The Rio scale was proposed in 2000 as a means of quantifying the significance of a SETI detection.[1][2] The scale was designed by Iván Almár and Jill Tarter to help tell policy-makers how likely, from 0 to 10, it is that an extraterrestrial radio signal has been produced by an intelligent civilization.[3]
The scale is inspired by the Torino scale, which is used to determine the impact risk associated with near-Earth objects. Just as the Torino scale takes into account how significant an object's impact on the planet would be, the Rio scale takes into account how much a public announcement of the discovery of extraterrestrial intelligence would probably impact society.
The IAA SETI Permanent Study Group officially adopted the Rio scale as a way of bringing perspective to claims of extraterrestrial intelligence (ETI) detection, and as an acknowledgement that even false ETI detections may have disastrous consequences.[4][5]
The scale was modified in 2011 to include a consideration of whether contact was achieved through an interstellar message or a physical extraterrestrial artifact, including all indications of intelligent extraterrestrial life such as technosignatures.[6] A 2.0 version of the scale was proposed in 2018.[3][7][8]
In its 2.0 version, the Rio Scale, R, of a given event is calculated as the product of two terms.
The first term, Q, is the significance of the consequences of an event. It is determined considering three factors: the estimated distance to the source of the signal (a value between 0 and 4), the prospects for communicating with the source (a value between 0 and 4) and how likely is that the sender is aware of humanity (a value between -1 and 2). The value of each factor is determined by answering a question and Q is calculated by summing the three values.
The second term, δ, is the probability that the event actually occurred. Its value is determined by first calculating a term, J, based on three factors: the probability that the signal is real, the probability that it is not instrumental, and the probability that it is not natural or human-made. The values for these factors are determined by answering a questionnaire and J is calculated by summing them. δ is then calculated using the formula δ = 10(10-J)/2.
The final R value, going from 0 to 10, is the likelihood that the observed event was produced by an intelligent civilization.
| Rio value | Importance |
|---|---|
| 10 | Extraordinary |
| 9 | Outstanding |
| 8 | Far-reaching |
| 7 | High |
| 6 | Noteworthy |
| 5 | Intermediate |
| 4 | Moderate |
| 3 | Minor |
| 2 | Low |
| 1 | Insignificant |
| 0 | Nil |
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