Dutch-born Christiaan Huygens is probably one of the most famous physicists you've never heard of. His work in the late 17th century straddled both the intangible and tangible realms of our Universe: the nature of light, and the mechanics of moving objects.

Among his many contributions, Huygens proposed a wave theory of light that would give rise to physical optics, which deals with the interference, diffraction, and polarization of light. He also invented the first pendulum clock; the most accurate timekeeper for almost 300 years, right through the Industrial Revolution.

Little has been made of the connections between these two seemingly disparate fields of optics and classical mechanics – until now.

A pair of physicists at the Stevens Institute of Technology in New Jersey have revisited Huygens' seminal work on pendulums, published in 1673, and used his 350-year-old mechanical theorem to uncover some new connections between some of the strangest, and most fundamental, properties of light.

"With this first study we've shown clearly that by applying mechanical concepts, it's possible to understand optical systems in an entirely new way," says physicist Xiaofeng Qian.

Qian and his colleague at the Stevens Institute, Misagh Izadi, considered two properties of light in their calculations: polarization and a form of correlation known as classical, or non-quantum, entanglement.

These two properties reflect the strange duality of light that permeates every pocket of our Universe. In a quantum sense, light – like all forms of matter – can be described as waves rippling through space, but also as discrete particles localized to a single point.

This isn't just a quantum phenomenon, however. In the classical world of cogs and springs and tick-tocking clocks, light waves rise and fall like physical ripples on an intangible ocean, with properties linked to their ever-shifting progress through space.

"We've known for over a century that light sometimes behaves like a wave, and sometimes like a particle, but reconciling those two frameworks has proven extremely difficult," said Qian.

"Our work doesn't solve that problem – but it does show that there are profound connections between wave and particle concepts not just at the quantum level, but at the level of classical light-waves and point-mass systems."

Most commonly considered a quantum phenomenon, entanglement simply describes correlations in the properties of objects.

For particles, it could be the spins of electrons, or the momentum or position of a pair of photons. Knowing something about one of these characteristics for one particle tells you something about the same characteristic for the other.

Classical entanglement also describes certain correlations, only without a need to consider the unsettled nature of an object prior to its measurement.

Polarization is the directional property of a light wave oscillating up and down, or left and right. Particles like photons, the packets of energy that make up a beam of light, can be polarized too.

If a light wave oscillates, and so does a pendulum, then Qian and Izadi thought they might be able to use the mechanics of the latter to describe the properties of the former.

"Essentially, we found a way to translate an optical system so we could visualize it as a mechanical system, then describe it using well-established physical equations," Qian explains.

Ordinarily, classical mechanics is used to describe the movement of large, physical objects like pendulums and planets. For example, Huygens' parallel axis theorem describes the relationship between masses and their rotational momentum.

Qian and Izadi envisaged light as a mechanical system to which Huygens' parallel axis theorem could be applied, and found a "profound" connection: the degree of a light wave's polarization was directly related to the degree of a recently recognized property called vector-space entanglement.

Qian and Izadi's calculations suggest that as one rises, the other falls, enabling the level of entanglement to be inferred directly from the level of polarization, and vice versa.

"Ultimately, this research is helping to simplify the way we understand the world, by allowing us to recognize the intrinsic underlying connections between apparently unrelated physical laws," Qian says.

The study was published in Physical Review Research.