In recent months we’ve been looking at science writer Matt Williams’s coverage of the many reasons (links below) that have been advanced as to why we do not see extraterrestrials except at the movies. Last Saturday, we considered the Percolation Hypothesis, whose beauty is its common-sense simplicity: The aliens can’t overcome the laws of physics, any more than we can. In the real world, barriers like years between communications even at the speed of light would take a toll on adventurousness.
The thesis has had a busy life in science media. Its earliest form originates with astronomer Michael Hart in “An Explanation for the Absence of Extraterrestrials on Earth” (1975) and Frank Tipler in “Extraterrestrial Intelligent Beings do not Exist (1980). Both papers are open access. Thus, the thesis originated as the Hart–Tipler conjecture, to which Carl Sagan and William I. Newman wrote a rebuttal in 1983, “The Solipsist Approach to Extraterrestrial Intelligence,” also open access.
But the current version of the hypothesis takes its name from iconic science fiction writer Kim Stanley Robinson’s Aurora (2015), an account of colonists on a distant moon who begin to die mysteriously from the ravages of a primitive prion, leading to conflicts among the settlers. It was first so named in a 2019 paper by, among others, Adam Frank and Caleb Scarf (open access).
Now here’s the key point from their paper, as set out by Williams:
After factoring all of this into a series of simulations, they reached several conclusions. First, they concluded that the time it would take for an exo-civilization to settle the galaxy is less than (or comparable to) the present age of the Milky Way (13.5 billion years). However, when one factors the “Aurora Effect” into the equation, it creates a scenario where only certain parts of the galaxy are settled.
Adding to that the notion that the lifetime of civilizations is finite, then it appears that certain clusters of the galaxy are destined to be settled and resettled while the surrounding areas will be unsettled. Last of all, if Earth is in a region of the galaxy that doesn’t correspond to a “resettlement cluster,” it’s entirely probable that we would not have been settled or visited for a long stretch of time – as much as 1 million years.Matt Williams, “What is the Aurora Hypothesis” at Universe Today
But, if that’s the view, it is very unlikely that intelligent extraterrestrials would have visited Earth at any time that humans were in a position to recognize the significance of their visit — at least in the way that we would understand it today. Williams goes on to discuss the possibility that myths and legends might capture evidence of such a visit.
One of the real advantages of the Aurora Hypothesis, along with the Percolation Hypothesis, is that they honestly confront the difficulties aliens would face, instead of just assuming that they have developed new technologies to overcome them all.
Have they indeed? Is faster-than-light (FTL) travel even possible? There are legitimate, physics-based doubts about that, in which case, it would take a very long time to go anywhere outside a given solar system.
We all want to hear the story starting from the point when the aliens make contact (or we make contact with them). Fewer of us follow the boring stuff like whether the Perseverance rover will find remnants of fossil bacteria on Mars.
So, never mind ET, this is where we are now: It took centuries of science to get to Perseverance today — via which we are systematically looking for remnants of fossil bacteria on the nearest planet to us. The remains of dead bacteria, not evidence of intelligent aliens.
Science happens faster now than it did centuries ago, it’s true. But things like the speed of light aren’t changing to suit us. Why should it be any different for the extraterrestrial intelligences? Their exploration chances might be worse than ours if their nearest planet isn’t a comparatively stable place like Mars.
One problem we face might be confusion between what we reasonably believe ought to be true and the constraints we face in taking our assumptions any further. We reasonably believe that in a universe so large there must be other intelligent life forms. The difficulty of getting any information about them or getting in touch with them is a separate issue that is easily confused with the first one.
Possibly that’s because, here on Earth, if we know that people exist somewhere, we can usually manage to get in touch with them somehow. If we know that they once existed, we can excavate their civilization. But Earth is not the universe so many of the assumptions of science fiction are best pursued as an art form, not as a prediction of the future. On the other hand, good art is worthwhile in itself.
You may also enjoy these accounts of why we do not see the aliens:
1.Are the Aliens We Never Find Obeying Star Trek’s Prime Directive? The Directive is, don’t interfere in the evolution of alien societies, even if you have good intentions. Hence the Zoo hypothesis. Assuming the aliens exist, perhaps it’s just as well, on the whole, if they do want to leave us alone. They could want to “fix” us instead…
2.How can we be sure we are not just an ET’s simulation? A number of books and films are based on the Planetarium hypothesis. Should we believe it? We make a faith-based decision that logic and evidence together are reasonable guides to what is true. Logical possibility alone does not make an idea true.
3.Did the smart machines destroy the aliens who invented them? That’s the Berserker hypothesis. A smart deadly weapon could well decide to do without its inventor and, lacking moral guidance, destroy everything in sight. Extinction of a highly advanced civilization by its own lethal technology may be more likely than extinction by natural disaster. They could control nature.
4.Researchers: The aliens exist but they are sleeping… And we wake them at our peril. The Aestivation hypothesis is that immensely powerful aliens are waiting in a digitized form for the universe to cool down from the heat their computers emit.
5.Maybe there are just very few aliens out there… The Rare Earth hypothesis offers science-based reasons that life in the universe is rare. Even if life is rare in the universe, Earth may be uniquely suited to space exploration, as the Privileged Planet hypothesis suggests.
6.Does science fiction hint that we are actually doomed? That’s the implication of an influential theory, the Great Filter hypothesis, as to why we never see extraterrestrials. Depending how we read the Kardashev scale, civilizations disappear somewhere between where we are now and the advanced state needed for intergalactic travel.
7.Space aliens could in fact be watching us. Using the methods we use to spot exoplanets. But if they are technologically advanced, wouldn’t they be here by now? The Hart-Tipler conjecture (they don’t exist) is, of course, very unpopular in sci-fi. But let’s confront it, if only to move on to more promising speculations.
8.Is the brief window for finding ET closing? According to some scenarios (the Brief Window hypothesis), we could be past our best-before date for contacting aliens. Of course, here we are assuming a law of nature as to how long civilizations last. Can someone state that law? How is it derived?
9.What if we don’t see aliens because they have not evolved yet? On this view, not only did we emerge during a favorable time in the universe’s history but we could end up suppressing them. The Firstborn hypothesis (we achieved intelligence before extraterrestrials) lines up with the view that humans are unique but sees that status as temporary.
10.The aliens exist—but evolved into virtual reality at a nanoscale. That’s the Transcension Hypothesis, the latest in our series on science fiction hypotheses as to why we don’t see extraterrestrials.
On this view, after a Singularity the ETs become virtual intelligences, exploring inner space at an undetectably small scale.
11.Is intelligent life in the universe living in interior oceans of planets and moons? The Ocean Planets Hypothesis is that intelligent beings may flourish in the interior oceans of the moons of gas giant planets — or within exoplanets — but they are trapped there.
If intelligent life forms are trapped in the interior oceans of rocky moons and planets, Earth is a special planet—much better suited to space exploration.
12.Is real-world space travel just too daunting for ET? That’s the Percolation Hypothesis as to why we don’t make contact with aliens. They can’t overcome the laws of physics, any more than we can. If there is a purpose behind the universe, maybe the aliens and we weren’t intended to meet. That’s worth considering, given the physics barriers.