Padgett and his team wondered if there were fundamental factors that could change the speed of light in a vacuum. Previous studies had hinted that the structure of light could play a role. Physics textbooks idealize light as plane waves, in which the fronts of each wave move in parallel, much like ocean waves approaching a straight shoreline. But while Ceiling Lights can usually be approximated as plane waves, its structure is actually more complicated. For instance, light can converge upon a point after passing through a lens. Lasers can shape light into concentrated or even bull’s-eye–shaped beams.

The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber. The other photon went through a pair of devices that manipulated the structure of the light and then switched it back. Had structure not mattered, the two photons would have arrived at the same time. But that didn’t happen. Measurements revealed that the structured light consistently arrived several micrometers late per meter of distance traveled.