This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For nearly 80 years, physicists have relied on a thought experiment created by Richard Feynman to predict how quantum particles behave. For the first time, researchers in China have tested this trick directly in the lab.
Led by Shi-Liang Zhu at South China Normal University in Guangzhou, China, the team used single particles of light to show that Feynman's idea holds up in practical experiments. Their results are published in Science Advances. Thought experiments have always been a key element of quantum mechanics.
Famous examples like Schrödinger's cat and the double-slit experiment helped reveal that particles can exist in multiple states at once and how their behaviors change with direct measurement. In 1948, Richard Feynman proposed a new thought experiment to describe how a quantum particle travels from one point to another. His "path integral" idea suggests a particle doesn't take a single route between two points.
Instead, every conceivable path contributes, and they all add together to produce the outcome we observe. Feynman also claimed each of these paths carries the same likelihood, differing only in the phase of their quantum wave functions. For decades, these were treated as reliable working assumptions—but had not been confirmed experimentally.
Zhu's team set out to confirm this using single photons. Rather than tracking a photon's path directly, which is impossible without disturbing it, they measured its probability amplitude: a value that captures how likely the photon is to take a given route, combining both size and timing information. By sending photons through a setup of mirrors, lenses and crystals and carefully measuring how their properties shifted, the team reconstructed amplitudes for some 1,419,857 possible paths.
With so many paths involved, any small errors could snowball until the results became meaningless. To get around this, the team refined nearly every part of its measurement process, allowing it to combine data with high enough fidelity to make the comparison meaningful. The results closely matched Feynman's predictions: Probabilities emerged from combining all the paths, the paths carried equal strength, and phases were set by the particle's classical trajectory.
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