¹ The difference between the average absolute magnitudes of stars (or galaxies or any other similar sources) in magnitude- and distance-limited samples, discovered in 1920 by K.G. Malmquist.

² The effect by which rare species seemingly disappear earlier than their numerous contemporaries (thus making extinction episodes more prolonged in the fossil record than they were in reality), discovered by P.W. Signor and J.H. Lipps in 1982.

³ For a summary of vast literature on observation selection, anthropic principles, andanthropic reasoning in general, see Barrow and Tipler (1986); Balashov (1991); Bostrom (2002a).

⁴ Parts of this section are loosely based upon Ćirković (2007).

⁵ For a more optimistic view of this possibility in a fictional context see Egan (2002).

⁶ I thank C.R. Shalizi for this excellent formulation.

⁷ Earth-like planets have not been discovered yet around Tau Ceti, but in view of the crude observational techniques employed so far, it has not been expected; the new generation of planet-searching instruments currently in preparation (Darwin, Gaia, TPF, etc.) will settle this problem.

⁸ Originally in Leslie (1989); for his most comprehensive treatment, see Leslie (1996). Carter did not publish on DA.

⁹ This is the original, Carter-Leslie version of DA. The version of Gott (1993) is somewhat different, since it does not deal with the number of observers, but with intervals of time characterizing any phenomena (including humanity’s existence). Where Gott does consider the number of observers, his argument is essentially temporal, depending on (obviously quite speculative) choice of particular population model for future humanity. It seems that a gradual consensus has been reached about inferiority of this version compared to Leslie-Carter’s (see especially Caves, 2000; Olum, 2002), so we shall concentrate on the latter.

¹⁰ It would be more appropriate to call it the Tsiolkovsky-Fermi-Viewing-Hart-Tipler Paradox (for more history, see Brin, 1983; Kuiper and Brin, 1989; Webb, 2002, andreferences therein). We shall use the locution ‘Fermi’s Paradox’ for the sake of brevity, and with full respect for the contributions of the other authors.

¹¹ The requirement that any process preventing formation of a large anddetectable interstellar civilization operates over large spatial (millions of habitable planets in the Milky Way) andtemporal (billions of years of the Milky Way history) scales. For more details, see Brin (1983).

¹² In the pioneering paper on GCRs/existential risks Bostrom (2002b) has put this risk in the ‘whimpers column, meaning that it is an exceedingly slow and temporally protracted possibility (and the one assigned low probability anyway). Such a conclusion depends on the specific assumptions about extraterrestrial life and intelligence, as well as on the particular model of future humanity and thus is of rather narrow value. We wouldlike to generalize that treatment here while pointing out that still further generalization is desirable.

¹³ Parsec, or paralactic second is a standard unit in astronomy: 1 pc = 3.086 x 10¹⁶ m. One parsec is, for instance, the average distance between stars in the solar neighbourhood.

¹⁴ A version of the ‘deadly probes’ scenario is a purely informatics concept of Moravec (1988), where the computer viruses roam the Galaxy using whatever physical carrier available and replicating at the expense of resources of any receiving civilization. This, however, hinges on the obviously limitedcapacity to pack sufficiently sophisticated self-replicating algorithm in the bit-string of size small enough to be received non-deformed often enough – which raises some interesting issues from the point of view of algorithmic information theory (e.g., Chaitin, 1977). It seems almost certain that the rapidly occurring improvements in information security will be able to clear this possible threat in check.

¹⁵ There may be exceptions to this related to the complex issue of reversible computing. In addition, if the Landauer-Brillouin bound holds, this may have important consequences for the evolution of advanced intelligent communities, as well as for our current SETI efforts, as shown by Ćirković and Bradbury (2006).

¹⁶ The ‘planetarium hypothesis’ advanced by Baxter (2000) as a possible solution to Fermi’s paradox, is actually very similar to the general simulation hypothesis; however, Baxter suggests exactly ‘risky’ behaviour in order to try to force the contact between us and the director(s)!