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westurner OP
From https://phys.org/news/2023-08-physicists-year-old-theorem-re... :

> The work, led by Xiaofeng Qian, assistant professor of physics at Stevens and reported in the August 17 online issue of Physical Review Research, also proves for the first time that a light wave's degree of non-quantum entanglement exists in a direct and complementary relationship with its degree of polarization. As one rises, the other falls, enabling the level of entanglement to be inferred directly from the level of polarization, and vice versa. This means that hard-to-measure optical properties such as amplitudes, phases and correlations—perhaps even these of quantum wave systems—can be deduced from something a lot easier to measure: light intensity.

> [..] Qian's team interpreted the intensity of a light as the equivalent of a physical object's mass, then mapped those measurements onto a coordinate system that could be interpreted using Huygens' mechanical theorem. "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," explained Qian.

> Once the team visualized a light wave as part of a mechanical system, new connections between the wave's properties immediately became apparent—including the fact that entanglement and polarization stood in a clear relationship with one another.

westurner OP
Does light have mass?

Photons cannot have mass in GR General Relativity because E=mc^2?

How do solar sails achieve thrust from photons and solar pressure if they are massless?

Photons are massless but have momentum.
westurner OP
So things with momentum but no mass can transfer such force though they are massless?
Yes (some people would say the photon has a "mass equivalence" but that's really just conceptual)! One of the greatest scientific discoveries of all time:

https://en.wikipedia.org/wiki/Radiation_pressure

"""Johannes Kepler put forward the concept of radiation pressure in 1619 to explain the observation that a tail of a comet always points away from the Sun.[9]

The assertion that light, as electromagnetic radiation, has the property of momentum and thus exerts a pressure upon any surface that is exposed to it was published by James Clerk Maxwell in 1862, and proven experimentally by Russian physicist Pyotr Lebedev in 1900[10] and by Ernest Fox Nichols and Gordon Ferrie Hull in 1901."""

westurner OP
"Bridging coherence optics and classical mechanics: A generic light polarization-entanglement complementary relation" (2023) https://dx.doi.org/10.1103/PhysRevResearch.5.033110
westurner OP
Christiaan Huygens > Legacy: https://en.wikipedia.org/wiki/Christiaan_Huygens

Horologium Oscillatorium by Huygens: https://en.wikipedia.org/wiki/Horologium_Oscillatorium

Parallel axis theorem: https://en.wikipedia.org/wiki/Parallel_axis_theorem :

> The parallel axis theorem, also known as Huygens–Steiner theorem, or just as Steiner's theorem,[1] named after Christiaan Huygens and Jakob Steiner, can be used to determine the moment of inertia or the second moment of area of a rigid body about any axis, given the body's moment of inertia about a parallel axis through the object's center of gravity and the perpendicular distance between the axes

...

Harmonic oscillator: https://en.wikipedia.org/wiki/Harmonic_oscillator

Quantum Harmonic Oscillator: https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

Pendulum (mechanics) https://en.wikipedia.org/wiki/Pendulum_(mechanics)

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