Search for Life in the Universe

1. Search for Life in the Universe Chapter 12 (Part 1) Search for Extraterrestrial Intelligence AST 248, Spring 2007

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, Spring 2007

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, Spring 2007

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, Spring 2007

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, Spring 2007

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, Spring 2007

7. AST 248, Spring 2007

8. AST 248, Spring 2007

9. 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, Spring 2007

10. 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, Spring 2007

11. 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, Spring 2007

12. 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, Spring 2007

13. AST 248, Spring 2007

14. AST 248, Spring 2007

15. AST 248, Spring 2007

16. AST 248, Spring 2007

17. AST 248, Spring 2007

18. AST 248, Spring 2007

19. AST 248, Spring 2007

20. 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, Spring 2007

21. AST 248, Spring 2007

22. 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, Spring 2007

23. 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, Spring 2007