Search for Life in the Universe

1. Search for Life in the Universe Chapter 11 Search for Extraterrestrial Intelligence AST 248, Fall 2005

2. Outline • What is SETI Searching For? • Drake Equation • Numbers, Numbers, Numbers • Intelligence: Rare of Common? • Indicators of Intelligence • Early SETI • SETI Begins • Categories of Signals • Other Ways of Searching • SETI Today • Radio SETI • Optical SETI • And If We Detect Something? • What Could We Learn? AST 248, Fall 2005

3. Drake Equation • Equation • NHP: number of habitable planets in the Milky Way Galaxy • flife: fraction that actually have life • fciv: fraction that have a civilization at some time • fnow: fraction that have a civilization now AST 248, Fall 2005

4. Numbers, Numbers, Numbers • An equation is better than vague talk • But it is only as good as the numbers that go into it • NHP • The “best” known number • Could be as high as ~1011 • Various fractions • Wild guesses AST 248, Fall 2005

5. Intelligence: Rare or Common? • Chance, rare evolution? • At a minimum, long development time • Chance events, e.g., the Cambrian explosion and the KT impact • Convergent evolutions? • Evolution often leads to similar results, e.g., eyes evolved independently at least 8 times • Natural selection favors intelligence, cf., predator-prey dynamics AST 248, Fall 2005

6. Indicators of Intelligence • Encephalization quotient (EQ), the ratio of brain mass to average for that body mass: humans : dolphins : chimps = 7 : 4 : 2.5 • Warm blooded: faster metabolism • Extended parenting: more time to teach • Social structure: learn from the community • Agile extremities: necessary for tools • Motion on land and in water AST 248, Fall 2005

7. SETI Begins • Guglielmo Marconi (18741937) and Nikola Tesla (18561943) • Thought they detected signals from Mars • No intelligent life on Mars • Radio frequencies observed not transmitted by the ionosphere • Giuseppe Cocconi (1914) and Philip Morisson (1915) • Search in a narrow bandpass • Search around the hyperfine line of neutral hydrogen at 1420 MHz • Project Ozma by Frank Drake (1930) • Search around two nearby G stars Epsilon Eridani and Tau Ceti (distance approx 12 ly) • Two-month search yielded no results AST 248, Fall 2005

8. Categories of Signals • Local communication • With our equipment, we could detect the total television power emitted on Earth at a distance ~ 1ly • Military radar more powerful, detectable at a distance of a few tens of ly • Communication between a home world and another site • Coherent communication, but weaker than the incoherent totality of television and radar • Intentional beacon • Best chance, if it exists and we are in the beam • In 1974 we tried to send a beacon to M13 (21 kly) AST 248, Fall 2005

9. Other Ways of Searching • Artifacts left by visiting aliens • On Earth • In orbit around the Earth, particularly at the stable Lagrange points of the EarthMoon system • Astroengineering • Planetary civilizations: we are not far from that, but the emission is weak • Stellar civilizations: utilize the total radiation of the star (Dyson sphere), most of which is radiated away, but there are many natural IR radiators, so how would we tell the difference? • Galactic civilizations: too advanced relative to us, so we may not know what to look for. AST 248, Fall 2005

10. Radio SETI • Types of searches • Targeted • Sky survey: random or deliberate • Observing • Narrow bandwidth: key to detection • Limited on the biggest telescopes • Need funds for dedicated telescopes • Interference • Telecommunication satellites • Radar, primarily military • Problem worsens with time AST 248, Fall 2005

11. Current SETI Projects AST 248, Fall 2005

12. Optical SETI • Disadvantages • Absorbed by interstellar dust: “half distance” ~ 3000 ly • Needs more energy  not chosen by a civilization • Counterarguments • Plenty of stars within 3000 ly • Energy limitation mitigated by highly focused and pulsed laser beams • Lick experiment: can detect a signal aimed at us from up to 500 ly • UV, X-rays, neutrinos, gravity waves … • More difficult • No advantage AST 248, Fall 2005

13. And If We Detect Something? • Differentiating from natural emission • Narrow bandwidth • Laser light pulses • Have we detected anything? • “Wow” event, never repeated, probably terrestrial • Chances in the future • Moore’s law: 2x better electronics every 18 months • Announcement • Careful verification • Public release to scientists and governments • Consensus reply, not by individual teams AST 248, Fall 2005

14. What Could We Learn? • Can we decipher it? • Not needed to identify signal as intelligent • Information intended for us best sent by a picture • Number of pixels should be the product of two prime numbers M x N, or better yet, the square of a prime number M2 • Information per pixel should be a bit, not a byte • Can we communicate with them? • They are probably too far for practical communications • If we cannot decipher the signal? • The signal may not be intended for us • We may not be able to decipher it, even if it is intended for us • But at least we know there is intelligent life out there AST 248, Fall 2005