During World War II the writer was temporarily associated with the aircraft industry. In those days aeronautical engineers and fliers were generally skeptical about the possibility of flying faster than the speed of sound. Many of them believed that if we should ever succeed in building a plane that would achieve this velocity—a little over seven hundred miles an hour—the results would probably be disastrous for the aircraft and its pilot.
The sound barrier was soon penetrated. Today many planes fly at more than twice the speed of sound, and a military plane has recently flown at three times that speed. Plans are far advanced for commercial planes to go as fast, and one no longer hears an arbitrary limit fixed to the velocity at which aircraft may eventually travel. Astronauts orbit the earth at speeds far beyond any human expectation a few years ago. It is now clear that the so-called sound barrier was never an actual barrier, but simply a limit erected in human thought beyond which men feared or doubted their ability to go; it was not unlike the belief in earlier times that if navigators sailed far enough out to sea, they would drop off the edge of the earth, which was thought to be flat.
Another example of a mental barrier overcome is the four-minute mile. Until a very few years ago no one had ever run a mile in less than four minutes, and it was widely believed that no one ever would. Yet today, at almost every major national or international track and field meet, the mile is run in less than four minutes by at least one runner and sometimes by more than one. The physical structure of the men who do this is the same as that of men in earlier times; but the mental barrier has been eliminated, and new goals have been set for this event.
