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THE FERMI PARADOX

by DUNCAN LUNAN

 

 

(Speculations in Science & Technology, January 1988;  revised Asgard, November 2002;  edited August 2013.)

 

 

[When it came to writing up ASTRA's first discussion project, on interstellar travel and communication, the idea was that I would write the first half of "Man and the Stars" and the late Chris Boyce would write the second.   In the end, Chris wrote only a guest chapter, and in turn invited me to contribute one to his book "Extraterrestrial Encounter".

 

Guest chapters, however, can include material which the author wants to cover elsewhere in his own words.   I had to top-and-tail Chris's chapter and in turn he cut out nearly half of mine;  I turned the rest into an article for Robert Temple's magazine Second Look, but a whole page of text and some key words went missing, though it was published twice in Europe in full translation  (see ref. 44 below).

 

Meanwhile I was running the Glasgow Parks Astronomy Project, which created the first astronomically aligned stone circle in Britain for at least 3500 years (see ‘The Stones and the Stars’ [link].   After it was covered in Omni in 1981, Leslie Banks of IBM, whose hobby was aerial archaeology, offered to photograph it from the air.   That led to an aerial archaeology flight which I organised, coordinated and navigated in 1982.

 

But Leslie was head of scientific public relations for IBM in the UK, and at the same time he was putting together the extremely prestigious 9-year series of Heathrow Conferences.   He invited me to the first one in 1982 and I attended more than half of the rest, speaking on 'The Fermi Paradox' in 1987.   I was the only amateur scientist ever to address a Heathrow conference and an updated version of my original guest chapter text was published with that title in Speculations in Science & Technology, January 1988.]

 

 

It seems that Enrico Fermi formulated his paradox during a lunch break at Los Alamos.  “Where is everybody?” he asked – meaning not his colleagues or the cafeteria staff, but the extraterrestrials who might have been expected to be present.  “Everyone knew what he meant”, the participants recall, but they had the advantage of prior discussion.1  The full question is, given the relative ages of the Galaxy, the Earth, and the human race, would we not expect that the galactic civilization would have already been established, and have encroached on us (benevolently, for preference) long since?

 

Until recently, the answer you preferred depended on the optimism you felt about the human race itself; or about the feasibility of interstellar travel; or about the rarity or otherwise of life, intelligence and technology.  In the last category, the numbers are nowadays outwardly more favourable to Fermi’s argument: we now place the age of the Earth at 4,600 million years, with primates established on it for ten million, while in the disc of the Galaxy there are Population I stars like our Sun but up to twice as old (Population I stars formed out of interstellar clouds enriched by supernova explosions, taking up the heavy elements necessary for the formation of Earth-like planets and the evolution of life.  Population II stars [discovered later] are older still, but when they formed, to make up the galactic nucleus, halo and global clusters, the only building materials available were hydrogen and the helium formed in the Big Bang).  Those who believe that interstellar travel is possible, believe that the Galaxy could be thoroughly explored in only ten million years.  If others attempted it, it could well have been accomplished long since.

 

But have there been any others?  Here opinions differ sharply.  After all, the idea that we might not be alone in the Universe is relatively new in human thought.  For most of human history we have thought that the sky was some kind of solid shell in which the stars were pierced or embedded, and for most of recorded history the planets were thought to be fixed on transparent but unbreakable concentric spheres.  Kepler’s demonstration of the ellipticity of Mars’s orbit metaphorically broke the crystal shells that enclosed us, and the plurality of worlds became established philosophical doctrine for a time.  Milton wrote of “every star perhaps a home of destined habitation”,2  the nebular hypothesis of Kant and Laplace gave an account of the Solar System’s origin which presumably could be true of almost any star; Herschel believed that all the heavenly bodies were inhabited, even the Sun.  At the turn of last century Tsiolkovsky, in “Dreams of Earth and Sky”,3 looked forward to discovering extraterrestrial life with as much enthusiasm as Burton in “The Anatomy of Melancholy” in the early 17th century, with the only difference that Tsiolkovsky knew just how man could “fly up…command the spheres and heavens, and see what is done amongst them”.4  Practical discussion of Communication with Extraterrestrial Intelligence (CETI), or Search for… (SETI), as NASA now prefers, might have followed, but instead the pendulum swung the other way.

 

The problem with the Kant-Laplace hypothesis was that it failed to account for the distribution of angular momentum in the Solar System; 98% of that angular momentum resides in the planets and only 2% in the Sun, whereas according to the Nebular Hypothesis the Sun should be rotating much more rapidly.  Sir James Jeans now suggested that the planets had formed and gained their angular momentum in a near-collision with a passing star.  The hypothesis had far more obvious mathematical flaws than its predecessor but, nevertheless, gained widespread acceptance – possibly because it restored man to his previous eminence at the peak of evolution in the known Universe.  A collision between stars was too rare an event to occur more than once or twice in the history of the Galaxy, and the fact that spiral nebulae were other galaxies had yet to be proved.  We could forget Tsiolkovsky’s alarming prediction that we might find “what humanity was like several thousand years ago – and what it will be like in a few million years”.

 

While few of them probably could have pinpointed its flaws, the emerging cadre of science fiction writers felt intuitively that the collision hypothesis was false.  In 1928, when Jeans’s reputation was about to make him the best-selling scientific author to date, E.E. 'Doc' Smith published an unreadable classic called “The Skylark of Space”, the first novel of interstellar travel.  Science fiction writers have been filling the Galaxy with inhabited planets ever since, and until 1960, the only discussion of contact with other intelligence in any form was within the pages of science fiction.

 

Meanwhile, the pendulum of scientific thought had been swinging again.  Growing understanding of nuclear fusion had been leading to a much better idea of what went on in the Sun and the stars, and what must happen during their formation.  Hannes Alfvén and Fred Hoyle evolved the hypothesis that during formation of the Solar System the Sun’s rotation had been slowed by magnetic interaction with the surrounding disc of ionised preplanetary material.  Angular momentum had been transferred to the planets from the Sun, and still more had been carried away by gases driven out of the Solar System altogether.  The exciting implication was that any slowly rotating star might have planets; and it was found that, whereas the more massive, short-lived stars like Sirius all rotated rapidly in a matter of hours, stars less massive than F5 (7) (just a little less massive than Procyon) had rotation periods on the order of a month, like the Sun.  The plurality of worlds was back, and seemed to be confirmed by announcements that 61 Cygni had a ‘superjovian’ planetary companion and the red dwarf Barnard’s Star had at least two planets less massive than Jupiter.   (Both discoveries are now suspect, but hundreds of exoplanets have been found around other stars.)

 

Astrodynamics is a tricky discipline, however, and for this article I shall invoke the principle of mediocrity and assume that planetary systems like our own are indeed common in the disc of the Galaxy.

 

Whether the same assumption can be made about the evolution of life is another matter.  That life will arise when it has the opportunity is generally agreed, but there are major disputes about the permissible range of conditions, and even wider-ranging ones about the evolution of intelligence.  In his ‘engineer’s approach to evolution’, for instance, Alan Bond has argued that while bacteria will be found on many planets, blue-green algae will be relatively rare….and so on to a prediction that at its present age, the Galaxy supports one intelligent species, maybe two.  Since Earth hosts humanity and the cetaceans, that would take care of the possibilities.5  In a later review of the question, Martyn Fogg’s model suggested by contrast that a million civilizations might have arisen independently in the history of the Galaxy thus far.6-8

 

The CETI issue came to a head in 1960 with Project Ozma, Frank Drake’s brief listening watch for intelligent radio signals from Tau Ceti or Epsilon Eridani.  By 1963 there was enough published scientific material for A.G. W. Cameron to collect and publish it as “Interstellar Communication”.9  “Extraterrestrial Civilizations” appeared in the USSR in 1965,10 and Carl Sagan collected the papers of the 1971 CETI Conference as “Communication with Extraterrestrial Intelligence” in 1973.11  Lunan’s Law was in operation: ‘the respectability of a scientific topic is directly proportional to the number of times it has been argued within hard covers’.

 

But along the line, as respectability was attained, a curious hardening of thought had taken place.  The moderate assumption underlying Project Ozma – that a nearby civilization might have detected the more powerful transmitters brought into use during the Second World War, and might be trying to attract our attention in return – had been elevated into what Freeman Dyson calls “philosophical discourse dogma”; insistence that the higher civilizations of the Galaxy communicate only by radio, refusing all other options, despite the difficulties of attracting attention unless they are near-neighbours12.  In the second volume of “Interstellar Communication”, for example, published in 1974, Cameron and Cyril Ponnamperuma reprinted the “Bibliography on Interstellar Communication” by Eugene Mallowe, Caren and the late Robert Forward; but as Mallowe justly complained in reviewing the book, “The category of interstellar transport and propulsion methods was regrettably omitted by editorial fiat.”13   As a philosophy graduate, I analysed and attacked the supposed ‘higher philosophy’ of radio communication in a guest chapter for “Extraterrestrial Encounter” by Chris Boyce14 and I do not believe that advanced civilizations will find “philosophical discourse dogma” binding.

 

The currently accepted view is that advanced civilizations are unlikely to be near-neighbours, unless by pure chance.  The most often quoted analysis is the Drake or Green Bank equation,

 

N = r* fsfpfeflfifcL

 

where N is the number of civilizations capable of interstellar communication, r*. is annual rate of star formation, and the f terms are the fractions of stars which resemble our Sun, stars which produce planets, planetary systems which have Earth-like worlds, fractions of those which produce life, intelligent life, and communicative technology – all multiplied by L, which is the average lifetime of a communicative civilization.  The lifetime term is typically put at several million years, and inserting a set of arbitrary but conservative values for the other terms gives a figure of one million advanced civilizations in the Galaxy, separated on average by 300 light years or so.15

 

Sebastian Van Hoerner has published a much more pessimistic assessment,16 giving four headings under which destruction could come to an advanced civilization: destruction of all life, destruction of higher life only, physical and/or mental degeneration or decay, or simple loss of interest in science and technology.  It has been all too easy for pessimists and cynics to conclude that high-technology civilizations destroy themselves in a few decades or centuries at most, just as (they argued) our own was about to do.  I have recently updated my own attempt to tackle this argument in detail.17  Such a Politics of Survival would list the ways in which we can be wiped out, and what could be done about them, under eight headings:

 

1.      Weapons of mass destruction

2.      Overpopulation

3.      Pollution of the environment

4.      Exhaustion of natural resources

5.      Genetic breakdown

6.      Giant meteor impact

7.      Sun change or nearby supernova

8.      Direct contact with other intelligence (not necessarily with harmful intent)

 

The worst case in any of those areas of catastrophe would wipe us out entirely, as could lesser cases of two or more of them in concert (synergy).  All the single possible causes of human extinction are on the list, although one or more headings may have to be modified to encompass genetic engineering, depending on the direction taken by that research.

 

The first thing to notice about the list is that the first five dangers are man-made and the last three are external, i.e. they cannot be countered effectively while we are restricted to a single planet.  Any politics of survival programme, therefore, whose final goal was the guaranteed survival of the human race, would have to include an ongoing space programme.  Specifically to deal with heading 7, we must have self-sustaining communities not located on open planetary surfaces, and drawing their materials and energy from sources other than Earth; we must also have the capability to reach and dismantle (mere shattering will not suffice) any asteroid which threatens to collide with Earth, that being the primary threat under heading 6.  Since the Moon lacks volatiles essential for life support, and those compounds can be found in the asteroids, to deal with those headings alone the space programme must be pursued as far as the industrialization of the asteroid belt, and that objective must be met even if purely terrestrial solutions can be found for the first five problems.  Since space technology can, in fact, play a major part in solving them, and is essential to counter the other three, space-oriented solutions should be the ones adopted.  Following an Association in Scotland to Research into Astronautics (ASTRA) project “Man and the Planets” it seems that extinction under any of headings 1 to 7 can be rendered impossible within 150 years of space development.18

 

The previous ASTRA project “Man and the Stars” had been concerned in large part with human survival on the interstellar stage.19  To be sure of surviving any catastrophe in class 7 or 8, we supposed that human settlements had to be spread  over a radius of at least 10 light years.  We were allowing only for the ‘ordinary’ Type 1 or Type 2 supernovae, rather than the more violent Type 3 event which might have wiped out the dinosaurs at a distance of over 1000 parsecs.20  The evidence now suggested that the dinosaurs were wiped out by impact – one big asteroid21 or a bombardment of comets.22  Since publication of “Man and Stars” in 1974, however, the whole concept of human expansion into space has changed radically.

 

It was in that same year that Gerard O’Neill published his proposals for large free-flying settlements in space, following earlier suggestions by Tsiolkovsky, Bernal and others.  O’Neill’s initial proposal was for paired, counter-rotating cylindrical habitats, stationed at the L4 and L5 points of the Earth – Moon system, and even the smallest models to be capable of supporting 10,000 people.  Artificial gravity would be simulated by the rotation and the pairing of the cylinders would cancel gyroscopic effects, so that the long axes could be kept pointing at the Sun and mirror systems would provide natural daylight.  Conditions inside the cylinders were to be as Earth-like as possible, including standard atmospheric pressure and full simulated Earth gravity.23

 

The habitats were to be built from lunar materials, which lack the volatiles needed for life-support, and the very large quantity of nitrogen required seemed to be the biggest problem.  For his early settlements O’Neill therefore produced a less ambitious “Bernal sphere” design.  Even in this, however, the rotation rates required for 1-g simulation would be high enough to cause vertigo, and a 1975 NASA/Stanford University summer study evolved the “Stanford Torus”, a ring-shaped habitat a kilometre in radius, built of aluminium or steel and rotating within a stationary radiation shield of rock.24  D.J. Sheppard later improved the concept, using prestressed concrete as the building material so that the radiation shield was the hull itself.25

 

Meanwhile, out on the frontiers of speculation, the conventional idea of the starship was taking some severe knocks.   From the British Interplanetary Society’s “Project Daedalus” study, of an unmanned probe mission to Barnard’s Star, it emerged that the collision effects in the interstellar medium were more severe than had been supposed; even at 12% of lightspeed, the cruise velocity of the Daedalus probe, a thick erosion shield would be needed for’ard.  The “photon drive” starship, which Eugene Sänger had supposed could approach the speed of light,26 requires a very large heat-emitting surface for its matter-antimatter engines and would presumably disintegrate long before it reached top speed. 

 

That problem might have been solved by the hydrogen ramscoop, proposed by R.W. Bussard,27  which swept up the interstellar hydrogen ahead to provide power and thrust.  The bursting effect of the very powerful magnetic fields required posed structural problems, but these perhaps might be overcome by sufficiently elaborate arrangements of magnetic and electrostatic fields putting the principal flow of plasma round the outside of the ship.  However, T.A. Heppenheimer’s analysis showed that in practice, at anything over 5% of lightspeed, the ramscoop would begin to function as a brake.28

 

(In itself that is not to be sneezed at.  Eliminating the need to carry fuel for braking manoeuvres greatly reduces the mass of launch mass of space vehicles, and most studies of manned Mars Missions assume some form of aerodynamic braking on arrival at Mars and on return to Earth.)

 

Our ideas about interstellar settlement have also changed radically.  Even in the “Man and the Stars” discussions, we had realized that integrating terrestrial lifeforms into the biosphere of another Earth-like world would be too formidable a task to accomplish with ‘foreseeable’ interstellar missions.  Ed Buckley illustrated a worked example in the book's Plates 1, 2 and 3.  Plate 1 showed the approach to an imaginary Earth-like world of Alpha Centauri, which, although the nearest star system to us, is the most likely one in our neighbourhood to have such a planet29 – unless, of course, double stars cannot have planets.

 

Even on approach his outwardly peaceful planet hinted at dangers on the surface.  Although the presence of liquid water and free oxygen in the atmosphere confirmed that the planet could support ‘life as we know it’, the large equatorial desert suggested more extreme conditions than Earth’s – perhaps reaching a peak when the two suns are close together.  Speaking of peaks, the cloud pattern in the Northern Hemisphere showed a very high plateau on the shoulder of the continent.  Ed Buckley had told us that this world is larger and more massive than Earth, so perhaps there’s violent volcanic activity with the associated dangers of earthquakes and tidal waves.  If surface gravity is higher than Earth’s, then storms too may be more violent, and the cyclonic patterns visible may develop into hurricanes even on this planet’s temperate latitudes.

 

The temperate zones are obviously the candidates for landing sites, and he suggested that the first settlement should be on a large island, or a peninsula cut off by a natural barrier such as a desert or mountain range, so that dangerous lifeforms could be contained and controlled if necessary.  An offshore island about the size of Britain would be ideal, allowing for expansion to the mainland later.  It was not until after the publication of “Man and the Stars” that we realized we had duplicated the reasoning of H.G. Wells in “The War of the Worlds”, where the Martians do indeed target an offshore island the size of Britain – Britain itself – in order to contain and control the most dangerous lifeform on the planet, human beings.

 

Plate 2 showed a cargo landing craft leaving a large colony ship over the island.  The plug-nozzle vehicle is designed to land on water because a coastal site is wanted: it allows retreat inland from storms or tidal waves, and escape by water from animals and brush fires.  The continental shelf allows fishing and sea farming, and the mountain range inland provides rainfall for farming in the coastal strip, as well as mineral resources and a high ground refuge if tidal waves are common.  To protect the landing craft and the settlement sheltered waters would be best, such as the sea-lochs at lower left, and the upper one seems ideal because it is also protected by offshore islands.

 

The same spot at ground level still looked peaceful in Plate 3, at first glance.  But the tree in the foreground had apparently been killed by parasitical growths, and in the colour reproduction the one behind did not look healthy.  The airborne specks clustering around the flowers (which are not quite flowers, just as the trees are not quite trees) could be pollen or could be insects, and might be harmless, but they were certainly clustering around the explorers in their biological isolations suits.  In the distance, flocks of birds (which as we saw from the nearest one are not quite birds) were taking to the air.  Perhaps something big and dangerous was approaching beyond the trees – but then again, the overall movement of the birds themselves was in the explorers' direction.  Our heroes might soon be wishing they had not strayed so far from their hovercraft.

 

In the foreground were the ‘nozzles’ – many more of them than were apparent at first glance.  It was not obvious how big, intelligent or dangerous they are, but they were all watching the explorers – as was another creature visible in the original painting, hiding in the grass which is not quite grass.  The whole outwardly friendly landscape is full of potential dangers, and those are only the visible ones.  Bacteria and viruses spring to mind, but as life on Earth uses only a few of the range of proteins available in nature, it may be that everything in Plate 3 would be either poisonous or inert to terrestrial life.  It could even be that life on that world is based on dextra-rotatory compounds instead of the laevo-rotatory ones used on Earth, in which case this blooming landscape would be as dead to us as the surface of the Moon.  (However, Phillips has suggested reasons why life throughout the Universe might prefer laevo compounds.30)

 

Trying in the “Man and the Stars” discussions to plan the exercise in detail, we swiftly realized that establishing a settlement of terrestrial life in such conditions would call for continuous links with Earth itself.  Only faster-than-light travel would allow that, and all the evidence to date suggests it can’t be done.  We nevertheless assumed for the sake of argument that an ‘acceptable’ method could be found, and went ahead to analyse in detail the practicalities and ethics of interstellar colonization and contact with other intelligences, in the sense of face-to-face meeting.  This drew some hostile reviews and the conclusions made no impression on spaceflight literature.  In Iain Nicolson’s “Road to the Stars”, for instance, it is repeatedly stated that nothing has been said or can be said about the issues to which we gave so much thought.  This is to say that you cannot discuss the ethics of the voyage unless you have a blueprint of the engines.

 

By the first appearance of this article, however, we could say that a blueprint was now available.  For their Project Daedalus study the British Interplanetary Society postulated that their probe should reach Barnard’s Star within 60 years, an adult human lifetime.  The engine would run on pulsed nuclear fusion (triggered by converging electron beams) of pellets of frozen deuterium enclosing liquid helium-3.  It would take 50,000 tons of propellant to accelerate the probe to its interstellar velocity of 12% of lightspeed, and the Daedalus team proposed to gather these from the atmosphere of Jupiter (see back cover, top).  In “Man and the Planets”, I pointed out  that just 200 tons of propellant would suffice to move a 50,000 ton payload from Earth to Mars in five days, 500,000 tons – a metal Stanford torus – could be moved from here to Mars in a fortnight for the same fuel expenditure, and five million tons (the Sheppard design) could make the trip in less than a year.

 

When I checked these figures with Gerry Webb of the Daedalus team, he replied, “Within the Solar System, Daedalus is pure Flash Gordon in its potential”.  On my visit to the Jet Propulsion Laboratory in 1986, I was surprised to have the same figures quoted back to me and attributed to the Lawrence Livermore Laboratory.  The Daedalus propulsion system (or a very similar variant) was then under development there, and initial results had been so promising that the Laboratory was allowed samples of lunar soil for destructive testing.  From this it emerged that the solar wind deposits on the lunar soil contain (relatively) large amounts of helium-3 – at first sight, enough to provide the means for us to open up the whole Solar System for development.  The programme could be financed by importing helium-3 to Earth, if deuterium/helium-3 fusion can be adapted for energy generation31  - but later publications suggested that the volumes of lunar soil to be processed would be prohibitive.

 

Nevertheless, the Daedalus concept tied in directly with another scenario I put forward to finance building O'Neill habitats, using nuclear waste disposal as the baseline of the programme.32  Developing the scenario, I later pointed out that building and fitting out habitats will be much easier if the requirement for full simulation of Earth gravity is relaxed.  In the Starseeds themselves, the mobile factories I envisaged being built out of space shuttle external fuel tanks, the maximum rate of rotation without nausea would be enough only to simulate lunar gravity, one sixth of Earth's.  But no other solid body in the Solar System has a surface gravity more than one-third of Earth’s – with the exception of Venus, whose otherwise hellish conditions rule out settlement in the foreseeable future.  If we were to settle on quarter-g as a compromise, giving acclimatized occupants access to every known solid world except Earth and Venus, then the slower rotation would allow the settlements to be built in cylindrical form as O’Neill originally envisaged, and would reactivate Matloff’s proposal to fit those habitats with Daedalus engines and make them mobile.

 

Matloff’s proposal was to use such habitats as space-arks, or world ships as Ed Buckley terms them, for self-supporting communities to cross interstellar space.  The 50,000 tons of propellant of the Daedalus probe would accelerate an O’Neill habitat to 1% of lightspeed, giving a 430-year voyage to Alpha Centauri or 600 years to Barnard’s Star.  Again, the resources of Jupiter would be required; but a mobile habitat, fully shielded as it must be against galactic cosmic rays, could enter Jupiter’s radiation belts with impunity to ‘mine’ Daedalus propellants from the planet while in close orbit – more efficiently than the Callisto based operation which the Daedalus team proposed (see front cover).

 

In the first instance, the habitats will be using Jupiter’s resources to gain access to the outer Solar System, and that is enough for the politics of survival objective.  Creating the first mobile habitats was enough to ensure that humanity could not be wiped out by events under Politics of Survival headings 6 and 7; but a habitat culture spread over the halo of comets, up to two light years from the Sun, would not only be safe from the dictates of central government,34 but would represent a virtually impossible “sweeping up” operation for even a deliberately malevolent other intelligence.  Guaranteed survival of mankind can be achieved over a baseline of only four light years, rather than twenty as suggested in “Man and the Stars”, and can be attained in two centuries from now or less.

 

By that time von Hoerner’s ways in which technological civilizations can collapse will be completely irrelevant.  We are considering in effect a civilization of mobile worlds, with a wide choice of locations to occupy from the inner Solar System to the outer and beyond, and no reason to stop expanding.  If Alpha Centauri has an envelope of comets, one can see the mobile world civilization expanding into it simply because, for the cometary habitats on that side of our envelope , they will be the easiest targets to reach.  But if there is anything worth having in the Alpha Centauri – planets, asteroids or what you will – the foreseeable combination of O’Neill and Daedalus technologies could send a mobile world there from the comets in only two hundred years, at a little over 1% of the speed of light.  Once you have mastered your own Solar System, the interstellar gulf has lost its terrors.

 

In his contribution to the “Man and the Planets” discussions, 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 the proposed Project Cyclops (a commitment equivalent in cost to the Apollo programme, specifically to pursue CETI by radio35).  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.

 

Our Galaxy was then estimated to contain 150 - 200 thousand million stars  (by 2013, now thought to be nearer 400 billion, mostly red dwarfs).  Taking 200 billion for convenience, and 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, short-lived and/or fast-rotating to have planets and 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 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 (Figure 7), 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.  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 combination.  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 this 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 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 CETI 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.

 

In the 1980s there was a new category of possible explanations for the Fermi paradox, triggered by the proof that the dinosaurs were wiped out by an impact or bombardment, and the suspicion that ‘megadeaths’ in the Earth’s biological record show a 26-million year periodicity.  Cramer suggested that the explanation could simply be that in other planetary system bombardments may be too frequent to allow intelligent life to develop, or else not frequent enough for evolution to be pushed along by periodic liberating of environmental niches.38  Brin suggested that if every third ‘megadeath’ is the really big one, the cause may be beamed radiation from some violent object orbiting the galactic centre at 2,400 parsecs, now hidden from us on the far side of the nucleus.39  Alternatively, the 26-million year period is close enough to the timescale for colonizing the Galaxy to suggest that high-technology visitors have taken over the Earth several times, only to wipe themselves out in industrial accidents.40

 

My own suggestion in this category was prompted by the suggestion that Cygnus X-3, a high-energy source of X-rays, gamma rays and radio waves on the rim of the Galaxy, might be responsible for the entire flux of high-energy cosmic rays which we receive from the Milky Way.  The nuclei are scattered in transit by the magnetic of the Galaxy and come at us from all parts of the sky, but the electromagnetic emission of Cygnus X-3 indicates that powerful beam-plasma interactions are taking place there, with enough spillover to account for the cosmic rays we observe even at 30,000 light years from the source.  Two different models have been suggested to explain this gigantic version of star wars:41  what affected the Fermi paradox was that in the more violent version, where one of the Cygnus X-3 stars is thought to be physically destroying the other, the whole process will take only a century.  Unless events like that are common in the Galaxy, cosmic rays might be an intermittent phenomena.42

 

The risk from galactic cosmic rays became apparent during the Apollo missions, when the astronauts began reporting strange flashes in their eyes.  These proved to be caused by high-energy nuclei passing through the eye43 and on through the head, killing every cell in their path.  It was estimated that on a two-week mission the astronauts lost 10-4 to 10-7 of their central nervous tissue, which does not regenerate.24  Three years exposure would be enough for an adult human to begin to show symptoms such as impaired vision, slurred speech, loss of memory and coordination: like being punch-drunk, and for much the same reason.  To block the cosmic rays effectively requires rock shielding five metres thick, or the equivalent in some denser material; and a little protection is worse than none, because a high-energy nucleus hitting a metal atom in a cabin wall releases a shower of less energetic secondary particles, which can cause cancers and mutations instead of killing cells outright.

 

It has been said that the trouble with the space age is that prophecy becomes history before it becomes current fact.  Books of the 1950s and 1960s were full of examples: metal-hulled spaceships, plastic domes on the open surfaces of the Moon and Mars.  We know now the settlements will be below ground, and the spaceships will be fast Daedalus couriers or armoured mobile worlds.  If cosmic rays were intermittent, then we would be very lucky to have been warned.  Others might have been less fortunate: a civilization which had been expanding into space for two or three centuries, following the politics of survival route in all other respects and becoming dependent on space technology, could be very hard hit.  The effect on children and embryos in unshielded space settlements hardly bears thinking about.  Some cultures would be further away from the cosmic ray source than we are, and the danger represented by the onset of the flashes would take longer to be recognized; others would be closer, bringing on the flashes with the intensity of a migraine attack and leaving hapless travellers unable to see their controls.

 

In 1979, I questioned the validity of the Drake equation.44  I pointed out that for all the f  terms on the right-hand side of the equation are assumed to have some fixed numerical value, over very long times in the history of the Galaxy.  They can therefore be multiplied together to give a constant, reducing the equation to N = r* KL, and since N too is supposed to be constant over long periods, it can be divided by K to give another constant, K2.  Rearranging for L, we find L = K2/r*, which is to say that the lifetime of high-technology civilizations is inversely proportional to the rate of star formation.  There’s no internal reason in the equation for that to be true:  seeking to justify it in his Gifford lectures at Glasgow University, Sagan found that it led to a statement that the average lifetime of civilizations is proportional to the average lifetime of stars – demonstrably wrong, whether you are an optimist or a pessimist about the true value of L.  If, however, cosmic rays were intermittent, then the more stars form, the more Cygnus X-3s would be generated, and the shorter becomes the time before the unlucky civilizations are hit.

 

In the 1980s the evidence from the Solar System was not very conclusive on this point.  Carbon-14 formation in the atmosphere does vary, as witness the recalibration of radiocarbon dates, but is affected by more factors than just the cosmic ray flux.  The evidence from lunar and meteorite samples is that cosmic rays have been around for millions of years, but in the case of the lunar cores, as Chris Boyce pointed out, the turnover of lunar soil is so slow that the cosmic rays need not have been continuously present.  Whether the same applies to cosmic ray tracks within meteorites I was not then sure, but it now appears from ongoing studies that cosmic rays are not intermittent.

 

Personally I would prefer to have it ruled out.  For Cygnus X-3s to have stopped the expansion of all previous spacefaring civilizations in the Galaxy, there would have to have been relatively few of them, perhaps ten or less.  Although we would reach them eventually and tell them that it is safe to come out, it was liable to take a long time and in the meantime we would be lonely.  I much 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.

 

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.

 

 

References

 

1.  Ben R. Finney & Eric M. Jones, eds., "Interstellar Migration and the Human Experience", University of California Press, 1985.

2.  John Milton, "Paradise Lost", Book 7, lines 621-622.

3.  Konstantin Tsiolkovsky, 'Dreams of Earth and Sky', in Tsiolkovsky, "The Call of the Cosmos", Foreign Language Publishing House, Moscow, 1960.

4.  Robert Burton, "The Anatomy of Melancholy", vols.1-3, Every­man's Library, J.M. Dent & Sons, 1932  (‘Digression of the Air’ in Vol. 2).

5.  Alan Bond, British Interplanetary Society Interstellar Studies Conference, 1979.   A more advanced model of  his ‘engineer’s approach’, in terms of gen­ome size, was advanced in ‘On the Improbability of Intelligent Extra­terr­est­rials’, Journal of the British Interplanetary Society 35, 5, 195-207  (May 1982), foll­owing his and Anthony R. Martin’s ‘A Con­servative Estimate of the Num­ber of Hab­­it­able Planets in the Galaxy’, JBIS, 31, 11, 411-415  (November 1978)  and Part 2, JBIS, 33, 3, 101-106  (March 1980).

6.  Martyn J. Fogg, 'Extra-solar Planetary Systems, II – Habitable Planets in the Galaxy', JBIS, 39, 99-109  (1986).

7.  Martyn J. Fogg, 'Extra-solar Planetary Systems, III – Potential Sites for the Origin and Evolution of Technical Civilizations', JBIS, 39, 99-109  (1986).

8.  Martyn J. Fogg, 'Temporal Aspects of the Interaction among First Galactic Civilizations:  the "Interdict Hypothesis"', Icarus, 69,  (1987).

9.  A.G.W. Cameron, ed., "Interstellar Communication", Benjamin, New York, 1963.

10.  G.M. Tovmasyan, ed., "Extraterrestrial Civilizations", Academy of Sciences of the Armenian SSR, 1965;  trans. Israeli Programme for Scientific Translations, Jerusalem, 1967.

11.  Carl Sagan, ed., "Communication with Extraterrestrial Intelligence", MIT Press, 1973.

12.  Freeman J. Dyson, 'Intelligent Life in the Universe', lecture, Astronomical Society of the Pacific, NASA, and the City College of San Francisco, 18th September 1972.

13.  E.F. Mallowe, 'Interstellar Communication:  Scientific Perspectives', JBIS, 28(3), 223-224  (1975).

14.  Duncan Lunan, 'Past Contact and the Moving Caravan', in Chris Boyce, "Extraterrestrial Encounter", David & Charles, 1979.

15.  Carl Sagan & I.S. Shklovskii, "Intelligent Life in the Universe", Holden-Day, 1966.

16.  Sebastian von Hoerner, 'The Search for Signals for Other Civilizations', reprinted in Cameron, ed., "Interstellar Communication", op cit.

17.  Duncan Lunan, 'Notes towards a Politics of Survival', Science & Public Policy, February 1987, translated in Katedra, Yugoslavia, January 1987, trans. in Samo Resnik, ed., "Fantazia", Casopis za kritiko znanosti, Llubjana, Yugoslavia, 1990;  ‘Introduction to the Politics of Survival’, ‘The Politics of Survival’, Asgard, March 2002.

18.  Duncan Lunan, “Man and the Planets”, Ashgrove Press, Bath, 1983.

19. Duncan Lunan, “Man and the Stars”, Souvenir Press, London, 1974.   (“Interstellar Contact” in USA, Henry Regnery Co., 1975).

20.  V.A. Hughes, D. Routledge, ‘An Expanding Ring of Interstellar Gas with Centre Close to the Sun’, Astronomical Journal, 77, 210  (1973).

21.  Michael Allaby & James Lovelock, "The Great Extinction", Secker & Warburg, 1983.

22.  Victor Clube & Bill Napier, "The Cosmic Serpent", Faber, New York, 1982.

23.  G. K. O’Neill, “The High Frontier”, Jonathon Cape, 1977.

24.  Richard D. Johnson, Charles Holbrow, "Space Settlements, a design study", NASA SP-413, US Government Printing Office, 1977.

25.  D.J. Sheppard, 'Concrete Space Colonies', Spaceflight 21, 3-8  (1979).

26.  Eugene Sänger, 'Zur Flugmechanik der Phonetenraketen', Astronautica Acta, 3, 89-99  (1957).

27.  R.W. Bussard, 'Galactic Matter and Interstellar Flight', Astronautica Acta, 6, 179-194  (1960).

28.  T.A. Heppenheimer, 'On the Infeasibility of Interstellar Ramjets', JBIS, 31(6), 222-224  (1978).

29.  Stephen H. Dole, "Habitable Planets for Man", Blaisdell Publishing Company, 1964.

30.  D. Phillips, 'A Chemist Looks between the Stars', IBM Heathrow Conference, 1987.

31.  'Lunar Initiative Updates', Lunar & Planetary Information Bulletin, 44, 11  (1986).

32.  Duncan Lunan, ‘Project Starseed:  an Integrated Programme for Nuclear Waste Disposal and Space Solar Energy’, JBIS, 36, 9, 426-432  (September 1983);  reprinted Infinity, 1985;  'Project Starseed, or, Nuclear Waste Saves the World', Analog, CV, 2, 54-73  (Feb­ruary 1985);  'Project Starseed', revised version, Settlers Sentinel, 1987;   ‘Pro­ject Starseed’, (fourth version), Asgard, Christmas 1991.

33.  Gregory L. Matloff, 'Utilisation of O'Neill's Model 1 Lagrange Point Colony as an Interstellar Ark', JBIS 29, 775-785  (December 1976).

34.  F.J. Dyson, ‘Human Consequences of the Exploration of Space’, in Eugene Rab­in­owitch & Richard S. Lewis, eds., “Men in Space”, Medical & Technical Publishing Co., Ltd., 1970.

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; 

'The Fermi Paradox', Speculations in Science & Technology, January 1988;  ‘Epsilon Boötis Revisited’, Analog, March 1998.[link]

38.  John G. Cramer, 'The Alternate View:  the Pump of Evolution', Analog Science Fiction/Science Fact, CVI(1), 124-127  (1986).

39.  David Brin, 'The Deadly Thing at 2.4 Kilo-Parsecs', Analog, CIV(5), 66-73, 1984.

40.  David Brin, 'Just How Dangerous Is the Galaxy?', Analog, CV(7), 80-94  (1985).

41.  D.H. Smith, 'Cygnus X-3:  Cosmic Ray Powerhouse', Sky & Telescope, 69(6), 497-500  (June 1985).

42.  B. Silcock, 'Space Rays:  a Starry Birth', Sunday Times, 3rd February, 1985.

43.  (Anon), 'Cerenkov Radiation Caused Apollo Flashes', New Scientist, 72, 1021, 14, 7th October, 1976.

44.  Duncan Lunan, 'Are Humans a Protected Species?', Second Look, June 1979  (incomplete);  (complete, translated) 'Is Man a Protected Species?' in W.A. Fuchs, ed., "Neue Beweise", Moewig Verlag, 1979;  reprinted in Spanish, Mundo Desconocido, Spain, 1980, and in P. Fiebag, ed., "Aus den Tiefen des Alls", Hohenrain, 1985.  

 

 

 

 

 

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