1. Extra-terrestrial Civilizations: Interstellar Radio Communications

2. 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

3. Are we alone? Contact …
Radio communication more likely possibility for contact
Electromagnetic radiation travels at the speed of light.

4. 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!

5. 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?

6. 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.

7. 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?

8. 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.

9. 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.

10. What frequency to choose?
Recall our discussion about electromagnetic radiation and the multitude of frequencies associated with it.

11. Wavelength and Frequency

12. 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

13. 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

15. 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.

16. 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.

17. Arecebo Observatory

18. 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

19. 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).

20. 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).

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

22. 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.

23. Doppler Effect: the wavelength is affected by the relative motion between the source and the observer