A world with a sudden limit on air travel would be tremendously different from the one we live in now.

In 1491 the Inka ruled the greatest empire on earth. Bigger than Ming Dynasty China, bigger than Ivan the Great’s expanding Russia, bigger than Songhay in the Sahel or powerful Great Zimbabwe in the West Africa tablelands, bigger than the cresting Ottoman Empire, bigger than the Triple Alliance (as the Aztec empire is more precisely known), bigger by far than any European state, the Inka dominion extended over a staggering thirty-two degrees of latitude—as if a single power held sway from St. Petersburg to Cairo.

Japanese maps tend to come in two varieties: small, schematic, and bewildering; and large, fantastically detailed, and bewildering.

Compared with U.S. cities, Japanese cities bend over backward to help foreigners. The countryside is another matter.

A smartphone links patients' bodies and doctors' computers, which in turn are connected to the Internet, which in turn is connected to any smartphone anywhere. The new devices could put the management of an individual's internal organs in the hands of every hacker, online scammer, and digital vandal on Earth.

Scientists have established huge numbers of links between particular diseases and snippets of DNA, but in the great majority of cases, this has not yet been translated into treatments that can help cure patients. These treatments will come - tomorrow, or the day after.

Smartphones can relay patients' data to hospital computers in a continuous stream. Doctors can alter treatment regimens remotely, instead of making patients come in for a visit.