Extra-terrestrial Civilizations: Interstellar Radio Communications

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Extra-terrestrial Civilizations Interstellar
Radio Communications
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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

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Are we alone? Contact

• Radio communication more likely possibility for
  contact
• Electromagnetic radiation travels at the speed of
  light.

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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?

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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?

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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.

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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.

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What frequency to choose?

• Recall our discussion about electromagnetic
  radiation and the multitude of frequencies
  associated with it.

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Wavelength and Frequency
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• 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

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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

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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.

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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.

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Arecebo Observatory
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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

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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).

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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).

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The spin-flip transition in hydrogen emits 21-cm
radio waves
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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.

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Doppler Effect the wavelength is affected by
therelative motion between the source and the
observer
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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.

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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!

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Common Transmissions from Earth
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Can we conclude ET from these signals?

• TV signals may well vary their frequencies
  periodically as a result of Earths rotation (on
  its axis) and revolution (around the Sun)
  Doppler shifts.

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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

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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.

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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

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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.

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SETI Searches to-date
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The Wow! Signal

• August 15 1977
• Ohio State University Radio Observatory (Big Ear)
• 72 seconds in length and VERY strong

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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)

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Data, data everywhere

• SERENDIP generates vast quantities of data that
  need to be searched for a signal (from ET).
• SETI_at_home links idle computers (like yours) from
  around the world to analyze data
  (setiathome.berkeley.edu

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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.

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The Flag of Earth