According to McLuhan, why is the telephone a "cool" medium and the radio "hot"?

A distinction made by McLuhan between media such as print, photographs, radio, and movies (hot media) and media such as speech, cartoons, the telephone, and television (cool media). Hot media are ‘high definition’ because they are rich in sensory data. Cool media are ‘low definition’ because they provide less sensory data and consequently demand more participation or ‘completion’ by the audience (a useful mnemonic is to imagine that hot media are too hot to touch). Note that McLuhan was not referring to the issue of the relative cognitive effort involved in the use of different media. Arguably, in McLuhan's terms, television has grown hotter since the 1960s as its technical picture quality has improved, so these terms are relative. Critics of McLuhan's concept have argued that it reifies the medium, underestimating differences within the same medium; the degree of audience engagement does not depend primarily on the medium itself (although its affordances may play a part), but on its content and the ways in which the medium is used on specific occasions within specific contexts. See also McLuhanism; compare high and low involvement.

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