A Hacker News thread discusses a study about shining near-infrared light through the entire human head to potentially measure brain activity. The original study detected photons on the opposite side of the head, but commenters are skeptical about its practical applications. One commenter explains that current fNIRS techniques use shorter light paths to measure oxygenated/deoxygenated hemoglobin fluctuations, a proxy for brain activity. However, the new technique's long light path may result in noisy data. It's also noted that the experiment had restrictive conditions (light skin, no hair). Other comments debate whether the technique could create 3D images like CT scans, but the diffractive medium may limit resolution. Concerns are raised about the technique's current limitations to fair-skinned individuals and whether it might become controversial if it became clinically useful. Some users entertain more speculative applications, like photon teleportation.