Research Paper The Temporal Aspect of the Drake Equation and SETI
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ASTROBIOLOGY
Volume 4, Number 2, 2004
© Mary Ann Liebert, Inc.
Research Paper
The Temporal Aspect of the Drake Equation and SETI
MILAN M. ĆIRKOVIĆ
ABSTRACT
We critically investigate some evolutionary aspects of the famous Drake equation, which is
usually presented as the central guide for research on extraterrestrial intelligence. It is shown
that the Drake equation tacitly relies on unverified assumptions on both the physicochemi-
cal history of our galaxy and the properties of advanced intelligent communities. In this man-
ner, the conventional approach fails to take into account various evolutionary processes form-
ing prerequisites for quantification of the Drake equation parameters. The importance of
recent results of Lineweaver and collaborators on chemical build-up of inhabitable planets
for the search for extraterrestrial intelligence is emphasized. Two important evolutionary ef-
fects are briefly discussed, and the resolution of the difficulties within the context of the
phase-transition astrobiological models is sketched. Key Words: Galaxy:evolution—Extrater-
restrial intelligence—History and philosophy of astronomy. Astrobiology 4, 225–231.
INTRODUCTION is the desired result of the analysis. We investi-
gate the following form (see, e.g., Shklovskii and
I T IS HARD TO DENY that the search for extrater-
restrial intelligence (SETI) is one of the major
scientific adventures in the history of humankind.
Sagan, 1966; Walters et al., 1980; Duric and Field,
2004):
At the beginning of the 21st Century it remains N R* fg fpne fl fi fc L (1)
the oldest and perhaps the most fascinating sci-
entific problem. However, the field is still largely while keeping in mind that other equivalent
qualitative and thus often not taken seriously forms exist. In this expression, the symbols have
enough. One of the attempts to overcome this cir- the following meanings: N the number of
cumstance is encapsulated in the famous Drake galactic civilizations with whom communication
equation, developed by Frank Drake for the first is possible; R* mean rate of star formation in
SETI symposium in 1961 (Drake, 1965). the Galaxy; fg fraction of stars suitable for sup-
The first problem any student of SETI faces is porting life; fp fraction of stars with planetary
that there is no canonical form of the Drake equa- systems; ne number of planets per planetary
tion. Various authors quote various forms of the system with conditions ecologically suitable for
equation, and it is in a sense dependent on what the origin and evolution of life; fl fraction of
Astronomical Observatory, Belgrade, Serbia and Montenegro.
225226 ĆIRKOVIĆ
suitable planets where life originates and evolves order to promote their respective views. How-
into more complex forms; fi fraction of planets ever, arguments for both sides are suspect if the
bearing life with intelligence; fc fraction of underlying relationship has serious deficiencies
planets with intelligence that develops a techno- for any practical application, e.g., for estimating
logical phase during which there is the capabil- the time scale for sustained SETI effort by which
ity for an interest in interstellar communication; we might expect to detect extraterrestrial intelli-
and L mean lifetime of a technological civiliza- gent signals (or artifacts).
tion. Almost all authors agree on the general In 4 decades of SETI projects there have been
meanings of various f parameters and ne (though no results, in spite of the prevailing “contact op-
the values ascribed to each differ by several or- timism” of 1960s and 1970s, motivated largely by
ders of magnitude!); on the other hand, the prod- uncritical acceptance of the Drake equation. Con-
uct R*L is sometimes written in the form ventional estimates of that period spoke about
106–109 advanced societies in the Milky Way
R* L n* L (2) forming the “Galactic Club” (Bracewell, 1975).
t0 Today, even SETI optimists have abandoned such
where n * is the current number of stars in the fanciful numbers, and settled on a view that ad-
Galaxy, and t0 is the age of our stellar system [cur- vanced extraterrestrial societies are much rarer
rently thought to be t0 12 Gyr (see, e.g., Krauss than previously thought. One of the important
and Chaboyer, 2003)]. This is useful since (i) R* is factors in this downsizing of SETI expectations
not a directly measurable quantity [The problem has been demonstration by “contact pessimists,”
of evolution of the star-formation rate in spiral especially Michael Hart and Frank Tipler, that the
galaxies has been recently been tackled from both colonization—or at least visit—of all stellar sys-
local (nearby galaxies) and the large-scale galaxy tems in the Milky Way by means of self-repro-
survey perspectives. These studies shed some ducing von Neumann probes is feasible within a
light on the mean star-formation rate during the minuscule fraction of the galactic age (Hart, 1975;
entire history of a spiral galaxy (see Hopkins et Jones, 1976; Tipler, 1980, 1981). In this light,
al., 2001; Panter et al., 2003; Hartwick, 2004).], Fermi’s legendary question, “Where are they?,” be-
while n* and t0 are, at least in principle, and (ii) comes disturbingly pertinent (Webb, 2002). In ad-
it enables direct comparison of two characteristic dition, Carter (1983) suggested an independent
time scales, cosmological (t0) and “astrosociolog- and powerful anthropic argument for the unique-
ical” (L). There is a catch in Eq. 2, however, since ness of intelligent life on Earth in the galactic con-
the star-formation rate is not uniform throughout text. It is generally recognized that “contact pes-
the history of the Galaxy, and thus in general simists” have a strong position. How then, one is
R* n*/t0. While this particular problem is not tempted to ask, does the discrepancy with our
acute from the SETI point of view, because of the best analyses of Eq. 1 arise?
metallicity effects (early epochs of intense star We show that there are two difficulties that
formation are characterized by low metallicity, make Eq. 1 much less practical from the SETI
preventing formation of habitable planets), it point of view than conventionally thought. The
points in the direction of similar difficulties fol- two have the opposite effect on N, and may well
lowing from unwarranted assumptions of unifor- partially cancel one another out; still, by a care-
mity. We argue below that the main shortcoming ful consideration those effects could be decou-
of the Drake equation is its lack of temporal struc- pled. Some other criticisms of the Drake equation,
ture, i.e., it fails to take into account various evo- from different points of view, can be found in
lutionary processes that form a prerequisite for Walters et al. (1980), Tipler (1980), Wilson (1984),
anything quantified by f parameters and ne. Mash (1993), Ward and Brownlee (2000), and
It is important to understand that we are criti- M.A. Walker and M.M. Ćirković (unpublished
cizing the Drake equation not as an expression data). Some of these accounts mention in passing
per se, but as a guideline for a rather specific set the difficulties arising from changing of one or
of programs, procedures, and (in the final analy- more parameters in Eq. 1 with time, e.g., Mash
sis) investments, known overall as SETI. In SETI (1993) relegates this problem to a footnote, but a
research proposals, Eq. 1 figures very promi- more elaborate treatment of these evolutionary
nently. Both supporters and opponents of SETI aspects is still lacking. Proponents of vigorous
invoke the same simple numerical relationship in SETI research, including Frank Drake himself,THE DRAKE EQUATION AND SETI 227
have also from time to time mentioned the lack tology (Adams and Laughlin, 1997; Ćirković,
of temporal structure as one of the disadvantages 2003). Any results from it, albeit very exciting and
of this equation (see, e.g., Drake and Sobel, 1991), interesting in their own right, are unimportant to
but avoided the discussion of systematic biases SETI because of the large disparity of the time
following such simplification. scales involved.
According to a recent study by Lineweaver
(2001), Earth-like planets around other stars in the
UPPER LIMIT ON CIVILIZATION’S AGE galactic habitable zone are, on average, 1.8 0.9
Gyr older than our planet (see also the extension
In principle, the parameter L in Eq. 1 could be of this study by Lineweaver et al., 2004). His cal-
arbitrarily large, thus offsetting any exceptionally culations are based on chemical enrichment as the
small value among different f parameters. Histori- basic precondition for the existence of terrestrial
cally, that was the conventional assumption of planets. Applying the Copernican assumption
“contact optimists” like Sagan, Shklovskii, or Drake naively, we would expect that correspondingly
in the earlier decades (1960s and 1970s) of SETI ef- complex life forms on those others to be on the av-
forts. It is reasonable to assume that after a tech- erage 1.8 Gyr older. Intelligent societies, therefore,
nological civilization overcomes its “childhood should also be older than ours by the same
troubles” (like the threat of destruction in a nuclear amount. In fact, the situation is even worse, since
war or through the misuse of nanotechnology) and this is just the average value, and it is reasonable
starts colonizing space, it has very bright prospects to assume that there will be, somewhere in the
for survival on time scales of millions or even bil- Galaxy, an inhabitable planet (say) 3 Gyr older
lions of years. Since it was intuitively clear (al- than Earth. Since the set of intelligent societies is
though quantified only recently; see below) that likely to be dominated by a small number of old-
most of the inhabitable planets in the Milky Way est and most advanced members (for an inge-
are older than Earth, it was hypothesized that civ- nious discussion in somewhat different context,
ilizations to be found through SETI projects will be see Olum, 2004), we are likely to encounter a civ-
significantly older than our civilization. However, ilization actually more ancient than 1.8 Gyr (and
it is a leap of faith from a reasonable estimate of probably significantly more).
the temporal distribution of civilizations to the as- It seems preposterous even to contemplate any
sumption that we would be able to communicate possibility of communication between us and
with them, or that they would express any interest Gyr-older supercivilizations. Remember that 1
in communicating with us using our primitive Gyr ago the appearance of even the simplest an-
communication means. Even worse, people of the imals on Earth lay in the distant future. [Edi-
“contact optimism” camp have been expressing acaran fauna—a kind of fuse on the the famous
hope that we would be able to intercept communi- Cambrian Explosion—is now being dated at
cations between such very advanced societies, “only” 565–543 Myr before the present (see, e.g.,
which seems still less plausible. [For a profound Conway Morris, 1990).] Thus, the set of the civi-
and poignantly ironic literary account of these is- lizations interesting from the point of view of
sues see Lem (1984, 1987).] SETI is not open in the temporal sense, but in-
Obviously, from Eq. 1 we have limL N , stead forms a “communication window,” which
which is senseless, for the finite spatial and tem- begins at the moment the required technology is
poral region of spacetime we are considering in developed (factor fc in the Drake equation) and is
practical SETI. And still, remarkably, it is not terminated either through extinction of the civi-
senseless to contemplate upon the possibility that lization or through its passing into the realm of
very advanced civilizations can exist indefinitely “supercivilizations” unreachable by our primi-
in an open universe (see, e.g., Dyson, 1979), i.e., tive SETI means. Formally, this could be quanti-
that the limit L makes sense. Whether an ad- fied by adding a term to the Drake equation cor-
vanced technological society can exist indefi- responding to the ratio of the duration of the
nitely—in accordance with the so-called Final An- “communication window” and L. Let us call this
thropic Principle of Barrow and Tipler (1986) or ratio ; we are, thus, justified in substituting L in
the Final Anthropic Hypothesis of Ćirković and Eq. 1 with L. Since is by definition smaller than
Bostrom (2000)—is still an open question in the unity (and perhaps much smaller, if the present
nascent astrophysical discipline of physical escha- human advances in communication are taken as228 ĆIRKOVIĆ
a yardstick), the net effect would be to drastically Exactly such a picture is presented by a class
reduce the value of N. Fortunately (from the SETI of phase-transition models (Clarke, 1981; Annis,
point of view) this is not the only evolutionary 1999; see also Norris, 2000), which assume a global
bias hidden in the Drake equation. regulation mechanism for preventing the formation
of complex life forms and technological societies
early in the history of the Galaxy. Such a global
SIMPLICITY OF UNIFORMITARIANISM mechanism could have the physical form of -ray
bursts, if it can be shown that they exhibit suffi-
A still more important shortcoming of Eq. 1 as cient lethality to cause mass biological extinctions
a guideline to SETI consists of its uniform treat- over a large part of the volume of the galactic hab-
ment of the physical and chemical history of our itable zone (Scalo and Wheeler, 2002; see also
Galaxy. It is tacitly assumed that the history of Thorsett, 1995; Melott et al., 2004). If, as main-
the Galaxy is uniform with respect to the emer- tained in these models, continuous habitability is
gence and capacities of technological societies. just a myth, the validity of the Drake equation
This is particularly clear from the form of Eq. 2, (and the spirit in which it was constructed and
as mentioned above. If, on the contrary, we as- used) is seriously undermined.
sume more or less sharply bounded temporal For illustration, let us assume that the para-
phases of the galactic history as far as individual meter fl has the following evolutionary behavior:
terms in Eq. 1 are concerned, and take into ac-
106, 0 t tp
count our own existence at this particular epoch
of this history, we are likely to significantly un-
fl { 0.9, tp t t0
(3)
derestimate the value of N. We consider such a
model below. (we put the zero of time at the epoch of the Milky
Uniformitarianism has not shone as a brilliant Way formation). Here, tp is the epoch of global
guiding principle in astrophysics and cosmology. “phase transition” (Annis, 1999), i.e., the epoch in
It is well known, for instance, how the strictly uni- which the lethal galactic processes became rare
formitarian (and from many points of view enough for sufficiently complex life forms to
methodologically superior) steady-state theory of emerge. Let us take t0 12 Gyr and tp 11 Gyr.
the universe of Bondi and Gold (1948) and Hoyle Naive uniformitarian application of the Drake
(1948) has, after the “great controversy” of 1950s equation would require us to find the average fl,
and early 1960s, succumbed to the rival evolu- in particular, as an example fl 0.072; if we as-
tionary models, now known as the standard (“Big sume ne 1, other f parameters all equal to 0.1 (a
Bang”) cosmology (Kragh, 1996). Balashov (1994) rather conservative assumption), and R* 5
has especially stressed this aspect of the contro- years1, we obtain N 3.6 105L, where L is
versy by showing how deeply justified was the measured in years, and is the relative duration
introduction—by the Big Bang cosmologists—of of the communication window discussed above.
events and epochs never seen or experienced. In fact, the true result is instead N 4.5 104L,
Similar arguments are applicable in the nascent more than an order of magnitude higher. Such a
discipline of astrobiology, which might be con- big difference is of obvious relevance to SETI; if
sidered to be in an analogous state today as cos- L is 105 years or less, it might as well be the
mology was half a century ago. difference between sense and nonsense in the en-
The arguments of Lineweaver (2001) are cru- tire endeavor. The discrepancy increases if the
cial in this regard, too. Obviously, the history of epoch of the phase transition moves closer to the
the Galaxy divides into at least two periods (or present time. The latter is desirable if one wishes
phases): before and after sufficient metallicity for to efficiently resolve Fermi’s paradox through
the formation of Earth-like planets has been built phase-transition models.
up by global chemical evolution. But this reflects In particular, Annis (1999) argues that the rate
only the most fundamental division. It is entirely of -ray bursts in an average galaxy declines as
plausible that the history of the Galaxy is divided exp(t/), where is 5 Gyr. Only a very slight
still finer into several distinct periods with radi- improvement in the “step” model above would
cally different conditions for life. In that case, only be to filter the same parameter fl in accordance
weighted relative durations are relevant, not the with this rate, by assuming that the (ensemble-
overall age. averaged) probability of evolving complex lifeTHE DRAKE EQUATION AND SETI 229
forms is inversely proportional to the -ray burst rather gradual transition between the uninhabit-
rate during the history of the Milky Way. A able and inhabitable part of the galactic history.
model with fl(t) f*(et/ 1), where with f* we This transition results from the metallicity build-
denote everything dependent on astrobiological up coupled with the decrease in frequency of su-
parameters other than time, normalized to the pernovae with time. This is not incompatible with
present-day fl(t0) 0.9 gives the time-average of the phase-transition models, in the same sense as
fl 0.28. The discrepancy is thus about half an continuous existence of life on Earth is not in-
order of magnitude. No specific conclusions compatible with the mass extinctions of biota oc-
should be drawn from toy models such as these, curing from time to time. The main argument for
except the general conclusion that in phase-tran- compatibility is that here we are concerned pri-
sition models the relationship between time and marily with the problem of the distribution of in-
abundance of life is more complicated than usu- telligent life—the subject matter of SETI—which
ally assumed. On the balance, this approach fa- clearly requires much sharper filtering of points
vors SETI more than the Drake equation. Deeper in space and time, being much less robust in re-
understanding of the specific effects of forcing in- sponse to adverse environmental changes (in-
fluences (for instance, the atmospheric chemistry cluding supernovae and -ray bursts) than life in
in the aftermath of a galactic -ray burst or the general. An additional important difference is
controversial issue of the cosmic-ray generation that smoothing the spatiotemporal distribution of
in -ray bursts) will bring us to the more satis- supernovae applied by Lineweaver et al. (2004) is
factory quantitative relationship between the probably not applicable to much rarer and much
galactic astrophysical evolution and the specific more destructive -ray bursts, which according
terms in the Drake equation, notably fl and fi. to Scalo and Wheeler (2002) can seriously impair
More realistically, we would expect several of biospheres in the entire galactic habitable zone.
the f parameters, as well as ne, to exhibit secular Intuitively, it is clear that in such phase-transi-
increase during the course of galactic history in a tion models it is a very sensible policy for hu-
more complicated manner to be elaborated by fu- manity to engage in serious SETI efforts: We ex-
ture detailed astrobiological models. Yet, steps pect practically all extraterrestrial intelligent
similar to the one in Eq. 3 seem inescapable at societies to be roughly of the same age as ours,
some point if we wish to retain the essence of the and to be our competitors for Hart-Tiplerian col-
phase-transition idea. Barring this, the only fully onization of the Milky Way. This class of models
consistent and meaningful idea for both expla- underlines the essential weakness in the “contact
nation of the “Great Silence” and retaining the pessimist” position; as Tipler (1980) wrote: “[pes-
Copernican assumption on Earth’s non-special simist] argument assumes that the . . . probabili-
position is the “Interdict Hypothesis” of Fogg ties of the Drake equation do not vary rapidly
(1987), as the generalized “Zoo Hypothesis” (Ball, with galactic age.” Phase transition is exactly such
1973), which still seems inferior, since it explic- a “rapid variation.” The price to be paid for bring-
itly invokes non-physical, e.g., sociological, ele- ing the arguments of “optimists” and “pes-
ments. simists” into accord is, obviously, the assumption
A recent important study by Lineweaver et al. that we are living in a rather special epoch in
(2004) analyzes the astrobiological evolution of galactic history, i.e., the epoch of phase transition.
the Galaxy in time versus parameters such as That such an assumption is entirely justifiable (by
metallicity and frequency of life-extinguishing an observation–selection effect) in the astrobio-
supernovae. The results confirm previous ones by logical context will be argued in a subsequent
Lineweaver (2001), but add several new features. study. Parenthetically, this is entirely in accord
The galactic habitable zone is an annular ring of with the tenets of the currently much-discussed
the Milky Way thin disk that widens with time, “rare Earth” hypothesis (Ward and Brownlee,
consisting of stars formed between 8 and 4 Gyr 2000).
ago. From the SETI point of view, this makes Note that in this case, the overall average age
Fermi’s paradox more serious, since 75% of the of a civilization (L) would give an entirely false
stars in the galactic habitable zone are older than picture of the outcome of the Drake equation. In
the Solar System. However, in seeming contrast the toy model above, any hypothetical civiliza-
to the phase-transition models discussed here, the tion age of (say) 10 Gyr is obviously irrelevant
solution of Lineweaver et al. (2004) indicates a (although possibly sociologically allowed). Thus,230 ĆIRKOVIĆ
the universally disliked sensitivity of N on L and Aleksandar Antić, and David Brin for useful and
all its “astrosociological” baggage is diminished, enjoyable discussions. This research has made ex-
at least in the limit of large values of L. This con- tensive use of NASA’s Astrophysics Data System
clusion is valid even if the width of the commu- Abstract Service, as well as of the services of the
nication window is very large or spans most of KOBSON consortium of libraries, supported by the
the lifetime of a civilization, as SETI pioneers Ministry of Science, Technology and Development
claimed ( 1). In these cases, the relevant time of the Republic of Serbia. The project would be im-
scale is not something inherent in the bio- possible to complete without the kindness, inspi-
sphere/noosphere, but the external forcing time ration, and support of Irena Diklić; any grain of
scale, e.g., the time elapsed since the last catas- merit in it can be undoubtedly traced to her.
trophe. Thus, we obtain a physically more desir-
able theoretical framework for the explanation of
Fermi’s paradox in which sociological influences ABBREVIATION
are much less relevant.
SETI, Search for ExtraTerrestial Intelligence.
CONCLUSIONS
REFERENCES
We conclude that the Drake equation, as con-
Adams, F.C. and Laughlin, G. (1997) A dying universe:
ventionally presented, is not the best guide for
The long-term fate and evolution of astrophysical ob-
both operational SETI and future policy-making jects. Rev. Mod. Phys. 69, 337–372.
in this field. The reason for this is its lack of tem- Annis, J. (1999) An astrophysical explanation for the great
poral structure and appreciation of the impor- silence. J. Br. Interplanet. Soc. 52, 19–22.
tance of evolutionary effects, so pertinent in the Balashov, Yu.V. (1994) Uniformitarianism in cosmology:
modern astrobiological discourse. If we wish to Background and philosophical implications of the
go beyond the “zeroth-order” approximation en- steady-state theory. Studies History Philos. Sci. 25B,
933–958.
capsulated by Eq. 1, we will need to account for
Ball, J.A. (1973) The Zoo Hypothesis. Icarus 19, 347–349.
evolutionary effects, such as metallicity build-up Barrow, J.D. and Tipler, F.J. (1986) The Anthropic Cosmo-
and “catastrophic” regulation of habitability. No- logical Principle, Oxford University Press, New York.
tably, the non-uniform history of the Galaxy—as Bondi, H. and Gold, T. (1948) The steady-state theory of
conceived in the phase-transition models—can the expanding universe. Month. Not. R. Astron. Soc. 108,
accommodate both the arguments of “contact 252–270.
pessimists” and the justification for SETI projects, Bracewell, R.N. (1975) The Galactic Club: Intelligent Life in
Outer Space, W.H. Freeman, San Francisco.
which have been deemed incompatible in the lit-
Carter, B. (1983) The anthropic principle and its implica-
erature so far. Future detailed modeling will tions for biological evolution. Philos. Trans. R. Soc. Lond.
show in which way we can best incorporate our A 310, 347–363.
knowledge of the history of the Galaxy in the Ćirković, M.M. (2003) Resource Letter PEs-1: Physical es-
overall astrobiological picture. chatology. Am. J. Phys. 71, 122–133.
Ćirković, M.M. and Bostrom, N. (2000) Cosmological con-
stant and the Final Anthropic Hypothesis. Astrophys.
Space Sci. 274, 675–687.
ACKNOWLEDGMENTS Clarke, J.N. (1981) Extraterrestrial intelligence and galac-
tic nuclear activity. Icarus 46, 94–96.
Two reviewers for Astrobiology are acknowl- Conway Morris, S. (1990) Late Precambrian and Cam-
edged for significant improvements upon a pre- brian soft-bodied faunas. Annu. Rev. Earth Planet. Sci.
vious version of this manuscript. The author 18, 101–122.
wholeheartedly thanks Alan Robertson, Saša Drake, F. (1965) The radio search for intelligent extrater-
Nedeljković, Duška Kuhlmann, Richard Cathcart, restrial life. In Current Aspects of Exobiology, edited by
G. Mamikunian and M.H. Briggs, Pergamon, New
Vjera Miović, Nikola Milutinović, Olga Latinović,
York, pp. 323–345.
Srdjan Samurović, Samir Salim, Branislav K. Drake F. and Sobel, D. (1991) Is Anyone Out There?, Simon
Nikolić, and Vesna Milošević-Zdjelar for technical and Schuster, London.
help. In addition, I am grateful to Nick Bostrom, Duric, N. and Field, L. (2004) On the detectability of in-
Robert J. Bradbury, Zoran Živković, Ivana telligent civilizations in the Galaxy. Serb. Astron. J. 167,
Dragićević, Mark A. Walker, Slobodan Popović, 1–11.THE DRAKE EQUATION AND SETI 231
Dyson, F. (1979) Time without end: Physics and biology Tough, Foundation for the Future, Washington, DC, pp.
in an open universe. Rev. Mod. Phys. 51, 447–460. 103–105.
Fogg, M.J. (1987) Temporal aspects of the interaction Olum, K.D. (2004) Conflict between anthropic reasoning
among the first galactic civilizations: The ‘Interdict Hy- and observation. Analysis 64, 1–8 (preprint available on-
pothesis.’ Icarus 69, 370–384. line at: http://www.gr-qc/0303070).
Hart, M.H. (1975) An explanation for the absence of ex- Panter, B., Heavens, A.F., and Jimenez, R. (2003) Star for-
traterrestrials on Earth. Q. J. R. Astron. Soc. 16, 128–135. mation and metallicity history of the SDSS galaxy sur-
Hartwick, F.D.A. (2004) Cosmic star formation history vey: Unlocking the fossil record. Month. Not. R. Astron.
from local observations and an outline for galaxy for- Soc. 343, 1145–1154.
mation and evolution. Astrophys. J. 603, 108–118. Scalo, J. and Wheeler, J.C. (2002) Astrophysical and as-
Hopkins, A.M., Irwin, M.J., and Connolly, A.J. (2001) trobiological implications of gamma-ray burst proper-
Global star formation history: A local group perspec- ties. Astrophys. J. 566, 723–737.
tive. Astrophys. J. 558, L31–L33. Shklovskii, I.S. and Sagan, C. (1966) Intelligent Life in the
Hoyle, F. (1948) A new model for the expanding universe. Universe, Holden-Day, San Francisco.
Month. Not. R. Astron. Soc. 108, 372–382. Thorsett, S.E. (1995) Terrestrial implications of cosmolog-
Jones, E.M. (1976) Colonization of the Galaxy. Icarus 28, ical gamma-ray burst models. Astrophys. J. 444,
421–422. L53–L55.
Kragh, H. (1996) Cosmology and Controversy, Princeton Tipler, F.J. (1980) Extraterrestrial intelligent beings do not
University Press, Princeton, NJ. exist. Q. J. R. Astron. Soc. 21, 267–281.
Krauss, L.M. and Chaboyer, B. (2003) Age estimates of Tipler, F.J. (1981) Additional remarks on extraterrestrial
globular clusters in the Milky Way: Constraints on cos- intelligence. Q. J. R. Astron. Soc. 22, 279–292.
mology. Science 299, 65–70. Walters, C., Hoover, R.A., and Kotra, R.K. (1980) Inter-
Lem, S. (1984) His Master’s Voice, Harvest Books, Fort stellar colonization: A new parameter for the Drake
Washington, PA. Equation? Icarus 41, 193–197.
Lem, S. (1987) Fiasco, Harcourt, New York. Ward, P.D. and Brownlee, D. (2000) Rare Earth: Why Com-
Lineweaver, C.H. (2001) An estimate of the age distribu- plex Life Is Uncommon in the Universe, Springer, New
tion of terrestrial planets in the Universe: Quantifying York.
metallicity as a selection effect. Icarus 151, 307–313. Webb, S. (2002) Where is Everybody? Fifty Solutions to the
Lineweaver, C.H., Fenner, Y., and Gibson, B.K. (2004) The Fermi’s Paradox, Copernicus, New York.
galactic habitable zone and the age distribution of com- Wilson, T.L. (1984) Bayes’ Theorem and the real SETI
plex life in the Milky Way. Science 303, 59–62. equation. Q. J. R. Astron. Soc. 25, 435–448.
Mash, R. (1993) Big numbers and induction in the case for
extraterrestrial intelligence. Philos. Sci. 60, 204–222.
Melott, A.L., Lieberman, B.S., Laird, C.M., Martin, L.D., Address reprint requests to:
Medvedev, M.V., Thomas, B.C., Cannizzo, J.K., Gehrels, Dr. Milan M. Ćirković
N., and Jackman, C.H. (2004) Did a gamma-ray burst
Astronomical Observatory
initiate the late Ordovician mass extinction? Int. J.
Astrobiol. 3, in press (preprint available online at: Volgina 7
http://www.astro-ph/0309415). 11000 Belgrade, Serbia and Montenegro
Norris, R.P. (2000) How old is ET? In When SETI Succeeds:
The Impact of High-Information Contact, edited by A. E-mail: mcirkovic@aob.aob.bg.ac.yuYou can also read
