Nearly eight decades after Nobel laureate Richard Feynman proposed that quantum particles navigate space by exploring every conceivable route between two points, experimental physicists have directly confirmed his cornerstone postulate for the very first time.
Formulated in 1948, the "Feynman path integral" has long served as a theoretical backbone of modern physics. The principle posits that predicting the most likely path of a quantum particle requires a calculation that effectively sums up all possible trajectories between two locations. While this "sum over all paths" became standard textbook material and indispensable for particle physicists, testing it directly in a physical experiment proved exceedingly difficult because interacting with quantum particles inevitably alters their properties.
To test Feynman's idea directly, a research team led by Shi-Liang Zhu at South China Normal University devised an optical experiment of unprecedented precision. The researchers directed individual photons—particles of light—through an intricate maze composed of tiny mirrors, lenses, and crystals.
As each photon traveled through the setup, the team measured five successive "propagators"—functions that describe how a photon's quantum state changes as it moves from one section of the experiment to another. By tracking properties such as polarization, the researchers reconstructed propagators corresponding to an astonishing 1,419,857 distinct trajectories through the optical maze. They then multiplied these propagators together and plugged the data into Feynman's path integral formula.
The calculation derived from the experimental data agreed with the photon's actual physical behavior.
The success of the experiment represents a major advance in quantum measurement and control. Managing so many trajectories presented an immense challenge, as cumulative experimental noise and minor errors could have easily corrupted the data into randomness. By demonstrating that Feynman's theoretical formulation directly aligns with empirical observations, the milestone confirms the solid foundation of quantum mechanics and opens new avenues for testing path integrals across different physical materials.