Analog Science Fiction and Fact 04/01/11
Page 20
Parsimony has implications for SETI. For transmitting time t, receiver detectability scales as t1/2. But at constant power, transmitter cost increases as t, so short pulses (“pings”) are economically smart (cheaper) for the transmitting society. A one-second pulse sent every 10 minutes to 600 targets would be 1/600 as expensive per target, yet only approximately 1/25 times harder to detect. Interstellar scintillation limits the pulse time to greater than 10-6 sec, which is within the range of all existing high-power microwave devices. Such pings would have small information content, attracting attention to weaker, high content messages.
Even if Earth economics generally works similarly in other technological societies, why should it apply to their transmitting beacons? Even on Earth, larger goals often override economic dictates, such as military security, aesthetics, religion, et cetera. But two aspects of SETI undermine this intuition:
1. SETI assumes long time scales for sender and receiver. Still, while cultural passions can set goals, economics determines how they get done. Many momentary, spectacular projects such as the pyramids of Egypt lasted only a century or two, then met economic limits. The Taj Mahal so taxed its province that the second, black Taj was never built. The grand cathedrals of medieval Europe suffered cost constraints and so, to avoid swamping local economies, took several centuries of large effort. Passion is temporary; costs remain.
2. We found that the optimum cost strategy leads directly to a remarkable cost insensitivity to the details of economic scaling. The ratio of costs for antenna area and transmitter power is about one. The two costs are usually equal and their ratio does not depend on the details of the technology and varies on Earth by only a factor of two. Both these costs may well be related principally to labor cost; if so, labor cost cancels out. This means fashions in underlying technology will matter little, and our experience may robustly represent that of other technological societies.
Our quantifying approach is sobering, as it forces trad-offs on otherwise open-ended speculations. But it also advances the subject, which many beacon ideas do not do. It’s simply much clearer to pick a major organizing principle—economics—than generalize from a special design, or guess at alien ideas.
What if we suppose, for example, that aliens have very low cost labor, i.e., slaves or automata? With a finite number of automata, you can use them to do a finite number of tasks. And so you pick and choose by assigning value to the tasks, balancing the equivalent value of the labor used to prosecute those tasks. So choices are still made on the basis of available labor.
The only case where labor has no value is where labor has no limit. That might be if aliens may live forever or have limitless armies of self-replicating automata. But even such labor costs something, because to support it demands resources, materials and energy, which are not free.
Smart SETI, we feel, should take account of this basic constraint.
Counting Costs
Since the early SETI era of the 1960s, microwave emission powers have increased orders of magnitude and new technologies have altered our ways of emitting very powerful signals. The highest peak power systems on Earth (peak powers over 1 GW) trade peak power for average power in order to get to a much stronger signal at distance at the lowest cost.
Fig. 1. Antenna, microwave power and total costs of Beacon with effective isotropic radiated power of 1017 watts, range 600 light years. Cost is in billions of dollars, and power in gigawatts (109 watts, the power of a large nuclear reactor). It radiates at frequency 1 GHz and has costs typical of ours today. Minimum total occurs when the antenna cost and power cost are equal.
Most of these high power devices operate in bursts of short pulses and for fundamental reasons are not extremely narrow band, having bandwidths D f/f, with f the frequency) of 0.01–1% of the beaming frequency. Economical beacons are also likely to be pulsed. Frank Drake, who started SETI in 1960, remarked in 1990, “The most rational ET signal would be a series of pulses that would be evidence of intelligent design.” This would be similar to the strategy of the lighthouse, pulsing and swinging the beam to get noticed.
To minimize cost, we wrote down the cost scaling of two major terms: the electrical power needed and antenna building cost, which depends on the antenna area. Quite generally, we found that minimum capital cost occurs when the cost is equally divided between antenna gain and radiated power. High power Earth systems show this general feature, no matter the application.
How could we send a broadcast? Arrays of antennas are the only means of producing the large radiating areas (~km2) that interstellar beacons require. They also have high reliability and degrade gracefully, as loss of a few antennas does not mean failure. Arrays are widely used in radio astronomy receiving and are being planned for the new Deep Space Network refit.
A typical case SETI broadcaster, by our calculations, has parameters that scale like in Figure 1.
This basic approach gives us many implications:
To attract attention, beam in pulses, not steadily. It’s cheaper. Steady signals are vastly more expensive.
High powers demand broadband emission. At very high voltage and currents, the electrical breakdown threshold is much higher for short pulses, so a machine of a given size can radiate much more powerfully.
Conventional SETI looks for narrowband microwaves near 1 GHz, steadily beamed. We find that on Earth, cost declines with frequency. The galactic background noise spectrum is flat between 1 GHz and 10 GHz. This is also the lowest-attenuation region of Earth’s atmosphere. The most favored spectral region is near 10 GHz, since this minimizes the cost of the beacon while imposing no increased cost on the receiver.
This is quite different from some SETI thought, which privileges the “water hole” region between 1 and 2 GHz. Indeed, the metaphorical resonance between the spectral lines of H I and OH with “meeting at the water hole” may be a classic case of anthropic reasoning. The secondary reasons given as early as the 1970s Project Cyclops—that the low end of this band demands less stringent frequency stability—vanishes if the beacon is broadband, as we argue is essential for high powers.
Since that era, detection of over 100 spectral lines in the interstellar medium, many of them organic, undermines the classic argument. Further, synchrotron radiation in the 1 GHz region increases going inward toward the galactic center, where the highest density of older stars peaks. A further benefit of higher frequencies for both beacon and receiver: Interstellar scintillation fades quickly with frequency and can be ignored around and above 10 GHz. As beacon builders we will prefer that the listener not be confused by scintillation.
Our conclusion is that cost, noise, and scintillation argue for radiating above the “water hole,” especially if space-based. In the atmosphere, the optimum will be below 10 GHz where atmospheric attenuation minimizes.
Cost-efficient beacons will be pulsed, narrowly directed, and broadband in the 1–10 GHz region, with a cost preference for the higher frequencies.
This means that SETI may be looking for the wrong kinds of signals.
Traditional SETI burdens itself with adjusting their receivers for narrow-band signals. This means they must account for Earth’s motion, and so introduce Doppler shift corrections. But at distances greater than 1,000 light-years, Doppler adjustment to offset relative motions, as nearby SETI searches do, becomes pointless; with many stars in the field of view, none is especially addressed. Further, distortion of signals from greater than1,000 light-years arises from interstellar scintillation. Such “twinkling” of the signal comes from both the dispersion of differing frequencies and delays in arrival time for pulses moving along slightly different pathways, due to refraction. Temporal broadening probably would limit bandwidth to greater than 1 MHz, as we know from the broadening of pulsar signals.
So there’s a gain from realizing that thrifty beacons will be broadband—we can ignore Doppler corrections and just look for quick, broad pulses.
Thrifty beacon systems would still be large and co
stly. They would have narrow “searchlight” beams and short “dwell times” when the beacon would be seen by an alien observer at target areas in the sky. They may revisit an area infrequently, perhaps only annually.
If this is right, what strategies should SETI change to?
Where to Look
A natural corridor to broadcast in lies along the galactic spiral’s radius or along the spiral galactic arm we are in.
To see beacons as we envision them, SETI should search in the plane of the spiral disk. From Earth, 90% of the galaxy’s stars lie within 9% of the sky’s area, in the plane and hub of the galaxy. This suggests a limited sky survey.
We will need to be patient and wait for recurring events that may arrive in intermittent bursts. Special attention should be paid to areas along the Galactic Disk where SETI searches have seen coherent signals that don’t recur in their limited listening time intervals. Since most stars lie close to the galactic plane, as viewed from Earth, occasional pulses at small angles from that plane should have priority.
Whatever forms might dwell farther in from us toward the center, they must know the basic symmetry of the spiral. This suggests the natural corridor for communication is along the spiral’s radius from Galactic Center or toward it, a simple direction known to everyone. This avenue maximizes the number of stars within a telescope’s view, especially by staring at the galactic hub.
A beacon near the center should at least broadcast outward in both directions, while societies at the far reaches may save half their cost by not emitting outward, since there is much less chance of advanced societies there. Radiating into the full disk takes far more time and power, so beams may only occasionally visit any sector of the radial plane. We listeners fairly far out (and fairly young) should look inward, within a narrow angle (about 10 degrees) toward the constellation Sagittarius. (Fig. 2)
We are newcomers. Most stars of our type lie inward and on average are about a billion years older than ours. Listening outward seems less efficient, since fewer life sites lie that way.
Fig. 2 We could see a cost-optimized beacon if it is part of a narrowly directed radial interstellar communication link. Art copyright Jon Lomberg 2009.
Life sites like ours will also know two rough time scales—a year and a day, from constraints on planetary habitable zones and biosphere mechanics. Observing every day over a year span might have a better chance of seeing intermittent bursts that revisit our part of the sky on a yearly time scale. To lower costs and have the best viewing range, sites near the equator seem optimal.
Have We Seen Beacons?
Our most important conclusion is that distant, cost-optimized beacons will appear for much less time than conventional SETI assumes. Many such signals may last for only fractions of a second.
If so, a receiver gets a short burst of pulsed microwaves and does not see it again until maybe a year later. Given the many possible local transient transmissions near a receiver (automobile spark plugs and other short-range machine time scales), a persistent signal for few seconds could be intuitively the best choice.
A beacon would linger a moment or two in our skies and be back within something like a year. No search we know could have been likely to see such an event. None checked back steadily over a year. Given the shortness of pulses in of such a strategy, perhaps cost-optimized beacons will be built to cover smaller, promising portions of the sky, and so revisit more often.
Earlier searches have seen pulsed intermittent signals resembling what we think beacons may be like, and may provide useful clues. We should observe the spots in the sky seen in previous work for hints of such activity but over yearlong periods. Perhaps newer search methods, directed at short transient signals, will be more likely to see the beacons we have described.
Have we already seen potential beacons? A provocative example is Sullivan’s survey of 1997, which lasted about 2.5 hours, with 190 1.2-minute integrations. With many repeat observations, they saw nothing that did not seem manmade. However, they “recorded intriguing, non-repeatable, narrowband signals, apparently not of manmade origin and with some degree of concentration toward the galactic plane . . .” Similar searches also saw one-time signals, not repeated. These searches had slow times to revisit or reconfirm, often days. Overall, few searches lasted more than hours, with lagging confirmation checks.
Another striking example is the “WOW” signal seen at the Ohio SETI site. Though its signal was strong, there was no electronic ability to search for a true message in this event. The check-back time was fairly long, and subsequent studies observed for short times. Further, the total time spent searching the WOW signal site, directly toward galactic center, is about 0.1% of a year. This fact illuminates the constraints that a Galactic Center Search Strategy imposes: A yearlong campaign will require more effort than SETI has employed over the last half century.
We conclude that SETI searches may have been looking for the wrong thing. SETI has largely sought signals at the lower end of the cost-optimum frequencies. They also may have taken needless care adjusting Doppler shifts, since broadband beacons will need none. Searches have seen coherent signals that are non-recurring on their limited listening time intervals. Those searches may have seen beacons, but could not verify them because they did not steadily observe over periods of years.
Transmission strategy for a distant, cost-conscious beacon may well be a rapid scan of the galactic plane, to cover the angular space. Such pulses will be infrequent events for the receiver. Such beacons built by distant advanced, wealthy societies will have very different characteristics from what SETI researchers seek. Future searches should pay special attention to areas along the Galactic Disk where SETI searches have seen coherent signals that have not recurred on the limited listening time intervals we have used so far.
Perhaps the galaxy does have many SETI beacons, but we haven’t been bright enough to see them.
References:
Our papers, with many references to these ideas, are:
http://arxiv.org/abs/0810.3966
http://arxiv.org/abs/0810.3964
http://arxiv.org/abs/1003.5938
About the Authors
James Benford is President of Microwave Sciences, which deals with high power microwave systems from conceptual designs to hardware. A Fellow of the Institute of Electrical and Electronic Engineering, he has written 135 scientific papers and six books on physics topics, including the textbook, High Power Microwaves, now in its second edition.
Gregory Benford is a professor of physics at the University of California, Irvine, working in astrophysics and plasma physics. A Fellow of the American Physical Society, his fiction has won many awards, including the Nebula Award for his novel Timescape.
Copyright © 2011 Gregory and James Benford
PROBABILITY ZERO
PROBABILITY ZERO
Small Penalties
Alastair Mayer
The helicopter cruised over the tundra at five hundred feet. In the passenger compartment, Agent Steve Grant gazed out the window. His prisoner, Samuel “Spam Lord” Walford, sat manacled to the...
Top of PROBABILITY ZERO
SCIENCE FACT READER’S DEPARTMENTS
PROBABILITY ZERO
Small Penalties
Alastair Mayer
The helicopter cruised over the tundra at five hundred feet. In the passenger compartment, Agent Steve Grant gazed out the window. His prisoner, Samuel “Spam Lord” Walford, sat manacled to the aluminum seat frame across from him. The pilot’s warning sounded in his headset.
“Ten more minutes!” Grant relayed to Walford, shouting over the noise of the chopper.
“This is cruel and unusual punishment!”
“Come on, Walford, your lawyers tried that. The Supreme Court upheld the sentence. You’re getting off easy. Five days in exile and you’re a free man.” Grant shouted to be heard; it took the sarcastic edge off his voice.
“Yeah, if I survive. It’s not fair. I didn’t hurt anyone
, just sent a few e-mails.”
“You were convicted of almost thirty million separate counts of sending unsolicited commercial e-mail. That was for just one day. That’s not ‘a few.’”
“James Atkins sent a hundred million a day. So did Koralev. I didn’t do so much.”
“Koralev got fined thirty seven million dollars . . . under the old laws. Thirty million is just what your prosecutors went with.” Grant looked out the window. The ground below was green with new spring growth, scattered with shallow pools of snowmelt. He turned back to the Spam Lord. “If I had a nickel for every thousand spams you sent during your ‘career,’ I’d be a millionaire. Oh wait, you do and you are. Or were. That’s billions of e-mails.”
“E-mail never hurt anybody. Don’t want it? Just delete it. Two seconds.”
“You stole their time. You stole everyone’s time. Two seconds per spam e-mail? That’s a lifetime per billion emails. How many lifetimes did you destroy? It’s like murder.”
“So you’re going to leave me to die.”
“No single thing out there will kill you. You can hike out in three or four days at a good pace. Plenty of daylight this time of year.”
“What about polar bears?”
“The coast is two hundred miles away. They don’t come this far inland.”
“There’s wolves.”
“There’s a paintball gun in your pack—”
“Paintball! What the hell? How about a real gun?”
“Not for a criminal. The pellets are skunk juice. Hit a wolf and, between the sting and the smell, it’ll back off.”