John C. Baez's Blog, page 36

US Environmental Policy (Part 4)

Biden’s executive orders are now going beyond revoking those of the previous president. He may really take climate change seriously. It’s starting to scare some people.

• Juliet Eilperin and Brady Dennis, As Biden vows monumental action on climate change, a fight with the fossil fuel industry has only begun, Washington Post, 27 January 2021.

Joe Biden had long promised to become the climate president, and on Wednesday he detailed far-ranging plans to shift the U.S. away from fossil fuels, cr...

Open Systems: A Double Categorical Perspective (Part 3)

Back to Kenny Courser’s thesis:

• Kenny Courser, Open Systems: A Double Categorical Perspective, Ph.D. thesis, U. C. Riverside, 2020.

Last time I explained the problems with decorated cospans as a framework for dealing with open systems. I vaguely hinted that Kenny’s thesis presents two solutions to these problems: so-called ‘structured cospans’, and a new improved approach to decorated cospans. Now let me explain these!

You may wonder why I’m returning to this now, after three months of sile...

US Environmental Policy (Part 3)

It’s begun! On his first day in office, the new president of the US signed this executive order:

ACCEPTANCE ON BEHALF OF THE UNITED STATES OF AMERICA

I, Joseph R. Biden Jr., President of the United States of America, having seen and considered the Paris Agreement, done at Paris on December 12, 2015, do hereby accept the said Agreement and every article and clause thereof on behalf of the United States of America.

Done at Washington this 20th day of January, 2021.

JOSEPH R. BIDEN JR.

He also ...

Categories of Nets (Part 2)

guest post by Michael Shulman

Now that John gave an overview of the Petri nets paper that he and I have just written with Jade and Fabrizio, I want to dive a bit more into what we accomplish. The genesis of this paper was a paper written by Fabrizio and several other folks entitled Computational Petri Nets: Adjunctions Considered Harmful, which of course sounds to a category theorist like a challenge. Our paper, and particularly the notion of Σ-net and the adjunction in the middle column relat...

Categories of Nets (Part 1)

I’ve been thinking about Petri nets a lot. Around 2010, I got excited about using them to describe chemical reactions, population dynamics and more, using ideas taken from quantum physics. Then I started working with my student Blake Pollard on ‘open’ Petri nets, which you can glue together to form larger Petri nets. Blake and I focused on their applications to chemistry, but later my student Jade Master and I applied them to computer science and brought in some new math. I was delighte...

Categories of Nets

I’ve been thinking about Petri nets a lot. Around 2010, I got excited about using them to describe chemical reactions, population dynamics and more, using ideas taken from quantum physics. Then I started working with my student Blake Pollard on ‘open’ Petri nets, which you can glue together to form larger Petri nets. Blake and I focused on their applications to chemistry, but later my student Jade Master and I applied them to computer science and brought in some new math. I was delighte...

This Week’s Finds (1–50)

Take a copy of this!

• This Week’s Finds in Mathematical Physics (1-50), 242 pages.

These are the first 50 issues of This Week’s Finds of Mathematical Physics. This series has sometimes been called the world’s first blog, though it was originally posted on a “usenet newsgroup” called sci.physics.research — a form of communication that predated the world-wide web. I began writing this series as a way to talk about papers I was reading and writing, and in the first 50 issues I stuck closely to ...

CP Violation

Here are two more open questions about physics. I have a question of my own at the end!

Why are the laws of physics not symmetrical when we switch left and right, or future and past, or matter and antimatter? Why do the laws of nature even violate “CP symmetry”? That is: why are the laws not symmetrical under the operation where we simultaneously switch matter and antimatter and switch left and right?

Violation of P symmetry, meaning the symmetry between left and right, is strongly visible ...

Applied Category Theory 2021 — Adjoint School

Do you want to get involved in applied category theory? Are you willing to do a lot of work and learn a lot? Then this is for you:

• Applied Category Theory 2021 — Adjoint School. Applications due Friday 29 January 2021. Organized by David Jaz Myers, Sophie Libkind, and Brendan Fong.

There are four projects to work on with great mentors. You can see descriptions of them below!

By the way, it’s not yet clear if there will be an in-person component to this school —but if there is, it will h...

Cosmic Censorship

I seem to be getting pulled into the project of updating this FAQ:

• Open questions in physics.

The more I look at it, the bigger the job gets. I started out rewriting the section on neutrinos, and now I’m doing the part on cosmic censorship. There are even bigger jobs to come. But it’s fun as long as I don’t try to do it all in one go!

Here’s the new section on cosmic censorship. If you have any questions or have other good resources to suggest, let me know.

Does Cosmic Censorship hold?  R...