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Lecture 37. SETI and the Drake Equation.
reading: Chapter 11;
The UFO Phenomenon
~50% of the US public believe intelligent ETs have
visited the Earth.
Many scientists readily accept a high probability of
life somewhere in the universe.
Few accept UFOs.
- photographs are not enough to support the extraordinary
claim that aliens have visited the Earth
- unsupported personal accounts
- “evidence” more logically explained by known earthly
- no one has provided unequivocal evidence
- explanations used to account for insufficient evidence
the government is hiding the bodies
scientists/government engaging in a conspiracy/cover-up
scientists are biased/can’t be trusted
ETs have modified people’s minds
witnesses abducted and no longer exist.
What Could Be Out There?
Nikolai Kardashev’s 3 types of civilizations:
Type1. Planetary Civilization.
Use the resources of their home planet only.
Type2. Stellar Civilization.
Can harness the resources of their home star.
Type3. Galactic Civilization.
Uses the resources of the entire galaxy.
A technologically adept civilization could capture all of the Sun’s light
energy by building a large sphere with solar cells
= Dyson Sphere
Would absorb all light of the star, emitting only infrared radiation (heat)
We’ve Been Sending Signals
Unintentional messages:
Beginnings of radio broadcasts in 1906.
Beginnings of TV broadcasts in 1927.
Intentional messages:
1974 Arecibo radio telescope, Puerto Rico
most powerful message deliberately beamed into space
omnidirectional broadcast 20 trillion watt message
aimed at the globular cluster M13 (21,000 lyrs away)
detectable anywhere in the galaxy given a radio telescope of
the size of Arecibo
bits transmitted by frequency shifting at 10 bits/sec
Simple, pictorial message containing our solar system, a radio
telescope, DNA, etc.
The Search for Extraterrestrial Intelligence
Goal: to identify a signal from another intelligent, communicating
Started with the early pioneers of radio:
Nikola Tesla in 1901
Guglielmo Marconi in 1901
picked up radio disturbances, thought they were from space
First scientific approach to SETI:
Guiseppe Cocconi and Philip Morrison in 1959
wondered how difficult it would be to send radio over interstellar
advocated looking for signals using large radio telescopes
Types of Signals
Type1. Signals used for local communication
(our own TV and radio)
we can’t detect (unless their signals are vastly more
powerful than our own signals)
Type2. Signals used for communication between civilization’s
home world and colonies/spacecraft
likely can’t detect
Type3. Intentional signal beacons designed to get the attention
of other societies
should be the strongest and easiest to interpret.
Sending Signals to ET
Radio has narrow bandwidth, transmits over a narrow part of the
electromagnetic spectrum.
TV has a broad bandwidth, carries sound and video images.
If you want to send a signal to get someone’s attention:
-confine it to a very narrow bandwidth
-all of the energy of the transmitter concentrated on one narrow
part of the electromagnetic spectrum
-this makes the signal easy to pick out from background noise
Cacconi & Morrison proposed 1420 MHz
-frequency where H2 produces natural radio static
-radio astronomers often observe this frequency for interstellar gas
The Drake Equation
1961 Frank Drake summarized the factors that
would determine whether any attempt to find
intelligent ETs would be successful.
Equation to calculate the number of civilizations in our
galaxy that we could potentially receive a signal.
N = R* x fPlanet x ne x fLife x fIntelligence x fCivilizations x L
N = the number of transmitting/communicating civilizations
R* = galactic birthrate of G/K/M type stars suitable for hosting life (~10/year)
fPlanet = the fraction of such stars having planets
ne = the number of those planets that are habitable
fLife = fraction of those planets on which life originated/evolved
fIntelligence = the fraction of inhabited worlds that developed intelligent life
fCivilizations = the fraction of inhabited worlds that developed civilizations
capable of interstellar communication
L = lifetime of those communicating civilizations
N = R* x fPlanet x ne x fLife x fIntelligence x fCivilizations x L
This is a simple equation, but most of the variables are unknown.
Get a huge range for N.
-can use to constrain our most optimistic and pessimistic estimates
-allows us to identify where we need more data/knowledge
Project Ozma
Frank Drake in 1960 independently came up with the same conclusion.
Used a 26m radio telescope at Green Bank, West Virginia
Searched for ETI for 2 weeks.
Called it Project Ozma (L. Frank Baum’s Land of Oz - “very far away,
difficult to reach, and populated by strange and exotic beings”)
Looked at Tau Ceti and Epsilon Eridani (stars about same age as the sun)
Looked for repeated series of patterned pulses or a series of
prime numbers.
False alarm caused by a secret military experiment.
Only found static and meaningless wiggles on the paper recorder.
Showed systematic searches are feasible and eventually
led to a small SETI program at NASA in the 1970’s.
1970’s a decade of radio astronomy and SETI searches.
Planetary Society: Project META
University of California: Project SERENDIP
Established SETI programs as NASA Ames AND JPL.
Can do targeted searches (focus on Sun-type stars -Ames)
Can do sky surveys (sweep across the sky looking for a signal -JPL)
NASA funding from 1988-1993 when funding cancelled by Congress.
Since then, scaled-down versions have been funded privately.
The “WOW” Signal
Received on August 15, 1977
Short duration very high energy pulse.
“Wow” written by a SETI volunteer indicating it was strange.
High likelihood it had an ET source. But is it ETI??
Was only picked up by one of the telescopes at Ohio State University.
The signal is not a message - is an unmodulated signal.
Perhaps it switched on just briefly?
It has never been seen again even with more sensitive equipment.
Radio SETI
Today’s searches piggyback on other projects:
-limited observing time
-can’t control where the telescope is pointed
-radio interference is becoming a big problem
Currently underway:
Project Phoenix
sky survey
Parkes, Australia sky survey
can detect at 100
light years:
100,000 watts
1 million watts
1 million watts
Project Phoenix 1995-present
Targeted search of local objects in the sky within 200 light years.
Examining 1,000 nearby stars using the world’s largest antennas.
Monitors millions of radio channels simultaneously
between 1,000-3,000 MHz.
Looking for narrow band signals.
By mid-1999 had examined 50% of stars on its “hit list”.
Listening done automatically.
Two billion channels examined for
each target star.
Optical SETI at Lick Observatory
Communicating by visual signals (e.g., lasers).
Requires more energy to send signal (than radio).
Tends to absorbed by dust grains in interstellar space.
Could use short bursts of laser light, into a Morse code.
Should be immune to “false positives” from Radio SETI.
Optical SETI
looking for laser pulses (billionth of a second long).
new technology makes this search possible.
uses a telescope and three photomultipliers, built by UC Santa Cruz
undergrad physics major.
examined 300 stars and a few star clusters.
Allen Telescope Array
Funding from Paul Allen and Nathan Myhrvold
Being constructed for SETI use and radio astronomy.
Joint effort by SETI and UC Berkeley.
Built at Hat Creek Observatory north of Lassen Peak.
Will expand Project Phoenix to 100,000-1,000,000 stars
Will cover 1,000-10,000 MHz
Will be 350 - 6.1m antennas, randomly placed in 1 km diameter area.
Lecture 38. The Fermi Paradox, Von Neumann
Machines, Galactic Colonization.
reading: Chapter 13