{
  "id": 16882284,
  "title": "The paradox of steady-state light diffusion",
  "url": "https://urgent.news/2026/10/28/the-paradox-of-steady-state-light-diffusion",
  "topic": "ai",
  "section": "AI",
  "published": "2026-10-28T08:48:30.000Z",
  "source": {
    "name": "Physics World",
    "slug": "physics-world",
    "url": "https://physicsworld.com/a/the-paradox-of-steady-state-light-diffusion/"
  },
  "original_language": "en",
  "account": "Diffuse optics is a field that examines how light travels through materials which scatter light strongly, such as biological tissues, powders, and disordered solids. Calculating the path of every individual photon is impractical in these materials, so the more complex radiative transfer equation is often replaced by the simpler diffusion equation, which should become more accurate as light travels further. The diffusion equation includes a coefficient known as the diffusion coefficient, which quantifies how quickly light spreads. For over three decades, researchers have been unable to agree on how to define this coefficient, despite its importance. The commonly used definition includes an explicit dependence on the material's absorption, which creates a problem since it contradicts a fundamental scaling property of the radiative transfer equation. This inconsistency has persisted for decades, with several alternative definitions proposed, but none gaining widespread acceptance. A recent study by André Liemert and colleagues aims to resolve this issue once and for all. The researchers found that the differences between competing definitions are extremely small, making them hard to distinguish using experiments or simulations. Instead, they used analytical methods, comparing solutions of the radiative transfer equation for both pulsed and continuous light. For a brief pulse of light, they showed that the long-term behavior aligns with the diffusion description only if the diffusion coefficient does not depend on absorption, as required by the radiative transfer equation. However, for continuous illumination, a paradox arises. Even at great distances, where diffusion would normally improve accuracy, no single diffusion coefficient can accurately capture both the light signal's decay rate and its amplitude. This means that while steady-state diffusion remains a useful approximation, it cannot be the exact long-distance limit of light transport in absorbing materials. Consequently, any future work in biomedical optics or materials physics that relies on diffusion models to estimate absorption and scattering properties must consider these findings. The full article is titled \"The paradox of steady-state light diffusion\" and was published in the journal Reports on Progress in Physics (89 087501) by A. Liemert et al.",
  "summary": "Researchers have resolved a 30-year debate by showing when the diffusion equation provides a consistent description of light transport, and when it fundamentally falls short The post The paradox of steady-state light diffusion appeared first on Physics World .",
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}