12a

Search for Life in the Universe: 

Search for Life in the Universe Chapter 12 (Part 1) Search for Extraterrestrial Intelligence

Outline: 

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?

Drake Equation: 

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

Numbers, Numbers, Numbers: 

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

Intelligence: Rare or Common?: 

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

Indicators of Intelligence: 

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

SETI Begins: 

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

Categories of Signals: 

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)

Other Ways of Searching: 

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.

Radio SETI: 

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

Optical SETI: 

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

And If We Detect Something?: 

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

What Could We Learn?: 

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