Spin may resolve century-old puzzle of light's momentum in matter

Amazing stuff! This paper (only 7 pages) was written by a single researcher employed at an university in Indonesia.

According to Google Scholar the lifetime citations of Adam B. Cahaya are only 358. Maybe this citation count will go up now. He published about 69 articles so far.

"When you shine a flashlight into a glass of water, the beam bends. That simple observation, familiar since ancient times, hides one of the oldest puzzles in physics: what really happens to the momentum of light when it enters a medium? ...

For more than a century, scientists have debated whether light's momentum inside matter is larger or smaller than in empty space. The two competing answers are known as the Minkowski momentum, which is larger and seems to explain how light bends, and the Abraham momentum, which is smaller and matches the actual push or pull that light exerts on the medium.

The controversy never went away because experiments seemed to confirm both sides. Some setups measured the larger Minkowski value, others supported Abraham, leaving physicists with a paradox. ...

What I found is that the resolution lies not in choosing between Abraham and Minkowski, but in recognizing that both are correct—once we include spin in the picture. ...

Spin is the intrinsic "twist" of light waves, a quantum property as fundamental as energy or momentum. By projecting momentum onto spin, I found a unified description: the Minkowski momentum corresponds to the magnitude of this spin-projected momentum, while the Abraham momentum is its expectation value—the average that actually drives motion and is directly tied to the Lorentz force on the medium.

This reframes the problem: Abraham and Minkowski are not rivals but complementary aspects of the same spin–momentum structure. ..."

From the abstract:

"The momentum of light in dielectric media has been a century-long controversy that continues to attract significant interest. In a linear dielectric medium with refractive index 𝑛, the momentum is predicted to be smaller by a factor of 𝑛 according to Abraham and larger by the same factor according to Minkowski.

By studying the coupled dynamics of electromagnetic waves and dipoles in a dielectric medium, we show that the change in momentum of the dipole, expressed by the Lorentz force, corresponds to the Abraham momentum and is given by the expectation value of the spin-projected momentum vector. On the other hand, the Minkowski momentum is obtained as the magnitude of the spin-projected momentum vector from the energy-momentum dispersion relation derived by diagonalizing the coupled Hamiltonian and determines the direction of refraction in accordance with Snell's law.

Our model also predicts a zitterbewegung-like oscillation due to helicity mixing between left- and right-handed wave components, mediated by dipole oscillation. These internal wave dynamics may be observable via wave-packet motion or polarization-sensitive measurements."

Spin may resolve century-old puzzle of light's momentum in matter (This article is also written by the scientist of the article below)

Zitterbewegung, momentum, and spin dynamics of electromagnetic waves in a linear dielectric medium (no public access)

Zitterbewegung, momentum and spin dynamics of electromagnetic waves in linear dielectric medium (preprint, open access)

When light enters a medium like water or glass, it bends, raising the long-standing question of how much momentum it carries inside. By projecting momentum onto spin, I found a consistent picture: the larger Minkowski momentum corresponds to the magnitude of this spin-projected momentum, while the smaller Abraham momentum is the expectation value of the vector.