Walker, Halliday & Resnick — Electromagnetic Waves: Problem 42

42 Frank D. Drake, an investigator in the SETI (Search for Extra-Terrestrial Intelligence) program, once said that the large radio telescope in Arecibo, Puerto Rico (Fig. 33-46), “can detect a signal which lays down on the entire surface of the earth a power of only one picowatt.” (a) What is the power that would be received by the Arecibo antenna for such a signal? The antenna diameter is 300 m. (b) What would be the power of an isotropic source 9000 ly away that could provide such a signal? A light-year (ly) is the distance light travels in one year.