December 2016 Media Thread

Some short papers worth reading.

A good introduction to topos theory, which in turn explains why the Yoneda embedding so so useful. I would not recommend this as an introduction to category theory or the Yoneda lemma.

A love letter to adjoint functors discussing their meaning and philosophical significance.

A paper on categorification, from which I will include a quote below in hopes of showing some of you heathens the light.

If one studies categorification one soon discovers an amazing fact: many deep-sounding results in mathematics are just categorifications of facts we learned in high school! There is a good reason for this. All along, we have been unwittingly ‘decategorifying’ mathematics by pretending that categories are just sets. We ‘decategorify’ a category by forgetting about the morphisms and pretending that isomorphic objects are equal. We are left with a mere set: the set of isomorphism classes of objects.

To understand this, the following parable may be useful. Long ago, when shepherds wanted to see if two herds of sheep were isomorphic, they would look for an explicit isomorphism. In other words, they would line up both herds and try to match each sheep in one herd with a sheep in the other. But one day, along came a shepherd who invented categorification. She realized one could take each herd and ‘count’ it, setting up an isomorphism between it and some set of ‘numbers’, which were nonsense words like ‘one, two, three, . . . ’ specially designed for this purpose. By comparing the resulting numbers, she could show that two herds were isomorphic without explicitly establishing an isomorphism! In short, by decategorifying the category of finite sets, the set of natural numbers was invented.

According to this parable, decategorification started out as a stroke of mathematical genius. Only later did it become a matter of dumb habit, which we are now struggling to overcome by means of categorification. While the historical reality is far more complicated, categorification really has led to tremendous progress in mathematics during the 20th century. For example, Noether revolutionized algebraic topology by emphasizing the importance of homology groups. Previous work had focused on Betti numbers, which are just the dimensions of the rational homology groups. As with taking the cardinality of a set, taking the dimension of a vector space is a process of decategorification, since two vector spaces are isomorphic if and only if they have the same dimension. Noether noted that if we work with homology groups rather than Betti numbers, we can solve more problems, because we obtain invariants not only of spaces, but also of maps. In modern parlance, the nth rational homology is a functor defined on the category of topological spaces, while the nth Betti number is a mere function defined on the set of isomorphism classes of topological spaces. Of course, this way of stating Noether’s insight is anachronistic, since it came before category theory. Indeed, it was in Eilenberg and Mac Lane’s subsequent work on homology that category theory was born!

Is Radical Life Extension Good for Society?

"Should we embrace our end, or should we cure aging? Are human lifespans long enough as is?

This was the central motion of a provocative debate recently hosted by Intelligence Squared. Pitting a philosopher and a sociologist against two scientists, the well-rounded debate delved into the ethical and social consequences of radically increasing human lifespan."

http://singularityhub.com/2016/12/01/is-radical-life-extension-good-for-society/

and this is interesting as well

Portions of the brain fall asleep and wake back up all the time, Stanford researchers find

https://www.eurekalert.org/pub_releases/2016-12/su-pot120116.php

"FOR SOME, he was one of the most subversive thinkers of his time — a 20th-century Nietzsche, only darker and with a better sense of humor. Many, especially in his youth, thought him to be a dangerous lunatic.

That Cioran is an unsystematic thinker doesn’t mean that his work lacks unity; on the contrary, it is kept tightly together not only by his unique writing style and manner of thinking, but also by a distinct set of philosophical themes, motifs, and idiosyncrasies. Among them failure figures prominently."

https://lareviewofbooks.org/article/philosopher-failure-emil-ciorans-heights-despair/

Newest issue of Compelling Science Fiction out, some good short stories on AI and cybers

http://compellingsciencefiction.com/stories/wearedestroyers.html

http://compellingsciencefiction.com/stories/allthatissolid.html

http://compellingsciencefiction.com/stories/thelittlegods.html

The solitude of prime numbers by Paolo Giordano. Interesting, unusual in its attitude to many things it contains, frightening when you think about solutions that the characters don't even take into consideration. I was hesitating over recommending it, but then I had A Sign From Above.

I work in a bookshop, and one of the perks is that I can borrow books like in a library. This one I liked, but was of two minds whether I should buy it, because one has to stop somewhere. So I asked my husband.

The first day he said it was cool.

The second day he said it was great.

The third day he said we were buying it.

'Why?' - 'I had A Sign From Above.' - '...' - 'I was reading it on the tube, and a drop of dirty water fell onto the page...'

I'm listening to Prince Igor by Borodin for the first time. Very grand.

https://www.youtube.com/watch?v=CzmIu-VjRCM

I heard this was almost the theme music for the anime Neon Genesis Evangelion, which is fun to imagine.

A note for gwern: please post a link to your juicy monthly link thread!

http://lesswrong.com/r/discussion/lw/o77/december_2016_media_thread/diq9