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Introduction

9006-001

 


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This study focuses on certain aspects of the Search for Extra-Terrestrial Intelligence, otherwise known as SETI.  It suggests that the scientific community may be looking for extraterrestrial communications beamed in the direction of our solar system at the wrong frequencies.  Moreover, that the rationale for preferring radio frequencies may be incorrect.

The present day ETI (Extra-Terrestrial Intelligence) programs were initiated by Giuseppe Cocconi & Philip Morrison's classic 1959 paper in Nature on Searching for Interstellar Communications.  Since then, the research activity has mainly centered on listening for signals in the microwave spectrum, particularly at or near the quiet "water hole", i.e., that part of the microwave spectrum between the hydrogen line at 1.420 GHz and the hydroxyl line at 1.662 GHz.

In 1958, Charles Townes co-invented the optical maser, later to be renamed the laser, for which he shared a Nobel Prize in 1964 with Schawlow.  In 1960, a year after Morrison's paper, Maiman demonstrated the first laser.  The following year, Townes suggested in a 1961 paper in Nature on Interstellar and Interplanetary Communication by Optical Masers, that lasers might be used to communicate between star systems.   Since that time, very little has been said about Optical SETI - the bulk of the research activity (and literature) has taken place in the low microwave region of the electromagnetic spectrum.


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The study reported here revisits Optical SETI, tries to discover why this approach and technology has been largely ignored by scientists and astronomers the past 30 years, compares the system sensitivities of the microwave and optical approaches, suggests that we should be looking for high power wideband signals and not the odd "bits/second", and shows that we may well be missing the signals we have been striving so hard to find.

No claim is made here that the idea of Optical SETI is in anyway original.  It was first suggested by Schwartz and Townes nearly 30 years ago.  In their 1961 paper on Interstellar and Interplanetary Communication by Optical Masers, they remarked that another civilization might have an inverted technical history, such that lasers might be developed and perfected before radio-wave systems.

Since this analysis indicates that the optical approach to SETI is sensible, it is very perplexing that it has been dismissed and largely ignored by the scientific community, though more recently Betz has written about a CO2 infrared laser approach.   It is shown that both visible and infrared wavelengths are appropriate frequencies for interstellar communications and that on a transmitter power comparison basis, a visible laser approach can produce improved signal-to-noise ratio.


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The literature on SETI is generally grossly misleading in its rare references to the optical approach.  For a advanced technical society, a laser transmitting telescope is only "slightly" more difficult to construct than a microwave transmitting dish, though Isaac Asimov appears to think otherwise.

Towards the end of his 1979 book Extra-Terrestrial Intelligence - The First Encounter (page 263) he says "With laser light we come closer to a practical signaling device than anything yet mentioned, but even a laser signal originating from some planet would, at great distances, be drowned out by the general light of the star the planet circles".  He goes on to say "One possibility that has been suggested is this--.  The spectra of sun type stars have numerous dark lines representing missing photons -- photons that have been preferentially absorbed by specific atoms in the stars' atmospheres.  Suppose a planetary civilization sends out a strong laser beam at the precise energy level of one of the prominent dark lines of the star's spectrum.  That would brighten that dark line . . .".

Asimov also went on to imply that a laser system was complicated, and that no civilization would be expected to use the harder method if a simpler (microwave) method is available.


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The arguments in support of the "search" in the microwave region of the electromagnetic spectrum, and the microwave water hole in particular, are as follows:

1. Microwave receivers are far more sensitive than optical receivers.  Quantum noise dominates thermal receiver noise at optical frequencies and produces a large sensitivity penalty.  Also, radio-frequency receivers allows for eavesdropping.
2. Because of resolution limits of both radio and optical telescopes and the manner in which such telescopes are used in a "search", it is not presently possible to separate (spatially) the transmitted signal and the Planck radiation from nearby stars.  Planck radiation is much greater at optical frequencies than microwave frequencies, and this will make it impossible to "see" the signal in the presence of the Planck background radiation.

We will show that the above arguments do not necessarily imply that microwaves are much better than light for communicating over vast distances, particularly for directed (beamed) communications at moderate and high data rates.


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There are some other arguments that are often left unsaid which may explain the resistance of the SETI community to consider the optical spectrum in its "search".

1. It is very difficult to "borrow" time on a large optical telescope for the "search" - getting access to a radio telescope is easier.
2. An optical approach is likely to be considerably more expensive than a radio frequency approach, and since funding for SETI has been somewhat of a political hot potato, effort has concentrated on the cheaper technology.
3. The frequencies to search in the optical regime are so huge, covering the far-infrared to the blue, i.e., nearly five orders of magnitude greater than in the microwave regime, that the task appears to be daunting.  The perception concerning the tremendous search effort required can be very off-putting psychologically.   If the Microwave Cosmic Haystack (MCH) can be considered to be large, the Optical Cosmic Haystack (OCH) is truly enormous.  For reasons which will later become apparent, the OCH is in reality no more than one hundred times the width of the MCH.

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The Optical SETI approach to be described uses coherent optical heterodyne detection to separate out the signal from the Planck background radiation.  Through the "light science" capability produced by an optical heterodyne front-end in the focal plane of a telescope, it may be possible to achieve something that has so far eluded astronomers, and is fundamental to all SETI approaches.  This is the positive detection of planets orbiting other stars, and the existence of planetary atmospheres.   While few scientists doubt that our solar system is in any way unique, except perhaps for intelligent life on the third planet, it would be reassuring to SETI researchers to know that most stars have planetary systems.  If we could also find out about their atmospheres, we would have a better estimate of the number of technical civilizations that may exist in the Milky Way galaxy, and throughout the universe.

The author considers that the findings on Optical SETI reported here may be considered as "A Guide to the Perplexed".  No issue has caused the author so much trouble during his photonics career than the seemingly inexplicable indifference by the ETI community to this approach.  What did they know that he didn't?  His conclusion, arrived at over a considerable period of time, is that the scientific community has suffered badly from myopic vision.

We will show:

1. That optical heterodyne front-ends allow for very narrow-band optical detection so that the Planck starlight in the background does not reduce the signal-to-noise ratio.
2. That for optical heterodyne receivers, the quantum shot noise spectral density due to the signal is likely to be much greater than the noise spectral density due to the Planck background continuum at a range of ten or more light years.  Bandwidths may be increased by many orders of magnitude if we assume spatial separation of the Alien transmitter and its star's light for nearby star systems.   This may be done with space-based or ground-based adaptive telescopes.
3. That it is not necessary to choose a wavelength that coincides with a Fraunhofer absorption line, though it does have certain benefits.

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4. That slightly less than ideal visible and infrared laser systems, can deliver higher signal-to-noise ratios or data rates than is possible at microwave frequencies for the same transmitted power.  Communications over distances greater than 3000 light years should be possible at visible wavelengths, and across the entire Galaxy at CO2 wavelengths.  The Optical SETI rational which presupposes signals of much greater bandwidth than proposed for the microwave regime, has the ability to target many planetary systems and transmit large amounts of data in short periods of time.  It is demonstrated that a symmetrical 10 meter diameter visible system with a 1 GW laser and heterodyning receiver, would have the capability of transmitting broadcast-quality video signals over distances in excess of 10 light years.
5. Two dimensional PIN photodetector heterodyning focal plane arrays are proposed for the simultaneous or rapid sequential sampling of a substantial fraction of the field-of-view. The area of each photodetector corresponds to the diffraction limited pixel size.  Planckian radiation from nearby stars and the possible signals from their alien planetary transmitters are detected by different photodiodes.

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6. That Optical SETI is the one branch of visible astronomy that with adaptive telescope techniques may be conducted during daylight hours under a clear blue sky, without loss in detection sensitivity.  This surprising capability comes about because optical detection bandwidths of 10 MHz or less ensure that skylight at each photodetector (pixel) is less than the quantum noise floor.  Thus, a symbiotic relationship between Optical SETI and conventional astronomy may arise which will allow the former to be conducted at many of the great ground-based telescopes.  Round the clock operation of such telescopes may then be possible.

 

In 1971, NASA made an extensive comparison of the merits of Optical SETI in its Project Cyclops study.  Unfortunately, that part of the analysis was flawed by an unnecessary conservative bias towards what was possible with optical telescopes.  In particular, it was assumed that the SETI transmitters and receivers had to operate within an atmosphere, and thus optical telescope sizes were severely compromised to fall within the limit set by the coherence cell size.  This is 10 cm to 20 cm at visible wavelengths, and 2 meters at 10.6 m.  No consideration was given to space telescope or adaptive telescope technology, which were then only ideas.


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The Cyclops study probably shares a great responsibility for the "bad press" so far given to Optical SETI.  This author holds that it is unreasonable to compromise optical telescope designs in this manner.  Some do not think is possible for an extraterrestrial civilization to be able to aim a laser with a beamwidth less than 1 arcsecond at a nearby target star system, and ensure that it "illuminates" the targeted planet or planets.  Confining beamwidths to greater than 1 arcsecond produces a SNR penalty between 30 and 40 dB.  Even if one doesn't think that advanced technical civilizations would be able to "hit the bull's eye", it is better to have a super-narrow diffraction limited beam, and de-focus (de-collimate) it when targeting nearby star systems, if the beam is too narrow for predictive (point ahead) targeting.  In this way, the long-distance performance, where the beam will have spread out to encompass entire star systems, will not be compromised.

It is shown that the performance of a symmetrical optical heterodyne communication system is a very sensitive function of telescope aperture, and that generally it is proportional to the fourth to sixth power of the diameter.  For small telescope systems it is proportional to the eighth power.  It is completely unrealistic to assume that aliens will be limited to using "toy" telescopes.

 

The Columbus Optical SETI Observatory
Copyright (c), 1990

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