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1.
D. V. Lukic 《Astrophysics and Space Science》2018,363(8):159
In the search for habitable planets, the ultimate aspiration is finding an extraterrestrial technical civilization. We already lost a half of century for an active search for extraterrestrial civilizations. Should we lose another half? If all civilizations in the Universe are only recipients and not message-sending civilizations, then no SETI (Search for Extraterrestrial Intelligence) searches make any sense. Detecting only leaked radio signals is a hard job with present resources. Fear from the extraterrestrials is unfounded, having in mind physical difficulties and requirements of the interstellar travel. If possible extraterrestrial civilizations are more advanced than ours then they can pick up life signs from Earth easier than we can from their planets at present. Here we propose a scientifically based METI (Messaging to Extraterrestrial Intelligence) program. 相似文献
2.
Carl Sagan 《Icarus》1973,19(3):350-352
Even with slow rates of technological advance, extraterrestrial civilizations substantially in our future will have technologies and laws of nature currently inaccessible to us, and will probably have minimal interest in communicating with us. If this communication horizon is ～103 years in our future, other crude estimates previously published imply that only ～10?4 of the technical civilizations in the Galaxy are accessible to us. The mean distance to the nearest such society is then ～104 light years. Radio detection of extraterrestrial intelligence seems to imply either (1) much larger telescopes or antenna arrays for the detection of civilizations within our Galaxy than now exist; or (2) attention to the nearer extragalactic systems, with smaller radio telescopes, to detect the very small fraction of very advanced societies which may choose to make their presence known to emerging civilizations via antique communication modes. 相似文献
3.
B.M. Oliver 《Icarus》1975,25(2):360-367
Approximate expressions are derived for the number of civilizations within a few tens of light years of each other since intelligent life first evolved in the Galaxy. The number is proportional to the square of the usual selectivity factors and to the first power of the longevity. Arguments are presented for expecting intelligent life in certain multiple star systems, and the number of coexistent civilizations in such systems is estimated. 相似文献
4.
The interstellar diffusion of galactic civilizations is reexamined by potential theory; both numerical and analytical solutions are derived for the nonlinear partial differential and difference equations which specify a range of relevant models, drawn from blast wave physics, soil science, and, especially, population biology. An essential feature of these models is that, for all civilizations, population growth must be limited by the carrying capacity of the planetary environments. Dispersal is fundamentally a diffusion process; a directed density-dependent diffusivity describes interstellar emigration. We concentrate on two models, the first describing zero population growth (ZPG) and the second which also includes local growth and saturation of a planetary population, and for which we find an asymptotic travelling wave solution. For both models the colonization wavefront expands slowly and uniformly, but only the frontier worlds are sources of further expansion. For nonlinear diffusion with growth and saturation, the colonization wavefront from the nearest independently arisen galactic civilization can have reached the Earth only if its lifetime exceeds 2.6 × 106 years. If discretization can be neglected, the critical lifetime is 2.0 × 107 years. For ZPG the corresponding number is 1.3 × 1010 years. These numerical results depend on our choices for the specific emigration rate, the distribution of colonizable worlds, and, in the second model, the population growth rate; but the dependence on these parameters is entrancingly weak. We conclude that the Earth is uncolonized not because interstellar spacefaring societies are rare, but because there are too many worlds to be colonized in the plausible lifetime of the colonization phase of nearby galactic civilizations. This phase is, we contend, eventually outgrown. We also conclude that, except possibly early in the history of the Galaxy, there are no very old galactic civilizations with a consistent policy of conquest of inhabited worlds; there is no Galactic Empire. There may, however, be abundant groups of ～105 to 106 worlds linked by a common colonial heritage. The radar and television announcement of an emerging technical society on Earth may induce a rapid response by nearby civilizations, thus newly motivated to reach our system directly rather than by diffusion. 相似文献
5.
For several decades, the search for extraterrestrial intelligence (SETI) has proceeded using advanced astronomical techniques.
Different strategies have been proposed for target selection for targeted searches with goals of improving the chances of
successful detection of signals from technological civilizations that may inhabit planets around solar-type stars, and to
minimize the chances of missing signals from unexpected sites. In this paper we demonstrate that these goals are best achieved
by observing star clusters. We show that standard open clusters are not appropriate for SETI scans because their disruption
time scale is shorter than the characteristic time scale for the development of a protective atmospheric layer on a habitable
planet. However, the old open clusters, those older than some Gy are optimal candidates for SETI surveys as their ages are
older than the likely time for intelligent civilizations to emerge and the probability of catastrophic orbital modification
as a result of close encounters with other cluster stars is, in general, rather negligible. The final performance of the proposed
survey can be significantly increased by using initially a radio telescope beam larger than the cluster apparent size so that
the entire cluster can be observed simultaneously. Globular clusters are also good candidates from the statistical point of
view but only if hypothetical civilizations located in these clusters have been able to develop astronomical engineering technologies
or have been involved in (rather speculative) cosmic colonization.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
G.L. Verschuur 《Icarus》1973,19(3):329-340
A search for narrow band radio signals, in the 21-cm wavelength band, possibly generated by other civilizations, has been made in the direction of ten stars. No such signals were found and upper limits to the power of possible transmissions toward the Earth are given. 相似文献
7.
A.V. Arkhipov 《Astrophysics and Space Science》1997,252(1-2):67-71
It is shown that the Earth is a natural collector of extraterrestrial nonsterile artefacts that could impact our planet. Artefacts from 1.2 × 106 nearby stars could have reached the Earth over its history, and could be agents for spontaneous interstellar panspermia, even if alien civilizations pollute space only at the current terrestrial rate. 相似文献
8.
V. A. Kotov 《Bulletin of the Crimean Astrophysical Observatory》2013,109(1):136-141
More than 840 exoplanets have been discovered and many people believe that on some of these planets there may be extraterrestrial civilizations. Astronomers, however, warn against contacts with aliens because of the possible dangers to humankind… In this paper I show that the solar system is a unique phenomenon in the universe and there cannot be any extraterrestrial civilizations. Being the “anthropic center” of the world, the earth and the sun are “designed” for the development of humankind and the cosmos as a supercomputer. This conclusion follows from an analysis of exoplanet orbits that is based on a coherent cosmic oscillation with a period of P 0 ≈ 9600.6 s (discovered in the sun and some extragalactic sources). The non-Doppler nature of the P 0 phenomenon is emphasized; this phenomenon appears to be related to the absolute time of the universe in the Newtonian sense. 相似文献
9.
Oliver BM 《The Planetary report》1987,7(6):23-25
On Earth intelligent life evolved as a natural consequence of the events set in motion when the planet formed over 4 billion years ago. Since chemical evolution and solar-system formation appear to be occurring throughout the universe, we theorize that our universe may be rich with planets populated by intelligent beings who, like us, can search for evidence of other technological civilizations. Terrestrial civilization now has this capability. But if we do not begin the search soon, we'll lose the opportunity to do it from Earth as interfering signals of Earthly origin rapidly close the microwave window. 相似文献
10.
The Green Bank Equation for the number of communicative civilizations, N, in our Galaxy is reconsidered in view of recent information on the multiplicity of solar-type stars and the possibilities of forming habitable planets around them. Our results are contingent upon the statistical validity of Abt and Levy's work of stellar multiplicity and upon features of Heppen-heimer's theory of planet formation in binary systems. It is concluded that N may have been overestimated by a factor of at least 50. 相似文献
11.
M.H. Hart (1975, Quart. J. Roy. Astron. Soc.16, 128–135) has argued that the absence of evidence for extraterrestrial visits to Earth indicates that there are no other advanced civilizations in our galaxy capable of interstellar colonization. If so, the possible success of any SETI program must be questioned. The authors propose that limited interstellar colonization may occur and an attempt is made to show the effects of such a journey on the Drake equation. 相似文献
12.
It is assumed that the density of sites of technical civilizations emitting suitable signals (whether purposeful or unintentional) is proportional to the stellar density at any location in our Galaxy, as modeled by Bahcall and Soneira (1980, Astrophys. J. Suppl. Ser.44, 73–110). A wide variety of possible radio luminosity functionsφ(L)for these civilizations is then assumed and for each the number of detectable signals per square degree over the sky is calculated. It has been found that most detectable signals occur at galactic latitudes of 10° or less and longitudes within 90° of the galactic center, a region which covers only 9% of the entire sky. This result holds for a wide range ofφ(L) types, including Gaussian distributions and power law functions with slopes less than 2.5, or any combination of these. The Milky Way is much less preffered, but still advantageous, for cases of steep power law functions (slopes greater than 2.5) or Gaussian functions with mean luminosities so low that any existing civilizations can only be detected at small distances(<0.5 kpc). The only cases where low galactic latitudes are not advantageous are (1) for frequencies of operation less than 600 MHz where the deleterious effects on signal-to-noise ratios of the natural galactic background emission become dominant, and (2) in searches for narrowband(<1 Hz) signals at frequencies less than 2 GHz where significant interstellar broadening of signals occurs over distances of? 10 kpc. Furthermore, all of the above results have broader applicability: they are equally valid for searches for any type of natural radio phenomenon if its probability of occurence is proportional to stellar density. Therefore, for a nontargeted search, a Milky Way strategy which concentrates on the inner galactic plane is preferred. The factor of 10 in time saved over an all-sky survey can then be used for, say, increased sensitivity or a survey of nearby galaxies. For a targeted search, rather than searching the nearestn solar-like stars, time is more profitably spent, for example, on the very nearest0.1n stars plus the next nearest0.9n stars which are also within 10° of the galactic plane. This picks up the Milky Way background while only sacrificing a factor of 2 in the average distance to the target stars. 相似文献
13.
D. R. Bates 《Astrophysics and Space Science》1978,55(1):7-13
The problem of, attempting to make radio contact, with other technological civilizations is examined. It is argued that there are few (perhaps indeed no) omnidirectional radio beacons in the Galaxy primarily because any such beacon, to be effective, would have to be designed for a call-response time of at least some 104 years, which is much too long to be likely to be acceptable to governments. A consequence, is that any search program by us would have little chance of detecting incoming signals.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30th May, 1978. 相似文献
14.
J. Lazio 《Astronomische Nachrichten》2008,329(3):330-333
Known classes of radio wavelength transients range from the nearby stellar flares and radio pulsars to the distant Universe γ‐ray burst afterglows. Hypothesized classes of radio transients include analogs of known objects, e.g., extrasolar planets emitting Jovian‐like radio bursts and giant‐pulse emitting pulsars in other galaxies, to the exotic, prompt emission from γ‐ray bursts, evaporating black holes, and transmitters from other civilizations. A number of instruments and facilities are either under construction or in early observational stages and are slated to become available in the next few years. With a combination of wide fields of view and wavelength agility, the detection and study of radio transients will improve immensely. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
Michael R. Rampino 《Icarus》2002,156(2):562-569
The largest explosive volcanic eruptions (supereruptions) produce >1000 km3 of ejected material and ≥1000 Mt (1015 g) of submicron atmospheric aerosols and dust. These eruptions may be capable of creating global climatic disturbances sufficient to cause severe problems for world agriculture and modern civilization. Supereruptions are estimated to occur on average about every 50,000 years, which is about twice the frequency of impacts by comets and asteroids ≥1 km diameter predicted to cause similar climatic effects. Prediction, prevention, and mitigation of global volcanic climatic disasters may be potentially more difficult than planetary protection from the threat of large impacts, so that explosive volcanism might limit the longevity of technological civilizations. 相似文献
16.
Guillermo A. Lemarchand 《Astrophysics and Space Science》1994,214(1-2):209-223
A passive and active strategy for the reception and transmission of call signals, from and to other possible extraterrestrial civilizations is suggested using the explosion of SN1987A. The reception area is an ellipsoid with the Earth and the supernova in its focuses, in the direction of SN1987A. Three hundred and forty objects have been identified inside this surface and four of them are stars with environments similar to our sun (e.g. necessary for the development of life and intelligence). A search strategy to look for fake pulsars signals, originated in other possible technological civilizations, in this direction, is examined.The active strategy consist in the transmission of terrestrial signals towards the hyperboloid surface (with the same focuses) in the antipodal direction of the supernova explosion. Thirty-three objects have been identified inside the hyperboloid and three of them are solar-type nearby stars. The transmission characteristics of a terrestrial origin fake pulsar (e.g. modulation, frequency, bandwidth, periods, duty cycle, etc.) are discussed in some detail. The possible use of available planetary radars to develop such proposal and the criterion of artificiality, without any implicit code or semantics is reviewed. A difference between active search and communication is established.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9–13 November, 1992. 相似文献
17.
18.
We consider the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. If a relatively small fraction of the nuclear resources present in the crust of a terrestrial-type planet were processed via breeder reactors, the resulting stellar spectrum would be selectively modified over geological time periods provided the star has a sufficiently shallow outer convective zone. Consideration of surface convective mixing and stellar lifetimes restricts the possible candidate stars to the approximate spectra range A5-F2. The abundance anomalies resulting from the slow neutron fission of plutonium-239 and uranium-233 are presented and it is argued that these anomalous distributions are unlikely to be duplicated by natural nucleosynthesis processes. Relative to solar system abundances, the elements praseodymium and neodymium are found to be the most overabundant. These elements, along with the radioactive elements technetium and plutonium, could be used to identify A5-F2 candidate stars in a preliminary spectral survey. 相似文献
19.
Horneck G 《Planetary and Space Science》1995,43(1-2):189-217
The primary goal of exobiological research is to reach a better understanding of the processes leading to the origin, evolution and distribution of life on Earth or elsewhere in the universe. In this endeavour, scientists from a wide variety of disciplines are involved, such as astronomy, planetary research, organic chemistry, palaeontology and the various subdisciplines of biology including microbial ecology and molecular biology. Space technology plays an important part by offering the opportunity for exploring our solar system, for collecting extraterrestrial samples, and for utilizing the peculiar environment of space as a tool. Exobiological activities include comparison of the overall pattern of chemical evolution of potential precursors of life, in the interstellar medium, and on the planets and small bodies of our solar system; tracing the history of life on Earth back to its roots; deciphering the environments of the planets in our solar system and of their satellites, throughout their history, with regard to their habitability; searching for other planetary systems in our Galaxy and for signals of extraterrestrial civilizations; testing the impact of space environment on survivability of resistant life forms. This evolutionary approach towards understanding the phenomenon of life in the context of cosmic evolution may eventually contribute to a better understanding of the processes regulating the interactions of life with its environment on Earth. 相似文献
20.
Daniela Comelli Massimo D'orazio Luigi Folco Mahmud El‐Halwagy Tommaso Frizzi Roberto Alberti Valentina Capogrosso Abdelrazek Elnaggar Hala Hassan Austin Nevin Franco Porcelli Mohamed G. Rashed Gianluca Valentini 《Meteoritics & planetary science》2016,51(7):1301-1309
Scholars have long discussed the introduction and spread of iron metallurgy in different civilizations. The sporadic use of iron has been reported in the Eastern Mediterranean area from the late Neolithic period to the Bronze Age. Despite the rare existence of smelted iron, it is generally assumed that early iron objects were produced from meteoritic iron. Nevertheless, the methods of working the metal, its use, and diffusion are contentious issues compromised by lack of detailed analysis. Since its discovery in 1925, the meteoritic origin of the iron dagger blade from the sarcophagus of the ancient Egyptian King Tutankhamun (14th C. BCE) has been the subject of debate and previous analyses yielded controversial results. We show that the composition of the blade (Fe plus 10.8 wt% Ni and 0.58 wt% Co), accurately determined through portable x‐ray fluorescence spectrometry, strongly supports its meteoritic origin. In agreement with recent results of metallographic analysis of ancient iron artifacts from Gerzeh, our study confirms that ancient Egyptians attributed great value to meteoritic iron for the production of precious objects. Moreover, the high manufacturing quality of Tutankhamun's dagger blade, in comparison with other simple‐shaped meteoritic iron artifacts, suggests a significant mastery of ironworking in Tutankhamun's time. 相似文献