«(Received 16 April 2003; Accepted 30 May 2003) Abstract. We consider the little-known anthropic argument of Fontenelle dealing with the nature of ...»
ON THE FIRST ANTHROPIC ARGUMENT IN ASTROBIOLOGY
MILAN M. CIRKOVIC
Astronomical Observatory Belgrade, Volgina 7, 11160 Belgrade, Serbia and Montenegro
(Received 16 April 2003; Accepted 30 May 2003)
Abstract. We consider the little-known anthropic argument of Fontenelle dealing with the nature of
cometary orbits, given a year before the publication of Newton’s Principia. This is particularly interesting in view of the rapid development of the recently resurgent theories of cometary catastrophism and their role in the modern astrobiological debates, for instance in the “rare Earth” hypothesis of Ward and Brownlee.
Keywords: comets: general, stars: planetary systems, extraterrestrial intelligence, history and philosophy of astronomy
1. Introduction The issue of stability of conditions prevailing on (at least potentially) habitable planets throughout the Galaxy is the central question of the nascent science of astrobiology (e.g., Darling, 2001). We are lucky enough to live in an epoch of great astronomical discoveries, the most distinguished probably being the discovery of dozens of planets orbiting nearby stars (for nice reviews, see Marcy and Butler, 1998, 2000; Ksanfomaliti, 2000). This particular discovery brings about a profound change in our thinking about the universe, and prompts further questions on the frequency of Earth-like habitats elsewhere in the Galaxy. In a sense, it answers a question posed since antiquity: are there other, potentially inhabited or inhabitable, worlds in the vastness of space? In asking that question, obviously, we take into account our properties as intelligent observers, as well as physical, chemical, and other pre-conditions necessary for our existence. The latter are the topic of the so-called anthropic principle(s), the subject of much debate and controversy in cosmology, fundamental physics, and philosophy of science.
Arguably the most signiﬁcant construal of the anthropic principle(s) is the one having to do with restrictions that the existence of human beings places on the observable features of the universe (Carter, 1974, 1983; Barrow and Tipler, 1986;
Bostrom, 2002). In other words, the anthropic principle acts as an observation selection effect in the set of all possible observations of the universe. However, the very deﬁnition of the universe evolved, of course (cf. Munitz, 1986). The universe of the standard (“Big Bang”) cosmological model is drastically different from the essentially static and eternal (or supernaturally created in essentially the same Earth, Moon and Planets 91: 243–254, 2002.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
state) universe of previous epochs, notably the one of Bruno, Descartes, Halley, Newton, Laplace, Lord Kelvin, and early Einstein. In attempting to reconstruct the evolution of cosmological and cosmogonical thinking, one should be very careful to avoid conﬂating various levels of cosmological discourse with which we are familiar today, with those known in earlier times. On the contrary, a conclusion of some generality arrived at in the times of much poorer level of understanding and description is certainly to be exceptionally praised from both the historical and the epistemological points of view.
In this note, it is our goal to show that one of the very ﬁrst instances of anthropic thinking, applied to planetary science and astrobiology (in modern terms), occurred very early, in the work of French philosopher and naturalist Bernard Le Bouyier de Fontenelle (1657–1757), published in 1686, a single year before the great scientiﬁc revolution inaugurated by Newton’s Principia. Fontenelle (Figure 1) was one of the most important precursors of the Enlightenment. In his long and productive career, he touched upon practically all aspects of human knowledge, other books of his bearing titles such as On the Usefulness of Mathematical Learning or A History of Oracles. Of course, the recent Copernican revolution in astronomy and cosmology could not escape his attention, and he supported the new paradigm with more than his usual eloquence and wit in Conversation on the Plurality of the Worlds, where the argument presently under scrutiny is located.
Fontenelle’s argument deals with the comets, bodies which presented an incredibly popular scientiﬁc topic in his day, and which have remained in the focus of interest of planetary studies until now. It was already known, through the great studies of Halley and others (see Yeomans, 1991; Schechen, 1997) that comets travel around the Sun on very elongated orbits which have high inclinations; that is, their orbital planes are at large angles from the orbital plane of Earth (and other planets known at the time). The late XVII century was in many ways obsessed with these strange celestial visitors and with good reason (see, for instance, a colorful “catalogue” of bright comets in Figure 2). For important epistemological and cultural implications of comet observations in the relevant period, see an excellent study by Barker and Goldstein (1988). The unusual orbital behavior of comets, in eyes of Fontenelle and his contemporaries (as well as of modern cosmogonists!) is strange and requires an explanation. He offered an unusual, ingenious, and even today controversial argument.
The remarkable argument put forward by Fontenelle in 1686, a single year before the appearance of Newton’s majestic Principia and ten years before Whiston’s
ON THE FIRST ANTHROPIC ARGUMENT IN ASTROBIOLOGY
A New Theory of Earth, is essentially contained in a single paragraph of his
Conversation on the Plurality of the Worlds. It reads (Fontenelle 1767):
In the next places, the reason why the planes of their [comets’] motions are not in the plane of the ecliptic, or any of the planetary orbits, is extremely evident; for had this been the case, it would have been impossible for the Earth to be out of the way of the comets’ tails. Nay, the possibility of an immediate encounter or shock of the body, of a comet would have been too frequent; and considering how great is the velocity of a comet at such a time, the collision of two such bodies must necessarily be destructive of each other;
nor perhaps could the inhabitants of planets long survive frequent immersions in the tails of comets, as they would be liable to in such a situation. Not to mention anything of the irregularities and confusion that must happen in the motion of planets and comets, if their orbits were all disposed in the same plane.
Figure 2. The obsession of late XVII century with comets is summarized in this vivid illustration from the book (or pamphlet) on “Surprizing Miracles” by Nathaniel Crouch (London, 1685).
Courtesy of Houghton Library, Harvard University.
ON THE FIRST ANTHROPIC ARGUMENT IN ASTROBIOLOGYThus, to the question: why are (observed) orbits of comets highly inclined, in contradistinction to the coplanar planetary orbits? Fontenelle offers a deceptively simple answer. We would not be here – to contemplate on the peculiarities of cometary trajectories – if these orbits were different (that is, similar to those of planets). This had been published 8 years before celebrated Halley’s suggestion of
December 12, 1694, that comets might collide with planets (Halley, 1726):
This is spoken to Astronomers: But, what might be the Consequences of so near an Appulse; or of a Contact; or, lastly, of a Shock of the Coelestial Bodies, (which is by no means impossible to come to pass) I leave to be discuss’d by the Studious of Physical Matters.
This famous idea has been followed up by such luminaries as Newton, Wright, Laplace, Lagrange, and others, in the vein of what is usually (and only partially justiﬁably) called “Biblical catastrophism” of the pre-Cuvier epoch. Fontenelle
wrote the passage more than 18 years before Newton wondered (in Opticks):
Whence is it that planets move all one and the same way in orbs concentrick, while comets move all manner of ways in orbs very excentric... blind Fate could never make all the planets move one and the same way in orbs concentrick, some inconsiderable irregularities excepted, which may have risen from the mutual actions of comets and planets upon one another, and which will be apt to increase, till this system wants a reformation. Such a wonderful uniformity in the planetary system must be allowed the effect of choice.
Thus Newton, as a great promoter of the Design argument in natural philosophy, failed to understand the power of Fontenelle’s argument, and went deeper into a blind alley (from the modern point of view) of seeking the supranatural Design and/or regulating mechanism. The same tension between the apparent design and the explanatory “ﬁltering” through various observation selection effects persists to this day, and is the source of innumerable debates and confusions. We shall return to this point below.
Unfortunately, in the subsequent long dogmatic slumber of uniformitarian domination in the entire realm of natural sciences, it has been too often forgotten what is and what is not “by no means impossible to come to pass”. In the present era of revived “neocatastrophism” (e.g., Schindewolf, 1962; Napier and Clube, 1979;
Clube and Napier, 1982, 1984, 1990; Clube, 1995; Raup, 1999), some of these early thinkers (like Halley) received a renewed attention, but Fontenelle is undeservedly rarely mentioned. It is one of the purposes of this note that this historical injustice is at least partially rectiﬁed.
Now, there are two arguments in the quoted passage of Fontenelle. The ﬁrst concerns the dynamical inﬂuence through impacts. Impacts of comets upon planets would have been much more frequent and destructive if their orbits lay in the plain of the ecliptic. Such collisions would be highly destructive to lifeforms (especially For colourful pieces of its history, see Clube and Napier (1990).
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MILAN M. CIRKOVICadvanced and intelligent ones). As we shall see below, its validity has been fortiﬁed by scientiﬁc ﬁndings in recent decades. The second argument concerns the effects of immersion of inhabited planets in cometary tails. The standard modern view is that this does not make sense, since such phenomena are thought to be completely harmless, due to the excessively low density of cometary tails. Still, there are some dissenters from this view, the most famous being the late Sir Fred Hoyle, who (with Chandra Wickramasinghe) argued, rather notoriously, that comets may be vehicles for panspermia, and even cause familiar cases of epidemic diseases (Hoyle and Wickramasinghe, 1979; Hoyle et al., 1986). Another suggested effect is a possible climatic inﬂuence due to depositing of cometary dust, with the zodiacal cloud as an intermediate reservoir, in the Earth’s atmosphere (Hoyle, 1987; Napier, 2001).
It would be only prudent to state that the effects of prolonged interaction of a cometary tail and the Earth have not been meticulously studied so far. In any case, there is obviously less interaction between Earth and comets due to this speciﬁc feature (high inclination) of the latters’ orbits in comparison to the counterfactual case Fontenelle considers.
Now, what could be counted as an explanation of this particular feature? When we come to the “grand questions” on the origin of the universe (or “world”, construed in some narrower sense!), we are left with surprisingly few viable options.
One of them, certainly unsatisfactory, is to claim some supernatural or Divine cause which is not accessible to further elucidation. Another, and it has become more and more interesting as modern cosmology progressed, is that any atypical particular feature can be made unsurprising and “natural” by embedding it in a set or distribution broad enough to include many (or all) cases of the phenomenon in question. This is the usual approach of the anthropic thinking. Thus, Fontenelle may appeal to a kind of “principle of fecundity” (Nozick, 1981): we explain the observed feature of a system by embedding its speciﬁc features into a wider system
where many (or all) possibilities are realized. As Nozick (1981, p. 131) writes:
The principle of fecundity is an invariance principle. Within general relativity, scientiﬁc laws are invariant with respect to all differentiable coordinate transformations. The principle of fecundity’s description of the structure of possibilities is invariant across all possible worlds. There is no one specially privileged or preferred possibility, including the one we call actual.... The actual world has no specially privileged status, it merely is the world where we are. Other independently realized possibilities also are correctly referred to by their inhabitants as actual.
Of course, Nozick speaks of it in the language of modern metaphysics, but we need to remember that the ontological construal of the locution “world” has undergone revolutionary changes since the epoch of Enlightenment. The universe of Fontenelle was essentially the universe of Bruno: an ensemble of different planetary systems taken as the self-contained ontological units. That Fontenelle’s worlds
ON THE FIRST ANTHROPIC ARGUMENT IN ASTROBIOLOGYwere planets (solar and extrasolar) and that his main interest was astrobiological is
testiﬁed by the following words from his Preface:
I have chosen that part of Philosophy which is most like to excite curiosity; for what can more concern us, than to know how this world which we inhabit, is made; and whether there be any other worlds like it, which are also inhabited as this is?