It’s based on an incident in 1950 when Italian physicist Enrico Fermi (1901–1954) asked colleagues at the Los Alamos National Laboratory a question: If space aliens are a sure thing, “Where is everybody?” As Seth Shostak puts it at Search for Extraterrestrial Intelligence (SETI),
The remark came while Fermi was discussing with his mealtime mates the possibility that many sophisticated societies populate the Galaxy. They thought it reasonable to assume that we have a lot of cosmic company. But somewhere between one sentence and the next, Fermi’s supple brain realized that if this was true, it implied something profound. If there are really a lot of alien societies, then some of them might have spread out.
Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire Galaxy. Within ten million years, every star system could be brought under the wing of empire. Ten million years may sound long, but in fact it’s quite short compared with the age of the Galaxy, which is roughly ten thousand million years. Colonization of the Milky Way should be a quick exercise.Seth Shostak, “Our Galaxy Should Be Teeming With Civilizations, But Where Are They?” at SETI Institute
Indeed, where are they? A flurry of explanations has resulted. Here are just a few:
The Perimeter Institute’s Adrian Kent explicitly invokes Darwinian theory to account for the aliens’ absence: Natural selection, he argues, favors quiet aliens, due to competition on a cosmic scale for natural resources. Similarly, Smith dubs his pessimistic view the “misanthropic principle,” a play on the “Anthropic Principle,” meaning that because we are probably alone, we must solve our own problems. As long as he puts it that way, he is mostly safe from charges of being “anti-science.”
At New Scientist (2011), Lee Billings utters the question: “Two decades of searching have failed to turn up another planetary system like ours. Should we be worried?” The magazine editorialized an answer pronto:
“The fact that there are few plausible Earth lookalikes among the 1200-plus candidate planets identified by Kepler is not hard to explain away. Such small lumps of rock are more elusive than bigger gassy bodies, and once they have been glimpsed it will take time to verify their existence. More perplexing is the apparent lack of any other solar systems that have the familiar qualities of our own, which we believe to have given rise to life.”
More often we are just told that we lack imagination, we have searched too narrowly: Moonless planets have been unfairly dismissed and sunless ones could maybe ferry life around the galaxy. Some argue that hardy Earth life forms could have made it to one of Jupiter’s moons and survived there. Jupiter’s moon Europa looks promising to many. NASA has talked of a “flying-saucer-shaped space boat” to Saturn’s moon Titan, some day. And the excitable word about another Saturn moon is, “Enceladus Now Looks Wet, So It May Be ALIVE!”
More exotically, some seek life around failed or dying stars. If that doesn’t work, dark matter could make planets habitable (though we don’t yet know what dark matter is). And, should the laws of physics vary from place to place, life elsewhere might follow different laws. In that case, should the physics term “constant” be changed to “local variant”?
Lastly, encountering hard, doubting hearts, alien life proponents resort to moralizing: An editorial preaches “Uniqueness seems rather too presumptuous a claim for one small planet in an undistinguished corner of a vast cosmos.” Our vaunted respect for evidence is a mere cloak for pride and presumption!Denyse O’Leary, “How Do We Grapple with the Idea that ET Might Not Be Out There?” at Evolution News and Science Today
Williams has analyzed seven popular explanations (so far), starting with key background information on the Fermi Paradox:
- The Zoo Hypothesis: “aliens are keeping their distance to allow humans to evolve without interference.” (August 31, 2020) This, he says, is the theory behind Star Trek’s Prime Directive:
That explanation assumes that the aliens are not only more advanced than us but also godlike in wisdom and restraint.
6. The Planetarium: Hypothesis: “humanity is in a simulation, and the aliens are the ones running it! In order to ensure that human beings do not become aware of this fact, they ensure that the simulation presents us with a “Great Silence” whenever we look out and listen to the depths of space.” (August 27, 2020) Williams traces it back to a 2001 paper by mathematician and engineer Stephen Baxter.
Possibly the best-known advocate of this theory today is Elon Musk:
- The Berserker Hypothesis: “This theory suggests we haven’t heard from any alien civilizations because they’ve been wiped out by killer robots!” (August 22, 2020) In a 2008 essay, philosopher Nick Bostrom expressed the hope that we don’t find any aliens on that account:
A disconcerting hypothesis is that the Great Filter consists in some destructive tendency common to virtually all sufficiently advanced technological civilizations. Throughout history, great civilizations on Earth have imploded—the Roman Empire, the Mayan civilization that once flourished in Central America, and many others. However, the kind of societal collapse that merely delays the eventual emergence of a space colonizing civilization by a few hundred or a few thousand years would not help explain why no such civilization has visited us from another planet. A thousand years may seem a long time to an individual, but in this context it’s a sneeze. There are planets that are billions of years older than Earth. Any intelligent species on those planets would have had ample time to recover from repeated social or ecological collapses. Even if they failed a thousand times before they succeeded, they could still have arrived here hundreds of millions of years ago.Nick Bostrom, “Where Are They? Why I Hope the Search for Extraterrestrial Life Finds Nothing” at MIT Technology Review , May/June issue (2008): pp. 72 77 (open access)
He has argued (2009) that even finding life on Mars would be a disaster for humanity:
- The Aestivation Hypothesis: “aliens are not dead (or non-existent), they’re just resting!” (August 7, 2020) Just as many life forms on Earth go into torpor during hot and dry conditions, they are awaiting more favorable conditions before they emerge.
It makes the most sense for the aliens to have become artificial intelligences, say many theorists:
Indeed, an increasing number of futurists, astrobiologists, and SETI experts are starting to think that advanced intelligence eventually transitions into a digital mode of existence. Living as digital beings within powerful supercomputers, post-biological aliens (or future posthumans) will demand unhindered access to powerful and efficient means of information processing—a hypothetical mode of existence known as “dataism.”
But as Anders Sandberg, Stuart Armstrong, and Milan Ćirković argue in their new JBIS paper, there’s a cost to information processing, particularly when the computer performing those calculations is temperature dependent. As computer scientists and information physicists know, the potential for information processing increases as temperature decreases (energy is required to cool a blazingly fast computer, after all). So rather than squander energy and resources in the current era, Sandberg and company believe it makes more sense for an advanced, computer-based civilization to aestivate and wait until the Universe is much colder than it is today.George Dvorsky, “Hibernating Aliens Could Explain the Great Silence” at Gizmodo
The paper, “That is not dead which can eternal lie: the aestivation hypothesis for resolving Fermi’s paradox,” is open access.
3. The Rare Earth Hypothesis: “the possibility that life-bearing planets like Earth are just very rare.” (July 29, 2020) It takes its name from the 2000 book Rare Earth: Why Complex Life is Uncommon in the Universe
by paleontologist Peter Ward and astronomer Donald Brownlee.
Williams wonders if this is not the most probable hypothesis but then cites a number of objections, noting that “humanity knows of only one planet where life exists (Earth). Having only this one template severely limits us when it comes to looking for life, which could exist in a range of environments and chemical conditions.” It seems that the Rare Earth hypothesis is not so much improbable as unpopular with alien hunters:
2. The Great Filter Hypothesis: “there must be “Great Filter” that prevents life from reaching an advanced stage of development.” (July 23, 2020) Originally posed in a 1998 online essay by economist Robin D. Hanson, the hypothesis is more general than some of the ones above; he specifies a nine-step filter rather than any one specific barrier (like killer robots). His argument does not assume that intelligent aliens are directing the process:
No alien civilizations have substantially colonized our solar system or systems nearby. Thus among the billion trillion stars in our past universe, none has reached the level of technology and growth that we may soon reach. This one data point implies that a Great Filter stands between ordinary dead matter and advanced exploding lasting life. And the big question is: How far along this filter are we?
To support optimism regarding our future, we must find especially improbable past evolutionary steps. And in fact we can find a number of plausible candidates for groups of hard trial-and-error biological steps: life, complexity, sex, society, cradle and language. Presuming there are about nine hard steps total here, the Great Filter could be explained if the expected time for each of these steps averaged (logarithmically) to about thirty billion years, if only one percent of stars could support such steps, and if we have only about a one percent chance of not destroying ourselves soon (or permanently banning colonization).Robin D. Hanson, “The Great Filter – Are We Almost Past It?” at George Mason University
Because the ninth step is “colonization,” Hanson’s assumption is that aliens must have failed one of the nine steps:
- The right star system (including organics)
- Reproductive something (e.g. RNA)
- Simple (prokaryotic) single-cell life
- Complex (archaeatic & eukaryotic) single-cell life
- Sexual reproduction
- Multi-cell life
- Tool-using animals with big brains
- Where we are now
- Colonization explosion
Or did they? Maybe we are a simulation because they didn’t. 😉
- The Hart–Tipler Conjecture: “Extraterrestrial intelligent beings do not exist” (1980). If they did, within 300 million years, they would have developed advance technology and be here by now.
Williams is not very happy with Hart and Tipler’s argument: “Why is it important that Hart’s argument wasn’t really also formulated by the eminent Enrico Fermi? Because Fermi’s name lends a credibility to the argument that it might not deserve”:
Besides assuming that interstellar travel is feasible, Hart’s argument is based on very specific and highly speculative ideas about how extraterrestrials must behave. He assumed that they would pursue a policy of unlimited expansion, that they would expand quickly, and that once their colonies were established, they would last for millions or even billions of years. If any of his speculations about how extraterrestrials will act aren’t right, then his argument that they don’t exist fails.Matt Williams, “Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture” at Universe Today
But wait! Virtually all the hypotheses about space aliens are speculations. Astrobiology is, famously, a science without a subject.
All these speculations have their vigorous detractors, of course. But that doesn’t matter. The aliens meet so many emotional needs that they are perhaps best seen as invented gods. All we know for sure is, They never write, They never phone, and for merely practical purposes, They may as well not be Out There.
But one thing that is always In Here is human imagination. So we will probably always enjoy science fiction about aliens. Maybe in that strict sense, they are always Out There.
You may also enjoy: Tales of an invented god