In his contribution to the Man and the Planets discussions (see Part 2), the late A.T. Lawton argued that CETI situations – contact or communication – were more likely to arise from practical activities than from radio searches. Another space-faring culture might well detect our astroengineering activities – building a new sun, say, from a cloud of interstellar dust on the gravitational ‘isocline’ between us and a neighbour star – rather than massive radio projects on Earth such as Project Cyclops (a 1970s proposal equivalent in cost to the Apollo programme, specifically to pursue CETI by radio(35). Alternatively, our own mobile worlds, while working a dust cloud or cometary halo far out on the gravitational fringes of our Solar System, may find someone else already at work out there. Within our own Solar System, just how soon and how near to us such an event may occur depends entirely on how many mobile worlds there are in the Galaxy, and how widely spread they are. We are now face to face with the Fermi Paradox.
As of 2022, our Galaxy is estimated to contain approximately 500 thousand million stars. Working in very round numbers, let us suppose that half of these are ruled out because they’re of Population II, and that half the remainder are too massive and short-lived to support the evolution of intelligent life. (Civilizations whose stars are near that mass limit have a particular incentive to move expand into space. A class F star with the same age as our Sun would now be leaving the Main Sequence, beginning to expand as the hydrogen in its core was exhausted. In the early 1970s, I seemed to have stumbled on to a message from such a civilization (36). It took two years to prove myself wrong (37), at least as regards the origin of the supposed message, but, nevertheless, the Galaxy may belong to the F-star civilizations because the pressures on them to become mobile are so great.)
In terms of the Drake equation, we have a situation where instead of remaining constant over very long periods, the number of civilizations N rises and its rate of rise accelerates, because hardly anybody dies out. Cultures may disappear in the sense that their units become so widespread and diverge so much that they are no longer recognizable, but the number of independent mobile units is growing all the time, at an accelerating rate.
Suppose arbitrarily that N rises to 50,000 before the first contacts become likely. At about that time the spacefaring civilizations become aware of one another and begin to exchange signals, then probes and finally mobile worlds. The average separation between them is 1000 light years, so if it is not practical to accelerate mobile worlds much beyond 1% of lightspeed, then from first detection to first contact could take as long as 100,000 years, but would probably take less as the two cultures built ‘bridges’ from star to star towards eventual meeting. As the Galaxy is only 100,000 light years across (some recent work suggests that figure is an overestimate), communication between the 50,000 parent civilizations will meanwhile become general. (This scenario is very different from ‘philosophical discourse dogma’. For example, there is no reason for the communications to be on frequencies which penetrate the atmospheres of Earth-like planets, and ground-based SETI programmes may be doomed to failure for that reason alone.) To complete the survey of life in the Galaxy, each of the 50,000 has on average a million stars to investigate. If each of them launches one mobile world a year, and each of those build another wherever it finds suitable resources, then the survey will be completed in full at about the time that the first face-to-face meetings take place. Altering the number of parent civilizations has surprisingly little effect on this conclusion: to take the extreme case where only one spacefaring civilization sets forth, in ten million years its offspring moving east and west will be meeting on the far side of the Galaxy.
As we are not members of such a loose-knit galactic culture or federation, only two possibilities rise. First, we are one of the first 50,000 spacefaring civilizations in the Galaxy; or second, Earth has already been visited and we are a protected species. In fact, since Earth would presumably have qualified for protected status for only the past five million years at most, after the appearance of the first hominids, then we can say with some confidence that either we are one of the first two spacefaring civilizations in the Galaxy, or we are already protected.
Other scenarios lead to similar conclusions. Suppose, for example, that faster-than-light travel is possible in spite of all the evidence against it. A culture possessing that technology might opt originally for starships rather than mobile worlds, and establish colonies only where habitable planets were found. But to explore the whole Galaxy that way would take a single culture a very long time indeed; so either it would develop much bigger ships which were in effect mobile worlds, or it would not complete its exploration programme before other cultures arose to join it, or it would remain confined to its own part of the Galaxy (as regards systematic exploration) for billions of years. But taking again the arbitrary figure of 50,000 FTL civilizations arising, 1000 light years apart on average, before they discover one another, then if they each keep 1000 ships in space and each checks one star a year then in 1000 years the galactic survey is completed. Not being a member planet of an FTL federation, we would deduce that:
We are one of the first 50,000 civilizations in the Galaxy; or
We are already a protected species; or
Faster-than-light travel is impossible.
If Earth had qualified for protected status for five million years, and yet was not protected, then we would be one of the first ten spacefaring civilizations. It is remarkable that if the number of civilizations is anything less than fifty, then exploring the Galaxy by faster-than-light takes longer than mobile worlds at 1% of lightspeed – unless the average FTL journey takes less than 73 days.
Another scenario is the beacon/probe one. Here I suppose cultures expanding across the Galaxy much as before, but impatient for CETI contact. Having checked out most of the nearby stars by space probe, while their first phase of mobile world expansion was under way, they would realize that the wavefront of their exploration would slow down with increasing distance unless they went on launching more and more probes per year up to impossible figures. They might then create some spectacular optical anomaly, such as creating or destroying a star in some obviously artificial way, as a beacon to be visible for one thousand light years or more. The beacon movement would sweep the Galaxy, like the watchfires of old, until all the cultures who could communicate at that time were in touch with one another. The beacons would be the switchboard of what Hoyle has called “the galactic telephone exchange”.
The whole beacon phase might take 250,000 years altogether, assuming that there are 50,000 civilizations 1000 light years apart on average and willing to pass on the word. In the same period, however, if each culture joining the net launches one space probe a year, each programmed to investigate ten stars, then at the end of the 250,000 years every Sun-like star will have been checked and all the life-bearing planets will presumably have been visited. Launching self-replicating probes of the type advocated by von Neumann would restore the timescale of the mobile world scenario. In this case playing about with the numbers is not very helpful unless we assume that the beacons can be seen for a great deal further than 1000 light years, in which case the numbers are probably too arbitrary to be of value. So the beacon/probe scenario gives us: First, we are one of the first civilizations in the Galaxy, where N is appreciably less than 50,000 and our nearest neighbours are thousands of light years away; or Second, the Galaxy is in a watchfire beacon phase which has either just ended (as regards the beacons) in this part of the Galaxy, or is going on but not yet recognized by ourselves. The odds are against that because the whole beacon phase lasts only 250,000 years, from beginning to end, and much less from the viewpoint of any one civilization. From our viewpoint, the total time from seeing ignition of the first beacon in these parts to seeing extinction of the last one might be 2000 years at most. Or, third, we are already a protected species. Summarizing all three scenarios, then, in increasing order of probability, we have: 1, Faster-than-light travel is possible but we are one of the first ten spacefaring civilizations. Probability: apparently nil. 2, the Galaxy is at present in a watchfire beacon phase. Odds against: two million to one. 3, We are one of the very first spacefaring civilizations in the Galaxy. If this is true, the odds are that we are either the first or the second. 4, We are already a protected species. When related to the age of the Earth versus the oldest Population 1 stars, all three scenarios overwhelmingly favour that possibility, unless terrestrial conditions are in some way absolutely unique.
If 1 is true it is unverifiable at the present state of our knowledge. If 2 is true it is verifiable only by chance. If 3 is true it is unverifiable until we have searched the Galaxy, i.e. for approximately the next ten million years. If 4 is true it implies that Earth has been visited and theoretically that could be verified at once, by finding traces of that visit.
Let us not, however, forget the currently favoured scenario, characterised by Dyson as ‘philosophical discourse dogma’. To date all SETI searches conducted on those assumptions have failed to resolve the Fermi Paradox, i.e. no signals have been detected. This situation generates three more possible scenarios: 5, philosophical discourse dogma is correct but we have yet to succeed in ‘tuning in’. Can only be verified by a long and costly, perhaps endless, radio search. 6, Technological civilizations do not opt for philosophical discourse but expand physically into space. Verifiable only as 2, 3, or 4 above. 7, All technological civilizations collapse before expanding into space. Unverifiable unless by clairvoyance.
Thus we find that of the propositions verifiable by systematic investigation, at least in theory, 5 is the most remote, time-consuming and difficult, while 4 is the most likely, easiest, cheapest and offers more or less immediate results if true. I find it remarkable that the scientific world at present fully supports 5 and denounces even the mildest enquiry into 4.
Personally I prefer the suggestion we are a protected species – known in SETI circles as ‘the Zoo Hypothesis’, though Martyn Fogg uses ‘the interdict hypothesis’(8) which is a little more flattering. It implies that the galactic civilization is comprehensive, but conservationist, and has a great deal of respect for individual development. That respect almost certainly extends to letting us destroy ourselves or be destroyed, if we take the wrong decisions. It may even extend to concealing our protected status from us, i.e. the traces of visits may have been carefully erased, but that seems a pointless exercise: once we have followed the Fermi Paradox reasoning this far, there seems no reason to deny us the answer to the crucial question.
Working with my colleagues Alan Evans and Jamie Bentley, we concluded that there could be four categories of possible evidence. Category A would be our objective, an artefact of unquestionably extraterrestrial origin. Category B would be optical or electromagnetic anomalies pinpointing such an object (like the Tycho monolith in 2001, A Space Odyssey); Category D would be the ‘von Däniken material’ of legends, drawings etc. which were no use except in suggesting areas to search for other types of evidence. Category C, included at the insistence of Alan Evans, would be anomalous astronomical alignments in man-made structures – anomalous because they revealed knowledge which the builders should not have had. For example, on high-resolution photographs of Stonehenge, he had identified markings which seemed to indicate Galactic alignments. If Category A stands for ‘artefact’, Category B for ‘beacon’ and Category D for ‘Däniken’, then Category C would be ‘circumstantial’.
In Intelligent Life in the Universe (13) Carl Sagan and I.S. Shlovskii suggested three tests which which might be applied to possible cases of Past Contact. These were (1) no attempt to conceal the extraterrestrial nature of the visitors, (2) a major impact on the society contacted, and (3) an account committed to writing at the time or soon after. These did not seem to us to be necessary or sufficient (what if the contacted society insisted on deifying the visitors, for example?), and instead we suggested (1) recognisable principles of technology, (2) rational purposes for the possible Contact; (3) information freely given, and (4) arising from (1), (2) and (3), the possibility of further investigation to find out what really happened. My paper ‘Epsilon Boötis Revisited’ (38) is a worked example of such an enquiry, to be discussed later.
Somewhere in our environment, then, almost certainly on Earth itself, there should be unequivocal proof that we have been visited. When we find it we will know we are not alone, that advanced civilizations are not doomed to destroy themselves, and that space technology does offer a route to long-term survival. If we cannot find proof that we have been visited, it does not follow that destruction is inevitable: of all the explanations for the Fermi Paradox that is one to be avoided, because it could so easily become self-fulfilling.
35. Bernard M. Oliver & John Billingham, eds., “Project Cyclops: a Design study of a System for Detecting Extraterrestrial Intelligent Life”, NASA Ames Research Center CR 114445, 1973.
36. Duncan Lunan, ‘Space Probe from Epsilon Boötis’, Spaceflight, April 1973; reprinted Pursuit, 1975.
37. Duncan Lunan, ‘Long-Delayed Echoes and the Extraterrestrial Hypothesis’, Journal of the Society of Electronic and Radio Technicians, September 1976.
38. Duncan Lunan, ‘Epsilon Boötis Revisited’, Analog, March 1998; updated Asgard, 2003, reprinted www.duncanlunan.com, August 2013; revised and updated, 2021.