W. L. Bragg, the pioneer in x-ray diffraction, gave the following loose but vivid interpretation of the wave-particle duality: _“The dividing line between the wave and particle nature of matter and radiation is the moment ‘now.’ As this moment steadily advances through time it coagulates a wavy future into a particle past. . . . Everything in the future is a wave, everything in the past is a particle.” If “the moment ‘now’ ” is understood to be the time a measurement is performed, this is a reasonable way to think about the situation._ The philosopher Søren Kierkegaard may have been anticipating this aspect of modern physics when he wrote, _“Life can only be understood backwards, but it must be lived forwards.”_