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In 1673, Christiaan Huygens wrote a book on pendulums and how they work. A mechanical theorem mentioned in the book was used 350 years later by researchers at the Stevens Institute of Technology to explain the complex behaviors of light, a university statement said.

Although known to us for eons, humanity has found it difficult to explain the very nature of light. For centuries scientists have been divided on whether to call it a wave or a particle and when there seemed to be some agreement on what light could actually be, quantum physics threw a new curveball by suggesting that it existed as both at once.

Researchers who were earlier working to disprove claims of the opposite faction have now been spending time explaining how light displays properties of both waves and particles at once.

To do so, a team led by Xiaofeng Qian, a professor of physics at Stevens Institute of Technology turned to a 350-year-old mechanical theorem that explains how objects like pendulums work.

Huygens proposed that light propagates in the form of waves through the entire universe. But the Dutch physicist also explained how the energy required to rotate an object depended on its mass and the axis around which it had to be turned.

This mechanical theorem could be used to explain the movement of objects such as pendulums as well as planets.

Applying this to light had one hurdle though. The theorem used the mass of the objects and light does not have any mass. Qian's team, therefore, used the intensity of light as the equivalent of physical objects' mass. It then became possible to map measurements on a coordinate system for interpreting Huygens' theorem, the statement added.

This enabled the team to visualize light as part of a mechanical system and connections between wave properties such as entanglement and polarization became clearer, the researchers stated.

Reconciling the two schools of thought on whether light is a wave or a particle has been particularly difficult. While the new research does not solve this problem, it demonstrates that there are connections between these two frameworks, which do not exist only at a quantum level but also at the classical physics level, where one is dealing with waves and point-mass systems.

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“What was once abstract becomes concrete: using mechanical equations, you can literally measure the distance between ‘center of mass’ and other mechanical points to show how different properties of light relate to one another”, said Qian in the statement.

Investigating these relationships further can help scientists in evaluating the properties of not just hard-to-measure optical systems but quantum systems as well. Deductions for these systems can now occur using light measurements which are not just much simpler to achieve but also more robust from a data collection point of view.

Further, researchers could also apply the same system to explore the complex behaviors seen in quantum wave systems. "Ultimately, this research is helping simplify the way we understand the world, by allowing us to recognize the intrinsic underlying connections between apparently unrelated physical laws,” Qian added.

The research findings were published in the journal Physical Review Research.

Abstract

While optics and mechanics are two distinct branches of physics, they are connected. It is well known that the geometrical/ray treatment of light has direct analogies to mechanical descriptions of particle motion. However, connections between coherence wave optics and classical mechanics are rarely reported. Here we report links of the two through a systematic quantitative analysis of polarization and entanglement, two optical coherence properties under the wave description of light pioneered by Huygens and Fresnel. A generic complementary identity relation is obtained for arbitrary light fields. More surprisingly, through the barycentric coordinate system, optical polarization, entanglement, and their identity relation are shown to be quantitatively associated with the mechanical concepts of center of mass and moment of inertia via the Huygens-Steiner theorem for rigid body rotation. The obtained result bridges coherence wave optics and classical mechanics through the two theories of Huygens.