Time

Philosophers have long argued about the nature of time. Some philosophers, notably German philosopher Immanuel Kant, have proposed that newborn babies may experience the passage of time. Others have proposed that the human mind must learn to construct time. For example, French philosopher Henri Bergson thought of time as something entirely derived from experience. In Bergson's doctoral dissertation, Time and Free Will (1889; translated 1910), he proposed that time is a matter of subjective experience. According to Bergson, an infant would not experience time directly but rather would have to learn how to experience it.

Time is not a physical constant. Motion and gravity effect time by dilating (slowing) it or by expanding its duration. In 1905 Albert Einstein described the effect of motion on time in his special theory of relativity. In 1916 he described the effect of gravity on time in his general theory of relativity.

Time dilation effects due to motion were experimentally observed in the early 1970s. Researchers placed atomic clocks on commercial airliners and observed the expected changes in time as measured by those clocks relative to similar clocks on the ground. In particular, when the planes traveled east, in the direction of Earth’s rotation, the clocks on the airliners were 59 nanoseconds (59 billionths of a second) slow relative to the atomic clocks on the ground. When the airplanes traveled west, the clocks were 273 nanoseconds fast. This discrepancy is caused by the rotation of Earth, which causes an additional time dilation. If the effect of Earth's rotation is removed, the time dilation produced by the motion of the airliners confirms Einstein's theory of how time changes with motion, as the dilation is in agreement with predictions made by the theory.

Time dilation effects due to gravity have been experimentally verified in many ways. For example, time on the Sun's surface runs about two parts in a million slower than on Earth because of the Sun's much higher gravity. In 1968 American physicist Irwin Shapiro confirmed this effect when he showed that radar signals (see Radar Astronomy) and their reflections from planets are delayed when the Sun is near the path of the signals.