Portrait of Jackie Appel

  • Researchers have proven that a fundamental law of physics applies in the quantum realm.
  • The principle of least action dictates that objects (barring interference) will always move along the route that requires the least action.
  • Not all the rules of everyday physics apply to quantum particles, but according to hard-to-make measurements from a new study, this rule certainly does.

The shortest distance between two points is a straight line, but the shortest distance doesn’t always mean the least work. What if that distance is straight uphill or in difficult terrain? If you’re looking to do as little work as possible, a straight line may not always be your best bet.

Humans may not always look for the easiest path. But when it comes to natural motions in systems, one of the fundamental laws of physics says that objects will always travel along the route requiring the least action. In physics, “action” has to do with things like energy, momentum, distance, and time.

Basically, without outside intervention, objects travel along the path of least resistance and least change. This is called the principle of least action. We know this applies to our everyday world, and now, thanks to a new study, we know this applies to the quantum world as well.

“A physicist’s ultimate dream is to write down the secrets of the entire universe on a small piece of paper, and the principle of least action must be on the list,” said Shi-Liang, one of the researchers from the project, in an article for new scientist. “Our ambition was to ‘see’ [the principle] in a quantum experiment.

Easier said than done. The research team from South China Normal University had to face the fact that not only is everything in the quantum realm small and hard to see, but the motions of quantum particles are complicated, really complicated. On the one hand, quantum states change when measured. And on the other hand, they can only be mapped using very complicated mathematics.

To best describe their behavior, scientists use a combination of two things: a wave function and a propagator. Wave functions describe the state of the particle and propagators describe how this state changes during the motion of a particle in a system. The problem is that wave functions and propagators are purely mathematical, and while they’re great at describing quantum particle behaviors, they often do so using imaginary numbers. Imaginary numbers are good at math, but are, by definition, impossible to measure.

In order to circumvent this problem, the team used a technique that had been established a few years earlier. In this technique, you essentially bounce and filter individual quantum light particles called photons through a maze of mirrors, crystals, and lenses. Eventually, the parts of the photon’s behavior described by the imaginary numbers will correspond to real measurable properties. Parts originally described by normal real numbers will also be measurable, and researchers will be able to reconstruct waveforms and propagators from actual measured data.


Once the maze was built, the researchers combined this technique with a new one they developed to avoid the “quantum state change when observed” problem. Then they sent individual photons through the maze and compared their behavior to the behavior predicted by the principle of least action and found that reality agreed with the theory, proving that quantum particles do in fact follow the principle.

“The measurements from this experiment are quite incredible and do not challenge our current understanding of quantum physics,” said Jonathan Leach, a quantum scientist not involved in the study. new scientist article. “It’s nice to see this theory brought to life in an experiment.”

There are many places where the quantum world and the everyday world don’t fit together. This is partly why researchers are always looking to improve the current standard model of physics. But in their desire to avoid action as much as possible, the quantum and the everyday are perfectly synchronized.

Portrait of Jackie Appel

Deputy News Editor

Jackie is a writer and editor from Pennsylvania. She particularly enjoys writing about space and physics, and enjoys sharing the weird wonders of the universe with anyone who wants to listen. She is watched in her home office by her two cats.

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