The classical principle of least action now exists in the quantum realm

quantum entanglement, conceptual artwork

Quantum particles take the easy way outVICTOR de SCHWANBERG/PHOTO LIBRARY OF SCIENCE – Getty Images

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  • Researchers have demonstrated that a fundamental law of physics applies in the quantum realm.

  • The principle of least action dictates that objects (unless they are interfered with) always move along the path that requires the least amount of action.

  • Not all the rules of everyday physics apply to quantum particles, but measurements that are difficult to make certainly apply, according to a new study.

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

Humans may not always be looking for the easiest path. But when it comes to natural motions in systems, one of the basic laws of physics states that objects will always travel along the path that requires the least action. In physics, action has to do with things like energy, momentum, distance, and time.

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

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

Easier said that done. The research team at South China Normal University had to come to terms with the fact that not only is everything in the quantum realm small and hard to see, but the motions of quantum particles are really complicated. For one thing, quantum states change when they’re measured. And for another, they can only be mapped using very complicated math.

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

To get around this, the team used a technique that had been established a few years earlier. In this technique, you basically bounce and filter individual particles of quantum light called photons through a maze of mirrors, crystals, and lenses. Eventually, the parts of photon behavior described by imaginary numbers will correspond to actual measurable properties. Parts originally described by ordinary 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 that technique with a new one they developed to primarily avoid the quantum state change when looking at the problem. Then, they sent single photons through the maze and compared their behavior with the behavior predicted by the principle of least action and found that reality agreed with the theory, proving that quantum particles do indeed follow the principle.

The measurements in this experiment are quite incredible and do not challenge our current understanding of quantum physics, said Jonathan Leach, a quantum science researcher who was not involved in the study. New scientist item. It’s nice to see this theory made real in an experiment.

There are a lot of places where the quantum world and the everyday world don’t intersect. It’s part of the reason why researchers are still trying to improve on 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.

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