Active SETI
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A representation of the 1679-bit Arecibo message.

Active SETI (Active Search for Extra-Terrestrial Intelligence) is the attempt to send messages to intelligent aliens. Active SETI messages are usually in the form of radio signals. But physical messages like that of the Pioneer plaque may also be considered an active SETI message. Active SETI is also known as METI (Messaging to Extra-Terrestrial Intelligence), or positive SETI. Active SETI is contrasted to passive SETI, which only searches for signals, without any attempt to send them.

The term METI was coined by Russian scientist Alexander Zaitsev, who denoted the clear-cut distinction between Active SETI and METI:¹

The science known as SETI deals with searching for messages from aliens. METI science deals with the creation of messages to aliens. Thus, SETI and METI proponents have quite different perspectives. SETI scientists are in a position to address only the local question “does Active SETI make sense?” In other words, would it be reasonable, for SETI success, to transmit with the object of attracting ETI’s attention? In contrast to Active SETI, METI pursues not a local and lucrative impulse, but a more global and unselfish one – to overcome the Great Silence in the Universe, bringing to our extraterrestrial neighbors the long-expected annunciation “You are not alone!”

Message construction

The lack of an established communications protocol is a challenge for METI.

Also characteristics of the radio signal such as wavelength, type of polarization, and modulation have to be considered.

Realized projects

• Arecibo Message (1974)
• Cosmic Call 1 (1999)
• Teen Age Message (2001)
• Cosmic Call 2 (2003)
• A Message From Earth (2008)

These projects have targeted stars between 20 and 69 light-years from the Earth. The exception is the Arecibo message, which targeted globular cluster M13, approximately 24,000 light-years away. The first message to reach its destination will be A Message From Earth, which should reach Gliese 581 in Libra in 2029.

Stars to which messages were sent, are the following:²³⁴

Perceived risk

Active SETI has been heavily criticized due to the perceived risk of revealing the location of the Earth to alien civilizations, without some process of prior international consultation. Notable among its critics is scientist and science fiction author David Brin, particularly in his article/"expose" Shouting at the Cosmos.

But some consider this as panic and irrational superstition, see: Sending and Searching for Interstellar Messages, Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization.

To lend a quantitative basis to discussions of the risks of transmitting deliberate messages from Earth, the SETI Permanent Study Group of the International Academy of Astronautics [1] adopted in 2007 a new analytical tool, the San Marino Scale [2]. Developed by Prof. Ivan Almar and Prof. H. Paul Shuch, the San Marino Scale evaluates the significance of transmissions from Earth as a function of signal intensity and information content. Its adoption suggests that not all such transmissions are created equal, thus each must be evaluated on a case-by-case basis before establishing blanket international policy regarding Active SETI.

Beacon proposal

One proposal for a 10 billion watt interstellar SETI beacon was dismissed by Robert A. Freitas Jr. to be infeasible for a pre-Type I civilization on the Kardashev scale.⁵ However, this argument, dated from 1980, assumes omni-directional beacons, which may not be the best way to proceed. As a result it has been suggested that civilizations must advance into Type I before the energy required for reliable contact with other civilizations.^{citation needed} Advances in consumer electronics have made possible cheaper transmitters.⁶ This can reduce the power and cost to levels that are reasonable with current (2008) earth technology.

Once civilizations have discovered each others' locations, the energy requirements for maintaining contact and exchanging information can be significantly reduced through the use of highly directional transmission technologies. In 1974, the Arecibo Observatory transmitted a message toward the M13 globular cluster about 25,000 light-years away, for example, and the use of larger antennas or shorter wavelengths would allow transmissions of the same energy to be focused on even more remote targets, such as those attempted by Active SETI.

References