STRATEGIES FOR COMMUNICATION WITH EXTRATERRESTRIAL CIVILIZATIONSCommunication is much less expensive than star travel. But still very problematic.Many choices to be made for “listening” strategy. We will discuss bandwidth, number ofchannels, sensitivity, etc. in class. Use “tuning the radio knob” analogy (discussed in class) toremember the problem.WHERE TO POINT?a. Search whole sky with low sensitivity -- could only detect strong signals.b. Point at the nearest (less than about 50 to 100 l.y.) stars roughly like the sun and cooler(recall conditions for habitable planets). Could detect weaker signals, i.e. would have highersensitivity.c. Supernova announcement strategy (not on exam) -- point "downstream" fromsupernova if transmitting, or toward supernova if listening. Use supernovae partly because othercivilizations are likely to be observing it. But this only gets civilizations along the line of sight,so low-probability.SN 1987A in LMC: Messages should have arrived a few years ago.WHAT FREQUENCY TO "LISTEN" TO OR SEND ON?Interstellar dust selectively blocks shorter wavelengths (higher frequencies), whichstrongly suggests we use the IR or radio parts of the spectrum (long wavelengths or smallfrequencies).For this reason we will discuss mainly radio searches; your text and Evans Ch. 9 havegood discussions of this material, especially current searches, and your text has a good discussionof “Optical SETI” searches which are a viable alternative.The other main consideration is noise: we should listen (or send) where the noise isminimized. (Figure discussed in class; surprisingly, not in your text! But in Evans Ch.9 reading)a. Cosmic background radiation (3 K primeval cosmic background) -- important at lowfrequencies.b. Receiver (quantum) noise -- increases at high frequencies.c. Galactic noise (e.g. synchrotron radiation from SNRs) -- increases at small frequencies.d. Earth's atmospheric noise. H2O, O2 emission bands, ...Comes in above 10 GHz; but unimportant if broadcasting or listening from space.This leaves frequencies between 1 and 10 GHz.But the signal is likely to be in one or more narrow frequency bands (to get the strongestsignal). Which frequency? Suggested “beacon frequencies” (or “hailing frequencies” or “magicfrequencies”):*HI (neutral hydrogen) 21cm (wavelength) line? Natural, abundant, but lots ofinterference by interstellar gas.*OH line at 1.7 GHz?*Inbetween H and OH? ("The Waterhole").*Some frequency based on fundamental constants of nature?*”Intergalactic” frequency standard based on temperature of cosmic background radiation?Many others have been suggested. Or should we be searching in the infrared? >> "Dyson spheres" ("Type IIcivilization" based on energy utilization; consider how anthropomorphic, and so typical ofWestern science and consumer culture, it is to think that "advancement" must mean "being able touse more energy".) Students should understand why a Dyson sphere would give a characteristicsignature in the spectrum of a star in a certain wavelength region. Text has good discussion ofthis, but fails to mention that many astronomical objects (e.g. old stars with circumstellar dustshells, dust in disks around protostars) have this signature, so there is no good way to distinguishDyson spheres from natural phenomena. How about “leakage radiation?” This was discussed in detail in class. Your textpoints out that signal would be very weak because of spreading and inverse-square law of light,but we discussed why it might also be unrecognizable as a signal, mostly because of theconfusion problem of so many radio and television stations broadcasting at once. Studentsshould understand what the signature of leakage radiation would look like as a pattern in time. Some modern plans for searches concentrate not on detecting a signal that is a message,but a signal that is a sign of advanced technology. An example would be gamma rays given off bynuclear fusion reactions if they had starships that could utilize controlled nuclear fusion. What about signals from alien probes that are emitting signals (or leakage!) in our ownsolar system? Your text describes why searching at the “Lagrange points” might be a goodstrategy in this case.Homework: Locate by an internet search a proposal to search for solar system probesignals or leakage by Bruce Cornet and Scot Stride from JPL using the Allen Telescope Array.Hint: A good interview with them is in the web magazine Astrobiology Magazine; a link to it isat the class web site. What objects to they propose to monitor? What seems to be the greatestuncertainty in the proposed search?CODESHow to recognize an alien signal, or to send one that aliens would recognize? Mostpeople think the problem is to devise a code that any other intelligence can decode.The simplest and most efficient way to encode a message (we think) is binary code. Useonly 2 characters, e.g. a 1 and a 0, or a + and a - , or "on" and "off", ... Each 1 or 0 (or whatever)is a "bit". Then the message can just be sent as a series of pulses.Party line: expect the message to be a two-dimensional picture that is encoded in a one-dimensional binary string that factors into prime numbers . e.g. 551 = 29 x 19 (or 19 x 29); 1679 = 23 x 73 (the 1974 Arecibo transmission).Example: We receive signal 1111100000101011010110101.This factors into 5 x 5, giving a picture of the greek letter "pi".Or try the letter "E", etc.But why would ETI send out signals that anyone could decode? Perhaps they send outsignals which could be understood only by others who are already "at the same level" as they are.What would be a difficult signal for us to recognize? Example: recognizing a deterministic,high-dimensional (complex) pattern mixed with "noise" (probabilistic, stochastic, randomprocess). Perhaps the test would be to recognize some sort of "meaning" in the message. (Thinkabout musical signals. At present, there is no viable theory of musical meaning in music analysis,philosophy, cognitive science, pattern recognition, or any other field that has approached theproblem. There is lots of (earth) music that most of us wouldn't even recognize as such [e.g.serial music of western composers (popular in 1950s-1970s), some music of other cultures])A deeper examination of the issues involved here require a discussion of the nature ofhuman languages. There are many important questions that are unanswered in
View Full Document
Unlocking...