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Looking for signs of alien civilisation
Image from C Costello and Steve Bowers
High Energy Emission Civilisations detected at long range in the Milky Way Galaxy to date

The Initial Failure of Radio Detection

In 1959 c.e. (10 BT) Giuseppe Cocconi and Philip Morrison outlined the possibility of a determined Search for ExtraTerrestrial Intelligence, or SETI, using the instruments available at that time, and radio astronomer Frank Drake made the first search the next year. The noisy technosphere of Earth made detection of weak signals very difficult, and so it became obvious that SETI would have to be conducted in orbit.

In 2009 CE (40 AT), the Kepler space observatory began to detect a very large number of exoplanets, including several Earth-sized objects. But at this time no more than a few erratic radio signals had been detected by Earth-based SETI searches, and these were attributed at the time to random noise and distorted transmissions of human origin. More recent analysis of this historical data suggests that some of these early signals may indeed have been of xenosophont origin, for instance intership relay communications of the type routinely intercepted in later years).

Before any interstellar probes were launched, the local stars had all been surveyed with enough resolution to detect a number of worlds which seemed to support a biosphere (the closest of which was in the Delta Pavonis system), and several candidates for terraforming. This gave a terrific spur to the colonisation movement, but still little evidence for data-bearing radio traffic had been found, although the search now covered the entire galaxy.

The first detection of an extraterrestrial radio civilisation was in 2230 c.e. (261 AT), by Julie Denley and a team of Space Adapted astronomers, using the PanTrojan Baseline between the L4 and L5 points of Jupiter. The artificial origin of the smear of modulated signals coming from 20,000 ly distant in Carina was soon accepted, although the data could not be decoded, and after only two hundred years ceased altogether.

This source (now known as Medium Energy Emission Civilisation One) has since been observed by more sensitive instruments and is still active, but the transient nature of the source indicated that many civilisations would not use broadcast radio or other wavelengths for an extended period in their cultural development, so other methods of detection would need to be utilized. MEEC1 was only detected because of its use of modulated signals in the radio band, but was not in itself a particularly high-energy civilisation. After a short period of radio transmission this civilisation progressed onto more sophisticated methods of communication (such as laser comms or comm-gauge wormholes), and may be on its way to HEEC status.

Before the Technocalypse several more distant radio civilisations had been discovered, all more than 20kly distant, and in 340 a.t. the superbright astrographer Sam Mwiraria detected artificial infrared emissions from postulated megastructures in a region of the galaxy about 40kly distant, in the direction of Cepheus. The First Federation restarted the search, with the advantages of much longer baselines, and a better idea of the quality of emission expected from a distant civilisation by observing the various colonies as they developed. Soon the Cepheus civilisation, today known as for classification purposes as High Energy Emission Civilisation Six, had been mapped in some detail and the detected megastructures analysed. These structures are now known to be efficient Dyson swarms surrounding red dwarf stars, but other evidence for civilisation is largely absent, so this civilisation is thought to be relatively isolationist in outlook, possibly an aioid civilisation like the Terragen Diamond Network.

Throughout the region known as the Terragen C-Horizon an increasing number of false detections of alien intelligence were occurring because of the independent spread of rogue artificially intelligent space probes. Entities aligned with the so-called Diamond Network and the Solipsist Panvirtuality often established themselves in systems well outside the reach of human colonisation efforts, and their emissions were often mistaken for alien civilisations until contact was made. When the first alien civilisation was finally contacted in 3831, expectations were once again proved wrong; after its unusual high CO₂ ecology was detected by interferometry a thousand years before, To'ul'h had been a target for exploration, but the low-tech civilisation discovered there would never have been found from any great distance.

During this period several worlds were discovered which held relics of long-vanished civilisations, including a significant number of worlds which seemed to have been seeded artificially with complex life. Indeed, in many parts of the Terragen Sphere, worlds which had been modified by xenosophonts (or their agents) to support biospheres were somewhat more commonplace than life-bearing worlds that had evolved naturally.

So, by learning about the incredible diversity that intelligent life could take, the search became more accurate; such species as the Cthonids and the Ultimates were detected first by their emission characteristics, while others such as the Paulans, the Kammerer sentient forest and the Whisperweed were only found by close range survey of Gaian planets, all of which had been accurately surveyed at great distances without any detectable signs of intelligence. One very high technology civilisation restricted to a single solar system was only detected by close examination of their homeworld; the Silent Ones have become withdrawn and self-absorbed, using very secure communication channels which were indistinguishable from random noise to an observer. Such withdrawn species may be widespread but undetectable in the Milky Way galaxy and elsewhere.

Beyond the C-horizon

Using the thousand-light year baseline of the Argus Array, the Milky Way galaxy and the other members of the Local group of galaxies have been comprehensively surveyed over the last seven thousand years. It is likely that all civilised intelligent species have been located except the well-hidden Low Energy Emission Civilisations (which may be as many as three-quarters of the total). The most spectacular results have been in the Triangulum Galaxy, assisted by an unsolicited data transmission giving many details of the society found there. Even more advanced emissions have been detected from the Andromeda Galaxy, but none at all from the Sagittarius Dwarf, for instance.

Within the Milky Way around a hundred medium emission civilisations have been found, all emitting energy somewhere betweeen Kardaschev Type I and II levels, comparable to the Solar System before the emergence of the Archailects. About forty more were found in the past but have disappeared over the period of observation, some to become more advanced societies with different characteristics, some perhaps to have retreated into solipsism or been destroyed.

High Energy Emission Civilisations

The High Energy civilisations are of particular interest, particularly HEEC2, located around the galactic core (a dangerous radiation flux environment) and also HEEC3 and HEEC5, all of which share some elements with the Meistersinger migration route adjacent to Terragen space, so may be associated with that race. The distant volumes HEEC7 and HEEC4 have displayed clear signs of high tech combat and weapons of great destructive force, while the closest advanced civilisation HEEC1 has apparently suffered conflict with the backscattering waves of its own expansion front and is considerably weakened.

None of these high energy civilisations appear to have existed for more than a few tens of millions of years, because proper motion effects would have spread their territory out around the plane of the galaxy. Each of the stars in these distant civilisations moves at a slightly different speed around the Galactic Hub; over time this would cause the volumes occupied by the civilisations to become elongated, smeared out in both directions. The only civilisation showing any signs of such smearing is HEEC6; this civilisation may be more than ten but less than a hundred million years old. If the civilisation deliberately avoids stars with high relative proper motion this effect would be reduced.

The civilisation HEEC2, around the galactic core, cannot be dated in this way; it already appears to have spread to cover the entire hub (although emissions originating from sites on the far side of the hub are not shown on the map above, as they are masked by the hub itself).

Medium Energy Emission Civilisations
Image from Steve Bowers
A small number of the closest MEEC civilisations have been contacted via directed radio and laser messages

A vast area of the galaxy in the Counterspinward direction seems to have no high or medium emission civilisation at all - this may be due to a depopulation event or a high civilisation now using advanced energy efficient technology such as reversible computation.

Five medium emission civilisations outside the current range of spacecraft have made contact with the Terragen civilisation by radio or laser, but the rest of the galaxy is outside our effective C-horizon; no light from here has reached them since the development of spaceflight. Unless (as is sometimes suspected) there exists a secret network of alien communication wormholes, the rest of the Universe is unaware of the existence of Humanity's mind children.

The following are dates at which Terragens are known to have first detected or discovered xenosophont civilizations by various methods.

Date
261 AT	First unequivocal xenosophont radio emissions detected (in Carina 20,000 ly distant, by Julie Denley, using the PanTrojan Baseline; source eventually known as MEEC1)
340 AT	First high energy civilizations detected (infrared emissions in the Cepheus direction, 40,000 light years distant distant, by Sam Mwiraria; eventually known as HEEC6)
1681 AT	First artifacts of an extinct xenosophont civilization (Meditating Idols, paleolithic or neolithic equivalent tech)
3831 AT	First direct contact (the To'ul'hs, by the MPA explorer transapient Fortunate Cloud)
5845 AT	First intergalactic HEEC signal intended for reception (Triangulum Transmission, unsolicited)
7480 AT	First detection of radio emissions leading to direct contact (Cthonids, by the Andian Mission)
10404 AT	First extragalactic object/envoy (Semblance of Eternity; possibly a hoax)