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