Last week we looked at another reason that has been advanced, as to why we do not see extraterrestrials except at the movies. Science writer Matt Williams has been looking at the reasons (see the links below.) Last Saturday, we looked at the possibility that Earth is unusual in that it is a rocky planet whose intelligent inhabitants live on the surface. Many rocky planets and moons with icy surfaces may have interior oceans that harbor life.: In that case, intelligent life may not think of space exploration.

Another hypothesis that Williams has examined is the Percolation Theory Hypothesis, that there are limits imposed by the laws of physics as to what intelligent life forms can do by way of exploring the universe. We and the extraterrestrials haven’t found each other because we are all subject to those laws. Geoffrey A. Landis argued that in a 1993 paper, “The Fermi paradox: an approach based on percolation theory.” There, he said, “A model for interstellar colonization is proposed using the assumption that there is a maximum distance over which direct interstellar colonization is feasible. Due to the time lag involved in interstellar communications, it is assumed that an interstellar colony will rapidly develop a culture independent of the civilization that originally settled it.”

Williams explains,

To summarize, an advanced species would not colonize the galaxy rapidly or consistently. Instead, it would “percolate” outwards to a finite distance, where increasing costs and the lag time between communications imposed limits and colonies evolved their own cultures. Thus, colonization wouldn’t be uniform but would happen in clusters with large areas remaining uncolonized at any given time.

Matt Williams, “Beyond “Fermi’s Paradox” XV: What is the Percolation Theory Hypothesis?” at Universe Today

Thus, we might not be on the radar of the few who are looking. Plus, due to the huge expense of space exploration and the time lag in communications with the home planet, space colonies would simply lose interest in a search for other intelligent beings in the universe.

The Curiosity Rover (2012) gives some sense of the time lag and frustrations even communicating with devices on the nearest planet:

As for interstellar travel, Williams examines in detail many proposals from the past seven decades and concludes:

Under the circumstances, and barring several major technological developments that would reduce the associated costs, it would be fair to say that any idea for interstellar crewed missions is simply impractical.

Matt Williams, “Beyond “Fermi’s Paradox” XV: What is the Percolation Theory Hypothesis?” at Universe Today

On the whole, as Williams allows, the Percolation Hypothesis is a hard-headed look at the problem of why we don’t hear from ET. It does not invent a past or a psychological profile for life forms whose existence we cannot even verify. It simply assumes that, living in the same universe, they face the same physical problems we do. The physical problems that would be a barrier for us would function the same way for them.

Most science fiction gets around this problem by positing that the aliens have solved problems like faster than light travel, which we have no idea how to solve.

The Percolation Hypothesis is a bit of a letdown after, for example, the Transcension Hypothesis, according to which the aliens exist—but have evolved into virtual reality at a nanoscale and are now undetectable. But good “literary” science fiction could come of this anyway. Imagine the aliens coming to terms with the fact that we are probably here but forever unreachable. And us coming to terms with the same thing about them. Almost like two people who have reason to believe that each other exist but can never meet.

If we believe that there is a purpose behind the universe and accept this hypothesis, we must conclude that we weren’t intended to meet the extraterrestrials. That was part of the storyline in C.S. Lewis’s space trilogy, in which an Earthling finds himself on Mars (Out of the Silent Planet) and Venus (Perelandra). He soon discovers that Earth is quarantined from other planets because of the violence and other bad nature of its intelligent inhabitants (us).

Whether we find that an appealing explanation or not, there is plenty of evidence that humans might be a bad influence on more innocent extraterrestrials.

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. 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 idea. 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 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, 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.