Scientists Used a 350-Year-Old Theorem To Explain Light

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A 350-Year-Old Theorem Can Help Explain LightFlavio Coelho - Getty Images
  • Researchers have used a 350-year-old theorem to better understand the strange nature of light.

  • The team took a theorem used for describing pendulums and substituted the brightness of light in for what would usually be an object’s mass.

  • In doing so, the researchers were able to map several properties of light and quantum systems onto the theorem, revealing a few entirely undiscovered relationships.

What is light, exactly? At a basic level, is it a wave or a particle? Well, if you ask a physicist you’re going to get a likely rather unsatisfying answer: yes. It’s a wave, it’s a particle, and on quantum scales it’s even both at once.

This isn’t new information. We’ve known about the strange behavior of light for some time now, but there’s still a lot about this seemingly dichotomous nature that we don’t understand. Recently, however, researchers got a little bit closer to fully understanding the puzzle that is the behavior of light.

It would be tempting—and fairly reasonable—to guess that the answer lay in some newly-discovered secret of the quantum world. But for this piece of understanding, the key was a 350-year-old theorem of classical mechanics.

“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,” Xiaofeng Qian, lead researcher on the study, said in a press release. “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.”

The central 350-year-old theorem in this work has long been used to describe the behaviors of pendulums and large, massive bodies like planets. It is meant to show how the energy needed to rotate an object can vary based on two main factors—mass and axis.

Generally speaking, this shouldn’t apply to light. Any theorem centered on the behavior of mass should have pretty much nothing to do with light, as light is massless. But the researchers saw the potential for explanative power in this theorem and made a leap—they used light intensity in place of mass.

“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 said in a news release.

Once that substitution was made, the team was able to map other properties of light onto the theorem, and some interesting relationships started to emerge. Central among them was the discovery that, apparently, there is an inverse relationship between polarization (the directionality of a light wave) and entanglement (the “spooky action at a distance” where particles are linked behaviorally across space). When light becomes more polarized, it becomes less entangled, and vice versa.

“This was something that hadn’t been shown before, but that becomes very clear once you map light’s properties onto a mechanical system,” Qian said in a press release. “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.”

The researchers anticipate that this and other relationships brought to… light… by this experiment will allow us to make better measurements of various hard-to-measure properties of light or quantum systems. If these relationships work the way the study implies, we should be able to measure such things as amplitudes and phases of light just by measuring the light’s brightness—which is much easier. The team’s application of the theorem may also—hopefully—allow us to use mechanical systems to accurately simulate and better understand quantum systems.

“That still lies ahead of us,” Qian said in a news release, “but 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.”

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