A Fascinating Paper on Optical SETI by
Dr. John Rather of NASA HQ
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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