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AST 251 Life on Other Worlds Lecture 11 Interstellar Communication Exolinguistics SETI Programs Duy Cuong Nguyen, P.Eng. Department of Astronomy and Astrophysics University of Toronto News Flash http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/release.shtml News Flash http://www.cnn.com/2005/TECH/space/03/23/craigslist.space/ Let’s move on to something more cheerful. Interstellar Communication Introduction Intelligent Life Life that is capable of building radio telescopes The Sky from the Ground All atmosphere 03 H2O, CO2, O2 Ionosphere For astronomers, the interference is communications. For interstellar messages, the interference is astronomy. Galactic synchrotron background 21 cm Big Bang Atmospheric absorption Water Hole λ 300 cm 30 cm 3 cm Galactic Galactic Background Background (synchrotron emission) (synchrotron emission) The “Water Hole” The “Water Hole” 3m 300 µm “Where shall we meet our neighbours? At the water hole, where species have always gathered.” λ 300 cm 30 cm 3 cm 3m 300 µm Water Hole Galactic Galactic Background Background (synchrotron emission) (synchrotron emission) The “Water Hole” The “Water Hole” – Bernard Oliver 1971 NASA design study for SETI Telescope – Basic Issues Wavelength λ Frequency ν = c/λ Diameter D θ Minimum Beam Size: roughly θ = λ/D (Maximum Resolution) … attained only if mirror surface has bumps no bigger than about λ/20 Telescope – Basic Issues Receiver noise or background emission Total amount of light received increases with area πD2 Wavelength λ Frequency ν = c/λ Diameter D θ = λ/D Arecibo: D = 300 m at λ = 21cm, θ = 2 arcmin (1/8th the size of the Moon) D=300m Beam on sky Image A Needle in the Cosmic Haystack 1. Location on sky 1010 stars, or 107 (2’) Arecibo beams 2. Depth into sky Signal strength – more sensitive receivers (bigger D) can see further. 3. Frequency of radiation (ν, or λ = c/ν) Consider the AM radio band: there are 210 channels (5 kHz wide) from 550 kHz to 1.6 MHz; at 1 sec/channel, scan across dial takes 4 minutes. In the SETI search, the dial stretches from 1 GHz to 10 GHz Scan across dial takes 104 years! (at one point on sky) 4. Polarization Perfect circular polarization might indicate intelligent origin – difficult to create in Nature. 5. Time – are we watching at the right time? A Needle in the Cosmic Haystack 1. Location on sky 1010 stars, or 107 (2’) Arecibo beams 2. Depth into sky Signal strength – more sensitive receivers (bigger D) can see further. 3. Frequency of radiation (ν, or λ = c/ν) Consider the AM radio band: there are 210 channels (5 kHz wide) from 550 kHz to 1.6 MHz; at 1 sec/channel, scan across dial takes 4 minutes. In the SETI search, the dial stretches from 1 GHz to 10 GHz Scan across dial takes 104 years! (at one point on sky) Big Problem! 4. Polarization Perfect circular polarization might indicate intelligent origin – difficult to create in Nature. 5. Time – are we watching at the right time? Search early, search often. Strategies for dealing with frequency uncertainties 1. Search “Magic Frequencies” that any intelligent species should know Molecule ν (GHz) λ (cm) H 1.4 21 OH 1.7 18 H2CO 4.8 6.2 µ+e- (muonium) 4.7 6 3He+ 8.7 3.5 CH3CH2OH (ethanol) 103 0.29 CO 115 0.26 e+e(positronium) 203 0.15 Perhaps the most important transition in radio astronomy: spin-flip transition of electron within an H atom. These transitions are absent or extremely rare in Nature. One should also consider, say, these frequencies times or divided by π or e, say… 2. Build detectors that can scan many frequencies at once BETA search (Harvard/Planetary Inst.): 8 million channel receiver Cuts search time down to 11 hours per beam, from 104 years. Strategies for dealing with frequency uncertainties Molecule ν (GHz) λ (cm) H 1.4 21 OH 1.7 18 H2CO 4.8 6.2 µ+e- (muonium) 4.7 6 3He+ 8.7 3.5 CH3CH2OH (ethanol) 103 0.29 CO 115 0.26 e+e(positronium) 203 0.15 Perhaps the most important transition in radio astronomy: spin-flip transition of electron within an H atom. What about Lucy? Humans have been broadcasting radio and television for about 50 years. Traveling at the speed of light, these signals have gone out 50 light years = 15 parsecs, passing about 500 stars along the way. A few of these, like ε Eridani and τ Ceti, are similar to the Sun. In fact, ε Eridani (a K star 10 light years away) has a planet much like Jupiter in a 3-AU orbit! It was one of the targets in the first SETI search by Frank Drake. Stars less than 25 light years (7.5 pc) away have had time to reply. Eavesdropping W.T. Sullivan, S. Brown, & C. Weatherill, Science January 1978 ( undergraduates! ) What could be determined from a nearby star about civilization on Earth? Civilizations on nearby stars will see a rapid rise in the amount of radio emission and will probably pay attention… Eavesdropping W.T. Sullivan, S, Brown, & C. Weatherill, Science January 1978 What’s the best indicator of Earthly civilization? Strongest signals: Military radar. Problem: not constant; frequency switched to evade jamming. (& not good ambassadors of goodwill.) FM Radio: Signal comes out in too broad a band AM Radio: Doesn’t get out of the ionosphere Best signal: Television carrier signal On (almost) constantly. Very narrow bandwidth. Eavesdropping How far away can it be detected? Arecibo-type detector: TV carrier detectable at 25 light years. What can you tell? Use the horizontal beam pattern of a television broadcast tower… signals get stronger as they cross the horizon as seen from afar. Eavesdropping From the pattern of red and blue shifts, you can tell In 1 day: the rate at which Earth rotates about its axis ... and more… In 1 year: the rate at which it orbits the Sun the orbital parameters of the Earth that the Earth is in the Habitable Zone of the Sun Eavesdropping In the pattern of Doppler shifts and the times of rising and setting of each transmitter, you can make a map of the planet If you pay attention to the timing and type of broadcasts, you can tell something about the number of broadcasting cultures… Eavesdropping In addition, You can tell the size of the radio towers from the width of their broadcast cones. You can tell something about the weather on Earth from the wobbling of the towers. You can tell something about… the power available to human civilization the sophistication of our broadcast technology And finally, if you want, you can try to decode the signals themselves… How far do we have to look? Simple model for galaxy: 1011 stars Radius: 1 5 kpc Width: 500 pc Volume of galaxy: π(15 kpc)2x(500 pc) = 3x1011 pc3 You are here. Volume per star: 3x1011 pc3 / 1011 stars = 3 pc3/ star … implying a typical spacing of about 1.4 pc between stars. How far do we have to look? Important issue: Disk geometry of galaxy creates some diminishing returns. 1. Search radius less than width of Milky Way (500 pc/2 = 250 pc) (N* < 20 million) N* increases as Rsearch3 2. Search must include more than 20 million stars. Now N* increases as Rsearch2, because the flattening of the Galaxy disk is apparent. Do we have a chance? Recall the Drake Equation as of 1961: the number of civilizations equals the lifetime of a civilization (in years). N ~ L / yr If that were true, How big would L have to be such that a “typical” civilization has time to find another one? ch ar se < 20 m on ill i *s Time enough for detection Time enough for 2-way communication se ar About 2000 years ch > 20 m illi o n* s Do we have a chance? If we can survive for more than 2 kyr, we have a chance. If we have a chance, then our current technology must look infantile to the typical civilization. (Our age is <1/40th of the typical age.) So it’s a safe bet that we are either alone or in the presence of civilizations far more advanced than ourselves. This affects the search strategy somewhat… 1. We must stay alive longer than 2 kyr. 2. We have fewer than 1010/2000 = 5 million stars to search. At some point we’ll have to start searching at least 2500 per year. 3. We can hope that these advanced aliens are broadcasting for us… “The probability of success is difficult to assess, but if we never search the probability is zero.” – Morrison & Cocconi SETI’s history so far N. Tesla, G. Marconi – both thought they heard ET signals. 1924: D.P. Todd – arranged radio silence with U.S. Military during Mars’ closest approach 1959: P. Morrison & G. Cocconi – Propose a search of the radio sky for extraterrestrials. 1971: NASA commissions a design study for the ultimate SETI receiver: Project Cyclops Arecibo: 0.3 km2 Square Kilometer Array: 1 km2 Cyclops idea: 3 km2 1979-1993: SETI search conducted in Soviet Union and in North America 1980, 1981: Pessimism (arguments by Hart & Tipler). Soviet search terminated. 1993: Congress cuts all NASA funding to the “Great Martian Chase” [Sen. R.Bryan] Jill Tarter starts the SETI Foundation and Institute. “Project Phoenix” begins; Berkeley’s SERENDIP project and others continue… Project Phoenix • Privately funded • Does not scan the entire sky • Listens to sun-like systems most likely to host long-lived planets capable of supporting life • Searches 1 GHz to 3 GHz signals with a resolution of 1 Hz Optical SETI • Proposed in 1961 • Current laser technology allows for beams with 5,000x brightness of the Sun • Less confusion with noise • Better beam focus • Higher bandwidth SETI@Home • • • • http://setiathome.ssl.berkeley.edu Distributed computing Piggy-backs off of observations made at the Arecibo Observatory Searches 21-cm line (1 418.75 MHz to 1 421.25 MHz) SETI@Home SETI@Home Amateur SETI Hydrogen Emission Lines along the Galactic Plane Intelligent Communication Exolinguistics How do you communicate with an alien? Note: Babies solve this problem all the time. Usual strategy (in a few broadcasts from Earth to star clusters): 1. Modulate the beam to send out a stream of bits (radio dashes and dots). 2. Repeat the stream so that it’s obvious how many bits are in the message. 3. Make the # of bits equal the product of two (or three?) prime numbers. 4. Prepare a message that can be viewed as a message if the bits are arranged as “pixels” in an image… 5. Try to ensure that there is only one way that the pixels of the image can be interpreted, given the facts of life in the Universe. - Signal wavelength, frequency: standard of length and time - Other common points of knowledge: chemistry, astronomy, physics 6. Pack as much information into it as you can without making it illegible. Exolinguistics This message was is similar to one sent in 1969 by Frank Drake from the Green Bank Telescope to the star cluster M13, 25,000 light years away. But this message is a fake signal Drake devised to test how well we could decode one like his. It is 551 = 19x23 bits long. It has five sections that each carry information. What do you think they mean? Exolinguistics 1. The exoplanets Host star four small (terrestrial?) planets Intermediate planet? Asteroid belt? Big planet – like Jupiter? Another? Like Neptune? Like Pluto? Exolinguistics 2. Important Elements for their biochemistry C: 4 valence electrons O: 6 valence electrons Exolinguistics 3. Numbers Exolinguistics 4. Population of planets? Apparently they’ve started to colonize their system… they’re just slightly ahead of us. (Miraculously.) Exolinguistics 5. Size and shape of the aliens. Proactive SETI… Radio Transmissions Frank Drake’s Message Globular Cluster M13 Automated Spacecraft Pioneer 10 & 11 Plaque Voyager 1 & 2 Voyager Golden Record Voyager Golden Record • • • • Greetings in 55 languages UN Greetings Whale Greetings Sounds of Earth •Music of the Spheres •Volcanoes, Earthquakes, Thunder •Mud Pots •Wind, Rain, Surf •Cricket (Teleogryllus Oceanicus) / Crickets, Frogs •Birds, Hyena, Elephant •Chimpanzee •Wild Dog •Footsteps, Heartbeats, Laughter •Fire, Speech •The First Tools •Tame Dog •Herding Sheep, Blacksmith Shop, Sawing, Tractor and Riveter •Morse Code •Ships, Horse and Cart, Train, Truck, Tractor, Bus, Automobile, F-111 Flyby, Saturn 5 Lift-off •Kiss •Mother and Child •Life Signs •Pulsar Voyager Golden Record • Sounds of Earth •J.S.Bach Brandenburg Concerto No. 2 in F, First Movement •(Java) Kinds of Flowers •(Senegal) Tchenhoukoumen •(Zaire) Pygmy Girls’ Initiation Song •(Australia) Morning Star and Devil Bird •(Mexico) El Cascabel •Johnny B. Goode •(Papau New Guinea) Men’s House Song •(Japan) Cranes in Their Nest •J.S.Bach Gavotte en Rondeaux from the Partita No. 3 in E Major for Violin •W.A.Mozart The Magic Flute, Queen of the Night aria, No. 14 •(Georgia) Tchakrulo •(Peru) Panpipes and Drum Song •Melancholy Blues •(Azerbaijian) Ugam •Stravinsky Rite of Spring, Sacrificial Dance •J.S.Bach The Well-Tempered Clavier, Book 2, Prelude and Fugue in C, No. 1 •Beethoven Symphony No. 5 in C Minor, First Movement •(Bulgaria) Izlel je Delyo Hagdutin •(United States) Najavo Night Chant •Holborne Fairie Round, from Paueans, Galliards, Almains and Other Short Aeirs •(Solomon Islands) Melanesian Panpipes •(Peru) Wedding Song •(China) Flowing Streams •(India) Jaat Kahan Ho • Dark Was the Night •Beethoven String Quartet No. 13 in B flat, Opus 130, Cavatina About Us Basic Math Basic Units Basic Astronomy Basic Chemistry Basic Biology Vertebrate Evolution Human Biology Human Development Human Education Human Society Human Society Human Science Human Science Human Art Human Progress Human Progress Caution Caution In Conclusion AST 251 Life on Other Worlds Next Time: Interstellar Travel Galactic Colonization End of AST 251 Lecture 11