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

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 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 M 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

The Wow! Signal August 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: 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). links idle computers (like yours) from around the world to analyze data

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