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Search for Life in the Universe

Chapter 11

Search for Extraterrestrial Intelligence

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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?

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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

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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

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Intelligence: Rare or Common?

• Chance, rare evolution?– At a minimum, long development time– Chance events, e.g., the Cambrian explosion

and the KT 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

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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

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SETI Begins

• Guglielmo Marconi (18741937) and Nikola Tesla (18561943)– 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

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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)

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Other Ways of Searching

• Artifacts left by visiting aliens– On Earth– In orbit around the Earth, particularly at the stable

Lagrange points of the EarthMoon 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.

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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

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Current SETI ProjectsProject Teles-

copeSearch Type

# of Channels

Channel Width

Total Band-width

Power @ 100 ly (100 m antenna)

Phoenix Arecibo 305 m

Targeted 56 x 106 1 Hz 1200 3000 MHz

105 W

SEREN-DIP

Arecibo 305 m

Sky Survey

168 x 106 0.6 Hz 1370 1470 MHz

106 W

Southern SEREN-DIP

Parkes 64 m

Sky Survey

58 x 106 0.6 Hz 1418.0 1420.5 MHz

106 W

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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

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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

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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