For a long time there has been an unspoken postulate of astrobiology that in the absence of confirmed alien technologies, it is necessary to come up with something for yourself.
This is far from a frivolous exercise. Rather, it is an attempt to resolve one of the more confusing aspects of human existence, commonly referred to as the Fermi paradox.
In 1950, renowned nuclear physicist Enrico Fermi came to an interesting conclusion. Given the size and age of the Milky Way, he said, any alien civilization only slightly smarter than humanity should have had enough time by now to explore and colonize it all.
Why, then, with the exception of a couple of dozen stoned people in the most agrarian states of America, has no one ever seen evidence of this?
Scanning the sky for the sounds of extraterrestrial radio broadcasts - the core of the long-term search for extraterrestrial intelligence (SETI) project - has so far proved inconclusive. Other lines of research, at least after Fermi's implicit challenge, have focused on finding technological evidence.
If the alien is there, logic dictates that he or she somehow arrived where it is and must somehow survive - and some type of alien machine leaving traces must be involved in order for that to happen.
So all one has to do is develop a way to discover the technology, which is not easy when the object of any such search is completely unknown.
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This led to some not-so-fantastic imagination. Flying saucers were an early example, although despite attempts to build them here on Earth, the engineering problems inherent in the design seem unsolvable.
The Dyson Spheres were and remain a much more truthful candidate. Named after the man who thought about them in 1960, the English mathematician and physicist Freeman Dyson, these spheres are composed of huge energy-absorbing panels arranged around entire stars.
Dyson Sphere.
Each sphere, the theory says, can capture, transform and transfer enough energy to power a far-reaching galactic empire. For the past two years, rumors have circulated that a real Dyson alien sphere has been discovered.
Such a thing, astronomers suggested in 2015, would explain the eccentric variations in light observed in a Star classified as KIC 8462852, but better known as Tabby's Star.
KIC 8462852.
The most recent study, unfortunately for enthusiasts, suggests that the irregular and sudden eclipse of KIC 8462852 is most likely caused by an orphaned moon - otherwise known as a plunet - getting in the way.
Hope, however, is eternal for hunters for extraterrestrial technology, and another most preferred hypothetical example is known as the von Neumann probe, named after the mathematician John von Neumann who came up with the idea.
These hypothetical machines overcome one of the fundamental objections to the Fermi paradox. Von Neumann probes allow aliens to explore vast distances while staying at home.
Essentially, they are self-replicating devices that explode and then make copies of themselves, thus rapidly - indeed, exponentially - increasing in number and range.
In terms of Fermi's concept, however, the von Neumann probes just kick the can further down the road. This idea may explain why humans have never seen an alien, but it does not explain why he never saw an alien machine.
Von Neumann probe.
Objections to the probe idea come in several forms. Some researchers note that machines will need materials to build their counterparts, and there may simply not be enough well-spaced asteroids or rocky planets to allow this to happen often enough.
Others cite evolutionary theory. Since the probes make copies of the codes necessary for their operation, errors are bound to occur. Some probes can thus become predators, hunting and exterminating others, or perhaps at some point in time, accumulated errors make most of them dysfunctional.
Recently, however, this idea has been adjusted.
In an article published on the arxiv preprint site, astrophysicist Zaza Osmanov of the Free University of Tbilisi in Georgia suggests that theorists were thinking about von Neumann probes on a completely wrong scale.
Using some very detailed calculations, Osmanov concludes that the probe idea works best if the machines are microscopic - about one nanometer long.
At this size, he notes, they would not require the significant resources of rocky planets to reproduce, but could instead feed on hydrogen atoms orbiting in interstellar dust. He calculated that this is generally more efficient and much, much faster since replication takes place over several years, rather than the somewhat longer timescales that were considered necessary for macro machines.
In addition, nano von Neumanns will very quickly - at least on a galactic time scale - become very numerous. Osmanov estimates that by the time the descendants of the original population of 100 have traveled one parsec - about four light years - they will number approximately 1,000,000,000,000,000,000,000,000,000 (or 1 x 10 33).
And this kind of mega-swarm, he suggests, could make them visible if only someone was looking in the right direction. Nanomachines, he says, will produce luminous radiation by meeting and collecting protons.
Mega swarm of probes.
Each individual radiation would be tiny, but collectively they would add up to something observable, given that a mature Roy von Neumann, assuming they are in a horizontal formation and constitute a “wave” at their leading edge, would collectively have “typical mass comet having a length scale of several kilometers”.
At least in the infrared part of the spectrum, Osmanov calculated, this is a goal worth looking for.
“All of the above results indicate that if a strange object with extremely high luminosity increments is detected, this could be a good sign for including the object in the list of extraterrestrial candidates for von Neumann sounding,” he concludes.
(It is also possible, of course, referring to Douglas Adams's The Hitchhiker books, that a huge and densely packed swarm of nano von Neumann had already swept through the atmosphere for a closer look at Earth, only to be swallowed by a yawning dog.)