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Agent of change
June 29, 2006
DeThreaded Comments
I love threaded comments. When I visit an old blog entry, it’s great to have the comments organized thematically, with responses threaded with the comments that inspired them.
On the other hand, while a comment thread is developing, there can be several parallel conversations happening at once. And it can be hard to figure out, from the threaded view, where the new comments are. A chronological view would be more convenient. Of course, you can always subscribe to my comment feed, but that’s not quite the same thing.
Fixing this has been a low-priority item on my TODO list for a long time. Finally, one of my commenters spurred me into action. Thanks to my previous experiments in DOM-wrangling, it was fairly easy to whip together something that would toggle between the default threaded-view and a chronological view (with trackbacks followed by comments, in chronological order).
I haven’t tested it yet in a wide variety of browsers. So, if you encounter problems with it, please let me know.
June 25, 2006
The LQG Landscape
Over at Cosmic Variance, in a long, and somewhat histrionic comment thread, Lee Smolin makes a new (to me, at least) claim about LQG
LQG easily incorporates most proposals for beyond the standard model unification including supersymmetry.
Since I’m afraid it will get buried over there, I thought I would drag the discussion of this rather important physics point over here. Hopefully, some LQG experts can chime in and explain Lee’s statement.
- What classes of quantum field theories can be incorporated in LQG and what classes cannot?
- In what sense do the former constitute “most”?
- In light of the fact that “most” can be coupled to LQG, how are we to deal with Georgi’s objection (which is discussed at greater length here) ?
In the same comment, Lee also says
… someone might earn a Clay prize by rigorously constructing quantum Yang-Mills within LQG. It will certainly not be me, but there are people working on exactly that program. The conjecture is that background independent QFTs are more likely to exist rigorously in 3+1 dimensions than Poincare invariant QFTs.
It would also be interesting for someone to chime in with an explanation of the intuition for why coupling to quantum gravity should make the problem of constructing quantum Yang Mills theory easier, rather than harder.
June 19, 2006
Metastable Vacua
The number of vacua in an supersymmetric field theory is, generically, dictated by the Witten index. This number is robust against perturbations, which makes dynamically breaking supersymmetry a challenge. One needs to start with a theory whose Witten index vanishes, and one needs to hope that the dynamical effect which break supersymmetry do not lead to runaway behaviour (like the runaway behaviour in supersymmetric QCD with ).
One of the nice insights of recent years is that we are not necessarily interested in true vacua (global minima of the effective potential). We might just as well be living in a metastable “false vacuum”, provided its lifetime is long enough. And, despite the paucity of true vacua (absent some symmetry), generic field theories can be chock-a-block with metastable vacua.
June 18, 2006
itex2MML 1.1.5
Bugs, bugs, bugs. It seems there’s always something to fix.
Since itex2MML 1.1.3, I’ve fixed the following:
June 10, 2006
Topological T-Duality
In a recent post, I talked a bit about what the mathematicians call “Topological T-Duality.” I posed the question as to how confident we were that their definition is right. A discussion ensued about what “being right” means in this context, and what the implications might be if the definition of Mathai, Bunke and others isn’t right.
June 9, 2006
Planetary Style
Aggregating and republishing over two dozen feeds, of varied provenance, is not without its pitfalls. Wordpress.com’s atom feed templates are somewhat problematic. As a consequence, some of Tommaso Dorigo’s posts were filled with doubly-escaped entities. Fortunately, Sam came through with a fix. Georg von Hippel’s Tex → PNG software plays poorly with Blogger. The latter inserts <br />s into the alt attributes of the created images, which can cause BeautifulSoup to cough up a hairball where the PNG image should be (hopefully fixed now).
But, for the most part, the results look pretty good1. Some further tweaks, however, made it look even better.
It is enlightened practice to separate styling information from the markup of your document, and place the former in an external CSS stylesheet. An aggregator, like Planet Musings takes blog entries from a variety of sources, and republishes them with a consistent style.
But, sometimes, something essential gets lost in ignoring the external stylesheets of the original sources. That’s where the “Accommodations” section of our stylesheet comes in.
For instance, Cosmic Variance uses CSS classes like class="alignright" and class="alignleft" to float images. Adding directives like
.alignleft { float:left }
made their entries “look right.” On similar grounds, adding
img.mathlogo, img.svglogo { float:right; border:0 }
makes the entries from Musings and The String Coffee Table look more pleasing.
More interesting, though, is the CSS styling of Math, tidbits of which have accumulated, over the years, in the stylesheets of Musings and The String Coffee Table. Now I had to think about which of those bits are actually important enough to merge into the “Accommodations” section of the Planet Musings stylesheet. So I thought I’d share some tips on styling MathML.
Browsers ha