Transcript Search for Life in the Universe

Search for Life in the Universe
Chapter 12 (Part 1)
Search for Extraterrestrial
Intelligence
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Outline
• What is SETI Searching For?
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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
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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 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
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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 (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
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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 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.
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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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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
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