The search for extraterrestrial life has fas-cinated humankind for centuries. But itwas only in the 1960s that the search

was put on a scientific footing by a youngastronomer Frank Drake, now president of theSETI Institute, the largest organization devotedto the search and promotion of life on otherplanets. Speculation and philosophy gave wayto science. In an unprecedented and bold movein 1960, the young Drake turned the GreenBank radio telescope onto two nearby stars –Tau Ceti and Epsilon Eridani – to search forsignals from extraterrestrial intelligence (ETI)(Drake 1961). Called Project Ozma, it searchedfor 150 hours but found no evidence of strongsignals from these stars. By performing thatexperiment Drake initiated new fields of studyin astronomy and the social sciences which arenow slowly finding their way into mainstreamacademic studies and university courses(Bhathal 1999).

Seminal papersAlmost 40 years ago Nature published twoseminal papers (Cocconi and Morrison 1959,Schwartz and Townes 1960) regarding the pos-sibility of searching for signals from extra-terrestrial intelligence. The papers advocateddifferent search strategies, radio and optical.Since large radio telescopes were coming online in the 1960s, Cocconi and Morrisonadvanced the view that the search for ETIcould be carried out using radio telescopes inthe region around the 21 cm (1420 MHz) lineor what has come to be termed the “waterhole” region. Their advocacy that “if we neversearch, the chances of success is zero” spawned

a number of microwave searches around theworld. Further advocacy of the water holeregion by Bernard Oliver in the Cyclops pro-ject (Sagan and Drake 1975) set the scene forsearch strategies confined mainly to thatregion. But after almost 40 years of fruitlesssearch, there is still no answer to Fermi’s ques-tion: where are they?

This has prompted new thinking amongstSETI researchers who believe that the searchstrategy needs to be expanded to include otherparts of the electromagnetic spectrum wherethere are clear windows for the passage ofelectromagnetic waves, such as the windows at12 000–10 000 nm, 1100–1000 nm and 900–400 nm. In fact, the optical strategy is not new.In the second and now almost forgottenNature paper, Schwartz and Townes arguedthat a search be made for interstellar laser sig-nals. They suggested two laser systems whichat that time had yet to be built. System A useda single continuous wave (CW) laser while Sys-tem B used a group of 25 lasers in a star con-figuration with the same laser characteristics asSystem A. System A used a laser with a powerlevel of 10 kW at a wavelength of 500 nm. Thediameter of the reflector at the source wastaken as 5 m (the maximum size of telescopeson Earth at that time).

A back-of-the-envelope calculation showsthat the apparent magnitudes of the ETI starand ETI CW laser at a distance of ten lightyears from us would be about 2 and 21 (smallastronomical magnitudes are brighter). Thelatter source could just be picked up by the200 inch telescope at Mt Palomar Observato-ry. However, to ensure that the ETI laser signal

could be picked up we need to increase thepower of the laser by several orders of magni-tude than that proposed by Schwartz andTownes. Their proposals were not taken up bythe scientific community.

Renewed interestSince Schwartz and Townes’ paper, three devel-opments have taken place. First, there has beena tremendous increase in radio frequency noiseand in the coming years it is going to becomeexceedingly difficult to carry out the search inthe microwave spectrum. Secondly, the discov-ery of a number of important microwave linesat various frequencies in interstellar cloudsmakes a choice of “magic frequencies” lessobvious. Thirdly, the power of lasers has beenrising exponentially from the milliwatt lasersof first-year university laboratories of the1960s to lasers in the megawatt power range inthe 1980s and 1990s. The development ofhigh-power lasers has been motivated by therequirements of the military and interest inimploding pellets of hydrogen to producenuclear fusion. Pulsed solid-state lasers haveachieved terawatts, albeit for about a nanosec-ond. It is expected that the National IgnitionFacility at the Lawrence Livermore NationalLaboratory will deliver 500 TW pulses lasting3 to 5 ns when it begins operating in the nextcouple of years. These developments in high-power lasers give credence to the search forETI signals in the optical spectrum. Incidental-ly, it will give Graham Bell’s terrestrial photo-phone a new lease of life in 21st century inter-stellar communication with laser light.

The high directionality and tight beam makes

SETI

1.25February 2000 Vol 41

The case foroptical SETI

After almost 40 years offruitless search forextraterrestrial life in theradio region, RaghirBhathal argues that it istime to expand the searchto other parts of theelectromagnetic spectrum.

The OZ OSETI Observatory in Sydney, Australia.

lasers ideal devices for attention-getting bea-cons and for communication purposes.According to Ross (1965), information theoryshows that at optical frequencies, narrowpulse, low duty-cycle systems can convey moreinformation per received signal photon thanradio waves.

In the 1980s, Townes (1983) made anotherappeal for searching for laser signals. It is nor-mally assumed that searches for ETI shouldconcentrate on attempts to receive signals inthe microwave region. The reason for this isthat communication there uses minimumbroadcast power. Townes questioned thisassumption. He said: “Such a conclusion isshown to result only under a restricted set ofassumptions. If generalized types of detectionare considered – in particular, photon detectionrather than linear detection alone – and ifadvantage is taken of the directivity of tele-scopes at short wavelengths, then somewhatless power is required for communication atinfrared wavelengths than in the microwaveregion.” Furthermore, for distances less than1000 light years, extinction and smearing ofpulsed laser signals due to interstellar grainsare not a serious problem (Cordes 1999).

Earliest proposalsDespite these developments and arguments foran optical SETI search there have been very fewsearches in the optical spectrum. Some of theearlier and more recent ones are reported below.

Some of the earliest proposals of using lightbeams were proposed by Karl Gauss andCharles Cross in the 19th century (Crowe1986). In 1822 Gauss had proposed using “100separate mirrors each of 16 square feet” to senda “good heliotrope-light to the Moon”. He feltthat if we could get in touch with our neigh-bours on the Moon, it would be a “discoverygreater that that of America”. Forty-seven yearslater Cross suggested that light could befocused by parabolic mirrors so as to be visibleto inhabitants on Mars. Periodic flashes couldbe used for sending messages to them.

In the 1970s, Shvartsman (1977) conductedexperiments to search for optical variability inpeculiar objects with a 6 m telescope. Calledthe MANIA (Multichannel Analysis ofNanosecond Intensity Alterations) experimentthe main aim was to study superfast variabilityof different objects using an extremely highresolution of about 10–7 s. The observationsalso included the search for extraterrestrial sig-nals. They observed about 60 peculiar andSun-like stars but did not find any ETI signals(Beskin 1997). Betz’s (1986) analysis showedthat for specified transmitter and receiver loca-tions, communications at infrared and radiofrequencies could be equally effective. On thisbasis he searched for extraterrestrial signals atinfrared wavelengths. He searched for CO2

laser signals from 200 nearby solar-type stars.Earlier, Demming and Mumma (1983) hadsuggested that CO2 in the atmosphere of Marsexhibited laser behaviour which could be chan-nelled into a powerful SETI beacon.

One of the most ardent advocates of the opti-cal SETI search has been Stuart Kingsley whoruns the Columbus Optical SETI Observatoryin the USA. More than anyone else he has beenplugging away to get more scientists both pro-fessional and amateur to be involved in theOSETI search. Conservative calculations car-ried out by Kingsley (1996) at a wavelength of550 nm show that light from a pulsed ETI laserwould outshine the ETI star by seven orders ofmagnitude. For a pulsed ETI laser beacon sys-tem with a 1 Hz repetition rate and a peakpower of 1018 W he predicted that the magni-tudes of the ETI laser and the ETI star at a dis-tance of 10 light years would be ~15 and 2.Sources of these magnitudes can be easilypicked up by small telescopes. However, itwould be advisable to use at least a 0.4 m orlarger telescope to ensure that one is able todetect the laser signal.

Kingsley’s advocacy for OSETI searches haspaid off. Over the past couple of years twomore groups in the USA (led by Dan Werthimerat the University of California and PaulHorowitz at Harvard University) and one inAustralia (the OZ OSETI Project) have takenup his challenge (Beatty 1999). All three groupswill be searching for nanosecond pulses withspecially built very fast detectors. The Universi-ty of California group will be searching in the300–700 nm range and will be looking for sig-nals from F, G, K and M stars including a fewglobular clusters and galaxies. The OZ OSETIproject will be targeting southern Sun-like cir-cumpolar stars, southern globular clusters anda few galaxies. It will be using two telescopesplaced 20 m apart to ensure that extraneoussignals are minimized when the same target staris observed. The search will be conducted in the300–700 nm range but centred on 550 nm. TheHarvard group is searching in the 160–850 nmrange with the search centred on 420 nm. Theyare using hybrid avalanche photodiodes andhave plans to carry out the search with twotelescopes at different locations. The Harvardgroup is well ahead of the other two groups,having already observed more than 1000 near-by stars but with no success in detecting anyETI laser signals. They are also planning tocarry out an all-sky optical SETI search.

Another search at the University of Californiaby Geoffrey Marcy and Paul Butler will search1000 stars for continuous laser signals whichwill manifest themselves in very sharp lines intheir high-resolution spectra of target stars. Inessence they will be mining the data from theirongoing planet search for any tell-tale signa-tures. In Arizona, Monte Ross is developing

plans to build two large collectors made up of18 (12.8 cm diameter) spherical mirror seg-ments on each collector. These will serve asphoton buckets. According to Ross, photonbuckets can be used in this experiment because:“The signal energy in a short pulse can over-come the generated noise by the star’s spec-trum. Exact optical frequency knowledge is notrequired because of the high background dis-crimination of the short-pulse approach.” Hehas proposed that we search for strong Fraun-hofer lines, especially the H α line (656.3 nm).

Rather than searching for pulsed laser signalsthere is a suggestion that we should look fortechnological activities that generate infraredradiation. This idea was suggested by Dyson(1960) who echoed Tsiolkovskii’s predictionsmade in the 19th century. He proposed thatintelligent civilizations could build thin shellsaround stars to trap radiation and extract theirenergy for use. The heat generated would radi-ate away in the infrared. Jugaku and Nishimu-ra (1997) searched for Dyson spheres associat-ed with 50 solar-type stars of spectral class F, Gand K within 25 pc of the Sun. However, theyfound no evidence of Dyson spheres.

ConclusionAfter several years of neglect it would appearthat optical SETI (both visible and infrared)may finally find a growing niche in ETI search-es. Over the next few years we will see theexploitation of a greater part of the electro-magnetic spectrum for ETI searches. Even if wedo not find ETI we may find very fast astro-nomical phenomena that have yet to be dis-covered. Who knows what the future holds. ●

R Bhathal is Project Director of the OZ OSETIProject at the University of Western Sydney inAustralia. He thanks Dr S Kingsley, Dr DWerthimer, A Howard, Prof. P Horowitz and MRoss for providing information on their projects.

ReferencesBeatty J K 1999 Sky and Telescope 97(6) 19.Beskin G et al. 1997 in Astronomical and Biochemical Origins andthe Search for Life in the Universe ed. C B Cosmovici, S Bowyer andD Werthimer, Editrice Compositori.Betz A 1986 Acta Astronautica 13(10) 623–629.Bhathal R 1999 Physics Education 34(2) 88–91.Cocconi G and Morrison P 1959 Nature 184 844–846.Cordes J 1999 Mercury March–April 14–19.Crowe M J 1986 The extraterrestrial life debate 1750–1900Cambridge University Press.Demming D and Mumma M J 1983 Icarus 55 356–368.Drake F 1961 Physics Today 14 40–46.Dyson F J 1960 Science 131 1667–1668.Jugaku J and Nishimura S 1997 in Astronomical and BiochemicalOrigins and the Search for Life in the Universe ed. C B Cosmovici, SBowyer and D Werthimer, Editrice Compositori.Kingsley S 1996 SPIE 2704 102–116.Ross M 1965 Proc IEEE 53(11) 1780.Sagan C and Drake F 1975 Scientific American 232(5) 80–89.Schwartz R N and Townes C H 1960 Nature 190 205–208.Shvartsman V F 1977 Soobsh SAO 19 5–36.Townes C H 1983 Proc Natl Acad Sci USA 80 1147–1151.

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