It has long been an unspoken tenet of astrobiology that in the absence of confirmed alien technologies its necessary to imagine some.

This is far from a frivolous pursuit. Rather, it is an attempt to resolve one of the more baffling aspects of human existence, generally summed up as the Fermi Paradox.

In 1950 the famous nuclear physicist Enrico Fermi came up with an interesting point. Given the size and age of the Milky Way, he said, any alien civilisation just a bit cleverer than humanity should by now have had ample time to explore and colonise all of it.

Why, then, with the obvious exception of a couple of dozen befuddled folk in Americas more agricultural states, has no one ever seen evidence thereof?

Scanning the skies for the sounds of extraterrestrial radio broadcasts the basis of the long-running Search for Extra Terrestrial Intelligence (SETI) project has thus far turned up squat. Other research avenues, at least since Fermis implicit challenge, have focussed on the search for technological evidence.

If ET is out there, the logic runs, she or he or it must have somehow arrived wherever there is, and must be somehow surviving and for those things to happen, some type of whizzbang alien machinery must be involved.

All that has to be done, therefore, is work out a way to detect the tech, which is not easy when the object of any such search is completely unknown.

This has led to some not entirely fanciful imagining. Flying saucers were an early example, although, despite attempts to build them here on Earth, the engineering challenges inherent in the design appear fatal.

Dyson spheres were, and remain, a much stronger candidate. Named after the man who thought of them in 1960, English mathematician and physicist Freeman Dyson, these bits of mega-kit comprise enormous energy-absorbing panels placed around entire stars.

An artists illustration of a Dyson sphere.

Marc Ward/Stocktrek Images

Each sphere, the theory goes, could capture, convert and transmit enough energy to power a far-flung galactic empire. Speculation grew over the past couple of years that an actual alien Dyson sphere had been located.

Such a thing, astronomers suggested in 2015, would explain the eccentric light variations observed in a star classified as KIC 8462852, but better known as Tabbys Star.

The most recent research, sadly for enthusiasts, suggests that the irregular and sudden dimming of KIC 8462852 is most likely caused by a rogue exomoon otherwise known as a ploonet getting in the way.

Hope, however, springs eternal for extraterrestrial tech hunters, and the other best-favoured hypothetical example is known as a von Neumann probe, named after a mathematician called John von Neumann, who came up with the idea.

These hypothetical machines overcome one of the principle objections to Fermis Paradox that it assumes ET would actually want to physically colonise the galaxy. Von Neumann probes allow aliens to explore across vast distances while staying at home.

Essentially, they are self-replicating devices that whizz off and then make copies of themselves, thus rapidly indeed, exponentially increasing in numbers and range.

In terms of Fermis notion, however, von Neumann probes simply kick the can further down the road. The idea might explain why humans have never seen an alien, but fails to explain why it has never seen an alien machine.

Objections to the probe idea come in several forms. The machines would need materials to build their doppelgangers, some researchers note, and there may simply be not enough well-placed asteroids or rocky planets to allow this to happen often enough.

Others cite evolutionary theory. As the probes make copies of codes needed for them to operate, mistakes are bound to occur. Some probes might thus turn into predators, hunting and destroying others, or perhaps at a particular point in time, the accumulated errors render most of them dysfunctional.

Recently, however, the idea has been tweaked, because, well, of course it has.

In a paper lodged on the preprint site arxiv, astrophysicist Zaza Osmanov of the Free University of Tbilisi in Georgia suggests that theorists have been thinking about von Neumann probes at entirely the wrong scale.

Using some very detailed calculations, Osmanov concludes that the probe idea works best if the machines are microscopic about one nanometre long.

At that size, he notes, they would not require the substantial resources of rocky planets to reproduce, but could instead power up using hydrogen atoms whirling around in the interstellar dust. He calculates that this is altogether more efficient and much, much faster, with replications happening in a matter of a few years rather than the rather longer timescales thought necessary for macro-scale machines.

Furthermore, nano von Neumanns would very quickly at least on galactic timescales become very numerous. Osmanov estimates that by the time descendants of an initial population of 100 had travelled one parsec about four light years they would number roughly 1,000,000,000,000,000,000,000,000,000,000,000 (or 1 x 1033).

And that sort of mega-swarm, he suggests, might make them visible, if only someone was looking in the right direction. The nano machines, he says, by encountering and collecting protons, would produce luminous emissions.

Each individual emission would be tiny, but collectively they would add up to something observable, given that the mature von Neumann swarm, assuming they are in a level formation and comprise a wave at their leading edge, would collectively have the typical mass of a comet having a length scale of several kilometres.

At least in the infrared part of the spectrum, Osmanov calculates, that constitutes a target worth looking for.

All the aforementioned results indicate that if one detects a strange object with extremely high values of luminosity increment, that might be a good sign to place the object in the list of extraterrestrial Von-Neumann probe candidates, he concludes.

(It is also possible, of course, referencing Douglas Adams Hitchhiker books, that a huge and tightly packed swarm of nano von Neumanns has already swooped down through the atmosphere to take a closer look at Earth, only to be swallowed by a yawning dog.)

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The galaxy might be full of micro-machines - Cosmos