Last week, we looked at the ominous “Great Filter” hypothesis about why the space aliens never show up: Something dreadful almost always happens to prevent a civilization from reaching the point of easy space-faring. Probably something to do with mutually self-destructive warfare.
But this week, let’s start with something different first. Let’s look at the possibility that the extraterrestrial intelligences could be alive and watching us right now, using the very same methods we use to spot exoplanets. A recent open-access astronomy paper tried to calculate which aliens could actually spot us by whether Earth dims the Sun when passing it:
These sudden drops in luminosity are very slight, but detectable nonetheless. These dimming events can yield other important data as well, allowing astronomers to determine the length of an exoplanet’s year, its temperature, and its chemical properties, the latter of which can be used to discern rocky planets from gas giants. Other detection techniques exist, such as the Doppler method, but the transit method continues to be the most reliable and straightforward.
The number of stars that we can observe through our telescopes seems almost endless, but the transit method means we’re caught in a rather glaring observational selection effect. With the transit technique, we can only spot exoplanets that pass in front of their host stars from our line of sight.George Dvorsky, “Aliens From These Worlds Could Be Watching Us Right Now” at Gizmodo
Of course, a life-bearing planet might be out of our line of sight but, as Dvorsky points out, astronomers have already spotted thousands of exoplanets, which enables some sort of analysis. And, he says, observers there might be able to detect whether an industrial civilization exists on Earth (composition of gases, for example). The projected James Webb telescope could do that for their planets too.
But now, on the other side, let’s look at the Hart-Tipler Conjecture: “Extraterrestrial intelligent beings do not exist” (1980). If they did, within 300 million years, they would have developed advanced technology and be here by now.” Or maybe by 650,000 years. Either way, they have had enough time.
Here are some of the actual constraints of space exploration:
As we may guess, a conjecture that the aliens must have reached or contacted us if they really existed is not popular among searchers of the skies. Consider a 2015 discussion at Universe Today by Toronto researcher and science writer Paul Patton of the Hart-Tipler conjecture.
As Patton notes, that hypothesis originated nearly a half century ago in a paper by astronomer Michael H. Hart. The paper doesn’t seem to be open access but an astronomy letter responding to it may be found here.
And how did eclectic Tulane physicist Frank Tipler become involved? Patton tells us that Tipler extended the argument in 1980, pointing out that even if they couldn’t visit Earth themselves, intelligent aliens could certainly have developed intelligent robots that could.
Curiously, Tipler made that point long before artificial intelligence (AI) triumphed in many strategic games like Go, chess, and StarCraft II. So if we want to say that Tipler is wrong, we need something more compelling than “We don’t like his idea.”
As it happens, those who persist in the search for extraterrestrial intelligences often become impatient with such skeptics. Patton, for example, cites alternative authorities like Carl Sagan (1934–1996):
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…Paul Patton, “Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture” at Universe Today
As might be expected, Carl Sagan and William Newman responded in 1981, offering a more hopeful analysis, based on the spread of animal populations:
For Newman and Sagan, the absence of extraterrestrials on Earth doesn’t mean that they don’t exist elsewhere in the galaxy, or that they never launch starships. It just means that they don’t behave in the way Hart expected. They conclude that “except possibly in the very early history of the Galaxy, there are no very old galactic civilizations with a consistent policy of conquest of inhabited worlds; there is no Galactic Empire”.Paul Patton, “Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture” at Universe Today
Well, we don’t really have a good reason for being sorry about the absence of a Galactic Empire, do we?
Maybe the aliens view the current turmoil here on Earth as a kind of distant soap opera. Maybe they’re just a studio audience and the show sells advertising in a distant galaxy. The audience can never get here, in the same way that we can’t intervene in a historical drama to save the heroine from the stake.
Can we prove that’s not true?
Anyway, next Sci-Fi Saturday, we will look at another account of why we don’t see the aliens!
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.
- 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.