Why can you turn clothing right-side-out?

http://math.stackexchange.com/questions/2755/why-can-you-turn-clothing-right-side-out

by ivoflipse • 14 years ago

145 33 14 years ago

math.stackexchange.com

sakai• 14 years ago

And this is why math is... awesome.

verganileonardo• 14 years ago

And this is why kids are awesome!

acheron• 14 years ago

Love this question and all the answers. I find topology fascinating even though I understand maybe 5% of it. I took a decent amount of math as part of my CS degree, but beyond basic calculus it was concentrated in probability, stats, and linear algebra; never came near topology. In hindsight I wish I had taken more math, but as a 19-20 year old student at the time, I was happy to be done with it.

If I invented a time machine, sometimes I think my second use of it would be to give my college self class choice and scheduling advice.

jacobolus2• 14 years ago

Topology is fun, and not especially mysterious if you give it the time. One of those subjects that is at the same time highly abstract and exercises the visual–spatial–kinetic thinking part of your brain. You can learn it whenever you like! (For a motivated student, I think self study of mathematics topics is better than a course with fixed problems anyhow, because you can move at your own pace, and take any path you prefer. Requires some focus though.) I don’t have enough experience with all the various textbooks to compare them, but I thought Munkres was alright.

xyzzyz3• 14 years ago

It depends, modern algebraic topology is frequently considered to be one of the most mysterious and abstract fields of math, along with things like algebraic geometry.

mturmon• 14 years ago

Nice observation, and 100% agree. Here's how one of the most painstaking explicators put it in the thread:

"I'm summarizing about 200 years of mathematics, almost none of which is standardly taught to undergraduates at almost any university."

which made me feel a little better.

oggy• 14 years ago

I majored in math with a minor in CS (undergrad). Of his entire technical explanation, I'm only familiar with the notions "automorphism" and "topology". To add insult to injury, I did have a course on topology, but it covered only so-called "point-set" topology, and not the algebraic variety thereof.

ctdonath2• 14 years ago

It's a matter of people imputing notions of "special" where they don't belong. Hence the importance of "proof" in math.

ETA: People are surprised that they can turn a sweater inside-out thru a sleeve or neck hole ONLY because they've imputed a "special" ability to the largest hole in the garment. The mathematical concept of "proof" strips away such imputations, leaving surprisingly unsurprising results - in this case, you can reverse a garment by pulling it thru one of its holes, be it the largest, smallest, or even a tear, because they're all just holes with nothing inherently topologically special about them. In a larger social concept: people tend to impute special attributes to various things where such attribution is not warranted; people who understand the concept of mathematical proof are less likely to get caught up in such incorrect imputation.

xyzzyz3• 14 years ago

I didn't quite get your comment (I'm a non-native speaker). Could you please be more elaborate?

karamazov2• 14 years ago

It's not because you're a non-native speaker - that comment doesn't make sense.

jacobolus2• 14 years ago

Ctdonath was claiming that many people notice that a shirt can be turned inside-out through the main hole at the bottom, or a glove through the wrist hole. Far fewer people try turning a shirt inside-out through a sleeve or neck hole, or a pair of pants through a rip in the knee. They then abstract the wrong way from this experience, inventing the idea that the size of the hole is important (or maybe just that the “main” hole is special).

I’m not sure how prevalent these misconceptions are. It would be interesting to do surveys of the general public to see what kinds of mental models people have here.

Aloisius• 14 years ago

Who are these people who are surprised that you can turn a sweater inside-out through other holes?

To verify, I just went around my office and asked a couple people if they thought you could. Every person answered yes.

olavk2• 14 years ago

A kid, according to the story.

raverbashing• 14 years ago

Actually, in real clothes it would be difficult to turn it inside out through a small tear

Topologically possible, but with a physical constraint (or more likely, you increase the tear trying to do it)

psyconn• 14 years ago

I would further argue that there is a special size of the hole beyond which turning a thing inside out takes less than optimal time.

I don't know the math, but empirical data seems to confirm the hypothesis so maybe someone can provide the math name of the special hole.

olavk2• 14 years ago

You cant turn a pair of trousers inside out through the belt loops though.

jcitme• 14 years ago

Yes you can, provided the belt loop is big enough.

pfortuny• 14 years ago

The real problem is: what would the world look like if the result were different?

That looks more interesting to me.

karamazov2• 14 years ago

For the result to be different, the real world would need to have a different sort of topology. For example, if we lived in a non-Hausdorff space, the result would potentially be different. A Hausdorff space is one where for any two different points, you can take a small ball around each point, and the two balls won't intersect. A non-Hausdorff space is just a space where this isn't true for at least one pair of points.

So, for example, if wormholes are real, the space we live in might not be Hausdorff. And if you had a wormhole sewn into your shirt in the right way, you wouldn't be able to turn it inside out.

adavies422• 14 years ago

thank you! i love asking that question of math results.

josefonseca• 14 years ago

Is this relevant?

https://upload.wikimedia.org/wikipedia/commons/5/55/Tesserac...

eru• 14 years ago

Not really. That looks like a 2-dimensional projection of a rotating 4-dimensional hypercube. The StackExchange question concerns 3-dimensional topology.

adavies422• 14 years ago

owww, my head....

jes5199• 14 years ago

a fun introduction to topology is this article "Using Asteroids [the game] to explain the topological classification of 2-manifolds"

http://everything2.com/user/sockpuppet/writeups/Using+Astero...

ars• 14 years ago

When I think of topology the first I do is remember that size means nothing in topology.

So the long sleeve of the shirt? Shrink it right down - you are left with just a hole, and no sleeve.

Then flatten out the curvature of the neck and other parts, and you are left with a flat sheet of clothing, with two holes in it. There is no inside or outside to this, meaning the two sides are interchangeable, and that's why you can turn the real shirt inside out.

Why can you turn clothing right-side-out?

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