Publications DepartmentCopyright © 1996 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.orgCopyright © 1996 Society of Photo-Optical Instrumentation Engineers
This paper was published in The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum II, Stuart A. Kingsley, Guillermo A. Lemarchand, Editors, SPIE Proc. 2704, May 1996, and is made available as an electronic reprint with permission of SPIE. Single print or electronic copies are allowed. Systematic or multiple reproduction, or distribution to multiple locations through an electronic listserver or other electronic means, or duplication of any material in this paper for a fee or for commercial purposes is prohibited. By choosing to view or print this document, you agree to all the provisions of the copyright law protecting it.
ABSTRACT
One of the arguments frequently made against conducting an Optical Search for Extra-Terrestrial Iintelligence (OSETI) deals with the propagation range limitations of optical signals. Examples abound, dating back to the Cyclops report, which show microwave signals to be easily capable of trans-Galactic communications, whereas optical signals reputedly are not. Could proponents of microwave SETI have perhaps overstated the capabilities of their systems, and unwittingly stifled the development of optical techniques in the process? The author, a strong supporter of microwave SETI, concedes the possibility. He reviews the assertion that Arecibo (Earth's largest radiotelescope) would be able to detect its theoretical twin across the Galaxy. This paper challenges the veracity of that widely held belief, through detailed range calculations. It traces the propagation of what the author calls the Arecibo Myth, and explores its impact upon public perceptions as to the validity of SETI activities in general, and optical SETI in particular.
Keywords: SETI, OSETI, Arecibo, microwave, communications.
1. INTRODUCTION
The electromagnetic Search for Extra-Terrestrial Intelligence (SETI) which was once conducted by NASA fell on hard times in 1993, with Congress terminating its funding. Fortunately, various groups, including the non-profit, membership supported SETI League, [1] are now involved in privatizing the effort. Some amateur groups are promoting SETI by non-professionals, justifying the viability of such an enterprise by citing the astounding communications range of Earth's largest radiotelescopes. If such "big guns" exist at the other end of the communications path, it is reasoned, then even small amateur stations have a reasonable probability of success in detecting their signals.
SETI pioneer (and former Arecibo director) Dr. Frank Drake teaches electromagnetic communications range, using as an example the highly capable Arecibo radiotelescope [2]. His numbers suggest that the existing 305 meter reflector, with currently available transmitting and receiving equipment, is able to detect a duplicate of itself situated near the center of the Milky Way galaxy. He further suggests that minor (and entirely feasible) system improvements would increase this range to the edge of the Galaxy. My personal range computations, which involve initial assumptions different from Dr. Drake's, fail to support that contention. This paper explores that apparent discrepancy.
2. ARECIBO: AN OPTIMISTIC ESTIMATE
For analysis of a hypothetical communications link (such as that between any Earth-based facility and a distant counterpart), we must make various assumptions as to the level of technology possessed by the distant civilization. A conservative approach is to invoke the Assumption of Mediocrity. [3] This pessimistic doctrine asserts that (a) Earth is a relatively young and undistinguished planet, not particularly technologically advanced for its age, and (b) other civilizations, likely being older than we, will at the very least be able to muster our present level of technological prowess.
Drake's application of the Assumption of Mediocrity in computing the communications range between two identical Arecibo-class facilities is summarized by me in Figure 1. His transmitter is Arecibo's existing 500 kW, 2380 MHz planetary imaging radar. The illumination efficiencies and noise temperatures assumed are realistic for the present system. Drake assumes a receiver resolution bandwidth of 10 mHz, the effective Drake-Helou limit [4] for the frequency used. A one hour integration time constant is assumed. At 7960 parsecs, Drake's estimated range, signal-to-noise ratio is on the order of +1.8 dB. This spreadsheet approach agrees well with Drake's own algebraic solution. The distance, however, is perhaps one quarter of the Galactic diameter.
Drake points out that true trans-Galactic range could be achieved merely by increasing Arecibo's transmitter power to 3 MW, or integrating for 6 hours. Either solution is feasible, he contends, and our current limits are financial, not technological. He states, "all the parameters used in the Arecibo numerical example are plausible. The point was to show that if one tried hard, one could detect an Arecibo anywhere in the Galaxy." [5] Arecibo transmitter engineer Bob Zimmerman adds, "the earlier 0.5 MW output power klystron transmitter has been completely removed, dismantled, and used in the construction of our new 1.0 MW output power transmitter, which is now at Continental Electronics in Dallas." [6]
3. ARECIBO: A PESSIMISTIC ESTIMATE
My personal Arecibo estimate is somewhat more conservative, as seen in Figure 2. Although Arecibo can indeed transmit at 2380 MHz, I have chosen the H1 frequency of 1420 MHz [7] as being more realistic for SETI, on the long-standing grounds that it may well be the obvious choice of other intelligent civilizations.[8] I have also set my receiver resolution bandwidth at 1 Hz. Current technology would certainly allow us to narrow our effective bandwidth down to tens of milliHertz, and similarly narrow multi-channel spectrum analyzers have indeed been used for SETI at Arecibo, and elsewhere. The cost of such receivers, however, is such that only a few currently exist in the world; they are not typical of what I consider practical communications systems. Recognizing that it will cost us dearly in range, let's see what kind of results are feasible with this wider bandwidth.
Radiotelescopes achieve gain in part through capture area, and partly through long integration of their received signals. Sensitivity goes up as the square root of integration time, so the longer we can view a target the better. By displacement of its feed, observations of over two hours are feasible at the Arecibo observatory. Thus we will assume here an 8000 second integration time. This value is intermediary between the integration times used in Drake's 8,000 parsec and 20,000 parsec range solutions. The selected integration time will preclude communicating information at data rates greater than one bit per 2.2 hours. On the other hand, we concern ourselves here only with signal detection, not communicating information.
We retain Drake's assumed overall system noise temperature of 20 Kelvins as consistent with the sum of three factors: noise temperature of the existing helium-cooled maser, Earth temperature contributed through antenna sidelobes, and expected galactic and cosmic noise sources at our operating frequency. Let's figure we have a 1 MW L-band transmitter available, since we will shortly have that power level available at Arecibo in S-band. The 50% illumination efficiency I have assumed for the reflector is consistent with a prime-focus dish which is spherical rather than parabolic, and is aimed by offsetting its feed. According to Zimmerman, "this number will change slightly with our new optics (Gregorian feed) but not significantly." [9]
My estimate yields a unity signal-to-noise range of about 3.2 kiloparsecs, or roughly 10,000 LY. This is perhaps an eighth of the Galactic diameter. By the constraints which my conservative estimates impose, communication at trans-Galactic distances between two Arecibos appears, if not impossible, then at least somewhat unlikely. The questions underlying these analyses are then: how hard are you willing to try, and how much are you willing to spend, to achieve trans-Galactic communications? The answers, clearly articulated by Congress in terminating NASA SETI, are "not very" and "not much".
4. WHAT ABOUT CYCLOPS?
The Cyclops array could well be capable of trans-Galactic communications. Designed as part of an interstellar communications feasibility study during the summer of 1971, Cyclops is the most powerful SETI receiver never implemented. Its phased array of 900 fully steerable 100 meter dishes would have given us truly impressive range. The Cyclops Report includes an example [10] which analyzes communications between two such systems, at a wavelength of 3 cm. A communications range of 450,000 LY was stated. I have repeated the calculation in Figure 3. Consistent with the Cyclops study, Figure 3 assumes a 100 kW transmitter and 20 K receive system temperature, as well as an antenna illumination efficiency of 90%. The Cyclops example used an integration time of a mere 1 second. Our results and those in the Cyclops report correlate within round-off error. Note that the maximum communications range exceeds the 26 kiloparsec diameter of our galaxy, by roughly a factor of five.
But we are mixing kiwis and kumquats here. In the interest of consistency, let us repeat the analysis for Cyclops operating at the hydrogen line (1420 MHz). We can assume that the lower frequency enables us to increase transmitter power back to the 1 MW level which we used in our Arecibo analysis. Similarly, we set antenna efficiency back to 50%, which we had previously assumed for Arecibo. Note in Figure 4 that effects of changing power and frequency nearly cancel, and that Cyclops still easily exceeds trans-Galactic range, even at a mere one second integration time.
5. A CURSORY COMPARISON
So Cyclops is clearly capable of interstellar communications on a trans-Galactic scale, even at the hydrogen line, and constrained to very conservative estimates of Earth transmitter and receiver technologies. Is it then logical to expect the same of Arecibo? Recall that communications range varies directly with the diameter of one antenna in the path. Since communicating with one's counterpart implies specifying the size of two antennas, range would be expected to vary by the square of antenna diameter.
Note that the diameter of the Arecibo reflector is roughly one order of magnitude less than the effective diameter of the full Cyclops array. All else being equal, we would expect the range between two Arecibos to be perhaps a hundred times less than that between two Cyclops systems. Logically, this should set the communications range between two Arecibo type antennas, given the assumptions we have made, at about one kiloparsec. The range computed in Figure 2 significantly exceeds our expected value, primarily because of the significantly greater integration time employed. Nevertheless, whereas Cyclops is a rather credible trans-Galactic communications facility, under similar conditions Arecibo clearly is not. And what of suggestions to the contrary? I have dubbed these The Arecibo Myth.
6. ORIGIN OF THE SPECIOUS
How does the Arecibo Myth influence public perception of SETI? A related question might be: how does the public come by its fragmentary knowledge of SETI? Any public interest in the topic must be attributed to the missionary zeal of one man: the late Bernard M. Oliver. The chief architect of Cyclops, Dr. Oliver devoted considerable energy over the last quarter-Century to lobbying for its implementation. He was dynamic, and his enthusiasm infectious. To a whole generation, Barney was SETI's most eloquent spokesman [11]. His hundreds of public presentations on the topic emphasized Cyclops' ability to communicate with its theoretical twin at trans-Galactic range.
Very well, but what has this to do with Arecibo? Precious little, and that's my point. "Cyclops," I often heard Oliver say, "as the world's largest radiotelescope, would be capable of communicating with itself across the galaxy." Here I must become speculative. I can easily imagine a journalist at one of Oliver's lectures, hearing the above statement but somehow missing the first two words. "World's largest radiotelescope? Why, that's Arecibo!" And citing Barney Oliver out of context, without malice aforethought, a myth is born.
Once an untruth becomes firmly planted in the public consciousness, it becomes almost impossible to dislodge. Magnified by repetition, legitimized by the press, the Arecibo Myth has now become reality in the minds of the masses. Though SETI enthusiasts would like to believe it, we would be better served by a healthy sense of skepticism.
7. IMPLICATIONS OF THE MYTH
SETI proponents have long dreamed of a fully-implemented Cyclops, while settling for whatever time could be begged, on whatever sub-optimal facilities came our way. In the spirit of make-do, some of us have even advocated small-scale amateur SETI. The SETI League's planned Project Argus all-sky survey, for example, will employ several thousand small (3- to 5-meter diameter) satellite TV dishes, geographically dispersed so as to maximize sky coverage. Our initial range estimates were, sadly, based upon optimistic estimates of Arecibo's range. TV dishes, it was reasoned, are about 100 times smaller than Arecibo, thus can communicate with a given facility at one percent of Arecibo's range. If Arecibo could hear Arecibo at 100,000 LY, we reasoned, then small TV dishes (equipped, of course, with state-of-the-art receivers and digital signal processors) should be able to hear Arecibo at a range of 1,000 LY. And there is certainly a substantial number of likely candidate stars within that range.
By questioning the practical range of Arecibo, we are forcing ourselves to re-examine the viability of amateur SETI, or any small-scale listening project. It is clear that the feat of hearing ourselves across the galaxy was never even a consideration for amateur SETI. It could easily be accomplished by Cyclops, but perhaps not so readily by Arecibo. Does that mean we can simply substitute "Cyclops" for "Arecibo" in any analysis which contemplates galactic range, and go on as before? Can we, for example, expect amateur SETI to hear a Cyclops at 1000 LY? Not quite.
The fallacy is that, whereas an amateur SETI dish is 100 times smaller than Arecibo, it is fully 1000 times smaller than the effective diameter of Cyclops. Which suggests, if Cyclops can hear Cyclops at a four hundred fifty thousand light years, then a satellite TV dish can hear Cyclops, but at a thousandth the range, or only abut 450 light years.
In fact, it's even a little worse than that. These rough approximations assume that amateur SETI can achieve efficiencies, noise temperatures and bandwidths on a par with those encountered in professional practice. Although some radio amateurs do indeed pioneer new technology, such a blanket assumption as to the capabilities of the average SETI station is probably unwarranted. Succeeding at SETI will require uncommon dedication and commitment, a little like winning the Indianapolis 500. Fortunately, those hams who choose to tackle SETI are anything but average.
Let's do a slightly different type of link analysis, factoring in the antenna efficiency, noise temperature and bandwidth which the dedicated amateur microwaver can actually hope to achieve. Such a SETI station is paired with a Cyclops in Figure 5. Over what distance could your serious ham SETI enthusiast hope to detect a Cyclops? Integrating for the full 10 minute transit time of these smaller antennas in drift-scan mode, the answer appears to be: on the order of 220 light years. This is on a par with the range over which the SETI Institute's Project Phoenix targeted search is being conducted.
Though certainly not futile, amateur SETI is definitely going to be range-limited. Its proponents are not deterred, however, because these range limits, like those I have implied for Arecibo, are predicated on quite conservative (even pessimistic) assumptions. Which seems to me the most responsible way to go about planning and promoting a SETI program.
8. WHAT ABOUT OPTICAL SETI?
Our final range example involves detection of a coherent CW optical source, using levels of technology typical of late 20th Century Earth. We expect optical communications range to be somewhat limited, and compared to such esoteric microwave links as Cyclops (or even Arecibo), it is. But optical SETI, it turns out, holds its own with the anticipated range capabilities of amateur microwave SETI.
Optical SETI champion Stuart Kingsley has described a hypothetical interstellar optical communications link, [12] which will serve as a benchmark for comparative range computation. His assumptions include a 1 kW visible CW laser transmitter at 656 nm, 70% efficient ten-meter diameter telescopes at both ends of the path, a heterodyne receiver with 50% quantum efficiency and a rather daunting (though probably realistic) 43,900 K noise temperature, an ultimate IF bandwidth of 1 Hz, and just 1 second of integration time. These figures yield a unity signal-to-noise range on the order of 250 LY, as seen in Figure 6. Dr. Kingsley believes his assumptions to be quite conservative, pointing out that "I have used a 1 kW power only for normalizing purposes. In practice, CW laser powers of MW and even GW may be available to ETIs!" [13] Nevertheless, we accept his 1 kW benchmark as typical of Earth's level of technological maturity, and analyze accordingly. The importance of the result depicted in Figure 6 is that optical SETI range, for the system described, rivals that of amateur microwave SETI, given a Cyclops at the other end of the link. This is quite impressive performance indeed, for "mere" 10-meter telescopes.
9. CONCLUSIONS
It has been said that if we integrate long enough, we can detect a flashlight across the universe. Trans-Galactic communications between two Arecibos is almost within our grasp. Given, for example, Drake's assumed 500 kW CW transmitter at 2380 MHz, his 10 milliHertz receiver bandwidth, and integrating for an hour, our signals will reach easily to the center of the Milky Way. Detection may be possible, but this should not be confused with communication, since our data rate would be limited to something like 278 microBaud. This is tens of millions of times slower than even the most sluggish PC modem. And since we presume the distant station is also situated on a rotating planet, we are faced with the challenge of making two antennas track each other for hours on end. Though the system works on paper, such communication requires great coordination, which is not likely to be present for SETI.
The Arecibo radio observatory is one of the technological wonders of the modern world. Within the limitations of its frequency and spatial coverage, its performance is truly incredible. As a radar telescope it is second to nothing ever built on this planet. But Arecibo was never intended, nor is it likely ever to function, as a trans-Galactic communications station. For that function, we're going to have to build a Cyclops.
We have seen (to paraphrase the simplified Drake equation) that given the capabilities of amateur microwave equipment, N ~ C : for amateur SETI, the number of communicative civilizations which we can hope to hear roughly equals the number of Cyclops arrays active within 220 light years. But then, how many Cyclops arrays exist within 220 LY, or even throughout the galaxy? For that matter, how many confirmed Arecibos? At present, we can count them on the thumbs of one hand.
On the other hand (thumbs notwithstanding), we might assume 1 kW lasers driving 10 m telescopes to be at least as likely to exist within 220 LY of Earth as Cyclops arrays, or Arecibos. Perhaps optical SETI has not received the consideration it's due. None of this should be taken to discredit microwave reception, which I still believe is the most likely way in which we will receive the existence proof which SETI seeks. Rather, this exercise is intended to show that proponents of microwave SETI may have overstated their case. By adopting a conservative (and realistic) view of the range limitations of microwave communications, we conclude that optical contact may prove no less feasible.
10. REFERENCES
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