Message in a bottle
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This document discusses two alternative strategies for communicating with extraterrestrial intelligence - sending electromagnetic signals or physical objects through space. It summarizes a recent paper arguing that sending physical packages of information across interstellar distances could be more efficient than beams of electromagnetic radiation for long messages. While radio telescopes have searched for electromagnetic signals, this article suggests SETI should also consider the possibility of finding physical artifacts containing information left by an advanced extraterrestrial civilization to enable one-way communication.
Message in a bottle
- 1. A re we alone? Although the Search for Extraterrestrial Intelligence (SETI) has yet to detect a signal, the efforts continue because so little of the possible parameter space has been searched so far. These projects have almost all followed the dominant paradigm — launched 45 years ago by Cocconi and Morrison in the pages of this journal1 — of using radio telescopes to look for signs of extraterrestrial life. This focus on electromagnetic waves (primarily at radio wavelengths, but also at optical ones) was based on various arguments for their efficiency as a means of interstellar communication. On page 47 of this issue, however, Rose and Wright2 make the case that, if speedy delivery is not required, long messages are in fact more efficiently sent in the form of material objects — effectively messages in a bottle. Although the sugges- tion itself is not new3,4 , it had never before been backed up by quantitative analysis. A fundamental problem in searching for extraterrestrial intelligence is to guess the communications set-up of the extraterrestri- alswhomightbetryingtocontactus.Inwhich direction should we look for their transmit- ter? At which frequencies? How might the message be coded? How often is it broadcast? (For this discussion I am assuming that the signals are intentional, setting aside the a priori equally likely possibility that the first signal found could be merely leakage arising from their normal activities.) Conventional wisdom holds that they would set up a beam of electromagnetic waves,just as we could do with, for example, the 305-metre Arecibo radio telescope in Puerto Rico, Earth’s most powerful radio transmitter, or a pulsed laser on the 10-metre Keck optical telescope in Hawaii. Rose and Wright conclude, however, that the better choice would be to send pack- ageslacedwithinformation. Unless the messages are short or the extraterrestrials are nearby, this ‘write’ strat- egyrequireslessenergyperbitof transmitted information than the ‘radiate’ strategy does. Cone-shaped beams of radiation necessarily grow in size as they travel outwards,meaning thatthegreatmajorityoftheenergyiswasted, even if some of it hits the intended target. A package,on the other hand,is not‘diluted’as ittravelsacrossspace(Fig.1),presumingthat it’s correctly aimed at its desired destination. For short messages, however, electromag- neticwaveswinoutbecauseof theoverheads involved in launching, shielding and then decelerating a package, no matter how small it is — FedEx charges you just as muchtosendamicrogramas100grams.For a two-way conversation with extraterres- trials, the light-speed of electromagnetic wavesisfarsuperior. As an example of a large message, con- sider all of the written and electronic infor- mationnowexistingonEarth:it’sestimated5 to amount to about one exabyte (that’s 1018 bytes,or1019 bits).RoseandWrightcalculate that, using scanning tunnelling microscopy, these bits could be inscribed (in nanometre squares) within one gram of material! But this precious package would still require a cocoon of 10,000 kilograms to accelerate it from our planet to a speed of 0.1% of the speed of light, protect it from radiation damage along a 10,000-light-year route,and thendecelerateituponarrival. Rose and Wright’s arguments are open to various criticisms. We do not understand howeconomicsworksonthisplanet,letalone for an extraterrestrial intelligence, so it is not clear that the key criterion in choosing a mes- sage’smediumwouldbeenergyexpendedper news and views NATURE|VOL431|2SEPTEMBER2004|www.nature.com/nature 27 bit,astheseauthorsassume.Furthermore,we donotknowifsuchpackages,evenifefficient- ly sent, would ever in fact be recognized and opened. Of course, we also do not know whether electromagnetic signals intended for us — and which may in fact be now bathing the Earth — will ever be recognized assuch.Inbothcases,repetitionanddiversity of communications media would seem to increasetheextraterrestrials’andourchances of success. So how should these results influence today’s SETI strategy? Short “we are here” messages would still seem to be most effici- ently sent by electromagnetic waves, and we should continue looking for the same. But perhaps some attention should be paid to the possibilityof onedayfindinginourSolarSys- tem an information-drenched artefact, sent by an extremely advanced extraterrestrial civilization interested only in one-way com- munication. This intruder might be orbiting theSunoraplanet,orrestingsomewhereona planet,moonorasteroid.Thescenarioisrem- iniscent of Arthur C. Clarke’s 2001: A Space Odyssey, in which a monolith discovered on the Moon has been left by extraterrestrials. If Message in a bottle Woodruff T. Sullivan III Extraterrestrial civilizations may find it more efficient to communicate by sending material objects across interstellar distances rather than beams of electromagnetic radiation. Figure 1 Is there anybody out there? Launched in 1977 and now at the edge of the Solar System, the Voyager spacecraft (one shown here in prototype) carry phonographic records of sounds and images from Earth, including the mathematical and biological information shown on the right. Rose and Wright2 suggest that sending such packages of information out into space is the most effective way for an extraterrestrial intelligence to send long messages to us — and that we should thus be alert to the possibility of finding similar objects, sent from another civilization, in the vicinity of Earth. JPL/NASA;J.LOMBERG/SPL 2.9 n&v NEW MH 27/8/04 5:28 pm Page 27 ©2004 NaturePublishing Group
- 2. once shared,and continue to share,regulato- ry elements. But although this idea might account for the preservation of some degree of organization, it seems inadequate to explain the extent to which the complex has been maintained. Another possibility is that the mechanism that allows the genes to be expressed in a strict anterior–posterior expression pattern requires some type of higher-level organization, involving the pro- gressive chemical or structural modification of alargecontiguousstretchof DNA. Theworkof Dubouleandcolleaguesover the past few years has added an extra dimen- sion to the issue of collinearity. They have shown that the vertebrate Hox genes show not just spatial but also temporal collineari- ty3 ;thatis,genesatoneendofthecomplexare expressed not only in the anterior of the embryo, but also relatively early in develop- ment. Hox genes located further along the complex are expressed both more posterior- ly and later. Duboule and colleagues4 have providedevidencethatitmaybetherequire- ment to maintain temporal collinearity that is responsible for keeping the complex together. A Hox gene experimentally moved aroundwithinthecomplexcanretainspatial information,but will have an altered tempo- ralexpressionprofile. Continuing this theme, Seo et al.1 provide a fascinating example of an animal in which the Hox complex has not stayed together yet appears to maintain some degree of ordered spatialexpressionalongtheanterior–posteri- or axis. Their studies focus on Oikopleura dioica(Fig.1).Oikopleuraisatypeof tunicate, but is quite distant from Ciona, the other well-studied representative of this group of animals. Tunicates are evolutionarily primi- tive relatives of vertebrates, and comparisons between living tunicates and vertebrates may help researchers to piece together the features of the common invertebrate ancestor that gave rise to vertebrates. Oikopleura also has a remarkable genome — it is very small (at 60–70 megabases) and compact (with one geneevery4kilobases)5 . Seo et al. find that Oikopleura has a com- plement of nine Hox genes. As expected, Oikopleura counterparts of the vertebrate anterior Hox genes are expressed in anterior regions of the developing animal,and coun- terparts of progressively more posterior vertebrate Hox genes are expressed in corre- spondingly more posterior regions. What is fascinating, however, is that the Oiko- pleura Hox genes retain this pattern of expressioneventhoughtheyarenolongerin any sort of complex. Seo et al. show that for at least eight of these genes, no other Hox gene is found within 250 kilobases on either side. It is not that these Hox genes are in a gene-poor region, however; each is sur- rounded by other genes at the usual high densityfoundinthisanimal. These results, then, would seem to indi- cate that spatial collinearity can be main- tained without requiring the organization of the Hox genes into a complex. Oikopleura, however, appears not to maintain temporal collinearity. The expression of its Hox genes does not seem to begin in a progressive temporal order, but rather at roughly the same time. Extensive splits within the Hox complex are also seen in the roundworm Caenorhabditis elegans and in Ciona, two othercasesinwhichtemporalcollinearityno longer applies. Even in fruitflies, the Hox complexissplitintotwo,andmanynon-Hox news and views 28 NATURE|VOL431|2SEPTEMBER2004|www.nature.com/nature astroarchaeologists were to find such an object, it would hardly be the first time that sciencefictionhadbecomesciencefact. ■ Woodruff T. Sullivan III is in the Department of Astronomy, University of Washington, Seattle, Washington 98195, USA. e-mail: woody@astro.washington.edu 1. Cocconi, G. & Morrison, P. Nature 184, 844–846 (1959). 2. Rose, C. & Wright, G. Nature 431, 47–49 (2004). 3. Bracewell, R. Nature 187, 670–671 (1960). 4. Papagiannis, M. Q. J. R. Astron. Soc. 19, 277–281 (1978). 5. Murphy, C. Atlantic 277, No. 5, 20–22 (1996). Evolutionary biology Time, space and genomes Nipam H. Patel In most animals, the Hox genes — which control development — are clustered together. But why? New evidence supports the idea that the requirement for a temporal order of expression keeps the cluster intact. S ome of the most striking discoveries in developmental biology over the past century concern the set of genes called homeotic (Hox) genes. Genetic studies in fruitflies first showed that these genes have a major role in producing the head-to-tail (anterior–posterior) pattern of tissues along the body axis. Then came the startling finding that the order of these genes along a chromosome correlates with the anterior– posterior position of the body regions they control, and with the domains in which the genes are expressed. It soon became apparent that the same relationship exists in other ani- mal groups, including vertebrates. Intrigu- ingly, however, it seems that somewhere in the evolutionary lineage leading to the tuni- cate Oikopleura dioica, the Hox complex has disintegrated, as Seo and colleagues report on page 67 of this issue1 . Evolutionary analyses have suggested thatthecommonancestorofbilaterallysym- metrical creatures — which include most animals, the main exceptions being cnidari- ans and sponges — probably possessed at leastsevenHoxgenes,organizedintoasingle complex.Withinthelineageleadingtoverte- brates,geneduplicationsledtoanexpansion of the cluster, and then the cluster itself underwent duplications, leading to the four copies of the Hox complex now found in humans and mice.All along, the collinearity of the genes — the correspondence between theirphysicalorderalongchromosomesand their domains of expression and function — wasmaintained. But why has collinearity been preserved? The ancestral bilaterian complex itself prob- ably arose through several rounds of local duplications, explaining how the genes first became organized as a cluster. In general, however, gene order is constantly shuffled by chromosomal rearrangements such as inversions and movements of large DNA segments.Giventherateatwhichthisprocess occurs, the maintenance of collinear organi- zation over at least 600 million years of evolution must not just be due to chance2 . One possibility is that different Hox genes Figure 1 Distant relative. Oikopleura dioica has a simple body plan reminiscent of that of a tadpole, hinting at its close affinity to vertebrates. Remarkably, the generation time for this organism is only four days. R.RUDOLF 2.9 n&v NEW MH 27/8/04 5:28 pm Page 28 ©2004 NaturePublishing Group