## November 25, 2013

### Kan Extension Seminar applications

#### Posted by Emily Riehl

A friendly reminder: applications for the Kan Extension Seminar are due at the end of the week. More information can be found in the initial announcement and on the seminar website.

For those who don’t enroll, watch this space. You’ll be hearing from us again soon after the new year.

Posted at 5:04 PM UTC | Permalink | Followups (1)

## November 21, 2013

### The 8th Scottish Category Theory Seminar

#### Posted by Tom Leinster

ScotCats 8 will take place in Edinburgh next Friday, 29 November, with an all-star programme featuring one of your $n$-Café hosts:

I’m particularly looking forward to this one: it promises to be an excellent afternoon of talks.

The seminar web page has practical details. Thanks to the Glasgow Mathematical Journal Trust and the Scottish Informatics and Computer Science Alliance for funding.

## November 19, 2013

### The Covariance of Coloured Balls

#### Posted by David Corfield

When you reach over 150 messages in response to a post, it’s probably time to start a new one, especially for those with inefficient browsers. In this post I want to see if I can make sense of discussions we were having there about general relativity, covariance, groupoids and sameness in a much more intuitively clear setting, though perhaps it will introduce its own problems.

OK, so say I have a set, $B$, of 5 indiscernible balls in a box, each of which can be coloured in 3 ways. Then I might take the state space to be $[B, 3]$, of cardinality 243. But perhaps while doing physics on these balls, I come to see that nothing about them matters aside from their colour. Since I can’t tell the difference between any two versions of 2 reds, 2 greens and a blue, I decide to reduce my state space to $[B, 3]/Sym(B)$, a set of equivalence classes of cardinality 21.

However, after a time this starts to strike me as odd. I have represented the fact that any colouring of 2 reds, 2 greens and a blue are equal, and yet were someone to swap a green and a red I’d notice in a way that I wouldn’t notice a swap of two reds. Any swaps in a state of 5 red balls would also go unnoticed. At the very least, there seems to be a difference between a monochrome state and a tricoloured state.

Posted at 12:35 PM UTC | Permalink | Followups (136)

## November 18, 2013

### Categories for the Working Philosopher

#### Posted by John Baez

Elaine Landry, in the philosophy department at U. C. Davis, is putting together a book called Categories for the Working Philosopher.

Posted at 4:09 AM UTC | Permalink | Followups (53)

## November 12, 2013

### Four New Talks

#### Posted by Tom Leinster

In October I did little but talk. Five talks in five locations in 23 days, with only one duplicate among them, left me heartily wishing not to hear my own voice for a while.

Having gone to the effort of making slides, I might as well share them publicly. All the talks are on topics that have come up on the Café before. Here they are:

Posted at 4:02 PM UTC | Permalink | Followups (4)

## November 10, 2013

### Severing Ties with the NSA

#### Posted by Tom Leinster

Updated on 11 Dec 2013: see end of post.

A letter from Chicago mathematician Sasha Beilinson in this month’s Notices of the American Mathematical Society calls for the AMS to sever all ties with the US National Security Agency, citing

the vast secret spying programs of the NSA that wildly exceed anything conspiracy theorists could imagine.

He lists some of the ways in which the AMS and NSA support each other, and issues a call for action:

What should be done is a question not only for US citizens but also for people all over the world: the NSA destroyed the security of the Internet and privacy of communications for the whole planet. But if any healing is possible, it would probably start with making the NSA and its ilk socially unacceptable — just as, in the days of my youth, working for the KGB was socially unacceptable for many in the Soviet Union.

Posted at 7:23 PM UTC | Permalink | Followups (69)