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Extra-terrestrial Civilizations:
Interstellar Radio
Communications
Are we alone? Contact …
• Direct contact through
traveling to the stars and their
planets
• Will be a challenge because
of the vast distances involved
and the (slow) speeds we can
travel
Are we alone? Contact …
• Radio
communication
more likely
possibility for
contact
• Electromagnetic
radiation travels at
the speed of light.
Radio contact: A test?
• If civilizations are common, then why have
we not yet ‘heard’ them?
• To find the signals from ET may involve
solving technology not yet known to us.
• Is the search for contact a test in itself …
are we worth talking to?
Direct or Accidental signals
• Realizing that signals from ET may well be
very weak, where should we look? … what
frequency?
• We may be lucky and detect signals not
beamed at us … eavesdrop on ‘Star Trek’,
‘Friends’ ,etc.
• What type of signal should we look for?
• What direction/star (planet) should we
listen to?
Where to look
• Closer civilizations if they are sending
signals will presumably have the strongest
signals and be easier to detect.
• Signal strength drops off as the square of
distance.
Type of Stars
• As discussed, stars like our Sun first targets.
• In the Milky Way galaxy, stars with similar
spectral types (F, G, K) constitutes 10% or more
of all stars (30 billion or more).
• Double, multiple, very luminous (and thus short
lived) stars not suitable targets.
• Specialization regarding how many planets
contain technologically advanced civilizations.
What frequency to choose?
• Recall our discussion about
electromagnetic radiation and the
multitude of frequencies associated with it.
Wavelength and Frequency
• Because of its
electric and
magnetic properties,
light is also called
electromagnetic
radiation
• Visible light falls in
the 400 to 700 nm
range
• Stars, galaxies and
other objects emit
light in all
wavelengths
Familiar Frequencies
• AM dial … radio stations tuned in with
frequencies 500 – 1500 KHz
• FM dial … radio stations tuned in with
frequencies 88 – 110 MHZ
• TV channels with frequencies 70 – 1,000
MHZ
ET listens to … CBC?
• How to decide what frequency ET will
listen to?
• Is there a galactic, common hailing
frequency?
• We assume that a civilization
technologically advanced enough to
send/receive radio signals will know the
language of science.
Considerations
• Economical to send a radio photon than
say, a (visible) light photon. If we are
sending to many stars, cost needs to be
controlled (low).
• The selected frequency must be able to
traverse significant distances without
interference or loss.
Arecebo
Observatory
Problems during transmission
• Photons of energy at the wrong frequency
will be absorbed … you cannot see
through a brick wall but your phone can
pick up a signal through the same wall.
• Long wavelength radiation can travel
further with less absorption … best for
sending/receiving signals
Natural background
• The galaxy is quote noisy … stars would
wash out a visible light signal (even if it
could travel a long way through the dust).
• The cosmic background radiation is an
echo/hiss left over from the Big Bang (high
frequency cutoff).
• Charged particles (mostly electrons) spiral
around the magnetic field lines producing
synchrotron radiation (low frequency
cutoff).
The water hole
• In between the upper and lower cut-offs in
frequency is a relatively radio quiet area
near where the hydrogen atom ‘flips’
giving a unique signal at 1420 MHZ or
21.1 cm (wavelength).
The spin-flip transition in hydrogen
emits 21-cm radio waves
The water hole … continued
• Near by is a
similar
transmission from
the OH
radical(1612,
1665, 1667, 1720
MHz).
• Thus the Water
Hole is a likely
spot to search for
a signal from ET.
Doppler Effect: the wavelength is
affected by the
relative motion between the source and
the observer
The question of Bandwidth
• The spread of frequencies examined
during a search for ET.
• A broad bandwidth (like for TV) has coned
the term ‘channel’.
• A bandwidth of as small as 1 Hz increases
the chances of detecting an artificial
signal.
• A 1 Hz bandwidth requires LOTS of
searching in a given frequency range.
Signal characteristics
• Narrow band can have more power
• Narrow can be dispersed by the
Interstellar Medium (ISM).
• Broad band carries more information.
• AM bandwidth: 10KHz
• FM Bandwidth: 200 KHz
• TV bandwidth: 6 MHz
• For all, half the power of signal confined to
1 Hz!
Common Transmissions from
Earth
Frequency
Range
Source
CB radios
(MHZ)
Fraction of
Effective
Time
Carrier,
Number of Transmitters Maximum Power Bandwidth
Transmitters
Emit
Radiated (watts)
(hertz)
2-7
10,000,000
1/100
5
2
20-500
100,000
1/10
20
1
1000-10,000
100,000
1/10010,000 to 1,000,000
1,000,000
Defenser Radarsa
400
2
1/10
10,000,000,000
1,000
FM radio stations
88-108
10,000
1
4,000
0.1
TV sound
40-850
2000
1
500,000
0.1
Professional mobile
radios
Weather, marine, & air
radars
Can we conclude ET from these
signals?
• TV signals may well vary their frequencies
periodically as a result of Earth’s rotation
(on its axis) and revolution (around the
Sun) … Doppler shifts.
The First Search: Project Ozma
• Frank Drake mounted the
first SETI search
• July 1960, 85 foot radio
telescope at Green Bank in
West Virginia
• Searched at a wavelength of
21 cm.
• Tau Ceti and Epsilon Eridani
were targets
Brief History
• Philip Morrison and Guiseppe Coconni
published Searching for Interstellar
Communication
• 1960 Project Ozma (Frank Drake)
• 1961, first SETI Conference, Order of the
Dolphin and the unveiling of the Drake
Equation.
• 1972-1973 Pioneer Probe Plaques.
History continued …
• 1973: Ohio State University begins a
major SETI project at its Big Ear
Observatory in Delaware
• 1974 Drake transmission to M13
• 1977 WOW signal
• 1977 Voyager probe disks
• 1979 Planetary Society founded (Carl
Sagan et al)
• 1984: The SETI Institute is founded
1974 Message to M13
• 20
trillion watt
transmission, lasting about 3
minutes
• Message 1679 bits,
arranged 73 lines x 23
characters (prime numbers!)
•DNA, a human being, the
Solar System, etc.
SETI Searches to-date
SCIENTIFIC
Investigator
Antenna
Diameter
(meters)
Frank Drake
V. Troitskii
B. Zuckerman & P. Palmer
G. Verschuur
S. Bowyer and others
R. Dixon and others
A. Bridle & P. Feldman
Frank Drake & Carl Sagan
T. Bania & R. Rood
P. Horowitz
NASA scientists
NASA scientists
S. Bowyer and others
D. Werthimer and others
SETI Institute scientists
26
14
91
43,91
26
53
46
305
43
26
305
26,34
305
305
64,22
Frequency
Observed (MHz)
Frequency
Resolution
(kHz)
Total
Frequency
Band (MHz)
1420
100, 1800, 2500
1413-1425
1420
variable
1420
22,235
1420, 1653, 2380
8665
1400-1720
1300-2400
1700,8300-8700
424-436
1370-1470
1200-1750
0.1
0.013
4
7
2.5
30
30
1.0
0.3
0.0005
1,7,28
0.019
0.0006
0.0006
0.001
0.4
2.2
12
20
20
0.4
3
320
1100
400
10
100
550
The Wow! Signal
• August 15 1977
• Ohio State University Radio Observatory
(Big Ear)
• 72 seconds in length and VERY strong
Current major SETI efforts
• Project Phoenix uses many radio
telescopes from around the world in
targeted searches (SETI Institute:
www.seti.org).
• The Allen Telescope Array of up to 500
radio telescopes in a linked array.
• Project SEREBDIP uses radio telescopes
‘piggy back’ to listen in to 1420 MHz.
(University of California at Berkley)
Data, data everywhere …
• SERENDIP generates vast quantities of
data that need to be searched for a signal
(from ET).
• SETI@home links idle computers (like
yours) from around the world to analyze
data (setiathome.berkeley.edu
Other search techniques
• Optical SETI assumes the use of lasers in
a pulsed manner to signal existence.
• Masers are microwave equivalents to
lasers and are being investigated as a
possible signaling medium.
The Flag of Earth