The fundamental purpose of this article is to catalogue the factors which would determine and/or predict the likelihood of contact with extra-terrestrial intelligent species (ETIS). The article attempts to proceed in a scientifically rigorous manner, beginning with factors which are widely believed to be realistic, and working its way toward several more extreme ideas and theories.
The battle lines in this debate are drawn between two basic groups: Contact Optimists (content that simple reasoning indicates a Universe in which life and intelligence must be relatively common) and followers of the Uniqueness Hypothesis (those who suggest that the Earth is unique).
With this in mind, I shall dive straight into the article and introduce the
'Drake Equation':
This equation only speaks of the number of sites in the Galaxy which harbor
life at any given instant. The equation does not take into account the
"encounter cross-section" or the probability that any such site will be
encountered by us. If we say
is the probability of contact between
mankind and ETIS, then a more appropriate equation would be (Equation 2):
For Equation (2), in which
is the probability of contact, the sum is
from 1 to
,
the total number of spontaneous detectable intelligent
technologies. In addition, for each
site:
=0
implies only the home planet is settled)
Unfortunately, the number of settled neighboring systems is equally
complicated. Brin, recognizing the dependence of
up a species'
expansion velocity, average life span, and recovery time before expanding
further, defines
as in equation (3):
species has
expanded
The concept is currently accepted to hold for physical conditions such as star formation, stellar types, planetary entourage (for the appropriate star types) and general chemical compostions.
The implication here is that life in general is a small perturbation from the general equilibrium of the universe and that the galaxy looks the same over long time periods.
- Average galactic rate of star production
- Fraction of created stars which are considered good
Good means the star in question is a stable dwarf (main sequence) star of spectral classs F, G or K.
- The number of "good" stars which have planets
Currently this is considered to be a large number for the following reasons. It seems that most stable dwarves rotate slowly. In our solar system, the sun, a stable dwarf, comprises 99.9% of our system's total mass and 1% of its total angular momentum. The theory here is that a forming star system must conserve the total angular momentum of its gas cloud progenitor. To conserve angular momentum and to have a low rotation rate would seem to require the existence of other bodies in the system (such as Jupiter and other planets), which would comprise the enormous angular momenta.
The factor
is assigned a value of 0.1.
- Fraction of planets which develop life
Current theories and experiments suggest that precursors to life might be created under mundane and non-biotic conditions. The universe might be a soup of pre-biotic material. However, the leap from organic soup to membrane-moderated, self-replicating DNA structures is a long one.
One theory suggests life may arise spontaneously in temporary liquid-filled chambers within water-ice comets. Another theory suggests that the 3-9 micrometer, infrared signatures of interstellar dust are perfectly matched by the transmission spectra of freeze-dried E. coli bacteria.
Statistical arguments suffer from the supposition that interactions were random, requiring aeons before the first ancester of DNA appeared. There are two rebuttals to this point. First, geological records indicate that the biological activity began within 700-900 million years after the planet formed (barely sufficient time for the surface to cool), and this indicates that a rapid process did in fact occur. Second, there may have beeen some sort of driver present which biased the early reactions in some particular direction.
This "driver" agent may have been adenine. Adenine is the only one of the DNA/RNA bases which pairs with two partners, and it is a very stable compound.
There appears to have been some sort of selection or biasing on Earth because, from the thousands of possible amino acids, only 300 occur naturally, and only 20 are coded for protein synthesis in your cells.
Cut to the chase: Life on Earth is of one "orientation", but this does not imply that some prebiotic miracle was responsible.
Other factors limiting
are; The possibility of a greenhouse
catastrophe, weaker geomagnetic fields leading to an ultraviolet holocaust,
'life-zones' around less massive stars being too narrow for habitable worlds
to be large in number.
Finally, 'panspermia': The idea that life on Earth was seeded by ETIS accidentally or purposefully. However, there is a 1.5 billion year time lapse between the appearance of life and the appearance of eukaryotic organisms. This huge window points against the idea of outside intervention - why wait so long?
Final words on
:
Nothing definite. SETI forces assign a value of
0.1-1.0.
- The odds of life evolving intelligence
Definition of intelligence: Active, mobile creatures, possessed of time-sense, discursive ability, curiosity, and powers of comparison and self-appraisal.
Some argue that intelligence is selectively advantageous. The concept of convergence support this idea. Convergence occurs when unrelated taxa evolve similar morphologies or behaviors. Examples of convergence on Earth are: the shapes of dolphins, tuna and the extinct icthyosarus; the similar lens-eyes of vertebrates and cephalopod mollusks; two totally unrelated species of wolves which evolved in Austrialia and South America; the North American flying squirrel and the phalanger of Australia; the porcupine and the echidna; and humming birds and humming moths.
Convergence does not imply that the converging taxa are identical. It has
been suggested that an average of 1000 descrete mutations separate neighboring
species. Noting that there are about a million known species on Earth, sheer
statistics put the odds at a billion-to-one against our current ecosystem and
odds of
against duplication of mankind. Duplication, however, is
not required; the supposition is that there are many paths to the same
end. thereby making it possible for many completely different, but similar, ETIS to evolve without outside intervention.
Chimpanzees and dolphins possess the ability to respond accurately to complex four or five element sentences. There is no longer much doubt that the higher cetacea are as bright as apes. Can the evolution of these two very distinct taxa to a similar level of threshold intelligence indicate that intelligence is the evolutionary norm?
So the concept of convergence seems to imply larger values of
.
One author suggests that the key factor in the advancement of a species
is the size of its genome (although salamanders and wheat germ have larger
genomes than man). He goes on to state that the only practical way to enlarge
the genome is via a rare, doubling mechanism called polyploidy. Assuming
this doubling were to occur at most every few hundred million years, the
mammalian genome should have a probablility of
or so.
However, polyploidy is almost universally fatal to vertebrates. In addition, the genome could simply enlarge gradually via translocation, inversion and chromosome fusion, and these processes are much more simple.
The final conclusion is that the value of
is not extremely
small, although no specific value is given. It is unlikely that this factor
is a limiting factor.
- The probability for the emergence of a technological culture
Even if there are selective advantages to developing the hunter/gatherer/farmer way of life, including language and tools, there is no explanation for mankinds' overshoot to a space-faring race. The basic implication being that natural selection does not point toward advanced technology, since advanced technology is not needed to be the dominant life form in an ecosystem - sticks, bricks and muskets will do just fine.
This factor is basically ignored because there is nothing that can be said
for sure about it, with one exception: No one contends that
could
be so small as to be the limiting factor in the contact equation.
- Home-world life span of technological species
This is the first factor that may actually be crucial. The mean lifetime
is crucial in determining the cross-section for interaction. Even if only one
in a hundred species survived some inital crisis, they would settle into a
long maturity and should dominate the galaxy at equilibrium. The other
possibility is that
is catastrophically low, implying that
technologically advanced species must pass through a survival crisis. (e.g.,
mankind's very real potential for self-destruction). However,
self-destruction is difficult to support with anything more than an amorphous
"aggressive instinct." The consensus is that loss due to self-inflicted
crisis is minimal or at least not limiting.
The following theories might also lower
by eliminating ETIS;
Resource exhaustion. (Straight forward)
Transcendence - the theory that an intelligence might abandon technology as we know it on purpose for some other realm of adventure or understanding.
The concept of surrogate robots - a species might send self-replicating
message robots in search of ETIS. There is little dispute that this could
conceivably work. There are two basic ideas for the robots' purpose; the "seek
out, contact, lend assistance" friendly robots, which would have a profound positive effect on the
of OTHER species and places a large burden on equation2,
(Remember, we think the contact ratio IS low, so large values mean something
else must be limiting contact...) or the concept of "deadly probes". Suppose
one in 10,000 ETIS are paranoid or xenophobic... 'deadly probes' is consistent
with all the facts and philosophical principles. We don't have to suppress
the Drake equations, we don't have to suggest the ETIS don't want to travel,
since all ETIS are destroyed shortly after discovering radio (and I Love Lucy
is past Tau Ceti. :)
- Star travel, effective expansion velocity
We shall assume Einsteinian travel restrictions remain in effect and that colonized areas expand under population pressure. A cycle of settlement, development, saturation and further exploration (decades to hundreds of thousands of years later). The rate depends on stable star site density, the species's growth rate and emigration rates. Assuming starship velocity below 0.1c (c being the speed of light) and ranges less then 5 parsecs, should be able to populate the galaxy in 60 million years. Including settlement pauses, one author calculated and expansion velocity about 0.016c. It turns out the expansion velocity is relatively independent of actual ship speed.
Other authors have suggested that population pressure is not the driving force and that dispersal is. Population pressure is preferred because it is seen here on Earth and is much easier to model. It also gives lower limits on diffusion rates.
Some authors suggest ETIS may simply not have arrived yet, claiming it may take up to 2 billion years for a species to populate the galaxy. However this extremely slow growth rate still requires only 500 million years if one takes galactic rotation into effect. The "not here yet" scenario is also a profound dis-equilibrium.
Suppression of
is still a favorite way to suppress
the contact cross-section.
- Approach and avoidance
When attempting to explain the apparently low value of
in Equation (2),
(i.e., why aren't they here yet...), authors generally take one of two
approaches. Either the ETIS are actually not here or they have chosen not to
contact us. So far, I have discussed reasons for their not being here right
now. The factor
concerns not wanting to contact us.
Avoidance: We may be avoided for the following reasons. Making us information consumers too soon would spoil our potential as information suppliers later. Or, if the "deadly probes" scenario is correct, ETIS may be cautious about leaking radio waves. Finally, if probes mutate as they replicate, they may change too much and ETIS may be cautious about sending out probes to begin with.
Search strategies: We may be looking via completely inappropriate methods. Argument over which radio frequencies to monitor continues. Communications traffic carried on narrow, coherent, directed beams would be undetectable.
Quarantine: We may be being kept as some type of "zoo" while some benevolent species awaits our physical (or intellectual??) maturity. We may be considered dangerous, or they may already be in secret contact for similar reasons.
Macrolife: Rapid social (and physical?) changes may make space dwelling
the prefered life style, leading to two scenarios; Space dwellers fragmenting
worlds for materials (directly suppressing
,
and
.)
or they might cherish "nursery" worlds, leaving them undisturbed
without contacting them at all.
Alternative lifestyles: ETIS may graduate to other interests; conservatism, ecological sensitivity, aversion to spaceflight dangers,...
- Settled spheres factor
Ship speed is generally assumed to be slow enough that ETIS would stop at the first acceptable site. Most discussions center on the leading surface of the expansion. However, population pressure, which is supposed to be driving the expansion, would likely cause conflict for resources within the planetary systems well inside the leading surface of the settled sphere.
To take this into account a survival factor is assigned to a site.
goes up as the cube of the radius reached. If
is small,
however, inner sites die off very rapidly, the outer shell dominates, and the
number of sites goes up as the radius squared (proportional to the surface
area). When the expansion encounters the galactic disc shape, the number of
sites goes linearly with r (proportional to the circumference.) Eventually,
R dominates and the expanding ring dies out like a circular grass fire.
This should lead to cyclic waves of migration and colonization. Planets are
settled, exploited and then abandoned. Unless extremely careful, tenant
species would suppress indigenous life forms. The recovery period after their
departure depends on their treatment of the adopted world. This implies
population pressure may also affect
and
. Not to mention
the possibility of counter migration.
So are there any signs of occupation of the Earth? There is a faint possibility that the cretaceous Catastrophe was a meteoritic impact. However, Brin suggests it may have been warring aliens. There are four other mass destructions which have the faint possibility of relation to ETIS population pressure. This might indicate that the Earth is the first nursery world to recover, according to Brin. (But I point out that this goes against the non-exclusiveness rule he so loves to use). Brin also suggests that the absence of ETIS may be a testament to the fate of species who let population pressure be their motavation to reach for the stars.
