In 1620, Francis Bacon wrote a book arguing that scientists should trust experiments instead of old philosophers.

And in that book he somehow found the time to mention that sugar emits light when you crush it. This non-sequitur is the first documentation of a phenomenon we now call triboluminescence.

Today, we know that his sugar wasn't special; tons of different crystals shine under stress like, those minty Lifesavers candies that spark blue when you chew them. And modern scientists are researching ways of working stuff that glows like this into all sorts of futuristic devices. Although they can't quite agree why it happens in the first place.

Triboluminescence, also called mechanoluminescence, is the release of some sort of light from some sort of crystal because of some sort of physical stress. And if that definition sounds pretty wishy-washy to you, that's honestly because it /is/. The most famous kind of triboluminescence is from Wint O Green Lifesavers.

If you take one into a dark bathroom and bite down on it, crushing the sugar crystals inside, you can often see a little flash of blue light in the mirror. But some crystals shine when they're fractured instead of completely broken, and others shine when they're stretched or squeezed. And sometimes, the color of the light that comes out depends on the gases surrounding the crystal, but sometimes it doesn't.

This isn't a rare phenomenon, either. Scientists estimate that between 30 and 50 percent of crystals triboluminesce. They even found x-rays shooting off a roll of clear tape when they unpeeled it under a vacuum back in 2008.

But all this variety has also made understanding the phenomenon pretty tricky. Many scientists think that if you can unify all these different effects under a single heading, something fundamentally similar is probably happening in all of them. And that “something” seems to be somewhat straightforward.

Breaking a crystal means breaking chemical bonds between atoms. In messier breakups, atoms on one side can lose electrons to atoms on the other. The side with extra electrons becomes negatively charged, which pushes those electrons back across the gap to the other piece of the crystal.

And as they move back and settle down around the right atoms, the electrons emit the light that we recognize as triboluminescence. This model helps explain why triboluminescence is more common in asymmetric crystals -- where the arrangement of atoms is different in different directions . That asymmetry can make it easier for electrons to move or pile up in one direction as opposed to another.

And scientists have even counted electrons coming from fracturing triboluminescent crystals. But it doesn't fully explain more complicated cases. Like, certain crystals shine in different colors depending on the kind of gas around them.

Crush sugar in nitrogen and it flashes blue, but crush it in neon and it's red. It's pretty clear that those electrons aren't just interacting with the crystals; they're also doing some sort of dance with the atoms around the crystals. But, the kind of gas doesn't always change the color of the light.

Whether it does or not depends on the crystal. And scientists are still working on why. Some crystals don't need to be totally broken to shine, either.

They'll shine every time they're stretched or compressed. Nobody's even positive what makes some fractures messier than others. Lots of things can make electrons find new temporary homes, from the fracture changing a crystal's symmetries to some extra charge that hitched a ride on whatever hit the crystal in the first place.

And things can get even murkier, like when crystals have impurities in them, unexpected atoms that change how electrons move around. That makes it really hard to study triboluminescence on its own, isolated from other effects. Like, let's go back to those Lifesavers.

You'll see light when you crush basically any sugar. But most of the light emitted isn't visible—it's UV. And it just so happens that the oil which makes Wint-O-Green Lifesavers taste the way they do is fluorescent: it absorbs ultraviolet light and re-emits it as blue light that we can see.

That's why the flash from them is so much stronger than from other candies: some of the light we see when the candies crack is straight triboluminescence, and some is the blue from their fluorescence being activated by the UV rays from triboluminescence. So it's hard to determine exactly what's happening at the atomic level. People are working to understand exactly what happens in all these complicated cases.

But we don't need to know exactly how they work to put them to good use, which is why some scientists are working on innovative ways to capitalize on these sparks. Like, there's the idea that doctors could inject tiny triboluminescent probes that act like lamps, lighting up fluorescent substances so they can map out what's going on inside a person's body. Other engineers see a future where crystals that get crushed by some natural process give off light that we can turn into electricity much like we do solar light.

And others have proposed a triboluminescent layer for equipment that can wear out over time. Once it's worn enough to expose this layer, it'll start glowing, and everyone will know right away that it's time to replace it. Similar tech could be used in building materials to warn engineers of weakening long before the structure becomes unsafe.

And those are just a few of the applications being explored. Triboluminescence is already all over the place, in the sense that it occurs in a good fraction of the crystals where scientists have looked for it. But give it a few years, and it really might be everywhere.

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