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Kardashev (1964) evaluated the feasibility of these ap- structure is unstable againstcollapse, as pointed by many authors proaches in radio and as a by-product developed a quantitative (e.g. Suffern 1977). This problem can be addressed without call- scheme to measure the stage of an ETC’s advancement via the ing for speculative technology or physics by breaking the sphere total amount of energy they have at their disposal. Undoubtedly, into a swarm of individual elements often called Dyson swarm. Kardashev’s classification is still useful for SETI because it helps Each of these elements is not unlike the space habitats proposed to define benchmark energy capabilities of the ETCs and from by O’Neill & Kraus (1979), but they must be quite numerous to these to set up sensitivity requirements for the SETI equip- provide a covering factor close to unity, so nearly the entire en- ment. However, recent years brought up some new considera- ergy of the star is captured – as required by the definition of the tions. For example, Benford et al. (2010b,a) argued about cost- Class II ETC. optimized means of interstellar communication, and proposed Dyson (1960); Kardashev (1964); Sagan & Walker (1966) some strategies to that effect (see more arguments for cost- realized that the most prominent signature of both the ETCs’ saving in Davies & Wagner 2013). Kardashev (1964) had not energy metabolism and of the Dyson sphere would be the in- taken these into account. He implicitly makes the assumption frared (IR) radiation and a number of searches for stars with that unlimited resources are available to the ETCs, allowing IR excesses have been carried out since, mainly at stars on unconstrained growth of the energy production and consump- the main sequence that are long-lived and are not expected to tion, at least up to galactic scale. Furthermore, the estimates of show IR excess, all with negative results (Slysh 1985; Tilgner & Kardashev are upper limits that give the maximum energy avail- Heinrichsen 1998; Timofeev et al. 2000; Jugaku & Nishimura able for interstellar communication for the given level of ETC 2004; Carrigan 2009; Wright et al. 2014b; Griffith et al. 2015; development. Osmanov 2016). Searches in the optical have also been consid- Our historic and modern experience can hardly support un- ered by Osmanov & Berezhiani (2018) who predicted anoma- constrained growth. Therefore, it is compelling to re-examine lous variability of the sphere’s structure due to oscillations. the ETC classification as a guiding tool for SETI strategies, Zackrisson et al. (2018) argued that a Dyson sphere with a aiming to optimize them and to arrive to a new priority scale covering factors less than unity can be recognized as a sub- for the different search methods. The next section reviews the luminous source, as long as an accurate parallax measurement Kardashev’s classification and its implications. Section 3 de- is available. They searched for such object combining the Gaia scribes a new quantitative scale proposed here, and the final Data Release 1 (Gaia Collaboration et al. 2016) with the Radial Section6 sumarizes this work. Velocity Experiment (RAVE; Kunder et al. 2017) and found a few stars with lower intrinsic luminosity than expected for their spectral Class and with no detectable IR excess. However, alter- 2. Quantitative classification: Kardashev’s scale native explanations such as unseen companions that might com- Kardashev (1964) introduced a classification scheme for ETCs promise the astrometric solutions or gray dust can not be fully based on the energy, available to them. This is a quantitative ap- excluded. proach, well-justified in the context of that study, aimed to deter- The last Class in the Kardashev’s classification is the most mine the technical feasibility of the communication between civ- speculative and our current technology gives little clues how an ilizations. He considered an isotropic radio emission, and esti- ETC could capture and utilize the energy of an entire galaxy. mated that transmitting with a data rate of 3×105 bits sec−1 at dis- One option is a simple quantitative expansion of Class II ETCs, tance of ∼3Mpc – this is just belowthe distances to the M81 and populating a galaxy with multiple Dyson spheres, whose total CenA groups of galaxies and encompasses a significant number number is comparable to that of the stars in the galaxy. Similarly of galaxies, increasing significantly the number of ETCs that can to the sphere-enshrouded stars, the galaxy will become fainter potentially be detected, in comparison with more modest emit- and redder and move away from the usual scaling relations such ting power that would limit an ETC search to nearby stars. The as the Tully-Fisher relation. Following this argument, Zackrisson estimate yielded a required transmitting power of 4×1033 erg s−1, et al. (2015)set an upperlimit of ≤0.3% to the local Kardashev’s comparable to the total solar luminosity. Class III disk galaxies. Kardashev (1964) concluded that the transmitting power is Wright et al. (2014a,b, 2016) and Griffith et al. (2015) the controlling parameter of the data rate and covered distance. searched for ETCs with large energy supplies, mostly by means This prompted him to build a classification of ETCs based on the of the Wide-field Infrared Survey Explorer (WISE; Wright et al. energy in their disposal: 2010). They identified some unusual objects, none of them fully matches the expected signatures of ClassIII ETCs. The authors – ClassI – an ETC in possession of all energy of its planet or converted the obtained observational limits into limits on the ∼ × 19 −1 4 10 erg s ETCs’ energy supply. Other teams also failed to detect Class III – ClassII – an ETC in possession of all energy of its star or ETCs (Annis 1999; Garrett 2015; Olson 2017) ∼4×1033 erg s−1 – ClassIII – an ETC in possession of all energy of its galaxy A possible explanation for the lack of detections is suggested or ∼4×1044 erg s−1 by Lacki (2019), who investigated the observational conse- quences if only a fraction of the stars is enshrouded. Presumably The first Class is the easiest to comprehend, because it im- it is easier to build Dyson spheres around low-mass stars than plies a technological level close to the present-day Earth’s. The around hot high-mass stars for which the habitability zone is fur- humanity itself is approaching this level of energy consumption. ther out. The model predicts no detectable effects if the limit is Right now we are still limited mostly to the Earth’s fossil fuel close to the Solar Class stars; it must be raised up to ∼30LS un to and atomic energy from some radioactive elements; the renew- make the presence of Dyson spheres apparent. able sources of energy are still underutilized, but their contribu- Lacki (2016) considers an alternative to the classical Dyson tion in the total energy budget of our civilization is increasing. spheres – enshrouding the entire galaxies with artificial dust that The second Class is more hypothetical. Kardashev (1964) would turn them effectively into black boxes, bright only in the gives as an example the Dyson sphere (Dyson 1960). Such a microwave spectral region. He searched the Planck Catalog of

2 V. D. Ivanov: Classification of extraterrestrial civilizations

Compact Sources (Planck Collaboration et al. 2016), with nega- A major argument for improving ourselves is to boost our tive result. adaptability – an important advantage in a world of nearly infi- Although the last two Class are purely hypothetical, the nite environmental variety. We can not tolerate the entire range Kardashev’s classification gained foothold in the ETC studies of temperatures and pressures without major protection mea- because of its convenience and the straightforward quantitative sures even on our home planet, let alone live on any of the other parameterisation. In his excellent review, Cirkovi´c(2015)´ shows planets in the solar System or potentially – on any exoplanet. with multiple examples that the scale had a strong effect on the Last but not least, adopting the strategy to modify ourselves many SETI searches over the last five decades, on the strate- removes the need to achieve consensus about how the environ- gies that these projects have adopted, and on the interpretation ment could be changed and our civilization has a remarkably of their results. poor record on agreement, as the two world wars in the last cen- tury demonstrate. We can bracket these two stages of ETC evolution. At the 3. Qualitative classification of ETCs low-end, we extent of the term civilization to include wild ani- The main motivation to re-examine the existing ETC classifi- mals that generally use the environment as it is. However, this is cation is the question how a hypothetical ETC would use the not always the case. First, in a broad sense any animal modifies available energy beyond the somewhat brute force approach of the environment – e.g. just because of its metabolism, and sec- emitting it in space or blowing things up and building artificial ond, there are well known examples of animal tool use (Goodall space habitats. In practical terms, we propose to measure this 1964; Van Lawick-Goodall 1971; McGrew 1974, among others) quality of use as the level of interaction with the Universe. which underlines the point that the new classes that we are about We can turn to the humanity’s own scientific and technolog- to introduce are not discrete bins but represent a part of a contin- ical progress, to trace the capabilities to manipulate matter: me- uous sequence. chanical – chemical – atomic – nuclear – etc. One can only spec- At the high-end side we speculate that the boundary between ulate what the next levels will be – Adams & Laughlin (1997) the environment and one’s self would eventually be diluted in mention the annihilation of CDM particles as a possibility. This the process of self-improvementto the point of merging the two. is similar to the ETC classification scheme of Barrow (1999), This is a natural consequenceif we assume that the ultimate goal who uses as metrics the level of manipulation of the microworld. of the intelligence is to spread, which in more speculative terms However, we can generalize further, combining these interac- may imply converting all the matter in the Universe into think- tions into a single process – of modifying our environment. The ing matter (but see Sandberg et al. 2017, for a reasoning why humanity entered this stage the moment the first tool was used. an advanced civilization may prefer to stay dormant during the From this prospective the next step will be to start modify- present cosmological era). ing ourselves, to match the environment. The modern medicine To underline, we use here the level of interactions with mat- is on the verge of this transition – from curing organisms to up- ter and the degree of integration of ourselves with the environ- grading them. It is one step form the gene therapies that pre- ment as near synonymous, because the latter follows from the vent a fetus from developing some dangerous diseases to im- former: historically, once our technological capabilities allowed proving it. Indeed, the CRISPR-Cas9 (clustered regularly inter- it, we tried to modify the environment, e.g. moving from natural spaced short palindromic repeats) technique for gene editing has caves to purpose build housing; we are already willing to accept recently been improved to allow simultaneous editing of multi- modifyingour kind as long as it is seen as upgrading,even if it is ple genes (Jinek et al. 2012; Strecker et al. 2019; Camps et al. ethically questionable – the tendency for selective abortions of 2019), bringing both medical and commercial applications of female fetuses in East Asia proves it (Hesketh et al. 2011). gene therapies within closer reach. Summarizing, we propose a new ETC classification scheme, Is modifying ourselves an improvement over modifying the containing the following three categories: environment? – Yes, for a number of reasons. First, because we, as a product of semi-random evolution, are far from optimal for – Class0 : the environment is used as is (animals) all the environments, even here on the Earth. We have evolved – Class1 : we modify the environment (clothes, buildings) for a short life in small groups, in the savannah. As a result, our – Class2 : we modify ourselves to fit the environment (geneti- brains have insufficient computing capabilities for the modern cally improved humans) life when we have to complete complex tasks that require func- – Class3 : we mergewith the environment,convertingthe dead tioning within large diverse groups. One of the unfortunate con- matter in the Universe into thinking matter sequences is that resort to typecasting – a major reason for the problems we face with various biases in the connected global Throughoutthis paper we denote the new classes with Arabic village of today (Kahneman 2011, and the references therein). numerals, including fractional classes such as 0.5, 1.3, 2.8, etc. Vast areas of Earth’s surface near the poles and the oceans are For clear separation for the Kardashev classes we use Roman marginally accessible to us. numerals, although there are strong arguments for fractional Next, our bodies wear out quickly and by age of 50-60 we Kardashev classes as well (see the discussion in Cirkovi´c2015).´ start facing problems with such basic components as bones, vi- The proposed new ETC scale is less strict than the classical sion and hearing. As of 2011 about 0.2-0.3% of people need Kardashev’s scale, as the example given above of tool use by hip joint replacement and 0.1-0.2% – knee replacement at some animals suggests. Furthermore, some of the modifications to the point of their lives (Wengler et al. 2014), and those numbers are environment that we apply right now can also be interpreted as increasing (Kremers et al. 2015). In some countries eight in ten modifications of ourselves – although this example is far from people wear glasses by age of 20 and the fraction has been rising the genetic manipulations mentioned earlier, a hand watch and a for ages, correlating with reading and education (Williams et al. pair of binoculars are modifications of the environment, but they 2015; Morgana et al. 2018). These are just a few easy problems, can also be though of as removable implants aimed to improve we are not discussing here the most serious ones such as cancer the internal time keeping and the eyesight of average humans. and various genetic disorders. However, the eye glasses or lenses despite being removable im-

3 V. D. Ivanov: Classification of extraterrestrial civilizations plants, aim to cure a disease, not to advance our capabilities and therefore are not indications of self-upgrade. Mechanical Atomic Sub-quark? ETC merging Some notions of the ideas proposed here can be found in Chemical ...... with matter? the works of Kecskes (1998, 2013) who considered a classifi- cation scheme adding to the energy resources the complexity of Class III

ETCs’ transport, communication and other resources. In another "Terraforming" pan-galactic supercivilization relevant work Nunn et al. (2014) considered the historic path of the humanity and concludes that it may have reached the point of creating an environmental utopia that removes the stimuli for Class II optimal health, but stops short of asking the question what is the Dyson sphere builders next step after that utopia – a question that we address with our

ETC classification scheme. energy available Total Class I The two classifications – Kardashev’s and the new one we Humanity propose – can be combined to form a single 2-dimensional present-day scheme that describes the ETC’s progress with two parameters: discovering fire first tool-making the quantity of the available energy and the quality of its use.

Figure1 demonstrates this scheme with a few examples. The Level of interaction with matter approximate locations of the humanity throughout history are shown on the upper panel: the first tool-making illustrates the mastering of the mechanical energy, the discovering of fire – the Class 0: use Class 1: Class 2: Class 3: widespread use of chemical energy, and our present day state environment modify modify merge with is characterized by use of atomic energy and an incomplete use as it is environment themselves environment of the entire energy available on our planet. Dyson sphere build- ing civilization and a conventional pan-galactic supercivilization Class III (e.g. one that expands though its home-galaxy by means of mul- "Terraforming" pan-galactic civilization "Computational" galaxy tiple Dyson spheres) are also shown. Presumably, the two last examples have not achieved the level of self-modification that characterizes our Class3 ETC; we describe such civilizations Class II Dyson sphere builders with the word terraforming with quotation marks to underline Not-energy-hungry our wider interpretation: adjusting the environmental condition pan-galactic "invisible" to ones needs in general, not necessarily on a planetary surface. energy available Total civilization Class I The bottom panel shows the Earth animals, with an offset Humanity Self-adapting pan-galactic from the pure Class0 to account for the tool usage, e.g. by biological civilization chimpanzees. The present-day humanity spans the regions of using and modifying the environment, but stops short of the Animals self-modifications. Barring major catastrophic events, we will Integration with the environment probably reach that level of technology in the foreseeable future. Again, we mark the loci of Dyson sphere builders and terraform- Fig. 1. Two-dimensional classification of the ETCs. Top: The ing pan-galactic supercivilization,and we add a hypothetical civ- horizontal axis expresses the capabilities of an ETC to interact ilization that has converted its host galaxy into a computational with its environment. The vertical axis quantifies the amount of environments – the heat losses of such ETCs can potentially energy available to them, as defined by the classical Kardashev’s be detected by the WISE searches (e.g., Wright et al. 2014a, scale. Bottom: Generalized two-dimensional classification. The among others). The largest, all encompassing class of civiliza- horizontal axis shows the level of integration with the environ- tions is that of the adaptable, self-modifying ones. Changing the ment. The approximate locations of the humanity throughout paradigm from physical change of environment to biological or history and of a few hypothetical civilizations are shown and even post-biological modification and optimization of the living labeled. For details see Sec.3. organisms changes the energy requirements. Indeed, the biolog- ical or computational research do not pose high energy demand. Our framework opens up a particularly interesting possibility – energy is used and what is its impact on the interaction with the a self-modifying civilization that does not need vast amounts of matter in the Universe. energy because it is fully adaptable to the environment. First, our classification scheme address in a new way the important question of detectability – the ultimate strength of 4. Implications for our notion of ETCs Kardashev’s work that was developed exactly to address this is- sue in the particular context of radio communications. Recently, The new classification proposed here is deeply rooted on hu- Lingam & Loeb (2019) concluded that the probability of de- manity’s own evolution and may be biased in ways that can not tecting advanced ETCs’ technosignatures may be two orders be evaluated as long as we only know of one intelligent species of magnitude lower than of detecting biosignatures from prim- – our own. Therefore, we assume that the lessons we learn from itive life. It is worth to remember that all SETI projects have the humanity’sevolutionand history are – at least to some degree explored an exceptionally low fraction of the Milky Way para- – typical for at least some ETCs. The most obvious advantage of metric space that can be inhabited by ETCs – only 10−21–10−18 our scheme is the novel way of thinking about the ETC: we ac- (Wright et al. 2018). The realization that the footprint of an ETC knowledge that the parametric space that SETI searches need to and its detectability – both dominated mainly by the energy – cover can not be described with a single parameter as Kardashev may not scale up with the available energy, makes this estimate (1964) proposed. We introduce the question of how the available optimistic.

4 V. D. Ivanov: Classification of extraterrestrial civilizations

Another point, underlined by the new framework is that the ment to more complex manipulation on chemical, atomic, nu- two classifications – Kardashev’s and the one proposed here – clear, etc. levels that allow it to achieve larger impacts and more are not directly correlated. In other words, the available amount importantly, impacts that were not possible earlier with the sim- of energy does not necessarily mean a more sophisticated inter- pler levels of interaction. However, this is not necessarily ac- action with matter and closer integration with the environment. complished by an ever increasing energy consumption – the bio- The Kardashev classes are separated by vast 11-12 orders of sciences show it, and the opposite notion is probably a bias, due magnitude but the humanity – estimated to be using still only to the fact that astronomers and physicists akin to Kardashev, about 70% of the energy at the disposal of a ClassI civiliza- Dyson and Sagan have been leading the SETI research, and they tion (Cirkovi´c2015)´ – does not seem too far from reaching the come armed with the idea that progress is embodied by a more adequate biotechnological development to improve itself and to powerful accelerator or radio transmitters. integrate with the environment. This is easy to understand if we The final and the most important consequence from the new keep in mind that the biological research is not as energy inten- framework is the weakening of the Fermi Paradox (Hart 1975) sive as the nuclear physics, as pointed above. – if ETCs’ progress does not always imply higher energy con- Furthermore, it is uncertain whether we can expect that more sumption and waste, then progress also does not imply higher available energy would only scale up our ability to modify the detectability of the ETCs. This explanation of the Fermi paradox environment. In other words, we still lack the understanding opposes the usual conclusion for the rarity of ETCs. In fact, they whether the building of a Dyson sphere is just a matter of may be common, but the low cross-section of ours and their level having more powerful mining equipment and heavier rockets of interaction with the Universe would account for the silentium or of some speculative technologies like nano-machines, self- universi. replicators, etc (see the discussion in Armstrong & Sandberg 2013). In the former case the total amount of available energy may play a role, but in the latter – less so, hereby removing any correlation between the two ETC scales described here. 5. Predictions and implications for the SETI These ideas are not entirely new among the SETI commu- strategies ´ nity. Indeed, the two-pointed arrow in his Fig.1 of Cirkovi´c The SETI programs search for ETC’s technosignatures – traces (2015) acknowledges the possibility, that the complexity is not of advanced technologies. On the other hand, the searches for directly related to the available energy. life at the crossing of the modern astronomical and biological The third direct consequence from the broader consideration research look for products form the natural life cycle (not neces- of ETCs’ properties proposed here is the invalidation of the ob- sarily advanced to the level of civilization, never mind how ill- vious statement that a ClassIII ETC does not exist in the Milky defined this level may be; Des Marais et al. 2002; Seager et al. Way (Hart 1975). Our searches of such advanced ETCs rely on 2005; Segura et al. 2005; Scalo et al. 2007). However, the dis- the conceptof detecting their heat leaks (e.g.Wright et al. 2014b; tinction between bio- and technosignatures may not be clear-cut. Griffith et al. 2015), on the observational consequences from the Raup (1992) considered hypothetical animals that communicate controlled disintegration of galaxies for resources (Tipler 2003), with radio waves. Indeed, the electric squids and rays (e.g. Raup or on searches for megastructures (Wright et al. 2016). The new 1992) use electricity, and direct electricity generation by bio- classification scheme allows for the existence of quiet advanced logical systems have been demonstrated Tanaka et al. (2016). civilizations that may co-exist with us, yet remain invisible to Therefore, signatures we commonly consider part of the techno- our radio, thermal or transit searches. The implicit underlying logical realm, may actually evolve naturally. For simplification assumption of Hart (1975) is that the hypothetical ETC is in- we will exclude this possibility from the following discussion, teracting with the matter on a similar level as us. We can not but we remind the reader that the most fundamental assumptions even speculate if it is possible to detect a heat leak or a transiting of SETI are not simple and straightforward. structure build by an ETC capable of interacting with the matter What does this new ETC classification scheme mean for at sub-quark level, but the answer is more likely negative and not the definition of future SETI projects? – Wittingly or not, the because that ETC would function according to some speculative searches so far have been fine-tuned to detect civilizations of physics laws, but because such an ETC would probably be vastly Kardashev TypesII and III, but only ones that follow the same more efficient than us controlling its energy wastes and minimiz- more-is-better philosophy as we do. The mechanistic transfer of ing its construction projects. Would such an advanced ETC even this power hungry reasoning across a range of available energy need megastructures and vast astroengineering projects? levels wider than 25 orders of magnitude could be why these It is also unlikely the on-going SETI project would success- searches fail. The searches for Dyson spheres and swarms, al- fully detect the Kardashev ClassII stellivorous ETC described though relatively easy with the present technology, also seem by Vidal (2016). Indeed, Heidmann & Klein (1991) noted that a less than promising. Somewhat more productive strategy may successful SETI search requires a match between the technology be to search for biosignatures, because that would accompany of the transmitting and receiving sides. life regardless from the technological development (Lingam& These consideration cast some doubts on the popular pes- Loeb 2019). However, as discussed earlier, we can not fully rely simistic conclusions about the lack of ETCs (e.g. Lacki 2016) even on the biosignatures, because an advanced ETC can (pre- (although some concerns for a Great Filter intrinsic to all civi- sumably) easily modify itself to survive beyond the limitations lizations appears to be still valid (e.g. Sotos 2019). of its original habitability limitations. Summarizing, the new framework leads to questioning the The possibilities of self-modification and for further inte- common assumption that progress is equivalent to ascending the gration of ETCs with their environment destroys the very idea ladder of energy consumption from ClassI to III, as suggested of separating natural and artificial phenomena and by definition by Dyson (1960) even before Kardashev came up with his clas- makes it impossible for us to detect with confidence any tech- sification. Indeed, an ETC can – as our own history shows it nosignatures, because – rephrasing A. C. Clarke – any advanced – progress from purely mechanical modification of its environ- ETC will be indistinguishable from nature. This idea of indistin-

5 V. D. Ivanov: Classification of extraterrestrial civilizations guishability has been discussed earlier in Cirkovic (2018)1. This References ff process of technological development and optimization is di er- Adams, F. C. & Laughlin, G. 1997, Reviews of Modern Physics, 69, 337 ent from the natural evolution of techno-like signatures proposed Annis, J. 1999, Journal of the British Interplanetary Society, 52, 33 by Raup (1992). Armstrong, S. & Sandberg, A. 2013, Acta Astronautica, 89, 1 Undoubtedly, these arguments would be well understood by Barrow, J. D. 1999, Impossibility - The Limits of Science and the Science of ETCs that have attained the levels of progress discussed here. Limits Benford, G., Benford, J., & Benford, D. 2010a, Astrobiology, 10, 491 Therefore, they would set up beacons to emit clearly artificial Benford, J., Benford, G., & Benford, D. 2010b, Astrobiology, 10, 475 signals – on the condition (addressing this major question is be- Bradbury, R. J., Cirkovi´c,´ M. M., & Dvorsky, G. 2011, Journal of the British yond the scope of this work) that they still wish to communicate Interplanetary Society, 64, 156 with less advanced counterparts such as us. Camps, C. C., Weisbach, N. R., Santinha, A. J., Danny, I., & Platt, R. J. 2019, These arguments do not close the possibility to find ETCs at Nature Methods, 16, 887 Carrigan, Richard A., J. 2009, ApJ, 698, 2075 similar level to ours, but given the limited energy resources avail- Cirkovi´c,´ M. M. 2015, Serbian Astronomical Journal, 191, 1 able to those ETC, such SETI programs are limited to smaller Cirkovic, M. M. 2018, The Great Silence: Science and Philosophy of Fermi’s space volume that can be searched with any hopes for success. Paradox Concluding, the new framework implies two strategies for Cocconi, G. & Morrison, P. 1959, Nature, 184, 844 Davies, P. C. W. & Wagner, R. V. 2013, Acta Astronautica, 89, 261 SETI: Des Marais, D. J., Harwit, M. O., Jucks, K. 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We thank the referees for the comments that helped to im- Sotos, J. G. 2019, International Journal of Astrobiology, 18, 445 prove the paper and for pointing at the number of relevant works in the field of Strecker, J., Ladha, A., Gardner, Z., et al. 2019, Science, 365, 48 Suffern, K. G. 1977, Proceedings of the Astronomical Society of Australia, 3, biology. We thank M.S. for the helpful discussions. D.M. acknowledges support from the BASAL Center for Astrophysics and Associated Technologies (CATA) 177 through grant AFB 170002, and Proyecto FONDECYT No. 1170121. CC ac- Tanaka, Y., Funano, S., Nishizawa, Y., et al. 2016, Scientific Reports, 6, 25899 knowledges support from DGI-UNAB project DI-11-19/R. Tilgner, C. N. & Heinrichsen, I. 1998, Acta Astronautica, 42, 607 Timofeev, M. Y., Kardashev, N. S., & Promyslov, V. G. 2000, Acta Astronautica, 1 The same idea can also be found in the 1971 essay The New 46, 655 Cosmogony by Polish writer and philosopher Stanislaw Lem. It can Tipler, F. 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