advent of Frank Drake's Project Ozma in the 1960s followed by the subsequent rise of more formal SETI radio searches. We've only been scanning the heavens for 55 years, and there's still plenty of territory to cover.

NRAO/AUI.

There's the possibility, of course, that no one is broadcasting. We ourselves are currently debating the pros and cons of Active SETI — the deliberate attempt to get ET's attention. It's quite possible that all aliens are xenophobic and radio silent. If that's the case, then SETI is pretty much doomed to failure.

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But if aliens are truly interested in getting our attention, they should have no difficulty doing so. Most logically, they could pepper the Milky Way with Bracewell probes — communication beacons that are parked in every star system just waiting for the first sign of intelligent life to emerge (such as a radio signal). The Bracewell probe would in turn communicate with us, while transmitting a confirmation signal back home via a series of communication way-stations, likely other Bracewell probes. Alternately, aliens could broadcast an exceptionally strong and directed radio or optical signal that couldn't possibly be missed, even by a seemingly primitive technological civilization like our own.

That said, this only applies to civilizations far more advanced than ours. SETI should continue to search for civilizations that are similar to our own in terms of their technological development. Given the Fermi Paradox, this may be as far as any civilization gets.

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