A foundational postulate of quantum physics has been directly verified by an experiment for the first time. Formulated by Richard Feynman nearly 80 years ago, it has long formed the backbone of much of modern physics. Quantum particles are notoriously difficult to analyse as interacting with them affects their properties.
But researchers have developed powerful mathematical tools for predicting what a particle is likely to do under particular conditions. One such tool is the “Feynman path integral”, which posits that the most likely path a quantum particle will take between two points can be predicted by performing a calculation that sums all the possible paths between those points in a special way. Since Feynman proposed it in 1948, his path integral had made its way into textbooks and is now invaluable for many scientists, including physicists who study particles.
Physicists used ‘dark photons’ in an effort to rewrite physics in 2025 Now, for the first time, Feynman’s idea has been tested directly in an experiment. Shi-Liang Zhu at the South China Normal University and his colleagues achieved this by building on their past work, in which they measured another quantum object that pertains to particles of light, or photons. This object, known as the propagator, predicts how a photon’s quantum state changes as it travels from one part of the experiment to another – in this case, as it travels through a maze of tiny mirrors, lenses and crystals.
Previously, the team had worked out how to measure different properties of photons, for instance polarisation, then reconstruct their propagators. Now, they realised that a path integral for a given photon could be constructed by measuring a succession of propagators then multiplying them together. Specifically, they measured five propagators for a given photon across the experiment.
Because there were so many different ways in which each of the five propagators could change as the photon moved through the maze, the researchers ended up measuring 1,419,857 distinct paths. They then plugged all of those paths into Feynman’s path integral formula to find out what it predicted about the photon’s behaviour – and found it agreed with what the photon actually did. But seeing it work … was still astonishing,” says Zhu.
The uncertainty inherent to quantum mechanics has long left physicists wondering whether the observations we make on the quantum level reflect reality – a new test suggests they do “We had to improve almost every aspect of the experiment simultaneously,” says Zhu. The team had to significantly improve the precision of their past experiment because had they used it as is, after the five propagator multiplications the data would have looked nearly completely random, he says. Jörg Götte at the University of Glasgow in the UK says that the precision of the experiment is a real scientific advance.
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