srean10 hours ago
Oh no! woe is me, they don't highlight my absolutely, ridiculously favourite fact/curiosity about a sheet of smooth paper:
If you fold it clean, the crease is a straight line. In fact I don't know of any other good way of obtaining a straight edge from scratch quickly, meaning without transporting one existing straight edge to another (*).
I remember spending a lot of enamored time coming up with different geometrical proofs of this fact. Perhaps the only time I have come close to jumping out of the proverbial bath tub.
The underlying reason is that paper does not stretch (**) (but, paradoxically, it does bend fine. It's a paradox because bending needs stretching).
I have to restrain myself from grabbing strangers off the streets to ask -- how cool is that.
Three other demonstrations that never fail to nerd-snipe me like this are Dirac's belt trick, that straight woven cloth rips usually at 90 degrees, and the working of a teeny tiny metacircular interpreter.
(*) Rope stretching is a close competitor, but the tension needs to be really really high and it is difficult to run a pencil along it to mark a straight line, lest you distort the st. line.
(**) of course, it does, but a tiny amount.
Coming back to straight line folds, this property holds beyond just Euclidean space, it holds for Riemannian geometry and probably for any continuous metric space.
roelschroeven8 hours ago
And once you have created such a straight line, you can fold the paper again such that the first crease lines up on both sides of the new crease, and then you have a right angle.
srean8 hours ago
Indeed !
One can create an axiomatic system of geometry through such coincident folds (as an alternative to straight-edge and compass) and it turns out to be more powerful than the Euclidean system.
One can construct cube roots, trisect angles.
Depending on the choice of paper folding axioms one can go beyond cube roots and k-secting angles to the entire set of algebraic numbers.
anax327 hours ago
I'm not getting any work done today
thank you!
srean6 hours ago
LoL. Most welcome.
My best (worst) HN days are like that :)
foobarian2 hours ago
One thing I find interesting about paper is that wetting and drying it turns it uneven. Even when drying it under a press.
And then another ridiculous process not involving paper, but super cool nonetheless is creating a flat surface by grinding 3 not-flat objects against each other in round-robin manner.
mjanx1232 hours ago
Three negatively curved surfaces (saddles) mate despite not being flat. You need to rotate the surfaces also when lapping. (See the famous Robrenz video)
sreanan hour ago
I did not know this, very interesting.
sreanan hour ago
I think wetting and drying of paper is a bit like heat treatment of steel. The fibres find a new local minimum stable position, prompted by the swelling and the shrinking.
wongarsu6 hours ago
If you want to use a rope to get a straight line, your best bet is to turn the rope itself into the pencil. Coat it in chalk or other powder, then put it under tension and snap it on to the desired surface
bena6 hours ago
This is actually a tool used in construction. A chamber filled with chalk and a coiled line. You hook the line to one end of your item, pull the chamber across, make it tight, snap the line.
https://www.homedepot.com/p/Milwaukee-100-ft-Bold-Line-Chalk...
mauvehaus2 hours ago
The Tajima ones [0] are phenomenal, though the hook leaves a longer blank stretch than I'd like. They make a super nice snap knife too. Highly recommend Tajima for anything they make. Annoyingly, they don't sell a rip saw, only crosscut.
[0] https://www.tajimatool.com/product_category/mt/#chalk-rite
UltraSane5 hours ago
Using one of these to snap a perfect reference line is extremely satisfying.
saagarjha8 hours ago
I'm a fan of tearing paper along a crease rather than cutting it for this reason, since the tear is straight and using scissors will invariably be all over the place.
srean8 hours ago
Yup and if you need a right angle, combine it with this
https://news.ycombinator.com/item?id=48538771
by roelschroeven.
I often wondered how to ensure that the corners of a sheet of paper make a right angle. You need that to form a square sheet, otherwise the standard trick of folding along the diagonal gives a rhombus, not a square.
arijun10 hours ago
> The underlying reason is that paper does not stretch
I don't think that's sufficient--tinfoil doesn't stretch, but it doesn't fold nearly as neatly as paper.
qsera8 hours ago
"Paper folds in a straight line" and I was like "duh! what else?" Until I read this comment, and it bought back all the memories where I tried to fold other things like plastic sheets and tin foils and how they never ended in straight line...damn. I never noticed...
srean10 hours ago
You are perhaps commenting about the force needed to fold, the persistence of the folded shape. My comment is about the shape of the crease once it has been folded.
Most metals are stretchier than paper. If it is thick it will resist folding, but once you have folded it, that is, the two flat boundary surfaces have coincided, the crease would be a straight line if the surfaces cannot stretch.
How much force you will need to exert to form a fold depends on material properties but the geometrical nature of the crease is dictated by stretching.
mjanx1235 hours ago
Paper is thin so the stretching needed to bend it is minimal
srean5 hours ago
Exactly.
jayd162 hours ago
rub 3 surfaces together and they end up smooth and flat. Fast in the across the world sort of way but not fast like folding a sheet of paper.
Still a sheet of paper is already made smooth and consistently flat. I'm not sure how well it works from hand made paper.
rusk7 hours ago
The shortest distance between two points is a straight line?
A sheet of paper approximates a Cartesian plane probably more closely than most things we can fold
Therefore a fold will always be in line with the theoretical 2D plane and thus will be the shortest (straight) line.
globular-toast7 hours ago
> In fact I don't know of any other good way of obtaining a straight edge from scratch quickly
A string made taut between two points is surely a better way? And works at much bigger sizes too (people build walls and foundations using this technique all the time). The paper is less useful in practice because any paper you find is probably straight and square anyway.
Still, I had fun thinking about this as I definitely hadn't considered it before.
dotancohen6 hours ago
Over long distances the string will sag in the middle. That's one of the reasons (not the only reason) construction uses lasers today.
gacgacgac4 hours ago
Folded paper won't?
boothby2 hours ago
If anybody has ever tried folding a very large paper (or, bedsheets, tarps, etc), they'll realize the wisdom of this comment. Our intuition from folding paper on the order of several to tens of centimetres does not scale to arbitrary size and precision. Paper is relatively rigid, but its rigidity is finite and ensuring local-to-global flatness becomes a painstaking endeavour.
srean2 hours ago
Then there is the puzzle of coming up with folds to intentionally increase the rigidity of the surface. Essentially, using Theorema Pizzarium
josefx3 hours ago
Folded paper has some structure, so not as much?
boothby2 hours ago
It depends what you need that line for -- if you're projecting onto a wall, that sag matters. If you're projecting onto the ground, it doesn't.
srean7 hours ago
I have an asterisk in my post addressing that :) Happy to have picqued your mind.
BTW, your method was the method of choice for the surveyors of the Nile, from the Egyptian civilization.
Paper is hi-tech and was not available until much later, and as you mentioned doesn't scale. But if I have misplaced my ruler ...
Peterz_shu6 hours ago
It has a lot of air gaps within its fibers.
We know that Solids CANNOT be compressed. So what's actually being folded is the air gaps.
Which is why you can't easily fold a piece of tungsten. It has less air gaps.
gruez6 hours ago
But you can fold aluminum foil just fine? That certainly doesn't have "a lot of air gaps within its fiber"
dotancohen6 hours ago
Actually I've never had success cleanly folding aluminium foil. There are always imperfections and creases.
borutz11 hours ago
Nice old video about this: https://www.youtube.com/watch?v=EKEavnS10HI
sampo6 hours ago
It was this paper folding video (and the guy's unapologetic Finnish accent) that launched Hydraulic Press Channel to fame.
aequitas9 hours ago
The Mythbusters proved you can fold it up all the way to 11: https://www.youtube.com/watch?v=65Qzc3_NtGs
arijun10 hours ago
Unless I'm missing a transcript somewhere, this is missing an [audio] tag.
yread11 hours ago
And why does it sink. Its basically squished wood!
flybrand2 hours ago
because the fibers are denser than wood and hydrophilic - so the pockets between the fibers fill with water that displaces the air.
Air pockets become water pockets = neutral.
Fibers denser than water = sink.
OJFord10 hours ago
I really can't imagine getting paper to sink any more easily than wood?
RetroTechie4 hours ago
Try toilet paper. Let it into the water vertically as to not catch air pockets.
tobyhinloopen9 hours ago
Because the air is squished out I suppose
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leebeef9 hours ago
This is exactly the sort of hard-hitting journalism that makes me proud to pay my TV licence.
tomalpha5 hours ago
It’s always hard to tell whether a comment like this is supportive or sarcastic so this is either me agreeing or disagreeing with you…
For me this kind of simple, straightforward, educational content is part of the reason I’m proud to pay my TV licence.
ern5 hours ago
I'm not from the UK, but the soft power of BBC Radio 4 in the late 90s and early 2000s (the Real Player era) made the UK seem like an advanced nation to my young and intellectually curious self. If lived in the UK at the time, I'd have been immensely proud of the quality of the programming.
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zeristorop2 days ago
A lovely little podcast on paper physics for origami.
mixseo9 hours ago
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