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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.
Types of civilizations
• Kardashev spawned the following classification scheme:
• Type 0 … inability to communicate with ET
• Type 1 … like us with limited technology
• Type II … almost 100% utilization of parent star’s energy (Dyson spheres)
• Type III … utilization of a galaxy of stars’ energy!
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?
Consider …
• You can see a cell phone but cannot ‘hear’ what it hears.
• Electromagnetic signals pass through your body all the time and you cannot detect them.
• Thus the human body is limited to what information it can process as is the cell phone.
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 cit-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.
• 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
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