The surprising scientific weirdness of glass

“There’s a myth about glass you might have read about in high school: If you go to a church that’s hundreds of years old and look at the glass windows, you’ll find that the panes are thicker at the bottom of the frame than at the top. That’s because, according to lore, glass is actually a liquid, just one that flows very slowly.
This is a myth for a lot of reasons. The simplest is that the thickness of glass at the base of the windows can be explained simply by how glass panes were manufactured in the olden days. Back then, flat windows were made by spinning a glass form into a flat disc, which left the finished product with uneven thickness.

But also as a scientific explanation, the myth does not do glass justice. Glass is so much weirder than a very slow-moving liquid. In fact, even though glass is one of the most common, most useful materials in the world — lining our windows, covering our phones, delicately holding our stems of roses — scientists still have deep questions about what it fundamentally is.

“It defies the very simple categories we have of liquid, solid, and gas,” says Camille Scalliet, a theoretical physicist at the University of Cambridge. She’s not the only scientist flummoxed by glass. All over the world, physicists, chemists, and other specialists are trying to unlock its secrets.

It’s true that glass does have some liquid-like properties. But remarkably, rather than flow, glass doesn’t move very much at all. In 2017, scientists analyzed the church glass myth in a paper, determining that, over a billion years, church windowpanes would flow a single nanometer. (That is one-billionth of a meter; it’s infinitesimally tiny. A piece of paper is around 100,000 nanometers thick.)

And this finding gets us closer to the deepest mystery of glass. The question scientists grapple with isn’t “why does it flow.” Instead, “we don’t really know why it’s solid,” Scalliet says.”

“Solids and liquids are both made up of atoms and molecules. Temperature changes how these components are arranged. Cooler temperatures solidify molecules; warmer temperatures make them juicy.

The important differences are seen on the microscopic scale of molecules. In liquids, the molecules are very disordered; they move around each other and flow. “If you could zoom in and see individual molecules, they would be packed randomly and they would be moving around very fast,” Scalliet says.

I think of a liquid like a crowd of people dancing at a club. They’re energetic, packed in, vibing. They can move around each other, bump and grind, dancing to the music. If you took a snapshot of the dancers, it would look like a chaotic, jumbled mess. That’s a liquid.

Solids are much more tame. As we typically think of them, they are made up of crystals, which are structured, orderly patterns of molecules. When the temperature cools down, the atoms and molecules line up in a regular geometric pattern. In the dance club metaphor, instead of undulating past each other, these ravers stop dancing and sit down in concert seats. They can still squirm a bit in those seats (as long as the thermostat in the theater isn’t set to absolute zero), but they’re mostly locked in place.”

“The simplest explanation for how glass forms is that it’s a liquid that cools too quickly for those crystals to form. So the molecules get locked in place in a chaotic liquid-like arrangement.

Imagine you’re in the crowded dance space, and you decide you need to use the bathroom. But when you try to get there, a lot of the dancers decide to stop moving. When that happens, it becomes harder and harder for you to navigate across the dance floor. “If you’re with your partner and you want to just trade places, you can’t do it because you’re so jammed, you need to get other people to move,” David Weitz, a Harvard physicist, says.

And when you can’t move, it makes it harder for other people to move around you. So gradually, and then very suddenly, the whole dance floor seizes up. You’re locked in place, and not in an orderly geometric pattern. It’s a mess. It’s glass. And you’re not going to make it to the bathroom in time (again, it might take some billions of years to move just nanometers).

This is the basic definition of a glass: a liquid that has been locked in place. Or, in science-speak: an “amorphous solid.” And it applies to a lot of materials, not just the silica-based glasses that hang in our windows or cover our phones.”

“Some plastics are considered glasses, as are natural materials like amber. And some parts of your cells are considered to be glass-like. Even foams like whipped cream can be described as glass-like, Weitz says. Finding out the underlying mechanics that connect all these forms of glass, that’s “the real challenge to me, the beauty of the whole science.””

“If you take a picture of the molecular structure of a glass and the molecular structure of a liquid, they look the same. So why does one flow and another is locked in place?

“There are currently different ways to explain this, why the glass is not moving,” Scalliet says. But no theory is universally agreed upon.

The various explanations involve some very math-heavy invocations of thermodynamics. But in short, scientists are in search of a deeper order to this system that we can’t see just in a snapshot — something to explain glass’s solidness like you could explain the solidness of table salt by pointing to its crystal structure. The secret is likely in the collective action of the molecules over time, and how they influence one another as the liquid seizes up.”

“In practical terms, it matters that scientists don’t have a complete theory of glass. For one, it means they simply don’t understand glass as well as they do crystalline solids.

With a crystalline solid, you can predict many of the properties of the solid just by looking at its simple crystal structure. Just by knowing the arrangement of the molecules in the crystalline solid, “you can understand, for example, how the solid will absorb heat,” Scalliet says, or “where it will break.” But in the case of glass, “you have basically an infinite number of arrangements. You don’t have this well-known underlying structure.””

” it’s hard to predict the properties of glass. We learn how glass breaks by breaking it, how it holds on to heat by heating it. That leaves the manufacturing of new types of glass to be a bit of trial and error. But the lack of a complete theory also leaves scientists with some fundamental — even existential — questions about what glass truly is.”

“glass will still flow a tiny bit over millions and billions of years. If we lived for that long, and experienced the passage of time more quickly, we might not think glass is very mysterious at all. We might think it was a liquid.

It could also be that, also over an immense period, glass will eventually crystallize and become a typical solid. In this light, glass is just liquid “that’s sliding on its way to being a crystal,” Mark Ediger, a chemistry professor at the University of Wisconsin Madison, says.”

“there’s another exciting possibility here: that instead of crystallization, over very long periods, glass can inch closer to the state of “perfect disorder,” as Ediger describes.”

The science is clear. So why can’t governments agree on vaping?

““It’s really a product that’s good for some people and bad for other people, which doesn’t feel like too complex of a statement, but actually feels like something that is difficult for many to grapple with,” said Hartmann-Boyce, who is an associate professor of evidence-based policy and practice at the University of Oxford.
She led a 2022 Cochrane review — considered the best type of analysis of the available evidence — which looked at studies of e-cigarettes for smoking cessation. It found the strongest evidence yet that vaping works better than traditional nicotine replacement tools such as patches or gum to help people stop smoking. For those advocating that vaping is an effective harm-reduction mechanism, it was a significant win.

But it’s also more complicated than that.

Hartmann-Boyce said that since Cochrane first started looking at the evidence nearly 10 years ago, things have changed dramatically. The devices themselves are different now and are much better at delivering nicotine. That’s good for people trying to give up smoking but creates a problem with non-smokers like kids who are trying these for the first time.

But not everyone is even convinced it’s good for most smokers in the long term.

Jørgen Vestbo, a clinician and emeritus professor of respiratory medicine at the University Hospital of South Manchester, who recently returned to his native Denmark, agrees that the randomized controlled trials show e-cigarettes can help people quit.

But he also points to data from clinical trials that show people given e-cigarettes were more likely to use them for longer than those using aids such as nicotine gum. Vestbo said population-level evidence shows that as long as you are addicted to nicotine you are more likely to start smoking again.”

Will diet soda, yogurt, and cereal disappear from stores?

“the World Health Organization’s International Agency for Research on Cancer (IARC) declared aspartame as “possibly carcinogenic.” Another WHO committee, the Joint FAO/WHO Expert Committee on Food Additives (JECFA), independently assessed the ingredient, too, but maintained its existing recommendation — suggesting not that people cut the substance entirely out of their diets but that they limit their daily aspartame consumption to about 40 mg per kilogram (or about 2.2 pounds) of body weight. Diet soda contains about 200 mg of aspartame per 12-ounce can. By that measure, an adult weighing 60 kg, or roughly 132 pounds, would need to drink about 12 cans of diet soda a day to exceed the JECFA’s recommendation, assuming they had nothing else containing aspartame.
Making matters more confounding, the Food and Drug Administration had yet another take. It told Vox in an email that it had reviewed the information used in WHO’s assessment and “identified significant shortcomings” in the studies the agency relied on. “Aspartame is one of the most studied food additives in the human food supply,” the agency added.”

Even the scientists who build AI can’t tell you how it works

“there’s two connected big concerning unknowns. The first is that we don’t really know what they’re doing in any deep sense. If we open up ChatGPT or a system like it and look inside, you just see millions of numbers flipping around a few hundred times a second, and we just have no idea what any of it means. With only the tiniest of exceptions, we can’t look inside these things and say, “Oh, here’s what concepts it’s using, here’s what kind of rules of reasoning it’s using. Here’s what it does and doesn’t know in any deep way.” We just don’t understand what’s going on here. We built it, we trained it, but we don’t know what it’s doing.”

“The other big unknown that’s connected to this is we don’t know how to steer these things or control them in any reliable way. We can kind of nudge them to do more of what we want, but the only way we can tell if our nudges worked is by just putting these systems out in the world and seeing what they do. We’re really just kind of steering these things almost completely through trial and error.”

Is weed safe in pregnancy?

“In a 2020 review of relevant studies published since the mid-1980s, the authors called out many of these studies for weak methodology. In particular, many researchers had failed to compare the outcomes they were measuring against any kind of a standard that would account for age and parental educational level. (That is: What if the kids of those who used cannabis during pregnancy were born to parents with lower levels of education, which could account for some differences?)
The review authors concluded that overall, “prenatal cannabis exposure is associated with few effects on the cognitive functioning of offspring.” What’s more, they noted, even when abnormalities were identified, almost all were still within the range of normal.”

“Despite the imperfect data, Mark suspects the risk of fetal harm with prenatal cannabis use is high enough to support recommending against purely recreational use.

But many aren’t seeking to get high.”

More States Are Using Science-Backed Reading Instruction. It Shouldn’t Have Taken This Long.

“Since 2013, Mississippi, Alabama, and Louisiana have all passed legislation mandating that teachers be trained in the “science of reading”—methods that typically center around phonics, an approach in which children are taught to read words by decoding the sounds that different letters or groups of letters make. Since these policies’ implementation, reading performance in these states has dramatically improved, even though reading scores there have historically been among the lowest in the nation.”