Transcript Slide 1

Week 13: Discovering SETI
The Drake List
Intelligence
SETI
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The Drake List (aka “the Drake Equation”)
Simplified math can often derive sophisticated results.
Example: How many undergraduates are at Harvard?
1) How many candidate undergraduates apply per year? ~ 24,000
2) How many of these candidates are accepted? ~ 7%
3) How long do students typically stay? ~ 4 years
24,000 applicants/year × 0.07 acceptances/applicant × 4 years = 6720 students
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The Drake List (aka “the Drake Equation”)
An interesting exercise from 1961 that helps, in concept:
— Calculate the number of advanced civilizations in the galaxy;
— Calculate the distance to the nearest extraterrestrial civilization.
In practice, it mostly…
— Encourages other sciences to dismiss astrobiology as fluff!
Three important considerations are included in this equation:
1) How rapidly candidate stars are being formed;
2) How many of these candidate stars could give rise to actual civilizations;
3) How long the civilizations persist.
(Yes, just like the Harvard calculation.)
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Input factors for the Drake list
How many civilizations are in the galaxy?
1.
2.
3.
4.
5.
6.
7.
What is the rate at which stars form? (R*, stars/year)
What fraction of stars have adequate life-spans? (FT)
What fraction of stars could have habitable planets? (FP)
How many habitable planets occur around each star? (NP)
What fraction of habitable planets develop life? (FL)
What fraction of planets with life-forms develop intelligent life-forms? (FInt)
What fraction of intelligent life-forms develop civilizations capable of
interstellar communication? (FComm)
8. How long does each planet
typically maintain such
civilizations? (L)
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Constructing the Drake equation
1. What is the rate at which stars form?
Galactic star formation is a subject of
considerable research efforts around the world.
Details differ, but to a general order of
magnitude, scientists feel that about 10-20 new
stars reach the main sequence each year.
R*=10-20 stars/year
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Constructing the Drake equation
2. What fraction of stars have adequate
lifespans?
Ruling out O, B, A, and F stars leaves about
97% of all stars.
FT = 0.97
Formation rate of suitable stars = R* × FT
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Constructing the Drake equation
3.
What fraction of stars might have habitable planets?
FP = ?? (~0 for the rare Earth hypothesis)
Kepler results suggest FP = 1.4-2.7% for sunlike stars
Formation rate of habitable star systems= (R* × FT) × FP
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Constructing the Drake equation
4.
How many habitable planets occur around each star?
For the Sun, NP = 1-2.
It will be several years before we can have reasonable information
about other star systems. Should we follow the Rare Earth
Hypothesis, or the Copernican Principle?
Since the majority of stars formed are K and M stars with tiny
habitable zones, might we infer that there should be room for very
few planets in the habitable zone?
Formation rate of habitable planets = (R* × FT) × FP × NP
This is similar to your book’s term “NHP”, which is the total number
of habitable planets currently formed.
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Constructing the Drake equation
5. What fraction of planets develop life?
How difficult is it for life to develop? Are the
conditions so specific that life almost never
develops?
Note that life developed relatively soon after the
planet became habitable (after the cessation of the
late heavy bombardment). This suggests that life
develops rapidly, i.e., relatively easily.
Furthermore, the chemical ingredients of life are
widespread through the universe, and readily
synthesized into more advanced compounds, as
demonstrated by the Miller-Urey experiment.
FLife = 1.0?
Formation rate of planets with life = (R* × FT × FP × NP) × FLife
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Constructing the Drake equation
6. What fraction of planets develop intelligent life-forms?
Treating the Cenozoic era (post K-T boundary) primates as one lineage, we
might wish to include cetaceans (whales and dolphins) in the ranks of intelligent
life. So, on our planet, we know intelligent life developed not once, but twice.
This suggests that intelligence occurs relatively frequently?
FInt = ?
Formation rate of planets with intelligent life =
(R* × FT × FP × NP × FLife ) × FInt
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Constructing the Drake equation
7. What fraction of intelligent life-forms develop civilizations
capable of interstellar communication?
Is such a step possible for fully aquatic organisms? If not, should cetaceans be
removed from the list?
FComm = ?
Formation rate of planets with technological civilizations =
(R* × FT × FP × NP × FLife ) × (FInt × Fcomm)
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Constructing the Drake equation
8.
How long does each civilization persist?
Perhaps the most variable and unknown of all the components of the Drake
List of parameters.
If advanced species go extinct
because they self-destruct, or
because of the environment, this
results in a decrease in the overall
number of species in the galaxy at
any time.
Vertebrate species typically survive for about 2 million years
L=2×106 yr
If advanced species overcome the obstacles to survival imposed upon them by
their environments, survival time is the age of the galaxy:
L=109 yr
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The Drake equation
Lets use the equation to determine the number of advanced,
communicating civilizations in the galaxy:
N = R* FT FP NP FLife FInt FCommL
R* 
FT 
FP 
NP 
FLife 
FInt 
FComm 
L
star formation rate
stars with adequate life spans
stars with planets
# of planets per system
planets that develop life
planets that develop intelligent life
planets with interstellar communications
survival lifetimes
— Or, as in your book—
N.B. See “Cosmic
Calculation 12.1”
to see how to
convert N to the
distance to our
nearest neighbor!
N = (R* FT FP NP × L) × (FLife) × (FInt FComm))
=
NHP
× FLife ×
FCiv
× FNow
Alas, N is what you want it to be! We are no closer to having N, than
we were when Drake developed the equation in 1961!
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FInt: Intelligence in the galaxy
Intelligence is important enough of a concept for us to examine this
more closely.
First, how do we measure intelligence?
“IQ” (intelligence quotient) was originally defined as a fraction, i.e., a
person’s “mental age” divided by their chronological age, times 100.
Now standardized IQ tests are created, with a difficulty level so that
on average people score 100, and 1σ = 15 points. Therefore, 67% of
people have an IQ of 85-115 points.
IQ tests are controversial, and may reflect cultural biases such as
social status, education, health care, etc.
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FInt: Intelligence in the galaxy
IQ tests cannot be given to other species, so we use “Encephalization quotient”
(EQ) instead. It is the average ratio of brain mass to body mass.
In general, bigger bodies require bigger brains to run them. But some organisms
have a larger-than-necessary brains. They are the smarty-pants organisms that
would contribute to FInt.
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FInt: Intelligence in the galaxy
If we think that intelligence is a common adaptation by life forms in
our galaxy, it must have an evolutionary reason to develop.
Hypothesis
Intelligence has an evolutionary advantage.
Predictions
We should observe intelligence being favored by evolutionary
selection pressures;
We should see the evolution of other intelligent species (i.e.,
intelligence is a strategy appearing multiple times, in convergent
evolutionary pathways;
We should see that intelligent species have a competitive
advantage over non-intelligent species.
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Intelligence as a strategy for survival
Do we see intelligent species evolving often?
There are not many smart animals. That doesn’t necessarily
mean that intelligence is not an advantageous adaptation—rather
it just might simply be limited in application.
If you are a grass-eating herd animal, you don’t have to be smart.
You just have to be able to eat, and evade predators long enough
to reproduce.
There are some surprisingly smart animals, such as corvids (i.e.,
crows and ravens).
However, intelligence seems to be a relatively uncommon
evolutionary approach. Is intelligence rare and unlikely?
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Intelligence as a strategy for survival
What factors favor intelligence?
While we do see a few cases in which the EQ of animal lineages
have increased in time, it seems to be a relatively uncommon
evolutionary approach.
Cetacean encephalization
(EQ=2-5) seems to have been
associated with the development
of sonar in toothed whales.
Similarly, human encephalization
(EQ=7) may have been
associated with the development
of verbal language.
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Intelligence as a strategy for survival
Why our large brains?
Taipans are astonishingly venomous, with neurotoxins and other toxic agents.
A single bite injects enough venom to easily kill a human several times over.
Why did taipans evolve such a toxic bite in a land without very large
mammals?
Similarly, humans seem to be far smarter than is needed from an evolutionary
perspective. Was it highly fortuitous humans just managed to evolve such an
unnecessarily large EQ?
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SETI—what is being sought?
1. Local communication signals used by aliens?
Probably very weak, nearly undetectable signals. Note that in human
civilization, our TV broadcasts have switched from 30MW stations to 20W
satellite direct-to-TV transmissions. We are going into stealth mode.
2. Communication between worlds?
Probably weak for communications between nearby planets—only distant
planets would have strong communication signals.
Communications between distant planets would probably be highly
directional, and unlikely to be detected.
3. Intentional signal beacons.
This third type of signal is what would be most likely to be observed and
understood, as it would be designed for our detecting it.
Optical or radio?
SETI has traditionally been conducted in radio wavelengths. Recently, some
SETI work has been conducted in optical wavelengths, i.e., seeking flashes of
laser light. An advantage is that there could be no interference from terrestrial
sources. A gregarious civilization would be visible with our naked eyes!
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SETI experiments: first attempts
1899
Nikola Tesla (who developed AC) thought he detected alien
broadcasts but these were later found to be whistlers (caused by
lightning) and possibly Jupiter’s magnetosphere in 1899.
“The thought flashed through my mind that the
disturbances I had observed might be due to intelligent
control…. The feeling is constantly growing on me that
I had been the first to hear the greeting of one planet to
another.”
Guglielmo Marconi (developed the radio telegraph) detected similar
false signals.
1924
The US army tried to detect aliens during a close approach to Mars.
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SETI experiments: early years
Green Bank, 1960:
Frank Drake used the 26m radio scope in Green Bank, West
Virginia, to look for 1420 MHz signals in two star systems. This
was called Project Ozma.
Continued in 1970s with “project Ozpa” and “Ozma II.”
Ohio State University, 1973-1998:
The “Big Ear” detected something on 15 August 1977,
“6EQUJ5”, the “WOW signal,” which lasted 37 seconds and
rated 30 sigma.
37 seconds was an integration time, and it was missing on a
return scan, only several minutes later.
RA= 19h 25m 31s ± 10s or 19h 28m 22s ± 10s,
Declination= −26°57′ ± 20′ ; Sagittarius
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SETI experiments: NASA
Two NASA searches began in 1992:
Arecibo, 1992:
This 300m telescope started to look for radio transmissions.
Goldstone Deep Space Complex, CA, 1992:
A 34m telescope in the
Mojave desert begins
simultaneous search.
Less than 12 months later,
NASA’s work was killed
by Sen. Richard Bryan
(NV).
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Current SETI projects
SETI institute, founded 1984:
Continues to search at various telescopes, including
the Parks 64m, Greenbank, VA, etc.
The Allen Array in Hat Creek (N California) has 42
antennae, and an ultimate construction target of 350
antennae.
SETI@home, 1999:
A “distributed computing project,” with 5 million
users, the world’s largest extended supercomputer.
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What if SETI ever became successful?
What would we do if we were successful? Protocols in the “Declaration
of Principles Concerning Activities Following the Detection of
Extraterrestrial Intelligence” are recommended.
In summary:
1. Verify it cannot be explained by conventional means.
2. Tell your funding agencies (so they can avoid embarrassment).
Governments should be informed. (Note: just “should”.)
3. Announce the news to all scientists, including many scientific
organizations and the UN.
4. Go public.
5. All data is made public.
6. Seek verification of the detection by further study.
7. Seek international agreement to protect the signal from terrestrial
pollution.
8. Plans to send a response would be studied carefully, after
appropriate international consultations.
Will these guidelines be followed by publication hungry scientists?
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CETI (note the C instead of the S)
SETI (Search for extraterrestrial intelligences)—passive.
CETI (Communication with extraterrestrial intelligence)—active.
CETI is by its nature potentially much more dangerous.
Unfortunately, many of these are science-lite PR stunts.
Ex: In 2008, on its 40th anniversary, an mp3 file of
the Beatles song, “Across the Universe” was beamed
to Polaris (433 LY) by NASA.
Some cautious scientists advise that it would be far
wiser for us to quietly listen in to possible conversations,
instead of bursting in, yelling at the top of our lungs.
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CETI
1974: “Arecibo Broadcast.”
A 3-minute signal was sent towards
M13, a globular cluster of a few×105
stars. If it were intecepted intact and
properly decoded, it could resolve to a
23×73 pixel graphic. Lucky inhabitants
will get this signal from Drake, Sagan,
and others in 21,000 years.
Results from CETI
After a 2001 transmission, a prank “response” was carved into a
wheat field by a group of crop circle aficionados…
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Further attempts at active CETI
Alexander Zaitsev, Chief Scientist at the Russian Academy of Sciences’
Institute of Radio Engineering and Electronics (Evpatoriya, Eukraine) is
continuing to use instrumentation to blast powerful signals to stars like
Gliese 581, which has several planets.
1999:
Cosmic Call 1—to four nearby stars. Mathematically more robust
than the Arecibo broadcast (it was less prone to degradation if
individual pixels are lost), it was marred by the inclusion of names
and addresses of 2000 donors to the program.
2001:
Teenage Message—to six nearby sunlike stars.
2003:
Cosmic Call 2—to five nearby stars.
2008:
A Message from Earth—to the single star, Gliese 581. To arrive in
2029.
2009:
Canberra Australia joins the silliness with “Hello from Earth”, a
stunt broadcast to Gliese 581, with little or no science behind it.
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Concerns about CETI
Remember item #8 in the protocol for detecting alien broadcasts?
“8) Plans to send a response would be studied carefully, after
appropriate international consultations.”
During a scientific meeting in 2007, this was modified by deleting the phrase
involving “after appropriate international consultations”
“8) Plans to send a response would be studied carefully.”
This has no teeth.
Various senior scientists at the leading international SETI study group have
recently resigned in disgust and protest.
Who are these few scientists to speak for all of humanity?
Should scientists be doing the talking, or should diplomats be involved? This may
seem funny today, but will it be funny tomorrow?
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