A Fascinating Paper on Optical SETI by

Dr. John Rather of NASA HQ

Radobs 15

In December, Dr. Jill Tarter (SETI Institute) informed me that there was a
scientist at NASA Headquarters, a Dr. John Rather, who was also very
enthusiastic about the optical approach to SETI.  Before Christmas I had a
very brief phone conversation with him, and I sent him copies of some of my
material.  On Friday, I received an advance copy of a paper on Optical SETI
by Dr. John Rather of NASA Headquarters, entitled "LASERS REVISITED: Their
Superior Utility for Interstellar Beacons, Communications, and Travel". 
This paper is scheduled for publication in the Journal of the British
Interplanetary Society (JBIS) in August of this year.  Even his title is
slightly similar to that of my first (not yet published) paper "OPTICAL SETI
REVISITED".  I am considering submitting a paper to JBIS.  Perhaps having
both our papers in the same issue could be useful, as that issue will be
devoted to "Interstellar Studies".  Here now is a summary of Dr. Rather's
paper, my comments, and some remarks about an Optical SETI paper published a
decade ago.

Major parts of his rationale and differences in approach:

1.   Advanced Technical Civilizations (ATCs) are not that common in the
     galaxy, and therefore, when we do receive a signal from such a
     extraterrestrial civilization, the source will be thousands if not
     millions of years more advance than us.  This is essentially the same
     as my rationale, and that of conventional SETI but assumes that ATCs
     are somewhat rarer than I would hope or believe.

2.   Dr. Rather bases his system on the assumption that the Signal-To-Noise
     Ratio (SNR) is always limited by Planckian starlight.  In my rationale,
     I assume that every attempt is made to ensure that the SNR is limited
     only by quantum noise, as this will maximize the SNR and require the
     least transmitter power.  Of course, if one is assuming an extremely
     high bandwidth, e.g. 10 GHz (see below for more details), it may be
     difficult to maintain quantum noise limited detection even with high-
     quality diffraction limited receivers that can spatially separate the
     star and transmitter.

3.   Because Dr. Rather assumes large optical transmission arrays, and when
     I say large I mean large - many kilometers in diameter - sort of
     Optical Cyclops, he can arrange things so that targeted star systems
     are always in the near-field Rayleigh range.  This ensures that at over
     reasonably large distances, the beam dimensions can be maintained at a
     relatively fixed level.  Not for John Rather puny 10 meter diameter
     transmitting and receiving telescopes, but vast phased arrays, trans- 
     mitting arrays that might be planet-sized, that can convert a large
     amount of the incident stellar radiation to laser light.

     While I have been very "conservative" in my assumption about ATC's
     optical technology other than in their interstellar ballistic
     capability (getting artificial photons to hit planets), being initially
     more interested in making the arguments as to why the present SETI
     rationale is suspect, he has let his imagination range unfeted by our
     Earth-bound prejudices.  His proposed large arrays means that he can
     arrange that the intensity of the received signal does not decrease
     with distance over his desired range of operation, i.e., the inverse
     square law does not imply.  In my system at large distances, a 10 meter
     diffraction limited receiving focal plane array would no longer be able
     to spatially separate the star and transmitter.  So in actual fact, the
     relative Planckian noise (per pixel) may actually remain constant with
     distance above a certain range.

     Dr. Rather's near-field beams result in the Planckian starlight
     background becoming negligible at large distances, even with wide
     optical bandwidth detection.  It seems almost incongruous to talk about
     the near-field of a transmitter stretching out thousands of light
     years!  For my state-of-the-art terrene 10 meter diameter telescopes,
     the collimated near-field at 656 nm only stretches out to about
     120,000 km (0.0008 A.U.) - a piddling distance (see RADOBS.14)!  In his
     cover letter to me, he remarked that I had failed to (explicitly)
     mention this important consideration for eliminating Planckian noise
     problems.  Because Dr. Rather is proposing the use of extremely large
     optical arrays, this option becomes available.  Thus, over a large
     range, the received signal intensity would be independent of range,
     with the Signal-To-Planck Ratio (SPR) actually increasing with distance
     as the Planckian noise level falls off inversely proportional to the
     square of the distance.  At some point, the Planckian noise would fall
     below the quantum noise, and hence be inconsequential.

     You will note that I myself had already suggested that ATCs would have
     the ability to "modulate" (see RADOBS.7C "OPTICAL SETI SURVEY - PART
     C", question 8, and RADOBS.12 "SETI SYSTEM PERFORMANCE COMPARISON
     TABLE) the collimation of their beams so that if the beam was too small
     for easy targeting to nearby star systems, they would slightly
     decollimate their beams.  I had put forward this suggestion in response
     to Dr. Bernard Oliver's Cyclops study (see RADOBS.2 "THE FLAWED PROJECT
     CYCLOPS STUDY"), which while technically excellent, was deficient in
     optical imagination.  The comparative part of the 1971/72 Cyclops study
     effectively crippled the transmitting telescopes because the beams were
     too small for nearby stars.

     By coincidence, I was just putting the finishing touches to document
     RADOBS.14 on "DIFFRACTION LIMITED BEAMS AND GAUSSIAN OPTICS".  This
     goes into much greater detail about Gaussian TEMoo single transverse
     mode laser beams, spot size, and telescope directivity patterns.  The
     document was written to demonstrate that to a fairly close
     approximation, a transmitter aperture fully illuminated with a beam
     having constant intensity across the aperture, produces a similar
     central lobe to that produced by a telescope aperture matched to the
     1/e^2 intensity points of the fundamental TEMoo single transverse mode
     of a laser.  The document also contains a section describing the near-
     field beam.

4.   Dr. Rather also doesn't think that interstellar absorption in many
     regions of the Galaxy is much of a problem in the optical regime.  So
     he considers that optical communications may be feasible over tens of
     thousands of light years, even in the visible part of the spectrum. 
     This would get us substantially across the Milky Way Galaxy. 
     Certainly, as I have previously indicated, infrared lasers could
     probably allow for communications across the entire Galaxy.

5.   Having dismissed the Planckian noise problem by ensuring very strong
     signals even over 5,000 or more light years, he does not even bother to
     consider the ability of his huge diffraction limited receivers to
     resolve the star and the transmitter.  I suppose that is just icing on
     the cake!  In line with this rationale, Dr. Rather also doesn't feel
     that operation within a Fraunhofer dark absorption line is necessary,
     though it can be helpful.

6.   His rationale assumes ETIs so far advanced of us, that they have a lot
     of information to download.  Not for him, kHz or MHz-type bandwidths,
     but bandwidths of the order of 10 GHz - and I thought a 30 MHz video
     signal was outrageous!  The late Dr. Shvartsman, the only person I am
     aware of that has actually done Optical SETI measurements, also had a
     rationale for extremely high bandwidths, bandwidths significantly
     greater than even I am proposing.  These signals might be encoded in a
     way that would allow low bandwidth signals to be demodulated first,
     perhaps giving the information to build bigger and better receivers
     that can extract the wideband information.  Because these bandwidths
     are so large, and the Planckian radiation so weak, he assumes that
     incoherent optical detection techniques will be used; employing
     conventional spectrographic gratings for filtering.  There is no
     discussion of the so-called Doppler shift or chirp problem.

7.   Although Dr. Rather did not use the expression "Signpost SETI", he
     believes it possible that we might detect low-bandwidth microwave ETI
     signals directing us to the optical regime.

8.   Dr. Rather doesn't stop at considering a 1 GW transmitter as being too
     powerful.  He considers 100+ GW laser transmitters are quite possible. 
     A phased array constructed on a planet like Mercury would have
     available an enormous amount of solar energy to power the lasers.

9.   He doesn't specifically mention the targeting problem with a near-field
     biosphere-matched beam only 21 light minutes (2.5 A.U.) in diameter,
     whatever the distance of the targeted star system.  This beam is
     significantly larger than assumed in my approach for nearby star
     systems, but smaller than for my beams at distant star systems. 
     Clearly for him this is "no sweat".  Dr. Bernard Oliver can only be
     appalled at such sweeping assumptions, even though there isn't any
     presumption for Dr. Rather of knowing where the planets are in their
     orbits!

10.  He also feels that a large optical array is ideally suited for spatial
     multiplexing many beams at many target star systems.

11.  Dr. Rather suggests that a small scale optical receiving system could
     be constructed with today's technology.

In my earlier brief conversation with Dr. Rather, I had obtained the wrong
impression as to how he was targeting stellar biospheres.  He was not
assuming fan-shaped or annular-ring beams just to increase the SNR by a few
dB, but as stated above, he was assuming near-field transmission systems. 
This mistaken notion of mine was featured as a question (No. 6) in the
recently uploaded OPTICAL SETI SURVEY (RADOBS.7C).  This will be corrected
in later versions.

In conclusion, I would like to say how impressed I am by the scope of
Dr. Rather's imagination.  As I have indicated previously to members of
RADOBS, when I first started investigating this area and "crunching the
numbers", I felt it important that initially I should not be "prejudiced" by
what other scientists had said about the subject.  It was only later, that I
discovered that there was in fact very little that had been said, and what
had been said was generally negative.  Now, six months after starting my
involvement, I have seen the thoughts of another like-minded scientist. 
Because of what he says and the fact that I was unaware of him until last
month, I can say that I now feel "less crazy"!  I was more concerned about
demonstrating how "simple" 10 meter diameter optical telescopes have the
capability of linking the Galaxy.  If my rationale can best be summed up as
"small is beautiful", Dr. Rather's rationale is "large is beautiful so long
as it is optical"!

I have asked Dr. Rather if he wouldn't mind me making a photocopy of his
paper and giving it to Bob Dixon.  I am sure that Bob will not hesitate to
express his opinion on this bulletin board, or elsewhere.

The latest on the progress of my paper is that so far, "Nature" has had one
response from a referee.  They are awaiting a report from a second referee. 
It is now about three months since submitting my paper to that journal, and
about four months since the earlier submission to Electronics Letters.  When
I revise the "Nature" paper I shall incorporate some references to
Dr. Rather's work.

Over the weekend, I received a large paper from Dr. Jill Tarter via Bob
Arnold.  The paper is entitled "Olbers paradox revisited and the future of
intelligence" and was written by Pierre Connes.  Jill sent me this paper
after I had inquired about a Frenchman I was vaguely aware of who was also
pro Optical SETI, though I don't know if this is the same Frenchman.  It
made quite interesting reading because it talks extensively about Optical
SETI, though his rationale assumes very low data rates, e.g. about 5 Hz,
similar to what the microwave SETI community assumes.  His "Prometheos" idea
assumes that ETIs have been flashing starlight or laser beams at us for
millennia at these very low data rates, in a way the their presence in the
extended cosmic haystack (microwaves to ultraviolet) would be obvious, even
to primitive man.

I generally discount the idea that there is much purpose in signalling in
this manner, so we should not be concerned that no one has ever spotted an
artificial flashing "star".  Such a means of signalling does not make the
best use of technology, and there would be no means of getting a reply until
the targeted civilization had reached quite an advanced state of technology,
and put large structures in space.  Today, even our crude technical
civilization has the means to communicate at low bandwidth across hundreds
of light years with our puny telescopes and lasers.

Pierre Connes does discuss the use of focal plane arrays for detection, so
in that respect it has some other similarities to my rationale.  In
Appendix I of Pierre's paper, which was presented at the 1979 "Conference on
Life in the Universe" and sponsored by UNESCO, he mentions that Shlovskii
and Sagan (1966) discussed Optical SETI.  I shall have to track that one
down - it was no doubt very negative.  It is possible that Jill Tarter's,
Edward Ashpole's, and Tom McDonough's idea that if ETIs were signaling to
us, their signals would have been seen, came from Connes's paper.

To paraphrase a well-known Presidential candidate, what we want is a kinder,
gentler Optical SETI, one not using brute force techniques.  Sophisticated
ETIs do not need to use extremely high powers, just some clever technology. 
Even today's primitive terrene optical technology is well capable of
"reaching out and touching some alien" - just aim a modulated 1 kW laser up
the barrel of the Mt. Palomar telescope.  Whether it would be wise to do so
is another matter!  Pierre Connes may have thought that ETIs would use as
much power as necessary to make their signals easily observable in the
bandwidth of the human eye.  Stuart Kingsley thinks otherwise; that there
should be no presumption that ETIs would want to talk to unsophisticated
civilizations who have yet to master technology, whether one believes or
doesn't believe in the "cosmic-zoo" hypothesis.  After all, the recipients
wouldn't have the means to respond.  It is more reasonable to assume that
detection of ETI signals will first occur at a period in the recipient's
history when the technology is mature or nearing maturity for sending a
reply, not thousands of years earlier.

For those in the SETI community concerned about the time it is taking to
discover the first ETI signal, take heart.  It doesn't mean there are no
ETIs, its just that we haven't even begun to scratch the surface of the new
extended cosmic haystack - I officially declare 1991 as the year of the
extended cosmic haystack!

January 9, 1991
RADOBS.15
BBOARD No. 315

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