In Understanding Media (1964), Marshall McLuhan distinguished between 'hot' and 'cool' media: hot media, like the radio, are 'high definition' but 'low in participation'; 'cool media', like the telephone, are 'low definition' but 'high in participation'. (In the early 1960s, TV was 'cool', compared to the 'hot' movies. Obviously that was long before the arrival of hi-def.) Predictions about the way technology is heading, whether made by SF writers or tech companies, tend to assume the future will be hot. Characters in Brave New World go to the Feelies. Thirty years ago, everyone (well, maybe not everyone) imagined that by now we'd be watching holographic movies and wandering around with virtual reality helmets on. But no one foresaw the rise of text-messaging or Twitter. Michio Kaku's whiggish Physics of the Future, published last month, follows the trend, confident that the future will be lived in high definition.

