The Morrison-Cocconi Conjecture was a pivotal suggestion, put forward by Philip Morrison and Guiseppe Cocconi, in a September 1959 paper,1 that the best frequency at which to search for signals from intelligent extraterrestrials is 1420 megahertz (MHz), corresponding to the 21-centimeter line of neutral hydrogen:
It is reasonable to expect that sensitive receivers for this frequency will be made at an early stage of the development of radio astronomy. That would be the expectation of operators of the assumed source, and the present state of terrestrial instruments indeed justifies the expectation.
They proposed that the search begin with 7 reasonably Sun-like stars within a radius of 15 light-years, and closed with this challenge:
The reader may wish to consign these speculations wholly to the domain of science fiction. We submit, rather, that the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now to hand.
Cocconi and Morrison's paper served to spark a debate among scientists that continues to this day. The year of its appearance coincided with the start of preparations for the first search for extraterrestrial signals, Project Ozma, by Frank Drake.
1. Cocconi, G., and Morrison, P. "Searching for Interstellar Communication," Nature, 184, 844 (1959).