A100
Life in the
Universe
Homework & Quiz 10 on Friday
Reading on Oncourse – “Essay 3”
Today’s APOD
The Sun Today
This week’s reading is available from the
Resource Tool on Oncourse – Essay 3
Final Exam…
Friday, 2:45 PM, here.
2 hours
100 multiple choice questions
Comprehensive
50% on chapters 10, 11 & Essay 3
50% on material covered in the first
three exams
Review sheet on Oncourse
Today’s Topics
How likely it is
that life exists
elsewhere than
Earth? (Drake
Equation)
Searching for
life elsewhere
Too hot!
Too cold!
The
Habitable
Zone
•The planet needs to be the right
distance from the star. WHY?
•The star needs to have the right mass.
WHY?
A planet needs the right star!
Constraints on star systems:
1) Old enough to allow time for evolution
(rules out high-mass stars - 1%)
2) Need to have stable orbits (might rule
out binary/multiple star systems - 50%)
3) Size of “habitable zone”: region in which
a planet of the right size could have liquid
water on its surface.
Even so… billions of stars in the Milky Way seem
at least to offer the possibility of habitable worlds.
You are
here
There are 400 Billion Stars in our
Galaxy. How many harbor life?
How common is life of any kind in the Milky Way?
Very Rare
Rare
Common
Very Common
How common is intelligent, technological life?
Very Rare
Rare
Common
Very Common
The Drake Equation
Start with 1011 stars in the Milky Way…
What fraction are similar to the Sun?
What fraction of sun-like type stars
have planets?
What fraction have planets in the HZ?
On what fraction will life emerge?
 On what fraction will intelligence
emerge?
What fraction will develop technology?
How long will a technological civilization
survive?
The Drake Equation
What are the odds that there are intelligent, advanced,
communicative civilizations out there? How many can we
expect to exist in all of the Milky Way Galaxy?
Make your own calculation of the
number of intelligent,
communicative, technologically
advanced civilizations in the
Milky Way.
All stars in the
Milky Way
fraction with planets?
fraction in the habitable zone?
fraction with simple life?
with technical society?
with intelligence?
with long-lasting technology?
= NHP  flife  fciv  fnow
We do not know
the values for the
Drake Equation
NHP : probably billions.
flife : ??? Hard to say (near 0 or near 1)
fciv : ??? It took 4 billion years on Earth
fnow : ??? Can civilizations survive long-term?
Search for Extra-Terrestrial Intelligence
SETI experiments look for deliberate signals from E.T.
Can We Find Extra-Terrestrial
Intelligence?
Looking for SIGNALS is the easiest way
We can also transmit a signal (but it’s a long
wait for the answer...)
Different kinds of signals to listen for:
local communication signals: on Earth, this includes
TV, radio, etc.
communication between the planet and another
site, such as satellites and spacecraft
A BEACON signal used to try to communicate with
other civilizations.
Searching for ET
NASA funded SETI until 1993
Present efforts all privately funded
SETI Institute (Frank Drake)
seti@home -- help analyze SETI data
Planetary Society
META (million channel extraterrestrial assay) -scans one million channels in the band
BETA (billion channel version of META)
84 ft. dish antenna at Harvard Univ.
connected to supercomputers that look for nonrandom patterns in the signals (most of the
signals come from natural sources such as stars)
250 megabytes of data each second
Your computer can help! SETI @ Home:
a screensaver with a purpose.
Can Earth Be
Heard from
Space?
 YES! Earth has been broadcasting TV and radio
communications for the last 50 years. ET
civilizations up to 50 light years away could be
picking us up.
 We can “listen” but radio wavelengths may be best
 Biggest collecting area - Arecibo telescope.
 The background sky is the quietest at wavelengths of
about 0.1 mm. At shorter wavelengths, emission from the
galaxy is loud, and at longer wavelengths, interstellar
clouds absorb the signals
Message to M13
Nov 1974
Message was beamed from the
Arecibo radio telescope
toward the M13 star cluster
24,000 light-years away
1679 (23 x 73) pulses and spaces
The message was transmitted
only once and was intended to
serve as a exercise in how we
might go about trying to contact
extra-terrestrials.
Message to
M13
Formed a picture showing
when arranged in a rectangle
numbers 1-10
elements, chemicals of life
a DNA molecule
a stick figure of a human
solar system
diagram of radio telescope
Visiting ET?
With foreseeable technology, we can achieve
speeds of 10% of the speed of light
We can travel 10 light years in 100 years
We can reach the nearest star in 43 years
Allow each new colony 5000 years to duplicate
the technology
Colonies could spread out about 50 light years
every 25,000 years
How long to colonize?
Assume 100,000 years
per 20 parsec hop
Total time to cover the
Galaxy:
1500 hops x 100,000 years
= 150,000,000 years
The Fermi Paradox




Emil
Konopinski
Enrico Fermi
Edward Teller
Herbert York
Emil Konopinski
LANL Tech
Area
LANL Fuller
Lodge Cafeteria
Enrico
Fermi
The Fermi Paradox
The Drake Equation – A few
hundred technical civilizations
150,000,000 million years
to colonize the Galaxy
WHERE IS EVERYBODY?????
Where is Everyone?
 Some factors in Drake equation may be
much smaller than we believe – is life,
or intelligent life, very rare?
 Do civilizations hide to avoid a “galactic
scourge?”
 Do technological civilizations selfdestruct?
 Is no one more advanced than we are?
 The Zoo hypothesis…
Possible solutions to the paradox
Civilizations are common but
interstellar travel is not.
Perhaps…
 Interstellar travel more difficult than
we think
 Desire to explore is rare
 Civilizations destroy themselves
before achieving interstellar travel
These are all possibilities, but not very appealing…
Possible solutions to the paradox
We are alone: life/civilizations much
rarer than we might have guessed.
•
Our own planet/civilization looks all the more
precious…
OR - There IS a
galactic
civilization…
… and some
day we’ll meet
them…
Difficulties of
Interstellar Travel
 Far more efficient engines are needed
 Energy requirements are enormous
 Ordinary interstellar particles become like cosmic rays
 Social complications of time dilation
Traveling to Another Star?
Distances between stars are much greater
than we can imagine
Sci-fi books and movies have dramatized
space travel to make it seem possible
Interstellar travel may never happen
Even the Voyager spacecraft (some of the fastest
ever flown) traveled at only 20 km/s through
space - not even 1% of the speed of light. They
would take 60,000 years to reach even the
nearest star
100.000000%
Maximum Speed Achieved
10.000000%
0.100000%
0.010000%
Space Shuttle
0.001000%
Automobile
0.000100%
Plane
0.000010%
Train
Horse
1000
100
10
1000
100
10
Years
Before
Present
YEARS BEFORE
1
Now
0.000001%
0.1
0.01
0.001
10
100
1000
Years
From
Now
YEARS
AFTER
% Speed of Light
1.000000%
Can we travel to new
worlds?
Within the lifetime of today’s children we will be
able to send robotic spacecraft to visit our
nearest neighbors
At 10% of the speed of light (30,000 km/sec)
travel time will be about 100 years
Then wait another 10-20 years for the data to
return
Dates to
Remember
 Reading, homework, quiz next week
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