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

Source

Frequency Range

(MHZ)Number of

Transmitters

Fraction of Time

Transmitters Emit

Maximum Power Radiated (watts)

Effective Carrier,

Bandwidth (hertz)

CB radios 2-7 10,000,000 1/100 5 2

Professional mobile radios 20-500 100,000 1/10 20 1

Weather, marine, & air radars 1000-10,000 100,000 1/10010,000 to 1,000,000 1,000,000

Defenser Radarsa 400 2 1/10 10,000,000,000 1,000

FM radio stations 88-108 10,000 1 4,000 0.1

TV sound 40-850 2000 1 500,000 0.1

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.

• 1972-1973 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 M13• 1977 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

SCIENTIFIC Investigator

Antenna Diameter (meters)

Frequency Observed (MHz)

Frequency Resolution

(kHz)

Total Frequency Band (MHz)

Frank Drake 26 1420 0.1 0.4V. Troitskii 14 100, 1800, 2500 0.013 2.2B. Zuckerman & P. Palmer 91 1413-1425 4 12G. Verschuur 43,91 1420 7 20S. Bowyer and others 26 variable 2.5 20R. Dixon and others 53 1420 30 0.4A. Bridle & P. Feldman 46 22,235 30Frank Drake & Carl Sagan 305 1420, 1653, 2380 1.0 3T. Bania & R. Rood 43 8665 0.3P. Horowitz 26 1400-1720 0.0005 320NASA scientists 305 1300-2400 1,7,28 1100NASA scientists 26,34 1700,8300-8700 0.019 400S. Bowyer and others 305 424-436 0.0006 10D. Werthimer and others 305 1370-1470 0.0006 100SETI Institute scientists 64,22 1200-1750 0.001 550

The Wow! Signal

• August 15 1977

• 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: 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)

Data, data everywhere …

• SERENDIP generates vast quantities of data that need to be searched for a signal (from ET).

• SETI@home links idle computers (like yours) from around the world to analyze data (setiathome.berkeley.edu

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