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EJASA - Part 9
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DISCUSSION
The thirty-year-old rationale which would have us believe that the
low frequency end of the microwave regime is the place to search for
ETI signals is seriously suspect. If the underlying assumption of
present-day SETI lore that the best ETIs could do would be to send us
very weak low bandwidth signals is swept away, then almost all the
so-called problems that are usually advanced to dismiss the optical
approach become insignificant. This is even more so if the use of
optical heterodyne reception is assumed. The increased immunity of
such systems to background noise means that the signal detectability
constraints set by Planckian starlight are essentially removed. In
addition, with dechirping of the local-oscillator to remove local
Doppler drift along the line-of-sight, problems from local Doppler
drift are eliminated.
Because of the very narrow field-of-view of a photodetector array,
Doppler drift compensation can be made simultaneously to all pixels in
the array to a very high degree. The larger bandwidths mean that the
effects of finite laser linewidths, Doppler shift and residual drift
are minimized, and the number of frequencies to search in the entire
optical spectrum is in reality no more than in the microwave spectrum.
Up to now, the SETI community has taken some comfort in the fact
that the obvious explanation as to why we have not detected ETI signals
is simply that they are too weak and that we need sophisticated
hardware and signal processing algorithms to extract this information.
An even simpler explanation for the lack of success so far is that
there are strong signals but they are elsewhere in the electromagnetic
spectrum. Of course, Tipler [39] has an even more simpler explanation.
It is the author's prediction that in years to come, it will be
hard to understand how anyone in the late Twentieth Century, e.g.,
people like Frank Tipler, could think it possible that humanity was all
alone - that Earth is atypical in that we are the "first civilization".
If anything, it is far more likely that the answer to "Where are they?"
is that we live in a "Cosmic Zoo". Tipler believes that ETI tech-
nologies only slightly superior to our own, if they exist, would have
produced self-replicating von Neumann machines (probes) that would have
rapidly populated the galaxy. Therefore, since we have not detected
these machines or they have not contacted us, ETIs do not exist.
The "Cosmic Zoo" rationale is probably the only viable alternative
explanation as to why ETIs do not appear to have colonized the entire
galaxy. We could just as easily be a typical or atypical civilization,
developing in a sector of the galaxy that is off limits for physical
contact, i.e, the Prime Directive so much loved by STAR TREK fans. As
was stated in the Preface, if the author has any doubts about the
efficacy of the Optical SETI, it surely has to do with the Kingsley
Paradox of "why communicate when one can just as easily travel?".
Nevertheless, the author is sufficiently convinced about the
plausibility of this Optical SETI rationale to believe it worthwhile to
construct his own Optical SETI Observatory and mount his own search.
He intends to start this project as soon as possible.
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The author has taken some pains to try and understand why he
believes Tipler is wrong. The author finds it very difficult to accept
that only once in the ten billion year history of our galaxy has
intelligent life arisen. Secondly, since the dawn of the Space Age,
i.e., since about 1957, he has thought that life was common throughout
the galaxy, and as a STAR TREK and science fiction fan, he has believed
that the future for mankind was in space. Thus, the idea that there
would be no one to meet out there is an anathema.
It is the author's intuitive feeling, that soon we will learn that
life appears relatively rapidly, given the right environmental
conditions. Life, rather than being the exception to the rule, is
the inevitable consequence of the mixture of certain elements,
temperatures, cosmic catastrophes, and time. In the roughly fifteen
billion year existence of the Universe, there will have been no
shortage of the latter. At the moment, we still have a very sketchy
picture of how life arose on this planet - that possibly, lines of
evolution were erased and new lines initiated several times during
Earth's history, due to bombardment by meteors, planetoids and comets.
If ETIs are operating in the visible spectrum we should not expect
to see flashing lights in the sky, for the power required to do this
and outshine their stars is much greater than required to establish a
decent communications channel. Free space optical communications will
be a mature technology for any spacefaring civilization. It seems
reasonable to assume that they will spinoff this technology for SETI
transmitters should they wish to contact emerging technical
civilizations. The fact that optical magic frequencies are hard to
identify at this time, save for 10,600 nm, is not an argument that such
frequencies do not exist.
Perhaps the only reasons for ETIs to build very large microwave
arrays would be to eavesdrop on radio frequency leakage from primitive
technical civilizations (like us), to beam microwave power, for astro-
physical research, or to communicate with other galaxies. Even this
author has some problems in believing that the civilizations of
extraterrestrials would be so altruistic and long-lived to attempt
electromagnetic communications across the intergalactic voids. The
interstellar eavesdropping scenario is also problematic, as it is
likely that a developing technical civilization only produces
substantial radio frequency leakage for a short period in its history.
In time, other technologies like fiber optics will replace high-power
radio and TV transmitters, and military radar systems will be
decommissioned. For this reason, if we attempt eavesdropping with large
radio frequency antennas ourselves, failure to detect such signals may
not imply very much about the existence or lack thereof of ETIs. Thus,
if the MOP does not detect ETI in the next decade, we should not jump
to the conclusion that we are alone in the Milky Way galaxy.
On the other hand, some civilizations may be continually threatened
by cosmic catastrophes in the form of bombardment by planetoids. These
races may have instigated powerful radar early warning systems for
planetary defense purposes. These comments are good examples of how
difficult it is to predict the future. Even Arthur C. Clarke and
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Stanley Kubrick appear to have been caught out by Pan Am going bankrupt
before it had a chance to ply the heavens between Earth and the Moon
(2001: A SPACE ODYSSEY, to the strains of the "Blue Danube"), or that
there would be no Soviet Union in 1992, let alone in 2001 or 2010! We
can only hope (and pray) that there will be a dynamic American Space
Program in 2001. We should not be too hard on Arthur Clarke, for
without his idea concerning "Extra-Terrestrial Relays" (Page 41), when
the word "Extraterrestrial" meant something completely different, the
Soviet Union might still be in existence. One notes in passing, that
the spaceship DISCOVERY, which was central to Arthur Clarke's 2001 and
its sequel 2010, used microwave dishes for its communication's link with
Earth. [94-96] Surely, the main (high-gain) link should have been a
laser-based system, notwithstanding the bright Earth background, and the
high solar background that might on occasion be viewed by the DISCOVERY
looking back towards Earth! A heterodyning telescope of several meters
diameter on the DISCOVERY, and a similar system on or near Earth, could
easily sustain a 1-10 Gbit/s data rate out to Jupiter and beyond.
We cannot even be sure that ETIs would want their signals to be
detected within an atmosphere or otherwise too easily. These are
prevalent assumptions among most SETI proponents. There might be
logical reasons for ETIs to think that only when a technical
civilization begins to "emerge" from its planet would it be truly
mature enough, and in a culturally receptive frame of mind, to receive
signals from ETIs. Thus, the recipients' atmosphere itself might be
used as an automatic protective blanket to avoid cultural shock. In a
way, the electromagnetic search for ETI is one of the greatest hunts
and detective stories ever. Unfortunately, there are still so few
clues.
CONCLUSIONS
The author feels that it is still an open question as to what are
the optimum electromagnetic frequencies for interstellar communi-
cations. As he concluded in his talk last year to the SETI Institute:
"The jury is still out as to whether ETIs are signalling with low-
energy microwave photons, or with high-energy optical photons". What
the author will say is that he feels a strong case has been made in
this paper for the SETI community and NASA to review their present
attitude towards the optical approach. This does not mean that the
Microwave Observing Project (MOP) should be abandoned or severely
modified, since clearly we need to do a exhaustive search in the
microwave spectrum. Some of the signal processing techniques developed
for MOP will also be applicable to the optical search.
In many ways the Cyclops Report may have become the cornerstone
upon which much of present-day SETI lore rests. While the report
itself was a very comprehensive study of Microwave SETI, and of high
technical quality, certain very conservative assumptions in that study
lead this author to consider the report flawed. Just like for NASA's
studies of the efficacy and cost of Microwave PowerSat technology back
in the 1970's, if we ask the wrong questions we are likely to get
incorrect answers. Attempting to lift all the material for PowerSats
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from the deep gravitational well of Earth is sure to make the
technology uneconomic and damaging to the environment. Sweep away the
inherent anthropocentric Assumption of Ineptitude of present SETI lore
and the problems associated with the optical approach disappear.
There appear to be some indications of group-think within the SETI
community, where it is easier to agree with the consensus than disagree.
The U.S. Space Shuttle CHALLENGER tragedy of 1986 is a classic example
of how group-think and the desire to conform can have immense ramifi-
cations. The issues may not be so acute here: Nevertheless, they
represent an impediment to the acceptance of new (or revisited) ideas.
Planning for an extensive optical search should be started now, so
that if by the year 2000 the results of the MOP are negative, we can
immediately initiate Professional Optical SETI activities. This would
be a natural extension to MOP so that the program could eventually be
renamed MOOP, the Microwave and Optical Observing Project. In the
meantime, amateur astronomers could be conducting a low-level
(low-sensitivity) optical search, helping to establish some ground
rules for a later high-sensitivity professional optical search.
It is believed that Professional Optical SETI with large hetero-
dyning telescopes is compatible with Professional Optical Astronomy in
that they can share most of the hardware, yet be undertaken at
different times so as not to interfere with each other's observations.
There is theoretical and experimental evidence to suggest that the new
adaptive telescope technology using Rayleigh or Sodium Resonance
Fluorescence laser guide stars [69] can be made to work during daylight
hours. This clearly has important ramifications for the concept of
Symbiotic (Serendip) Optical SETI. The idea of modifying Earth's Great
Optical Telescopes for Symbiotic (Serendip) Professional Optical SETI
has many attractions; where the scientific endeavors of conventional
and SETI astronomy could be of mutual benefit to each other.
There is probably a case here for an automated retrospective
historical study of stellar spectrographic plates to see if ETI signals
actually exist and are on record. It is quite possible that anomalous
spectral lines will be found in the record, signifying laser trans-
missions, but which had previously been overlooked, fogged the film,
saturated the recording media, been mistaken for natural bright
emission lines, or put down to "technical problems with the spectro-
graphic equipment". It would not be the first time that a major
scientific discovery had been missed for lack of attention and
curiosity. There does appear to be some doubt as to whether C.W.
ETI signals, if present, would have been accidently detected during
conventional optical astronomy and recognized for what they were. This
is the crux of the matter as far as the efficacy of Amateur Optical
SETI is concerned.
It is left as an exercise for others to determine the probability
of missing an ETI signal at any particular flux level. It is the very
concept that ETIs are supposed to be rare which makes it plausible to
suggest that the historic accidental discovery of ETI by optical
astronomers would be unlikely.
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Initially, to reduce the optical search time, we would concentrate
on efficient laser transition frequencies presently known to humanity,
and Fraunhofer dark lines. It is suggested that we must keep an open
mind here. For thirty years we have been digging relatively deep
trenches in a very small corner of our electromagnetic backyard. Was
it prudent to do this without at least turning over the topsoil in the
rest of the electromagnetic garden, particularly in that part of the
spectrum where solar output peaks, and which tells us and ETIs most
about our Universe?
The study also seems to indicate that the amateur SETI enthusiast
could make a useful contribution to the search using medium-size
amateur optical telescopes with photon-counting receivers. It is
certainly more debatable whether Optical ETI signals are present at
sufficient flux intensities to be detectable by small incoherent
telescopes. However, although the theoretical SNRs described for small
photon-counting (direct-detection) receiving telescopes are not
particularly impressive, even if very high mean EIRPs are assumed, it
must be remembered that ETI signals are likely to be pulsed and far
more detectable than the C.W. signals assumed here for the simplified
analyses. This would be particularly true for detection systems with
optical bandwidths greater than 100 GHz.
Today, the technology is available to construct efficient,
highly-sensitive photon-counting receivers for the visible and near-
infrared regimes. For several thousand dollars, top-of-the-line
amateur optical telescopes could be equipped with the instrumentation
to make unattended frequency searches of selected targeted stars. If
this new scientific endeavor really takes off, market growth will lead
to considerable reductions in hardware and software costs, making this
activity more affordable.
Not only would it be possible to slave many amateur telescopes
together at one site, to produce the equivalent of a larger telescope,
but it may also be possible to slave telescopes at different sites and
average the data. This would, of course, require accurate time
synchronization between the telescopes, though this should not be much
of a problem. However, the requirement to match the wavelength
accuracy of the optical filter or monochromator to within 100 GHz is
probably a more severe obstacle. In the case of co-site slaving, where
pre-detection combining of photons would occur, the SNR would increase
at a rate proportional to the number of identical telescopes. For
remote site slaving, where only post-detection electrical signal
combining could be employed, the SNR would increase at a rate
proportional to the square root of the number of identical telescopes.
While it is the author's view that Professional Optical SETI ought
not to required the use of more sophisticated signal processing
algorithms like KLTs [73,86] for extracting very weak pulsed signals
from noise, Amateur Optical SETI may well benefit from its use.
Perhaps one of the interesting aspects of the Amateur Optical SETI
concept using incoherent detection is that not only may there be a
useful contribution made by the enthusiast, but that such activities
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may occur before Professional (Visible) Optical SETI and its coherent
detection systems get established. A low-level search by amateurs
might help set some of the criteria for later professional searches,
even if the results are negative. Amateur optical SETI has the
potential to bring SETI to the masses, something that has not really
been possible at microwave frequencies, except in a limited way for a
few enthusiastic radio hams with modified satellite receivers
(AMSETI). [26] It also has the power to cause a renaissance in public
interest for astronomy and the night sky. It is an activity in which
amateur optical astronomers who live in big cities can participate,
unincumbered by light pollution, the bane of conventional amateur
astronomers. This could be the opportunity to dust off those old
telescopes and put them to use again.
It is clear, [27-29] that today there is an enormous interest in
SETI amongst the population. Professional SETI scientists could tap
into that interest to receive increased SETI funding and the cooperation
of enthusiastic amateurs.
It does not appear that Amateur Optical SETI at the infrared Carbon
Dioxide (CO2) wavelength of 10,600 nm would be very sensible because of
the limitations set by the essentially 24-hour day, 300 K temperature
background of the atmosphere, particularly for small apertures. As we
have seen, Professional Optical SETI in the visible and near-infrared
can use coherent or incoherent optical receivers. The coherent
approach is generally more sensitive but far more complex and
expensive. However, based on performance considerations, both ground-
based Professional and Amateur Optical SETI in the infrared would have
to be restricted to coherent receivers. This represents a complexity
and cost problem for the amateur. Of course, there could be very
powerful CO2 ETI transmitters present, as powerful as conjectured for
Visible SETI that have so far escaped detection, for we may not been
looking in the right direction at the right moment, with suitable
detection equipment. The CO2 observational work now being undertaken by
Dr. Albert Betz and Professor Charles Townes [57] is addressing this
issue.
Presently, Dr. Jill Tarter and Deborah Schwartz-Koyler of the SETI
Institute are involved with a NASA project (NASA NCC 2-407) titled:
"Supporting Research and Technology Activities in the Preparation of a
Three-Dimensional Map of the Infrared Sky". The goal of this project
is to construct a detailed three dimensional model of the infrared sky,
which will enable us to reconsider the question of the "best" frequency
at which to conduct a search for electromagnetic radiation, which is
indicative of the existence of an extraterrestrial technological
civilization. Thus, despite the general consensus that Microwave SETI
has the greatest likelihood for success, others are even now beginning
to probe deeper into the infrared part of the optical spectrum.
Since the start of modern-day SETI thirty two years ago, a strong
Microwave SETI constituency has developed. It will be understandable
if this author's views are attacked by that community, for many SETI
researchers have much at stake - decades of work invested in the
microwave regime and professional reputations. I would council the
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following thought: The public, and by that I mean the taxpayer, and
Members of Congress, should clearly be informed that it is quite
possible that the search for ETI in the microwave spectrum will be
unsuccessful - not because such signals are not present, but because we
are presently tuned to the wrong frequencies. It may well be necessary
to extend the search into the optical regime before we can be sure
whether electromagnetic ETI signals do or do not exist. It will not
look good for the SETI community if, in ten years time, they have to go
back to Congress, cap in hand, and ask for more funding to extend the
search into the optical regime after decades of maintaining that the
optical approach was useless. Note that these sentiments have
previously been expressed privately to both the SETI Institute and
NASA. It is the author's contention that SETI has been "hijacked" by
radio astronomers. It should now be clear to the reader that for
humanity to have devoted less than 5 percent of its SETI observation
programs to the optical regime, and an even smaller percentage to basic
Optical SETI research, was probably unwise. The author hopes that
readers will urge the SETI Institute, NASA, and Congress to rectify
this omission.
NASA should be able to put an end to recent problems in deploying
large high-gain microwave antennas in space, e.g., on the Galileo
probe, by moving to fixed high-gain optical antennas as soon as
possible. During the next few decades, other lights (visible and near-
infrared) will appear in the sky of terrene origin: they will be the
advanced laser communication systems of GEO and LEO satellites, along
with signals coming back to Earth from NASA's next generation of deep
space probes. [63-66] Sometime next century, humans will be seen
walking on the planet Mars. These HDTV television signals are likely
to traverse most of the distance between Mars and Earth via laser, be
relayed around the globe via laser-based geosynchronous satellites, and
arrive in people's home via optical fiber. When humanity sends out
(non-relativistic) interstellar probes to investigate nearby star
systems, the data and pictures of those encounters (hopefully with
other planetary systems) will come back to Earth via laser. The
computer technology of the day will also be substantially dependent on
photonics. See the January 13, 1992 issue of NEWSWEEK (pp. 56-57) for
the article on "The Highway to the Future", describing a fiber-optic
multi-gigabit data highway system being proposed for the United States.
Also see the January 9, 1992 issue of ELECTRONIC DESIGN (pp. 73-80) for
the article on "The World of Communications is Moving to Fiber Optics".
The author has seen the future, and it is photonic.
Truly, the superior communications and computing technology of the
future will be photonic, a technology that is likely to be around for a
while. Indeed, in the future, one of the main uses for low-gain
microwave space communications might well be the "acquisition" of the
party at the other end of the link, so that the high-gain laser
communications system can be locked on! The amateur SETI enthusiast,
with the right photonic receiving equipment, will be able to tune in on
these Earth-bound optical transmissions. How ironic, that next century
the complaint will surely arise, that terrene optical transmissions are
interfering with our ability to carry out Optical SETI free of false
alarms! Now where have we heard that before?
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We end as we began. If we look at the basic beliefs that
differentiate the proponents of the Microwave and Optical SETI
rationales, or the belief in Unidentified Flying Objects (UFOs), an
area that has been even more controversial than SETI, we find that the
respective convictions hinge on our assumptions about the technical
abilities of ETIs. In the case of Microwave SETI, the proponents
believe that while intelligent life within in galaxy is not rare, that
ETIs do not have the technical wherewithal to get the full benefits of
the superior optical technology for interstellar communications. If
the reader subscribes to the Optical SETI rationale, they will
additionally believe that ETIs have the technical prowess to use the
superior laser technology in an effective manner. Advocates of UFOs
essentially accept that ETI technology is so superior to our own that
rapid interstellar travel is easy for them, and if these ETIs actually
wanted to make contact, they would make physical contact. In the end,
the reader's belief will be limited only by their own vision.
The theoretical results quoted in this paper are based on standard
text book relationships, familiar to students of electrical
engineering, physics, and astronomy. Please refer to Appendix A for a
list of most of these formulas and specimen calculations. Perhaps the
main reason for the difference between the conclusions of this analysis
and many previous comparative SETI analyses, is that the author has
shown a bit more imagination.
A few additional closing statements. It may appear from the
author's comments throughout this document that he does not hold high
regards for the efforts over the past thirty two years of many noted
(microwave) SETI scientists. This would be far from the truth. It is
the nature of science that for every two steps forward, it may often
take a step back in the light of new discoveries or new ideas. It is
very easy with hindsight to criticize those who have gone before, but
without their predecessors' work and developments in other scientific
fields and technologies, it is unlikely that the new discovery or idea
would ever have seen the light of day. Each generation of scientists
and engineers builds on the foundations laid by earlier generations.
Readers are reminded that there is little which is innovative about
the contents of this document which have not previously been described
by Charles Townes [46-47,80] and others - the author has just been a
bit more forceful. Innovative ideas, like good wines, take time to
mature. The author hopes that the effort he has expended in this
revisiting of the optical approach to the search for extraterrestrial
intelligence will at last cause Optical SETI to be seriously considered
by the scientific community as warranting closer study.
This paper could be the start of an exciting new chapter in both
SETI and professional/amateur optical astronomy. One thing which can
be said for certain is that should a professional or amateur
astronomer discover electromagnetic (radio or optical) signals from
ETIs, neither they nor humanity will ever be the same. There is no
doubt that a Nobel Prize will await the discoverer. Perhaps now is the
time to get familiar with those Post-Detection SETI Protocols! [25]
See Appendix B for a description of these protocols.
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