English: Animation depicting the sum of probability amplitudes, as conceived by Richard Feynman, using little clocks timing each event, instead of the formal complex numbers.

A light source S emits light, which bounces off the mirror (in blue) and reaches the detector P. An obstruction between S and P (gray rectangle) avoids a direct path between source and detector. The graph below represents the time it takes for light to traverse each path.

The probability amplitude of all paths (the little spinning arrows) must be summed to find the vector S. The magnitude of this vector, which is the sum of all amplitudes, is squared to reach the final probability of light reaching P.

Paths closer to the path of least time (at the center) contribute the most to the total amplitude, which is consistent with the expected behavior in classical physics.

Adapted from QED: The Strange Theory of Light and Matter