22 ms·
Glass viscosity calculations debunk the myth of flow in medieval windows (2017)
- nikisweeting 6y agoFinally! I always knew my 7th grade science teacher was full of shit on that one, she was so adamant about some things that turned out to be wrong. This one was maybe more excusable because it was common to teach at the time. Maybe it ended up making us better students in the end because we learned to fact-check our teachers, and not to be adamant about things we only think are true (though I'm still guilty of that myself sometimes)...
- kace91 6y agoWhat other unsubstantiated claims did she make?
- zamfi 6y agoI'm not the original poster, but I was shocked by how many of the otherwise super smart friends I had in college were taught common falsehoods in high school. For example: the idea that your veins look blue through your skin because the deoxygenated blood in them is blue, and that it only turns red outside your body because it's exposed to oxygen.
- jcomis 6y agoOr the whole taste bud taste map of the tongue.
- nikisweeting 6y agoI think a lot of them have only been solidly disproven in the last ~15 years, and it takes some time for that science update to propagate through the world.
- elihu 6y ago...especially in a time before Internet access was common.
- monk_e_boy 6y agoOr how wings work.
- lozaning 6y agoFor anyone thats interested in what the parent is talking about: http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-tu... I've had people get like legit upset at me when i correct this.
- tidenly 6y agoMy god, thank you. I remember as a kid this explanation "the top has to move fast to meet the bottom stream at the trailing edge" always confused the hell out of me. Why do the top and bottom have to meet? How would they know? Even as a 10 year old it confused me no-one could answer this.
- monk_e_boy 6y agoEspecially for hydrofoils. Makes no sense
- GuB-42 6y agoThat's the now famous Bernouilli vs Newton debate. Personally, I am in camp Newton. Not that Bernouilli doesn't work, they both give the same results in the end, and they are both a simplification of the "truth" which is the Navier-Stokes equations. The reason I like the Newton explanation better is that if you start with the Bernouilli principle, it is much easier to come up with the wrong idea. The Newton is straightforward: push air down, plane goes up. A bit shallower but at least, the basic idea is hard to get wrong.
- rcxdude 6y agoIt's reasonable to phrase the working of a wing either in terms of momentum or in terms of a different in pressure between the top and bottom. What's incorrect is when trying to explain the action through bernouilli it's often asserted that the air above the wing must take the same time to transit the wing as the air on the bottom. Not only is this completely unjustified to assume when trying to solve the problem, it's also false in practice: in general air above and below will not pass the wing in the same time, even though the air over the top is travelling faster.
- parsecs 6y agoOr how reentry heating is apparently caused mostly by friction. (Compression of air is the main cause in actuality)
- pfdietz 6y agoNo, actually friction is closer to the truth. It's air running into air at a shock, a process that generates entropy, just as friction does. Compression is a process that is adiabatic, generating no entropy. And in an ideal gas the density increase across a shock reaches a finite limit even as the mach number goes to infinity.
- bernulli 6y agoI am having trouble seeing how compression in a shock would be isentropic? Care to elaborate? And why passing through a shock (compression) is akin to friction, but not to compression?
- pfdietz 6y agoBecause compression, as normally considered, does not generate entropy. It causes gas to become hotter because the temperature has to be higher for entropy to remain constant, as the volume is reduced. This is a reversible process. But shocks are inherently irreversible. But as my last sentence said, as the mach number goes to infinity the density increase across the shock is finite. Therefore, compression cannot be the (primary) source of the heating; dissipation is.
- bernulli 6y agoI have never seen compression to be considered inherently reversible, certainly not in the aerospace or hypersonics/reentry communities. I assume you come from a different background? Care to explain your second argument? Even when the density reaches a limit, the change in pressure over the shock does not, so you can pump in energy arbitrarily (of course your gas would dissociate at some point). The total pressure loss can be complete if your initial Mach number is high enough.
- jiggawatts 6y agoOne of my favourites is the myth that Iodine doesn't melt: https://www.youtube.com/watch?v=dPIaEWd8zf4 https://www.youtube.com/watch?v=dPIaEWd8zf4
- nikisweeting 6y agoTo be fair it was usually small, inconsequential things, not major parts of the curriculum. The only major one I remember is that water is conductive by itself. And that "C++ is the best programming language" (hah).
- p1necone 6y agoI'm sure you could make the argument using some definition of best. Perhaps best in it's impact via software created in it rather than the language itself. Although then I think C would win over C++ because of Linux. I'm imagining a slightly tongue in cheek 'best' alongside an implication of subjectivity. But given this was a highschool teacher I wouldn't be surprised if it was instead a multi choice test question with a single "correct" answer? Which of course would be ridiculous.
- nikisweeting 6y ago"make the argument using some definition of best" I won't take the bait to dive into that discussion! ;) Luckily she never tested us on it, I think she was understandably biased from working in C++ in an earlier job.
- wombatmobile 6y ago> To be fair it was usually small, inconsequential things I’ve always wondered about this bit: “One nation, under God, with liberty and justice for all”
- K2h 6y agoOnly 1 set of ‘primary’ colors
- jeffbee 6y agoThat traits of human physiology are heritable binaries. https://udel.edu/~mcdonald/mythintro.html https://udel.edu/~mcdonald/mythintro.html
- pilsetnieks 6y agoI think there's only so much time children can spend on learning all their subjects, so they learn Mendelian genetics, Darwin's evolution, Rutherford's atomic model because they're easy to understand, and close enough approximations for everyday life for non-specialists.
- jeffbee 6y agoSure, but some of those things they used to teach us about aren't heritable traits at all. The article recommends studying examples that fit Mendel's model.
- pjmorris 6y agoNot the OP, but a fourth grade substitute teacher made the claim that we always saw the same side of the moon because it didn't rotate. I don't know what got to me the most: that she didn't understand, that my attempt to explain it didn't go over, or that the other kids in the class didn't get it either. In any case, I've been working on improving my explanations ever since.
- hervature 6y agoWhat's your explanation now? I think people just get tripped up thinking about rotation of an object in circular motion and just ascribe one motion for the other. For example, if you imagine a newly reunited couple and they hug each other while simultaneously spinning, this is exactly what the moon and Earth are doing. I think most people would also say that the outside person isn't rotating. However, they would also say that the inner person is obviously spinning. Describing in words is often not the best way to impart explanations of physics, but the easiest way to show this concept for me is to imagine both people separately and consider a half turn. The inner person, rotates 180 degrees, simple rotation. What does the outside person have to do to get to the right position? They have to walk around the inner person to the other side and then rotate 180 degrees to see their partner again.
- solipsism 6y agoAnd your beef is that it does rotate, at a rate of one rotation per revolution? Sounds kind of pedantic to me, and confusing (certainly to kids).
- mercutio2 6y agoThe fact that we live our lives in a non-intertial reference frame is pretty mind blowing to everyone, I suppose. Doesn’t seem like a small point to me.
- pjmorris 6y agoI'll grant that I was a pedantic fourth grader. And I'll grant that the other kids didn't care. Still, seems to me that the teacher could at least skip the topic if it's confusing.
- nemo44x 6y agoAlmost everything we think (including me) is probably trash. We just need to accept we are mostly idiotic. Unless we put the time in to understand something that _maybe_ is somewhat understood by people that have put their lives into it. Truth is (heh) we are all mainly full of shit. We think we know more than we do. We confuse truth for BS constantly. We spend countless breaths debating unfalsifiable opinions as if they’re true - like we really know something. We are all mainly dumb at most things.
- ordu 6y ago> Almost everything we think (including me) is probably trash. We just need to accept we are mostly idiotic. I do not like this attitude. Everything is false, because the truth exists only in ideal condition of math. But it doesn't mean that we are all idiotic and know nothing worthy knowing. The trick is to understand the relation between knowledge and the reality, to feel this relation in your guts. It helps to know a lot of falsehoods, and to be smart, even wise and knowledgeable. > We are all mainly dumb at most things. I could agree with this, because to really know something one needs to spent all his life researching the topic. And even the life full of the dedicated research might be not enough. But as I see, to be dumb is not so about having no knowledge, as about having no idea of limits of one's knowledge.
- nemo44x 6y agoNo - truth doesn’t only exist in an ideal condition of math. I mean, yes maybe literally in some ways, maybe. But not really. Math doesn’t prove the sky is blue on a sunny day. My main point is that of falsifiability. You don’t need math to make a falsifiable argument. However, much of our discourse can’t even meet this basic qualification.
- ordu 6y ago> Math doesn’t prove the sky is blue on a sunny day. Yeah, it doesn't. So it is not a truth, but a belief. It is pretty strong belief, so strong that it almost indistinguishable from the truth. But nevertheless it is just a belief. > My main point is that of falsifiability. You don’t need math to make a falsifiable argument. However, much of our discourse can’t even meet this basic qualification. Even falsifiability is a too rigid property for some discourses. My main point is that attempts to find a silver bullet to shoot "bad beliefs" are no-go. Knowledge couldn't be bad or good, true or false. It is the way knowledge applied could be good or bad. For example, even "the sky is blue on a sunny day" could be misapplied. Take the sky of a Moon or a Mars for example, which are not blue in a sunny day. Or let us imagine atmosphere of Earth with a lot of dust after collision of Earth and an asteroid: on such Earth sky could be orange, or gray or of some other color. I mean that even "true" statements have limits of their truthiness. As an opposite example, a "false" statement could be applied in a "good" way. Earth is flat? Oh, yes, it is convenient to think so when dealing with a relatively small area of the Earth surface. Or when communicating with people, who believe that Earth is flat. You need only to remember the limits of this belief, to not misapply it accidentally. In math the situation a little different. Math statements also could go wrong when assumptions change (the sum of angles of a triangle could be not equal to PI, if triangle drawn not on a plane but on some other surface), but mathematicians make sure to strictly state all the assumptions as axioms and definitions. It is possible in ideal conditions, but not possible in real ones.
- elliekelly 6y ago"You won't always have a calculator in your pocket." - Every math teacher before 2005
- askvictor 6y agoCentrifugal force is a common one: https://xkcd.com/123/ https://xkcd.com/123/
- causality0 6y agoI had an electronics professor who in 2012 was adamant that in five years smartphones would be using x-ray transceivers to communicate with cell towers.
- deelowe 6y agoI think your professor read about millimeter wave and misunderstood. I've had others confuse the two in conversation as well.
- bigbubba 6y agoConfusing millimeters and nanometers? How? Visible light sits between the two, they're not even close.
- ClumsyPilot 6y agoWell, i once confused packaging and ordered ~100 kg of toiler paper
- coldtea 6y agoYou could have made a fortune selling your stock in the covid era
- kevin_thibedeau 6y agoGo brag on the prepper forums.
- coldtea 6y ago>Confusing millimeters and nanometers? How? Visible light sits between the two, they're not even close. Compared to kilometers and parsecs, they're extremely close...
- dheera 6y agoLots of grade school teachers are full of shit. I had one that claimed that a pentagon had 1 line of symmetry because "the top is bigger than the bottom".
- elmomle 6y agoAllow me to be dumb for a second--isn't that the case? I mean, if you label the vertices/edges then yes a regular pentagon has five distinct lines of symmetry, but typically people talk about lines of symmetry in a context where vertices are unlabeled, no?
- MacsHeadroom 6y agoA regular pentagon has 5 lines of symmetry. One from the center of each edge to the point on the other side.
- pgreenwood 6y agoThere are 2n symmetries for an n-sided polygon. In this case five rotations and five reflections. https://math.stackexchange.com/questions/490054/symmetries-of-a-pentagon https://math.stackexchange.com/questions/490054/symmetries-o...
- ordu 6y agoIn spite of an argument "the top is bigger than the bottom", it seems to me that he/she meant that there is no horizontal line of symmetry, so there is only vertical one.
- simongr3dal 6y agoI agree, but I think in most cases where grade schoolers are made to draw mirror symmetries on work sheets that all symmetries are included, even if they are simply the same symmetry but rotated.
- nemo44x 6y agoMost school teachers are full of it, but they don’t mean to. They are simply low level clerics in the church of truth. They recite a very lossy account of what they were ordained with at a likely low level institution. It’s not really their fault - people need jobs, the ministry needs repeaters, and the pay is bad. Your teachers weren’t the disease - they were just a symptom.
- bosswipe 6y agoThis idea of a shadowy unified people that control everything is a poor way to try to make sense of the world.
- nemo44x 6y ago“They” refers to the teachers in this context. Sorry to assume you spoke English as a first language.
- chowned 6y agoWho hurt you? Seriously, what's your problem?
- dang 6y agoWe've banned this account for repeatedly breaking the site guidelines. Please don't create accounts to do that with. If you don't want to be banned, you're welcome to email hn@ycombinator.com and give us reason to believe that you'll follow the rules in the future. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
- dang 6y agoIf you continue to post personal attacks and name-calling like "You are full of it" to HN, we will ban you. We've already had to warn you about this. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
- ars 6y agoThe article says the glass does actually have a viscosity, so maybe she was not quite that wrong - just about the magnitude. I think glass has a viscosity, but other solids do not, but will creep. I hope my understanding is correct - if not, can anyone help explain?
- aoki 6y agoI believe you are correct. I had to shorten the title, and removed “observable” because the word “medieval” already refers to the science class meme that the viscous flow is at human history timescale (as opposed to ~1 nm/10^7 years).
- GuB-42 6y agoThat's the reason why I love the internet. Things are so easy to fact check now, at least compared to the way it was before. Many people talk about the problem of "fake news". What they may not realize is that falsehood have always existed, but we now have unprecedented tools to debunk them. When I was a kid, I learned loads and loads of bullshit, the glass thing was one of it. The problem was, how could I know it was wrong? Obviously, I didn't know enough about material science to figure it out by myself, and I didn't know any material scientist. Books were mostly useless for that purpose, searching for the answers would have taken days at best, and I didn't have that much time to debunk something that insignificant. Now just searching "glass thicker at the bottom" on the web yields exactly what I need. In fact, I am quite sure, there is much less bullshit going on today, but it looks like there is more because that these are so much easier to figure out.
- abeppu 6y ago> In fact, I am quite sure, there is much less bullshit going on today, but it looks like there is more because that these are so much easier to figure out. You're "quite sure" but how would one fact check this? Other possibilities: - There are more new claims being made and circulated, even if the same proportion of them initially are false, the fraction that any given person can follow up on to validate or debunk decreases, and then the share of propagated claims which is false is higher. - The share false "facts" is higher because the ability to disseminate information has been democratized, but us amateurs have a lesser commitment to fact-checking than press professionals a generation ago. - The share of bullshit is greater today than when you were a kid because the internet makes it financially rewarding to churn out content regardless of accuracy.
- Kye 6y agoSee also: the myth of taste buds with specific functions neatly layered on the tongue.
- onion2k 6y agoThe problem is that it's obviously wrong with a simple experiment - just look at a few really old windows. I live about 1/4 mile from a church that was built 1300 years ago (https://en.wikipedia.org/wiki/St_Peter%27s_Church,_Monkwearmouth https://en.wikipedia.org/wiki/St_Peter%27s_Church,_Monkwearm...). There's not much of the original glass left but where there is the thicker part is occasionally at the top of the pane because that's how it was originally mounted. Teachers (and everyone else) will be wrong about things. That's OK. The problems happen when people don't bother to do any simple checks themselves, and just accept things as fact without doing any critical thinking or experimentation of their own.
- lookalike1974 6y agoI would not disagree that your teachers failed you or vice-versa. But making grand pronouncements about people ("everyone else") and their problems as you see them ("without doing any critical thinking or experimentation of their own") and teachers too - and apparently based solely upon your personal experience in school and your thoughts on one church that just happens to be physically nearby you - isn't gonna convince anyone. Your comment is ignorant and offensive to teachers.
- onion2k 6y agoI didn't suggest they're always wrong, or that all of them are wrong, or that some of them won't have done any checks on what they're teaching. I said that teachers will be wrong about things. That only means they're fallible and don't get things right all the time. If any teacher, or literally anyone else, is offended by the suggestion that they're human and could be wrong sometimes then I think I'm OK with offending them. If you think you're perfect and will never be wrong then frankly you deserve to be offended as often as possible.
- Anther 6y agoI’ve often said that it is not whether or not you offend someone; it’s whether or not someone is offended. Offence is up to the receiver after all.
- tinus_hn 6y agoI seem to remember research has found some windows were placed upside down so these would now support the story that glass flows up.
- JoeAltmaier 6y agoI was taught in physical sciences that friction was proportional to weight and surface area. We were given blocks and a 'force meter' that was a bendy springy length of metal in a wooden calibrated frame. So pull blocks on a surface with the meter to measure friction. Try different surface area, stack them for multiples of weight, plot on a graph. Mine weren't coming out on a straight line. Other people noticed theres' weren't either, so they fudged their data points. I put down the honest ones. I got it 'wrong', they got it 'right'. Of course that old myth was nonsense, my experimental method was correct, and I've felt the injustice for decades now!
- eitland 6y agoOur physics teacher were clearly happy when we got weird results and actually wrote down what we saw and wrote a comment on the difference between expect and observed results.
- Jakobeha 6y agobut tarmac does flow at an observable rate, although only observable over the course of years
- nikisweeting 6y agoEspecially if you live somewhere where the sun heats it over 50ºC! Then you get huge ruts from the trucks and potholes...
- deleted 6y ago[deleted]
- aoki 6y agoThe related example that I always remember is: https://en.m.wikipedia.org/wiki/Pitch_drop_experiment https://en.m.wikipedia.org/wiki/Pitch_drop_experiment
- Cactus2018 6y agoWikipedia - List of Common Misconceptions https://en.wikipedia.org/wiki/List_of_common_misconceptions https://en.wikipedia.org/wiki/List_of_common_misconceptions
- ternaryoperator 6y ago>Their measurements reveal that medieval glass has a much lower viscosity than expected at room temperature—16 orders of magnitude less than previous estimates It's pretty rare to read an article in which the degree of previous error is sixteen orders of magnitude.
- 4ad 6y agoThe way viscosity is measured doesn't follow naive expectation though. Water at 0C has over six times the viscosity of water at 100C, yet to the untrained human they seen to flow about the same. Olive oil is 100 times more viscous that water. Yoghurt about 10000 times more viscous than water.
- chabad360 6y agoFair, but keep in mind that it's orders of magnitude we're taking about.
- supernova87a 6y agoWhile this was specifically about glass from the middle ages, is there certain window glass more recently that does flow measurably? The soda lime glass referenced? Or when you see waves in window glass in 1930s buildings, is that too from the original imperfections in the glass fabrication itself?
- JimTheMan 6y agoI wonder if the craftsmen also knew that the windows were a little wedge shaped from manufacture and put the thick bit at the bottom. That's what I'd do at least, makes sense structurally. Then later, random people made guesses as to the 'flow' of glass because all the windows are thick on the bottom... If anyone can confirm this theory, you and I will feel eternally smug. Best of luck!
- mensetmanusman 6y agoHow does this viscosity compare to gold or silver? Those also have brownian motion under the force of gravity. I bet it’s higher than 1 nm / billion years.
- heikkilevanto 6y agoYeah, I learned this "fact" in school as well. Later I found a simple refutation: If a church window pane would deform by a measurable amount in 1000 years (say 0.5mm), then a 100-year old telescope or camera lens would be totally useless, and yet they seem to function all right.
- gattr 6y agoTo add some detail: a common minimum quality criterion for optical surfaces is λ/4 (divergence from an ideal surface), where λ is the wavelength used. For visible light this gives about 100 nm (though large professional telescopes are "figured" to more like λ/20). Any alleged glass creep visible in window panes over a few hundred years would quickly ruin any astronomical mirror (which definitely does not happen).
- Traster 6y agoI mean... the obvious answer to that is that we don't use the same techniques to manufacture a telescope or camera lens that we used to manufacture church windows. In fact we don't use glass for large telescopes specifically because they'll deform under their own weight. I'm not saying that original fact is correct, I'm just saying I don't think what you're saying disproves the idea that that glass could deform over time.
- gattr 6y agoThat's incorrect. We do use glass for large mirrors and it works just fine ([1], [2.1], [2.2]). Do not confuse these two effects: 1. Viscous liquid-like creep (does not occur). 2. Elastic deformation (sagging) under gravity - this of course takes place, and is compensated for by multipoint support and, in larger professional telescopes, "active optics" (actuactors that counter the deformation depending on mirror's orientation; not to be confused with "adaptive optics", which deform a mirror with high frequency to counteract atmospheric image distortion). [1] https://en.wikipedia.org/wiki/Hale_Telescope https://en.wikipedia.org/wiki/Hale_Telescope [2.1] https://en.wikipedia.org/wiki/Large_Binocular_Telescope https://en.wikipedia.org/wiki/Large_Binocular_Telescope [2.2] https://www.newswise.com/articles/steward-observatory-mirror-lab-to-cast-2nd-84-meter-lbt-mirror https://www.newswise.com/articles/steward-observatory-mirror...
- yters 6y agoWell, glass is still a liquid, which was the most counter intuitive claim to me, whether or not the flow is humanly perceptible. In fact it is even more counter intuitive now, since glass is an even more solid liquid.
- Pulcinella 6y agoIt makes more sense (to me at least) to think of glass as an amorphous solid and expand the concept of “states of matter.” It doesn’t have a repeating crystalline structure. I think expand the concept of states of matter is key. It really helps make sense of a lot more things when you move beyond just solid, liquid, and gas (and plasma). A good example are liquid crystals, in LCDs yes but also cell membranes. What state of matter is the cell membrane? It makes more sense to create a new phase to categorize it. The lipid molecules are arranged in a certain orientation to each other (crystalline) but are also free to shuffle past each other (liquid). This is not just academic. Your body uses cholesterol to modify the fluidity vs. crystalinity of your cell membranes so they don’t “freeze) (stiffen too much) in the winter or melt in the summer.