Upper limit on the fraction of alien civilizations that develop communication technology

Luis A. Anchordoqui1, 2, 3 and Susanna M. Weber4 1Department of Physics & Astronomy, Lehman College, City University of New York, NY 10468, USA 2Department of Physics, Graduate Center, City University of New York, NY 10016, USA 3Department of Astrophysics, American Museum of Natural History, NY 10024, USA 4Mamaroneck High School, 1000 Boston Post Rd., Mamaroneck, NY 10543, USA (Dated: August 2019) We re-examine the likelihood for alien civilizations to develop communication technology on the basis of the general assumption that life elsewhere could have a non-carbon chemical foundation. We particularized the discussion to a complex silicon-based biochemistry in a nitrogen solvent, and elaborate on the environment in which such a chemistry is feasible, and if so, on what scales. More concretely, we determine the region outside the habitable zone where such organisms can grow and flourish and after that we study how our findings impact the recently derived upper limit on the fraction of living intelligent species that develop communication technology ξ . We also h bioteci compare this new restriction on ξbiotec with that resulting from the extension of the habitable zone to accommodate subsurface exolife,h originatingi in planets with subsurface (water) oceans.

I. INTRODUCTION where Γ? is the average rate of star formation, fp is the fraction of stars with planetary systems, ne is the number of planets (per solar system) with a long-lasting ( 4 Gyr) he day on which the Earth began to move is a seminal ∼ Tmoment in world history. In the XVI century, Coper- ecoshell, f` is the fraction of suitable planets on which nicus’ theory of heliocentrism transformed a millennia- life actually appears, fi is the fraction of living species old worldview with the shocking revolutionary idea that that develop intelligence, fc is the fraction of intelligent we humans are not at the center of the universe. We may species with communications technology, and Lτ is the be again at the verge of an exciting moment in the his- length of time such civilizations release detectable sig- tory of mankind. In the last few decades, the so-called nals into space (i.e. the lifetime of the communicative “exoplanet” revolution has shown us that the universe phase) [3]. is awash in alien worlds and that we humans may not be A more compact form of (1) can be obtained by consid- alone. The heedful quest for “exolife” has been dubbed ering separately its astrophysical and biotechnological the cosmic modesty conjecture: “The richness of the uni- factors

verse teaches us modesty and guides us to search for N = ζastro ξbiotec Lτ , (2) both primitive and intelligent forms of life elsewhere h i h i where ζ = Γ f n represents the production rate of without prejudice” [1]. h astroi ? p e In its most restrictive incarnation the cosmic mod- habitable planets with long-lasting ecoshell (determined through astrophysics) and ξ = f f f represents esty conjecture calls for carbon-based organisms, oper- h bioteci ` i c ating in a water-based medium, with higher forms (per- the product of all chemical, biological and technologi- haps) metabolizing oxygen. All forms of life on Earth cal factors leading to the development of a technological civilization [4]. indicates average over all the mul- share this same basic biochemistry. Indeed, the concept h· · · i of life based around anything other than carbon cer- tiple manners civilizations can arise, grow, and develop tainly seems outlandish at first. However, our present such technology, starting at any time since the formation knowledge of physics does not guarantee this fact, rather of our Galaxy in any location inside it. This averaging falling into what Sagan referred to as “carbon chauvin- procedure must be regarded as a crude approximation ism” [2]. In this work we re-examine the likelihood because the characteristics of the initial conditions in a for alien civilizations to develop communication tech- planet and its surroundings may affect f`, fi, and fc with nology. We adopt an extreme viewpoint of the cosmic high complexity. In a recent study we estimated the production rate of exoplanets where carbon-based or- arXiv:1908.01335v1 [physics.pop-ph] 4 Aug 2019 modesty conjecture and argue that life is a sort-of “nan- otechnology phenomenon” of “molecular automaton” ganisms operating in a water-based medium can flour- style, and that it is the liquid nature where life evolves ish and the rate of planetary catastrophes which could what determines the biochemistry of non-terrestrial life threaten the evolution of life on the surface of these (rather than a certain biochemistry being required for worlds [5]. Armed with these estimates we used our life’s existence). current measurement of N = 0 to set an upper limit on ξ . In this work we extend our study by consider- The number of intelligent civilizations in our galaxy h bioteci at any given time capable of releasing detectable signals ing that life elsewhere could have a non-carbon chemical of their existence into space can be cast in a quite simple foundation. functional form, The layout of the paper is as follows. Webegin in Sec. II with an overview of the circumstellar habitable zone. In N = Γ? fp ne f` fi fc Lτ , (1) Sec. III we explore wether life may develop outside the 2 habitable zone. In Sec. IV we combine the odds of finding we do not use the whole surface area of the spherical exolife different from us with the non observation of planet, since the whole planet does not intercept the artificial signals from beyond Earth to revise the upper sunlight all at the same time. We also do not use half limit on the average fraction of living intelligent species of the whole surface area of the spherical planet, since that develop communication technology. We conclude even though half of the planet is in “daylight”, the areas in Sec. V with some implications of our results. that are not perpendicular to the rays of sunlight should not be given as much importance as the areas that are perpendicular to the rays of sunlight. We need to use II. THE HABITABLE ZONE the two dimensional area blocked out by the spherical planet, and since a sphere always casts a circular shadow, Life needs of stars for at least two reasons: we need only find the area of a circle of radius Rp. The stars are required to synthesize heavy elements power input to the planet is given by • (such as carbon, oxygen, , iron) out of which ··· 2 rocky planets and the molecules of life are made; Solar power incident on planet = Frp π Rp , (6) stars maintain a source of heat to power the chem- • istry of life on the surface of their planets. and so for planet Earth we have For example, the Earth receives almost all of its energy 2 Solar power incident on Earth = Fr π R from the Sun. We can think of the Sun as a black- ⊕ ⊕ body radiator at temperature T 5, 777 K. The Stefan- 1.74 1017 W . (7) Boltzmann law gives us a way of' calculating the power ' × radiated per unit area, However, not all of this radiation is absorbed by the planet, but rather a significant fraction is reflected. The 4 Sun0s emitted radiation per unit area = σT , (3) ratio of reflected to incident solar energy is called the albedo, αp, which depends on the nature of the reflecting 8 2 4 where σ = 5.67 10− Wm− K− . At the present time in surface (it is large for clouds and light surfaces such as its evolution the× Sun emits energy at a rate of deserts, snow, and ice), see Table II. Under the present terrestrial conditions of cloudiness and snow and ice Solar luminosity L = 4π R2 σ T4 cover, on average a fraction α 0.30 of the incoming ≡ solar radiation at the Earth is reflected⊕ ∼ back to space. The 3.87 1026 W , (4) ' × solar radiation absorbed by a planet is then where we have assumed that the Sun is a sphere of radius 2 8 Fp = (1 αp) Fr πR . (8) R = 6.95 10 m. ,abs − p p The flux× of solar energy at the Earth, generally refer Estimates of exoplanet’s albedos are given in [6]. In equi- to as the “solar constant”, depends on the distance of librium, the total flux radiated by the planet into space the Earth from the Sun, r , and is given by the inverse ⊕ must balance the radiation it absorbes. Now, we as- square law sume that the planets emits in all directions like a black- L body of uniform temperature Tp (known as the “effective Solar constant Fr = . (5) planetary temperature”, or “emission temperature”) the ≡ ⊕ 4πr2 ⊕ Stefan-Boltzmann law gives:

Obviously, due to variations in the orbit of the Earth the 4 Planet0s emitted radiation per unit area = σT , (9) solar constant is not actually constant. The estimate set p 2 out in Table I, Fr = 1, 367 Wm− , corresponds to the and so the total radiation emitted by the planet is average value which⊕ results from the average Earth-Sun distance of r = 1.5 1011 m. 2 4 Planet0s emitted radiation Fp,em = 4π RP σ Tp . (10) Since water⊕ is essential× for life as we know it, the search ≡ for biosignature gases naturally focuses on planets lo- Note that (10) predicts the temperature one would in- cated in the habitable zone of their host stars, which is de- fer by looking at the planet if a black body curve were fined as the orbital range around the star within which fitted to the measured spectrum of outgoing radiation, surface liquid water could be sustained. Each star is and therefore this can be taken as the definition of the surrounded by a habitable zone. emission temperature. We can now equate (8) and (10) To estimate the orbital range of the habitable zone in to obtain the solar system we consider the energy balance of any " #1/4 planet intercepting the solar energy flux and radiating Frp (1 αp) Tp = − . (11) energy away. Suppose that a planet of radius Rp absorbs 4σ all incident light (100%) from the Sun. The planet blocks 2 out an area of πRp, and so this is the area we will use to It is noteworthy that the radius of the Earth has cancelled find the power that is absorbed by the planet. Note that out. This means that Tp depends only on the planetary 3

TABLE I: Characteristics of some planets in the solar system. Frp is the solar constant at a distance rp from the Sun, αp is the planetary albedo, Tp is the emission temperature of the planer estimated using (11), Tpmeasured is the measured emission temperature, and Tpsurface is the global mean surface temperature of the planet. The rotation period τ is given in Earth days. 9 2 planet rp (10 m) Frp (W m− ) αp Tp (K) Tpmeasured (K) Tpsurface (K) τ (Earth days) Venus 108 2632 0.77 227 230 760 243 Earth 150 1367 0.30 255 250 288 1.00 Mars 228 589 0.24 211 220 230 1.03 Jupiter 780 51 0.51 103 130 134 0.41

be without this atmosphere. Radiatively active gases TABLE II: Albedos for different surfaces. (i.e., greenhouse gases) in a planet’s atmosphere radiate type of surface albedo (%) energy in all directions. Obviously, such a greenhouse ocean 2 - 10 effect is not the same on all planets, and differs dra- forest 6 - 18 matically based on the thickness and composition of the cities 14 - 18 atmosphere. Three planets that show how dramatically grass 7 - 25 the conditions of a planet can change with the differ- soil 10 - 20 ent levels of the greenhouse effect are Venus, Earth, and Mars. For example, T is nearly 40 K cooler than the glob- grassland 16 - 20 ally averaged observed⊕ surface temperature. Without desert (sand) 35 - 45 the greenhouse effect, the temperature on Earth would ice 20 -70 be too cold for life. cloud (thin, thick stratus) 3, 60 -70 There are four main naturally occurring gases that are snow (old) 40 -60 responsible for the greenhouse effect: water vapor, car- snow (fresh) 75 - 95 bon dioxide, methane and nitrous oxide. Of these gases, water vapor has the largest effect. Once these gases ab- sorb energy, the gas particles begin to vibrate and they albedo and the distance of the planet from the Sun. Sub- radiate energy in all directions, including approximately stituting for the values given in Table I we find that the 30% of it back towards Earth. Earth has an emission temperature of T 255 K. The other two important greenhouse gases are ozone ⊕ ∼ Using (4) and (5) we estimate the flux of solar energy and halocarbons. Despite the fact that most of the green- for any planet in the solar system, house gases occur naturally in the atmosphere, some are man-made and the most well-known of these are fluo- R2 σT4 rocarbons. Since the industrial revolution, human activ- Frp = 2 . (12) rp ities have also resulted in an increase in natural green- house gases, especially carbon dioxide. An increase in Substituting (12) into (11) and rearranging we obtain these gases in the atmosphere enhances the atmosphere’s ability to trap heat, which leads to an increase in the av- 2 p R T 1 αp erage surface temperature of the Earth. r − p = 2 . (13) 2 Tp For a really strong greenhouse effect, we should look at Venus. Venus is similar to Earth in terms of size and Using (13) it is straightforward to estimate the range of mass, but its surface temperature is about 760 K. This is distances that determine the habitable zone by requiring hot enough to melt lead! The Venusian atmosphere is 273 < Tp/K < 373. mainly made up of carbon dioxide, a greenhouse gas. In Table I we show several parameters for some of the In summary, the habitable zone is the orbital range planets in the solar system and we compare the approx- around a star within which surface liquid water could imate measured values with those computed using (11). be sustained. Since water is essential for life as we know In general, there is a good agreement. Jupiter, however, it, the search for biosignature gases naturally focuses on is an exception due to the fact that about one-half of the planets located in the habitable zone of their host stars. energy input comes from the gravitational collapse of The habitable zone of the solar system looks like a ring the planet. around the Sun. Rocky planets with an orbit within A point worth noting at this juncture is that the tem- this ring may have liquid water to support life. The perature of a planet as derived in (11) has small correc- habitable zone around a single star looks similar to the tions due to atmospheric effects. More concretely, the habitable zone in our Solar System. The only difference so-called “natural greenhouse effect” is the process by is the size of the ring. If the star is bigger than the Sun which radiation from a planet’s atmosphere warms the it has a wider zone, if the star is smaller it has a nar- planet’s surface to a temperature above what it would rower zone. It might seem that the bigger the star the 4 better. However, the biggest stars have relatively short that support life on Earth, making silicon a strong lifespans, so the life around them probably would not contender. have enough time to evolve [7]. The habitable zones of Silanols, the silicon containing analogues of alco- small stars face a different problem. Besides being nar- • hols have surprising solubility properties, with di- row they are relatively close to the star. A hypothetical isobutylsilane diol being soluble in water and hex- planet in such a region would be tidally locked [8]. That ane [20]. Solubility is another crucial factor in the means that one half of it would always face the star and development of life, since having a solvent and a be extremely hot, while the opposite side would always substance is the model for early development of be facing away and freezing. Such conditions are not life that we see on Earth. very favorable for life. Silicon’s chiral properties. All life on Earth is made The frequency η of terrestrial planets in and the hab- • of molecules that twist in the same direction, that is itable zone of solar-type⊕ stars can be determined using they have an inherent handedness. In other words, data from the Kepler mission [9–11]. Current estimates each of life’s molecular building blocks (amino +0.13 +0.07 suggest 0.15 0.06 < η < 0.61 0.15 [12, 13]. acids and sugars) has a twin: not an identical one, − ⊕ − but a mirror image. On Earth, the amino acids characteristic of life are all “left-handed” in shape, III. LIFE OUTSIDE THE HABITABLE ZONE and cannot be exchanged for their right-handed doppelganger.¨ Meanwhile, all sugars characteris- tic of life on Earth are “right-handed.” The op- It has been pointed out that life outside the habit- posite hands for both amino acids and sugars ex- able zone may be possible on planets with subsurface ist in the universe, but they just are not utilized oceans [14]. Allowing for the possibility of subsurface by any known biological life form. (Some bacte- ocean worlds yields a frequency of planets η 1. Now, ria can actually convert right-handed amino acids because the planets with subsurface oceans outside∼ the into the left-handed version, but they cannot use habitable zone are more common than rocky planets in the right-handed ones as is.) This phenomenon of the habitable zone, one may wonder why do we find biological shape selection is called chirality – from ourselves on the latter. The answer to this question the Greek for handedness. We say that both sugars most likely stems from the fact that “we” refers to an and amino acids on Earth are homochiral: one- intelligent, conscious and technologically sophisticated handed. species. In other words, albeit the probability of life Though we are still unsure why it is that the on subsurface worlds may be non-negligible, it is quite molecules of carbon-based life choose only one ori- plausible that the likelihood of technological life could entation, it seems reasonable to require that in or- instead be selectively lowered. In this section we ex- der for silicon to replicate the processes that orig- plore the possibility that life elsewhere could have a non- inated life on Earth the molecules must also be carbon chemical foundation; e.g., in the spirit of [15, 16] chiral, and exist in a left- or right-handed forms in we envision a race of intelligent silicon-based life forms. potential living environments. There is certainly Considering that Earth is the only reference point we reason to be optimistic: an observation of chiral- have when studying life, it is unsurprising that biochem- ity in noncrystalline silica chiral nano-ribbons has istry has always been connected to the elements of car- been reported in [21]. bon, hydrogen, oxygen, and nitrogen. Moreover, carbon Silicon’s high reactivity is a barrier to forming com- can form bonds with many other non-metals, as well as • plex structures on Earth, as this high rate of reac- large polymers. These unique qualities have led many tion leaves little time for construction. However, to argue that carbon is a pre-requisite for the existence this only holds true for environments with a cli- of even very simple life. However, this must not nec- mate similar to earth. On the outskirts of the solar essarily be true. It has long been suspected that silicon system, where the reactivity of carbon is severely and germanium can enter into some of the same kind impacted by the drop in temperature, silicon’s high chemical reactions than carbon does [2]. Recent research reaction rate could be the key to the development in both chemistry and astrobiology has shown that it is of life in these cryogenic environments, allowing it theoretically quite feasible for silicon to form complex, to flourish where carbon based life would be im- self-replicating systems similar to the ones that produced possible. the first, simple forms of life on Earth [17, 18]. More con- One probable environment for silicon life is liquid cretely: nitrogen [18]. Nitrogen is one of the few substances Silicon is able to form stable covalent bonds with • that can still dissolve silicon at very cold temper- itself, as well as stable compounds with carbon atures, as solubi. Additionally, silicon is able to and oxygen [19]. These structures can form many form stable covalent bonds with nitrogen, as well diverse systems, including ring systems, which as with itself. could be analogs to sugars, a key component of biochemistry on Earth. This stability is a prerequi- The habitable zone for silicon life would then depend site for building the complex chemical structures on the area around a star in which nitrogen is a liquid. 5

Neptune’s moon, Triton, has been considered a candi- exoplanets relative to their counterparts around solar- date for surface level nitrogen lakes [22]. Triton is the type stars [31]. Nevertheless, this may not be the case only large satellite in the solar system to circle a planet for silicon-based alien life forms. Herein, we evaluate the in a retrograde direction, i.e. in a direction opposite TRAPPIST-1 system as representative of ultra cool stars, to the rotation of the planet. The retrograde orbit and for which T? = 2, 511 K, and R? = 84, 179.7 km [32]. Triton’s relatively high density suggest that this satel- We generalize (13) substituting T by T? and R by R?, lite may have been captured by Neptune as it traveled to find that the habitable zone for ultra-cool dwarf stars through space several billion years ago. If this were the encompasses a distance range between 1.6 million km to case, tidal heating could have melted Triton in its orig- 3.0 million km from the planet’s star, whereas for silicon- inally eccentric orbit, and the satellite might have been based life on nitrogen lakes the habitability circumstellar liquid for as long as one billion years after its capture by region spans the orbital range within 28 million km and Neptune. However, presently Triton is quite cold, with 42 million km. This seems to indicate the frequency a surface temperature of 38 K, and an extremely thin at- of planets hosting any form of life must be extended. mosphere (the atmospheric pressure at Triton’s surface As for subsurface ocean worlds, we may take η 1 for is about 14 microbars, 1/70,000th the surface pressure on intelligent, conscious and technologically sophisticated∼ Earth). Nitrogen ice particles might form thin clouds a species. few kilometers above the surface. Hence, even though the surface temperature is below the freezing point of liquid nitrogen it is reasonable to assume that the albedo IV. UPPER LIMIT ON hξbioteci of a hypothetical planet that could support silicon life will be similar to that of Triton, α 0.6 [23]. Triton ∼ Next, in line with our stated plan, we derive an up- Next, using (13) we determine the habitable zone of per limit on ξ . The present day star formation h bioteci silicon-based life for a main sequence star like our sun, rate in the Galaxy is estimated to be M˙ ? = 1.65 1 ± with temperature T and radius R . We take the plan- 0.19 M yr− [33, 34]. This estimate has been derived as- etary surface temperature in between the boiling and suming the Kroupa initial mass function (IMF) [35, 36]. freezing point of liquid nitrogen, 63.15 < Tp/K < 77.36. The shape of this IMF is lognormal-like and exhibits Plugging in these values in (13) we find that for a main a peak around M/M 0.4 [37], suggesting there are ≈ sequence star like our sun, the habitable zone of silicon- roughly 2 stars per M . Altogether, this yields Γ? 1 ≈ based life stretches from 1.24 billion km to 1.85 billion km 3 yr− . Before proceeding, we pause to note that the av- from the star. We can now estimate what planets within erage star formation rate in the Galaxy could be about 4 the solar system fall into the silicon habitable zone dur- times the current rate [38]. Now, only 10% of these stars ing all parts of their orbit. The two planets closest to are appropriate for harboring habitable planets. This is the silicon habitable zone are Saturn and Uranus. Sat- because the mass of the star M? < 1.1M to be suffi- urn has a perihelion of 1.35 billion km and aphelion of ciently long-lived (with main sequence lifetimes larger 1.51 billion km, meaning that it is within the proper dis- than 4.5 Gyr) and M? > 0.7M to possess circumstellar tance range for silicon biochemistry. However, Saturn is habitable zones outside the tidally locked region [8]. The mostly a gas planet, and thus unsuitable for supporting production rate of habitable planets is then any life. Uranus, on the other hand, has a perihelion !   of 2.75 billion km and aphelion 3.00 billion km, making Γ? η 1 ζ 0.045 yr− , (14) it too cold for surface lakes or oceans of nitrogen. This 1 ∼ 3 yr− 0.15 result is also in agreement with the commonly accepted 1 surface temperature of Uranus, roughly 57 K [24], which with 0.15 < η < 1 and 3 < Γ?/yr− < 12. is below the freezing point of nitrogen. Now, a habitable planet must survive and remain in We expand our focus to include ultra-cool stars, such a habitable zone to present day. The potential hazard as TRAPPIST-1A, as they are the most common stars in of nearby gamma-ray bursts (GRBs) has been estimated the Milky Way, and thus their orbiting planets are rep- elsewhere [5]. Long GRBs are associated with supernova resentative of “average” star systems. More concretely, explosions. When nuclear fuel is exhausted at the center M-dwarfs like Proxima Centauri and TRAPPIST-1 are 10 of a massive star, thermal pressure can no longer sustain times more abundant than the Sun [25, 26] and have stel- gravity and the core collapses on itself. If this process lar lifetimes that are about 100 to 1000 times greater [27– leads to the formation of a rapidly spinning black hole, 29]. Furthermore, exoplanets around these stars are eas- accreted matter can be funneled into a pair of powerful ier to detect (the transit signals produced by Earth-sized relativistic jets that drill their way through the outer lay- planets are 80 times stronger than the signal produced ers of the dying star. If the jet is pointing towards Earth, by similar planets transiting a Sun-like star) and their at- its high-energy emission is seen as a GRB. mospheres can be analyzed via transit spectroscopy, thus The luminosity of long GRBs – the most powerful ones enabling the ready detection of biomarkers [30]. How- – is so intense that they are observed about once a day ever, various physical mechanisms could act in concert to from random directions in the sky. The physical condi- suppress the likelihood of Earth-based life on M-dwarf tions in the dissipation region produce a heavy flux of 6 photons with energies above about 100 keV. If one GRB we cannot use the usual standard error estimate for the were to happen nearby, the intense flash of gamma rays confidence interval. Instead, we use a small sample con- illuminating the Earth for tens of seconds could severely fidence interval for the estimate, based on the exact prob- damage the thin ozone layer that absorbs ultraviolet ra- abilities of the Binomial distribution. In the absence of diation from the Sun. Indeed a fluence of 100 kJ/m2 background, N < 3.09 determines the 95% confidence would create a depletion of 91% of this life-protecting interval [49]. layer on a timescale of a month, via a chain of chemical Models with associated parameters in the right-hand- reactions in the atmosphere. This would be enough to side of (2) predicting more than 3.09 events are excluded cause a massive life-extinction event [39–45]. at 95% CL. Then substituting (16) into (2) with L τ ∼ The critical time tc for life to arise and evolve becomes 0.3 Myr we obtain the dominant uncertainty on estimating the probability of having at least one lethal GRB, with a critical fluency 1 ! !   3 3 yr− 0.15 0.044 100 kJ/m2. Life has been evolving on Earth for close to ξbiotec < 5 10− (17) h i × Γ? η /ζ 4 Gyr [46, 47], but complex life is well under 1 Gyr old, and intelligent life is only a Myr old at most. In what at the 95% CL. follows we adopt tc = 1 Gyr and 4 Gyr as critical time intervals for life evolution [29]. With this in mind, the probability for a GRB to destroy an entire alien race /ζ is estimated to be [5] V. CONCLUSION

0.044 . /ζ . 0.22 . (15) We have re-examined the likelihood for alien civiliza- tions to develop communication technology on the ba- To derive the upper limit on ξ we must take h biotecti sis of the general assumption that life elsewhere could as fiducial parameters those giving the possible lowest have a non-carbon chemical foundation. We derived a value of ζ ζ/ζ, i.e. h astroi ∼ conservative upper bound on the average fraction of liv- !     ing intelligent species of Earth-like planets that develop 3 Γ? η /ζ 1 3 ζ 2 10− yr− . (16) communication technology: ξbiotec < 5 10− at the astro 1 h i × h i ∼ × 3 yr− 0.15 0.044 95% CL. The upper limit can be up to a factor of 6 more restrictive if we assume there may be non-carbon based Finally, to determine the upper bound on ξ we h bioteci living organisms. must decide on the possible minimum Lτ. Herein we The Breakthrough Listen Initiative, announced in July consider Lτ > 0.3 Myr such that cLτ propagation dis- 2015 as a 10-year 100M USD program, is the most com- tances of Galactic scales ( 10 kpc). This would provide ∼ prehensive effort in history to quantify the distribution enough time to receive electromagnetic (and/or high- of advanced, technologically capable life in the uni- energy neutrino [48]) signals from any advanced civi- verse [50–52]. The search for extrasolar biomolecular lization living in the Milky Way which is trying to com- building blocks and molecular biosignatures of extinct municate with us. extraterrestrial life will soon be extended to the lunar The non observation of artificial signals from beyond surface [53]. The ability to detect alien life may still be Earth prevents an estimate of the event rate. Our best years or more away, but the quest is underway. estimate of the proportion of cases which have a signal (i.e. an event) is zero, but there will be uncertainty in this estimate. Just because we have not seen an event yet does not mean we will never see one. We need a Acknowledgments confidence interval for this estimate. If a corresponding hypothesis test is performed, the confidence level (CL) This work has been supported by the by the U.S. is the complement of the level of statistical significance, National Science Foundation (NSF Grant PHY-1620661) e.g, a 95% confidence interval reflects a significance level and the National Aeronautics and Space Administration of 0.05. Because the number of events observed is zero, (NASA Grant 80NSSC18K0464).

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