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Back   Could we communicate with possible inhabitants?  
The Nancay radiotelescope
figures/grilleradio.jpg
Several radiotelescopes have been used in the SETI program.
Copyright : Paris Observatory.

The communication with possible inhabitants of an exoplanet require that (1), these inhabitants are sufficiently developed (both from an intellectual and a technological point of view), and (2), they want to communicate.

If we were trying to contact them, a first step would be to send signals in different wavelengths (visible, infrared, radio,...). These signals should be strongly different from natural signals.

The researchers of the SETI (Search for Extraterrestrial Intelligence) program have proposed criterion for this  : the signals resulting from natural processes have a high product length-frenquency width compared to the limit fixed by the laws of physics (the uncertainty principle). Conversely, the detection of a signal for which this product is close to the limit would be a remarkable event, likely to draw attention.

We will not discuss here what one would say in this conversation  ; we only note that such a communication requires a long time compared to the duration of a human life. Indeed, the time necessary for a message exchange would be twice the time of the propagation of light between Earth and the exoplanet, i.e. at least nine years if there was a planet around Proxima Centauri (the closest star to the Sun), or one hundred years in the case of 51 Pegasi.