Download Lecture 39: Life in the Universe The Main Point Simple Life vs

Lecture 39: Life in the Universe
The Main Point
• Life in the Universe
– Extrapolating from our solar system experience...
While simple forms of life could be expected to be
relatively common in the Universe, the odds of
finding intelligent life “out there” are probably much
lower. Still, we are compelled to search…
• The Search for Extraterrestrial Intelligence (SETI)
– Is anyone else out there?
– How can we find out?
– What would it mean?
• Reading: Chapters 24.424.4-24.5
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Simple Life vs. Complex Life...
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Why Care About Extraterrestrials?
• Pros:
• Simple organic molecules are abundant in the
solar system and beyond
• Simple, bacteria-like life forms dominate life on
Earth (Lecture 37)
• Focus of Mars and Europa exploration is on
uncovering evidence for simple life forms
• So why, then, should we even consider extending
the search towards more complex life forms, and
ultimately ones that are intelligent?
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– Increased awareness of our place in the Cosmos
– Answers the question "Are We Alone?"
– Practical knowledge gained over time?
• Cons:
– "Common wisdom" that complex life is rare
– Two-way communication takes too long
– We have enough trouble dealing with each other and
other species on this planet...
– Searching for them costs money
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But we already care...
The Science of “SETI”
• The Search for Extraterrestrial Intelligence (SETI)
can be addressed using the methods of science
• But the search takes resources like money, people,
time, equipment, ...
• How much should we devote to this search??
• Fundamentally, the answer depends on society’s
and individual people’s balance between looking
inward and looking outward
• Science (fiction) sells!
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Books
Games
Movies
TV
Magazines
Web
NASA
...
Billions
of
$$$$
industries
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Is SETI even practical
to think about?
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– The Sun is a common type of star, and there are
probably > 100 billion Sun-like stars in the Galaxy
– Planetary formation appears to be a common process
around single Sun-like stars (theory and observations)
– So: do the math:
• 100 billion stars • 30% in single-star systems • 10 planets/star
• = 300 billion planets!
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Some SETI math...
• Do we have valid reasons to believe that anyone
else is out there?
• Perhaps...
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But how many of these putative planets contain life?
Is our solar system typical? If yes: 10%
Is life on Earth a freak accident? If yes: ~ 0%
So out of ~300 billion planets:
– could be from just 1 to >30 billion in our Galaxy with life
• But how many planets have intelligent life?
• Is that intelligent life technological?
• And just how good are these kinds of estimates???
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The Drake Equation
Estimating N...
• Astronomer Frank Drake's attempt to
quantitatively estimate SETI's potential success
Factor
Optimistic Pessimistic Best Estimate
R*: Star formation rate
50
20
30
fS: Fraction of Sun-like stars
1
1/15
0.3
Np: Number of planets per star
20
5
10
fe: Fraction of habitable planets
1
1/1,000,000
1/40
fL: Fraction of those with life
1
1/1,000,000
0.5*
fi: Fraction with intelligent life
1
1/1,000
0.75*
L: Lifetime of that civilization
?
?
?
= 0.85L
From Goldsmith & Owen (1992)
*Wild Guess
N = R* × fs × Np × fe × fL × fi × L
• Where:
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N = Number of civilizations in the Galaxy capable of communicating with us
R* = The rate of star formation in the Galaxy (stars/year)
fs = fraction of stars that are Sun-like
Np = number of planets per star
fe = fraction of "environmentally correct" planets
fL = fraction of planets where life develops
fi = fraction where intelligent & technological life develops
L = lifetime of an intelligent & technological civilization (years)
N = R* × fs × Np × fe × fL × fi × L
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For reasonable estimates, N ≈ L
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Caveats...
Evidence for intelligent life on Earth
The Drake Equation is an estimate and not a true scientific derivation
• We know there is intelligent life here...
• Could we be detected by extraterrestrials?
– Call it "back of the envelope" or "handwaving"
•
There is much uncertainty in the various factors in the equation!
•
The general relationship that N~ L implies that:
– Optimism about L implies optimism about N
– Optimism about L implies optimism about SETI
For us, L ~ 100 years (so far) as a transmitting and receiving species
The number of civilizations in the galaxy is closely related to:
– (N = 1 to 1,000,000 ! )
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– (a) how long a technological species survives
– (b) how long any single technological phase lasts
– (c) the duration of technological phases that are compatible with our
current search methods
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– Directly?
• Images
• Electromagnetic "leakage" (radio, TV, military, ...)
– Indirectly?
• O2 detected in Earth's atmosphere
• Pollutants, man-made chemicals in the atmosphere?
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Views of Earth as seen from a lowresolution orbiting satellite platform
(tens of km resolution)
Views of Earth as it
could be seen from an
extra-terrestrial
telescope (hundreds of
km resolution)
(Top): Global view of
Earth in visible light (left),
infrared light (middle), and
within a water vapor
absorption band (right)
Views of Earth as seen from a higherresolution orbiting satellite platform
(a few km resolution)
(Right): Global infrared
view of Earth from the
Galileo spacecraft during
an Earth gravity assist
flyby
No unambiguous signs of
intelligent life at these scales...
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Spectra of
Earthlike Planets
Views of Earth as seen from a very high
resolution orbiting satellite platform
(a few tens of meters resolution)
Views of Earth as seen from an ultra high
resolution orbiting satellite or an airborne
platform (a few meters resolution)
• Greater sensitivity to Earth-like
"disequilibrium" atmospheres can be
obtained by measuring the spectra of
extrasolar planets
• Hard! Requires TPF or beyond...
We start to see regular, geometric
patterns (cities, farms, other
structures) at these scales
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Some "messages" already sent...
SETI Today
• SETI today is primarily
• A few directed radio telescope messages
• Pioneer, Voyager plaques and records
– Passive: Listening at "magic" frequencies
• Assume "they" know physics and math
1974 Arecibo
radio signal
directed at M13,
21,000 l.y. away
– Targeted: to nearest Sun-like stars
• Highest probability of Earthlike worlds?
– Privately funded: little NASA, government $
• No Congressional support for "little green men"
• Lots of public skepticism...
Pioneer Plaque
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Voyager Record
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Main Parts of Carl Sagan's
Baloney Detection Kit
Healthy skepticism is good!
• Much of what we have seen and learned this
semester is based on theory and/or interpretation
• Some of what we've "learned" is no doubt wrong!
• Science is a self-effacing, self-correcting,
incremental, and ultimately human endeavor
• Be critical and skeptical of "facts"!!
• Carl Sagan, in his book "The Demon Haunted
World", advocates that everyone carry around a
baloney detection kit to help maintain skepticism
http://setiathome.ssl.berkeley.edu/
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Wherever possible there must be independent confirmation of the facts.
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Encourage substantive debate on the evidence by knowledgeable proponents of all
points of view.
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Arguments from authority carry little weight (in science there are no "authorities").
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Spin more than one hypothesis - don't simply run with the first idea that caught your
fancy.
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Try not to get overly attached to a hypothesis just because it's yours.
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Quantify, wherever possible.
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If there is a chain of argument every link in the chain must work.
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Occam's razor - if there are two hypothesis that explain the data equally well choose the
simpler.
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Ask whether the hypothesis can, at least in principle, be falsified (shown to be false by
some unambiguous test). In other words, it is testable? Can others duplicate the
experiment and get the same result?
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Reminder: Final Exam! (2 hour exam)
Some Final Thoughts...
• Friday May 9, 2-4 p.m., Barton Hall
• Astronomy matters!
– Daily stuff like navigation and timekeeping
– But also plenty of “Big Picture” stuff
– And we also learn about our planet by studying others
• Climate changes
• Impact hazards
• Origin and evolution of life
– By learning and exploring, we grow as individuals and as a species
• You are now armed for some awesome party conversations!
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Closed book, independent work
Final exam approximately twice as long as prelims
Concepts from entire course emphasized
Some additional emphasis on Part IV of course
Study practice exams and old exams posted online
Review chapter summaries
Review the problems at ends of chapters
Review your lecture notes and section notes
Review your homework assignments
Bring #2 pencils
Calculators or other electronic devices NOT allowed
No bathroom breaks for the first hour
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