The year is 1603. An Italian shoemaker (and amateur alchemist) by the name of Vincenzo Casciarolo tried smelting some stones he'd found on the slopes of Mount Paderno, near Bologna. Unfortunately, he didn't hit the jackpot. No gold or silver as he'd hoped. But once the stone had cooled, he noticed something surprising: the stone glowed in the dark.
This is the Bologna Stone—the first of many artificial, persistently luminescent substances. Apart from fascinating natural philosophers, it wasn't particularly useful at the time. But now it's not just some wack nerd stuff: you've seen it in party bracelets, emergency lighting and road markings—and might start seeing them on streets in the future.
This isn't a gimmick, someday they might give us glowing cities that stay cooler and use less electricity.
These luminescent materials work by absorbing energy from visible light, which reduces the heat that would otherwise be dissipated in the environment, and re-emitting this light during nighttime. If you're interested, here's a bit on how they work:
Photoluminescent materials work by "trapping" the energy of a photon (a particle of light) and then re-emitting it as lower-wavelength (i.e. different colored) light. For fluorescent light bulbs, this energy is emitted immediately; whilst in persistently luminescent materials (the glowy stuff), store the energy for longer and emit it more slowly.
By substituting our light sources, it will cut down on electricity use by removing street lights and road markers from the grid, and replacing them with glowing sidewalks, street signs and even glowing buildings. Some cities in Europe are testing the technology with glowing bicycle lanes, and research is being conducted to create other implementations of these luminescent technologies. This could cut more than 1.6% of European electricity demand—according to the 2021 Annual Review of Materials Research.
New classes of engineered materials could be applicable in other, non-lighting, applications. For instance, "quantum dots" (semiconducting particles that can be made to glow and that are already being used in biological imaging) or perovskites (prime material for solar panels) materials used in solar cells that are also being studied for their luminescent properties.
Let's see how our lives will glow in the future.