Transcript: Why has the SETI strategy focused so heavily on radio waves for communication? There are simple reasons to do with fundamental physics. Radio photons are preferred over optical photons because they have low energy and so are less costly to produce in energetic terms, and they travel unimpeded through space, being unaffected by dust and gas. Also, there are regions of the electromagnetic spectrum where there are minimum amount of cosmic noise from other energy sources, and the radio part of the spectrum from one to a hundred gigahertz is just such a range. Also the sensitivity of our technology to radio detection is extraordinary. The Pioneer 10 satellite, currently at a distance of more than six billion miles from the Earth was detected by the Arecibo Radio Telescope when it was five billion miles from the Earth, yet it had the equivalent energy in its transmitter of a one Watt radio light bulb. The Arecibo dish itself, a thousand feet across, is the world’s largest radio telescope. In an interesting thought experiment, the Arecibo radio dish can not only listen but can send pulsed microwaves. If Arecibo-type radio technology existed anywhere else in the Milky Way galaxy, we could detect it if it were transmitting pulsed signals. Presumably the converse is true, and they could detect us. Thus radio waves, in principle, allow communication across the entire Milky Way galaxy.