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KEYWORDS: Astrobiology, anthropic principle, gamma-ray bursts, philosophy of astronomy
It is the thunderbolt that steers the course of all things.
Heraclitus of Ephesus, cca. 550 BC
INTRODUCTION
Key Anti-SETI Arguments
It is hard to deny that the Search for Extra-Terrestrial
Intelligence (SETI) is one of the major scientific adventures
in the history of humankind. However, four decades of serious
SETI projects have not given results, in spite of the prevailing
"contact optimism" of 1960s and early 1970s, motivated
largely by uncritical acceptance of the Drake equation. Conventional
estimates of that period spoke of about 1E6 - 1E8 (!) advanced
societies in the Milky Way forming the "Galactic Club"
(Bracewell 1975). Nowadays, even the greatest SETI optimists have
abandoned such fanciful numbers, and settled to a view that advanced
extraterrestrial societies are rarer than previously thought.
Today, it is widely recognized that the "contact pessimists"
have a rather strong position (Hanson 1999; Bostrom 2002); most
of recent scholarly monographs on the subject are strongly skeptical
towards the possibility of finding complex intelligent life elsewhere
(e.g. Ward and Brownlee 2000; Webb 2002). Why is that so?
Anti-SETI "rollback" of the late 1970s and
1980s has been essentially based on two widely discussed arguments:
(1) Tsiolkovsky-Fermi-Viewing-Hart-Tipler's question "Where
are they?" in its modern von-Neumann-probe rendering1, and
(2) Carter's (1983) "anthropic" argument.
Tsiolkovsky, Fermi, Hart, and their supporters argue from the absence of extraterrestrials on Earth and in the Solar System, and the fact that they have, ceteris paribus, more than enough time in the history of the galaxy to visit, either in person or through their self-replicating probes. Characteristic time for colonization of the Galaxy, according to these investigators is 1E6 - 1E8 years, making the fact that the Solar System is (obviously) not colonized hard to explain, if not for the absence of extraterrestrial cultures. On the other hand, Carter's "anthropic" argument ("argument from ignorance" would be a better label here) tries to infer conclusions from the possible relationships between the alleged astrophysical (t*) and biological (t1) timescales. In the Solar system, t* is approximately equal to t1, within the factor of two. However, in general, it should be either t1 >> t* or t* >> t1 for two uncorrelated numbers. In the latter case, however, it is difficult to understand why the very first inhabited planetary system (that is, the Solar System) exhibits t* ˜ t1 behavior, since we would then expect that life (and intelligence) arose on Earth, and probably at other places in the Solar System, much earlier than they in fact did. This gives us probabilistic reason to believe that t1 >> t* (in which case the anthropic selection effects explain very well why we do perceive the t* ˜ t1 case in the Solar System). Thus, according to Carter, the extraterrestrial life and intelligence have to be very rare, which is the reason why we have not observed them so far. It seems clear that any general astrobiological scenario capable of dissolving these two crucial anti-SETI arguments would not only undermine the position of "contact pessimists", but also offer a valuable methodological guideline for the SETI projects themselves. With this in mind, in the rest of this paper we suggest a "catastrophic" astrobiological scenario based on the phase-transition of Annis (1999), which is capable of both refuting the anti-SETI arguments and give us significant rational basis for optimism in our searches for intelligent life elsewhere in the Milky Way.
EVOLUTIONARY APPROACH TO THE ASTROBIOLOGICAL STATUS OF OUR
GALAXY
First, it is necessary to mention that both Fermi's "paradox" and
Carter's argument have been criticized so far. Thus, for instance, the colonization
timescale is still largely uncertain; for instance, diffusion models of Newman
and Sagan (1981) give the relevant timescale as ~1E9 years, which would correspond
to the naive answer one is expected to give on the Fermi's question: They are
still on the way! Second, the issue of motivation of colonizers, and particularly
their von Neumann probes is much less clear and unambiguous than the "contact
pessimists" would have us believe. Notably, as suggested by Brin (1983)
in his seminal review, the "deadly probes" scenario (the idea that
the dominant behavior of self-replicating probes is destruction of nascent civilizations,
not colonizing) is one of just a few theoretically satisfactory explanations
of the "Great Silence". In a similar vein, Kinouchi (2001) has recently
argued that the phenomenon of persistence, well-known from statistical physics,
holds the key for explanation of the apparent absence of extraterrestrial civilizations;
in this picture, Galactic colonization by advanced ETIs could have already last
for quite some time without influencing the Solar System. Wilson (1994) has
persuasively criticized Carter's usage of the anthropic principle to show that
life is rare in the universe.
However, the most important line of thought which can
easily defeat both Fermi's and Carter's arguments lies in investigation
of hidden temporal assumptions in these arguments. Fermi et al.
suppose that the history of the Galaxy is uniformitarian, in the
sense that advanced technological communities could arise at any
point in the Galactic history (except, perhaps, the first billion
years or so, when the metallic content was too low; see Lineweaver
2001). Similarly, Carter assumes that the only relevant astrophysical
timescale is the Main Sequence stellar lifetime. Uniformitarianism
has not shown brightly in astrophysics and cosmology, at least
since the demise of the classical steady-state theory in mid-1960s
(Kragh 1996). Today we are certain that evolutionary properties
of astrophysical systems are from time to time guided by processes
either rare or unique (like the primordial nucleosynthesis or
the reionization of intergalactic medium) or occurring at timescales
so much vaster than the timescales of human civilization that
the probability of actually observing them is nil (like the recently
computed evolution of M-dwarf stars2 ). For this fascinating
subject in theoretical astrophysics, see Laughlin, Bodenheimer,
and Adams (1997). In this specific case, if the phase-transition
model sketched in a brilliant short paper of Annis (1999; see
also Clarke 1981) is correct-as we have more and more reasons
to believe-the relevant timescale is the one describing intervals
between the major galaxy-wide catastrophes, precluding the increase
of complexity of planetary biospheres and, consequently, the development
of intelligent observers. There are several plausible candidates
for this global regulation mechanism. The strongest, as suggested
by Annis in his study, are gamma ray bursts (GRBs), which accompany
either a coalescence of binary neutron stars or explosions of
super-massive stars, also known as the hypernovae (for a review
of GRB mechanisms, see Piran 2000).
Astrobiological effects of GRBs have been investigated
recently in a number of papers (Thorsett 1995; Dar 1997; Scalo
and Wheeler 2002), and much of the older literature dealing with
effects of supernova explosions is useful in this case too (after
appropriate scaling, of course; see, for instance, Tucker and
Terry 1968; Ruderman 1974; Clark, McCrea, and Stephenson 1977).
It seems that each GRB is surrounded by a "lethality zone"
in which its effects are deadly for complex life forms (eukaryotes);
according to Scalo and Wheeler (2002). The radius of this zone
is ~14 kpc, rather large in comparison to the galactic habitable
zone. The exact effects of a GRB within a "lethality zone"
are still somewhat controversial, but it is clear that there will
be at least two deadly effects capable of causing mass extinctions.
(I) Creation of nitrogen-oxides (usually denoted by NOx) in the
upper atmosphere, which will destroy the ozone layer for thousands
of years, thus enormously increasing UV radiation at planetary
surface. (II) Creation of a longer delayed pulse of cosmic rays,
which penetrate the atmosphere (and even rocks and soil up to
several km of depth) and cause various sorts of damage to biological
materials. Both these effects are prolonged in comparison to the
GRB itself, thus affecting more than the hemisphere directed toward
the source. In fact, the consequences in biological domain may
last many generations, especially when one considers such effects
as increase in frequency of cancers, and occasionally very long
interval needed for a species to die out when its population decreases
below the so-called minimum viable population.
Other suggested regulation mechanisms are the climatic
change due to interaction with galactic spiral arms (Shaviv 2002),
neutrino-induced extinctions (Collar 1996), or galactic tides
leading to the Oort comet cloud perturbations (e.g. Clube and
Napier 1990; Rampino 1998)3. Their common property is that they
are global, i.e. influencing the entire Galactic habitable zone,
or a large portion of it. Moreover, they reinforce each other;
in other words, the total risk function is a sum of risk functions
for each specific threat to biological systems. Further research
might confirm that GRB-regulation has another desirable property:
quantifiable secular evolution, which explains our existence at
this particular epoch of the Galactic history. Notably, cosmology
suggests the rate of GRBs behaves, on the average, as a exp(-t/t),
with the time-constant t of the order of 1E9 yrs (Annis 1999).
Namely, as noticed by Norris (2000), we have to ensure that there
is no "overkill" as far as the regulation mechanisms
are concerned, and that our own existence is explicable in naturalistic
terms. This is readily achieved within the framework of the GRB-dominated
phase-transition picture: cosmology assures us that the average
rate of GRBs increases with redshift, i.e., decreases with cosmic
time. When the rate of catastrophic events is high, there is a
sort of quasi-equilibrium state between the natural tendency of
life to spread and increase complexity and the rate of destruction
and extinctions governed by the regulation mechanism(s). When
the rate becomes lower than some threshold value, intelligent
and space-faring species can arise in the interval between the
two GRB-induced extinctions, and the galaxy experiences a phase
transition: from essentially dead place, with pockets of low-complexity
life restricted to planetary surfaces, it will, on a very short
Fermi-Hart-Tipler timescale, become filled with high-complexity
life. We are living within that interval of exciting time, in
the state of disequilibrium (Almár 1992), on the verge
of the galactic phase transition.4 In Heraclitus' aphorism quoted
above, galactic GRBs are the "thunderbolt" which steers
the course of all things astrobiological.
It is clear that this class of models effectively removes
the threat to ETIs from both Fermi-Hart-Tipler and Carter's arguments.
Elsewhere in the galaxy, there are other planets with a level
of complexity more or less similar to the terrestrial one. There
simply was not enough time for them to come to us, since the astrobiological
history-as far as complex metazoans are concerned-is different
and significantly shorter from the history of dark matter, stars,
and gas clouds which constitute the physical structure of the
Galaxy. Local astrobiological clocks can tick at various rates,
but they are all from time to time reset by the global regulation
mechanism(s). But Fermi's question is rapidly becoming pertinent,
when we realize that during the phase transition many advanced
intelligent societies are bound to develop, but they are not all
bound to expand to their utmost limits (that is, to colonize the
galaxy) within the same interval of time.
On the other hand, the very existence of well defined
astrophysical and biological timescales is an unwarranted assumption
of Carter's argument. This assumption is wrong in the context
of the phase-transition models. The real timescales are specific
to each planetary system, depending on such factors as the location
of the system in the galactic habitable zone (GRB distribution
having a spatial, as well as temporal aspect!), peculiarities
of the local environment (notably the density and distribution
of cometary and asteroidal material presenting an impact hazard),
and-of crucial importance-the epoch of galactic history. In other
words, there is no physical reason why on planet A, at galactocentric
distance RA and at epoch tA we could not have t1 >> t* while
on planet B (characterized by RB, tB, and probably some other
astrobiological parameters) we could have t* >> t1. The
dependence on the epoch is particularly important; to paraphrase
the title of the controversial book by Ward and Brownlee (2000),
Earths might be rare in time, not in space. This sort of models
can also shed some new light on the Drake equation (Walker and
Cirkovic 2003; Cirkovic 2003).
LESSONS: ASTROPHYSICS OF OPEN SYSTEMS
The phase-transition models represent a natural extension
of the contemporary pictures developed in Earth sciences. We have
recently learned that impacts of extraterrestrial bodies had tremendous
influence upon the evolution of the biosphere (Raup 1991, 1999;
Clube and Napier 1990). Even the mundane phenomena like cloudy
skies are influenced by such previously un-dreamt of factors like
the low-energy cosmic ray flux (e.g. Carslaw et al. 2002). In
general, this is a part of the wider cultural tendency to relinquish
the notion of a closed-box system in favor of open (complex) systems.
Even in the connection with such rather well understood
threats to life on Earth as collisions with comets or asteroids,
a wider connection is sought in investigation possible influences
from outside the Solar System (Matese and Whitman 1992; Matese
and Whitmire 1996; Rampino 1998). For instance, it has been investigated
whether the giant molecular clouds, concentrated toward the galactic
plane, or even the tidal force of the galactic disk itself can
produce sufficient perturbations of the Oort cloud in order to
cause impacting comet showers. Crossing of galactic spiral arms
has been repeatedly argued to be dangerous from the planetological,
climatological, and biological points of view (e.g. Marochnik
1983; Shaviv 2002). There is no reason to think that this tendency
will be arrested or rejected in the years to come, as both astrophysicists
and planetary scientists on one side and biologists and ecologist
on the other, probe more and more subtle and minuscule effects,
and their computer simulations become richer and richer with seemingly
disconnected phenomena. Even the classical Gaia hypothesis (e.g.,
Lovelock 1988) is rather moderate in this respect, when astrobiological
issues are concerned. We envisage a tight interconnection of astrophysical
and astrobiological aspects of reality on scales as large as the
Milky Way galaxy. The closed-system innocence (or "splendid
isolation") of Earth and its biosphere is forever lost.
Two important practical conclusions for SETI projects
are the following. (1) Phase-transition models suggest that extraterrestrial
civilizations are vastly more likely to be of appropriate age
for communication than could be inferred by naive application
of the Drake equation. That is, we do not need to worry further
about the sensibility of attempting to communicate with beings
billion or more years older than us. Phase-transition models predict
that all galactic civilizations will be of similar age, i.e. the
time elapsed since the last "resetting" of the astrobiological
clock. (2) Future detailed numerical phase-transition theory (Cirkovic
and Sandberg, in preparation) will enable assigning statistical
weights to various SETI target stars and planetary systems depending
on their ages and orbital elements in the galaxy. For instance,
it is intuitively clear that we are more likely to encounter advanced
biospheres in the outer parts of the galactic Habitable Zone than
in the inner parts. The density of GRBs and supernovae declines
sharply with the galactocentric radius, so outer regions are more
likely to stay undisturbed for sufficiently long time to allow
for biogenesis and noogenesis. However, additional effects of
metallic content, spiral-arm crossings, etc. have to be taken
into account in the future numerical work in this direction.
CONCLUSIONS
The phase-transition models offer a hope of reconciling
both our astrophysical knowledge and negative SETI results on
one hand, with naturalistic explanations for biogenesis and noogenesis
and the Copernican Principle on the other. Thus, Fermi's paradox
is explained as a part of much wider astrobiological paradigm,
which is certainly desirable from the methodological point of
view.
The price paid is obviously high, but not in usual epistemical
sense, but rather in the ethical one. It means that enormous destruction
of life is taking place in what is conventionally portrayed as
peaceful and hospitable universe. GRBs occur once per day on the
average, and they sample practically all galaxies within our particle
horizon. Thus, if the picture suggested by the phase transition
model is correct, we are witnessing immense destructions of complex
life forms (some of them undoubtedly intelligent, but not reaching
the "immunity level") throughout the universe on a daily
basis! This cannot fail to be a depressing thought. Our past light-cone
is full of slaughterhouses on an unprecedented level. Statistically,
a staggering majority of all life forms ever emerging are exterminated
by random and violent catastrophes of astrophysical origin. The
universe might be a significantly more cruel and inhospitable
environment than it's usually assumed. It is quite possible that
simple organism analogs to Deinococcus Radiodurans (Battista 1997)
are by far the most prevalent form of life in the universe on
the average.
But on the balance, it is still hard to find this scenario
worse than any naturalistic alternative. As noted by many biologists,
the role of mass extinctions in the history of life here on Earth
was ambiguous, both destructive and constructive (e.g., Raup 1994).
We would almost certainly not have been here if a comet or asteroid
had not struck Earth 65 Myr ago causing, among other things, the
end of the epoch of giant reptilians. It is far from certain that,
had this event not happened, intelligence would have developed
on Earth (by now, at least).
This may mean, on the other hand, that those who survive
are even more valuable than conventionally thought. On the background
of natural disasters, even a single anthropogenic disaster (like
the looming ecological dangers, nuclear warheads, or nanotechnology)
is simply the one disaster too many. If properly understood, the
astrobiological models can teach us the rarity of values - the
ultimate products of generic creative and intelligent minds. In
futures, these values will almost certainly (by one or another
roughly human-comparable civilization) be spread to all corners
of the galaxy, and possibly even beyond. It is these values that
we ought to seek in our peers through the SETI programs, and to
advance in ourselves through the entire scientific and artistic
endeavor of humanity.
Acknowledgements. The author thanks Larry Klaes, Robert J. Bradbury, Ivana Dragicevic, Nick Bostrom, and Richard Catcart for useful and friendly discussions. Technical help of Vjera Miovic, Ivana Dragicevic, Branislav Nikolic, Ivan Almár, Vesna Miloševic-Zdjelar, Saša Nedeljkovic, Srdjan Samurovic, and Mark A. Walker is also kindly appreciated.
APPENDIX:
1. Stephen Webb, in his recent monograph, so far the best historical
introduction into the "Great Silence" problem (Webb
2002), dubs the relevant question Tsiolkovsky-Fermi- Viewing-Hart's.
We find it only just to add Tipler to list, since his von Neumann
probe setup gives the whole problem a completely new flavor (Tipler
1980). Of course, it is best known simply as "Fermi's paradox"
(which we shall use, for the sake of brevity, in the rest of the
paper).
2. For this fascinating subject in theoretical astrophysics, see
Laughlin, Bodenheimer, and Adams (1997).
3. The idea of Clarke (1981) that nuclear outbursts-similar to
the ones observed in Seyfert galaxies-from the core of the Milky
Way can lead to devastation of habitable planets throughout the
galaxy has been, historically, the first global regulation mechanism
proposed. However, it seems to be abandoned as we learn more about
the center of our galaxy.
4. (For variations-now of "only" historical importance-on
the same theme see Clube 1978; LaViolette 1987.) 4 Notice that
the anthropic selection effect (cf. Bostrom 2002) readily explains
why that is so, in spite of the very low a priori probability.
Humans could not arise prior to the phase transition, since there
was no time for high-complexity life to evolve without being destroyed
by cosmic rays and other detrimental consequences of GRB regulation
(or cumulative effects of impacts, close SNs, spiral-arms crossings,
and other calamities). On the other hand, we could not arise later
from the phase transition epoch for the same reason one does not
expect to find a previously unknown stone-age tribe in the present-day
Europe: high-complexity ecological niches do not allow spontaneous
emergence of new lower-complexity life forms.
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