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A Quantitative Assessment of Communicating Extra-Terrestrial Draft version November 10, 2020 A Typeset using L TEX twocolumn style in AASTeX62 A Quantitative Assessment of Communicating Extra-Terrestrial Intelligent Civilizations in the Galaxy and the Case of FRB-like Signals Bing Zhang1 1Department of Physics and Astronomy, University of Nevada Las Vegas, NV 89154, USA ABSTRACT A formula is proposed to quantitatively estimate the signal emission rate of Communicating Extra- Terrestrial Intelligent civilizations (CETIs) in the Galaxy. I suggest that one possible type of CETI signal would be brief radio bursts similar to fast radio bursts (FRBs). A dedicated search for FRB-like artificial signals in the Galaxy for decades may pose a meaningful upper limit on the emission rate of these signals by CETIs. The Fermi-Hart paradox is answered in terms of not having enough observing times for this and other types of signals. Whether humans should send FRB-like signals in the far future is briefly discussed. Keywords: fast radio bursts – astrobilogy 1. DRAKE EQUATION AND ITS MANIPULATION In this equation, the first three parameters, R∗, fp, and In 1961, Frank Drake wrote the famous equation to ne, can be constrained from astronomical observations, estimate the number of actively Communicating Extra- but the remaining four parameters are related to biology, Terrestrial Intelligent civilizations (CETIs) in the Milky sociology, and even philosophy, which cannot be quan- Way galaxy. The equation reads (see, e.g. Vakoch et al. titatively assessed based on observations of the sole in- 2015) telligent life as we know it: our own (e.g. Burchell 2006; Vakoch et al. 2015). It is worth emphasizing that fl, fi, N = R∗ · fp · ne · fl · fi · fc · L. (1) and fc as appeared in the Drake equation should mean The meaning of each parameter, as defined in the book the fractions that life, intelligent life and communicative “The Drake Equation” (Vakoch et al. 2015), reads intelligent life that can eventually appear (even if in the far future), not the fractions already appeared at the • R∗: Rate of formation of stars suitable for the current age of the universe (which will be discussed as development of intelligent life; ′ ′ ′ fl , fi , and fc later). To avoid ambiguity, I believe that • fp: Fraction of stars with planetary systems; it would be better to define them as • ne: Number of planets, per solar system, with an • fl: Probability of suitable planets that eventually environment suitable for life; develop life; • fl: Fraction of suitable planets on which life actu- • fi: Probability of life-bearing planets that eventu- ally appears; ally develop intelligent life; arXiv:2006.08493v2 [physics.pop-ph] 6 Nov 2020 • fi: Fraction of life-bearing planets on which intel- • fc: Probability that intelligent life that eventually ligent life emerges; develop technology and become CETIs. • fc: Fraction of civilizations that develop a technol- In the following, I define ogy that releases detectable signs of their existence into space; ceti ne = neflfifc, (2) • L: Length of time such civilizations release de- which has the physical meaning tectable signals into space. • ceti ne : Number of planets, per solar system, with an environment suitable for life and eventually de- [email protected] velop CETIs. 2 Zhang According to the so-called “astrobiological Copernican the average in the logarithmic space. This way, one can principle” assumption (e.g. Westby & Conselice 2020), write the three fraction parameters defined above as an Earth-like planet that are suitable for developing life ′ Ll will eventually develop life, intelligent life, and CETIs, fl = , (4) ceti Lp so that flfifc should be close to unity, i.e. ne . ne. The star formation rate R∗ in Eq.(1) was introduced ′ Li fi = , (5) to describe a steady-state process. Logically it is not Ll straightforward to see how it enters the problem and how ′ Lc fc = , (6) L is used to cancel out the “per unit time” dimension Li introduced in R∗. Since one is interested in the CETIs so that the product f ′ · f ′ · f ′ can be simply written as who are sending off signals to Earth “now” (corrected l i c the ratio Lc/Lp. The logic behind Eqs.(4)-(6) is that in for the light travel time from the source to Earth), it is a steady state and a random observing time, the prob- more reasonable to start with the number of stars in the ability of seeing a short-duration event during a long- galaxy now, i.e. N∗, rather than R∗ to estimate N, so duration event should be the ratio between the durations that of the former and the latter. · · ceti · ′ · ′ · ′ N = N∗ fp ne fl fi fc, (3) Plugging Eqs.(4)-(6) into Eq.(3), one can derive where the three new f parameters are defined as · · ceti · Lc N = N∗ fp ne • ′ Lp fl : Fraction of planets that can in principle de- velop CETIs and have actually developed life now; LMW · N∗ · · ceti · = fp ne Lc Lp LMW • f ′: Fraction of the above-defined life-bearing plan- i ≃ LMW · · · ceti · ets that have developed intelligent life now; R∗ fp ne Lc Lp ceti • ′ ∼ R∗ · fp · n · Lc (7) fc: Fraction of the above-defined intelligent life e that have become CETI now. where R∗ ≃ N∗/LMW is the average star formation rate Notice that they are different from fl, fi, and fc that throughout the MW history (with the assumption that enter the Drake equation (1). Let us further define the the number of dead stars is much smaller than the num- following four time scales: ber of living stars, which is justified since low mass stars that contributed to the vast majority of the total num- • Lp: the average lifetime of Earth-like planets (from ber have not died yet, see Westby & Conselice 2020 for its birth to death, likely associated with the death detailed calculations), and LMW ∼ 13.5 billion years is of the host star). For Earth, it is at least ∼ 4.54 the current lifetime of the Milky Way galaxy. Since the billion years; average lifetime of earth-like planets Lp is ∼ LMW (e.g. for Earth it is the lifetime of the Sun, i.e. ∼ 10 Gyr), the • Ll: the average lifetime of life (from its birth to last step in Eq.(7) has a ∼ sign. One can see that last death - probably associated with the death of the step roughly reproduces the Drake equation (1) noticing ∼ planet). For Earth, it is at least 3.8 billion years; Eq.(2). • Li: the average lifetime of the intelligent life (from 2. A QUANTITATIVE ASSESSMENT OF THE its birth to death - could be associated with the CETI SIGNAL EMISSION RATE death of the planet, but could be sooner due to its The number N of CETIs is not a direct measurable self-destroy). For Earth, it is at least ∼ 104 years; quantity. A human observer may be more interested in • L : the average lifetime of CETIs (from its birth the signal detection rate of CETI signals (e.g. in units c ˙ to death - again could be associated with the death of # per year all sky), which I define as Ns,o, where of the planet or its self-destroy). For Earth, it is the subscripts ‘s’ and ‘o’ denote ‘signal’ and ‘observer’, at least ∼ 102 years, i.e. since humans have devel- respectively. Ultimately, one cares about the average ˙ oped the technology to send off artificial signals to signal emission rate per CETI, which I define as Ns,e, space. where the subscript ‘e’ denotes ‘emitter’. The emitted signals of CETIs may not always be detected by humans The term “average” in the above definitions refers to on Earth. I therefore introduce a parameter ξo to de- the geometric mean rather than the arithmetic mean, or note the average fraction of the CETI signals that are 3 detectable by humans on Earth. One can then write single stars are likely the ones to harbor stable planet orbits, we assign fp as the fraction of single ˙ · ˙ Ns,o = N (ξoNs,e) stars, which ranges from 1/2 to 2/3 (Lada 2006). · · ceti · Lc · ˙ = N∗ fp ne (ξoNs,e). (8) • ceti Lp ne : This is the number with a large uncer- tainty. ne may be determined with precision where the first line of Eq.(7) has been used. This equa- when survey observations of planets from nearby tion is more helpful than Eq.(7) for a quantitative as- stellar systems are carried out, e.g. with the sessment of CETI signals. We now break down the Transiting Exoplanet Survey Satellite (TESS) mis- terms introduced in Eq.(8) and discuss how they may sion (Ricker et al. 2015). Based on the Kepler be constrained astronomically. The discussions on var- (Borucki et al. 2010) observations, the fraction of ious parameters of the Drake equation and other mod- GK dwarfs with rocky planets in habitable zones ified forms can be also found in Vakoch et al. (2015) may be around 0.1 (Burke et al. 2015). This may ceti and many papers in the literature, e.g. Burchell (2006); be considered as the upper limit of ne when con- Westby & Conselice (2020) and references therein. sidering other factors (planet mass, metallicity, ex- istence of a magnetosphere, existence of a Jupiter- • N∗: the Sun’s distance from the Galactic center like large planet to deflect comets, etc., see, e.g. (d⊙ ∼ 8.0 ± 0.5 kpc) and its proper motion veloc- Lineweaver (2001)) that might be relevant for pro- ity (v⊙ ∼ 220 km/s as measured from the dipole ducing CETIs. Since it is far from clear what phys- moment in the cosmic microwave background) al- ical conditions are essential for the emergence of lows one to estimate that the total mass within 2 11 CETIs, this number should allow for a large uncer- the solar orbit is M = d⊙v /G ∼ 10 M⊙.
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