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Low-functioning pinhead...
April 29, 2004
itex2MML 0.8
A small update to itex2MML. This one covers ellipses.
- \ldots,\dots
- horizontal ellipsis, … ()
- \cdots
- centered horizontal ellipsis, ⋯ ()
- \ddots
- descending diagonal ellipsis, ⋱ ()
- \udots
- ascending diagonal ellipsis, ⋰ ()
- \vdots
- vertical ellipsis, ⋮ ()
As always, a MacOSX binary is included in my source distribution. Thanks to Abiola and James, updated Windows and Linux binaries are also available.
April 28, 2004
Nima2
One of my favourite young physicists, Nima Arkani-Hamed, was in town today. He gave two talks.
One was about his work on spontaneously-broken diffeomorphism invariance. Specifically, consider a theory in which spatial diffeomorphisms are preserved, but time-translations are spontaneously-broken. There’s a scalar field which, in a really horrible pun, they call the ghostino, whose expectation value satisfies Expanding the field about its VEV,
under an infinitesimal diffeomorphism,
so transforms as a scalar under spatial diffeomorphisms, but transforms inhomogeneously under temporal ones, as befits a Goldstone boson. We also impose a shift symmetry under . The naive time-translation symmetry of a static spacetime is broken in this background, but the combination
is unbroken.
If you write out a general symmetry-breaking effective Lagrangian for (compatible with the shift symmetry and, for simplicity, with ), and expand it about a minimum, you find something like (after rescaling to give it a canonically-normalized kinetic energy)
The dispersion relation is a nonrelativistic one (unsurprising, since the symmetry-breaking has picked out a preferred Lorentz frame) and power-counting is a bit unconventional. should have mass dimension , should have mass dimension and should have mass dimension . The leading interaction term is
and is irrelevant in the infrared, so there’s a good perturbative effective field theory description.
Anyway, if you take eV, the coupling of this theory to gravity modifies gravity at cosmological distance scales, with interesting ramifications for cosmology.
There’s a bit of a swindle here, since the theory just described breaks down above the scale , and requires some ultraviolet completion there. However, if couples only gravitationally, they argue that it doesn’t really matter what the ultraviolet completion is. While there remains a challenge to embed this in a “real” theory, their effective Lagrangian analysis indicates that it’s not completely crazy to try to do so. You might not have expected it, but the long-distance physics makes sense.
Nima’s other talk was about “high energy” supersymmetry, some as yet unpublished work of his with Savas Dimopoulos, in which supersymmetry is broken at a relatively high scale and, of the superpartners, only the gauginos are light.
I’ll talk about that in more detail some other time…
April 24, 2004
Internationalization
Say you want to tag some text on a web page as being in a language other than the main language of the page (English, in the case of this blog). In HTML 4, you would slap a <span lang=".."></span> around it. In XHTML 1.1, the lang attribute is gone, and you’d write
<span xml:lang="fr">ma vie en rose</span>
instead.
And therein lies a small problem. No matter how you set your Sanitize Spec in the blog preferences, MovableType will strip out the xml:lang attribute from any sanitized text like, say, the comments on your blog. It can’t handle attributes with colons in them.
Fortunately, the fix for this is easy.
--- lib/MT/Sanitize.pm.orig Fri Apr 23 08:40:27 2004 +++ lib/MT/Sanitize.pm Fri Apr 23 08:41:42 2004 @@ -98,7 +98,7 @@ (exists $tag_attr->{$name} && $tag_attr->{$name} eq '/')) { if ($inside) { my @attrs; - while ($inside =~ m/(\w+)\s*=\s*(['"])(.*?)\2/gs) { + while ($inside =~ m/([:\w]+)\s*=\s*(['"])(.*?)\2/gs) { my $att = lc($1); if ($ok_tags->{'*'}{$att} || (ref $ok_tags->{$name} && $ok_tags->{$name}{$att})) {
That takes care of easy languages, like French. But say you want to comment in Hebrew. Hebrew’s a Right-to-Left language. If you want to use a phrase in Hebrew in the midst of an English paragraph, you’d paste the Hebrew text into a <bdo dir="rtl" xml:lang="he"></bdo>.
“<bdo>” stands for “BiDirectional Override”, which temporarily reverses the direction of the text. If you want an entire paragraph in Hebrew, you’d paste the text into a <p dir="rtl" xml:lang="he"></p>.
[Update (5/11/2004): According to the W3C Draft on Handling Bi-Directional Text, you can mostly get away without using the <bdo> element, thanks to the Unicode Bi-Directional Algorithm and the super-secret character entities, ‏ (Right-to-Left Mark) and ‎ (Left-to-Right Mark), which let you control how neutral characters, like punctuation marks are treated. E.g. compare 1705 רחוב בן יהודה. (typed straight) with “1705 רחוב בן יהודה.” (uses some astutely-placed ‏s). Note: Safari screws this stuff up pretty badly; there are serious bugs in WebCore’s bidi implementation. There are also useful documents on Specifying the Language of Content and the ever-popular subject of Character Encodings (via Phil). ]
All these tags and attributes are allowed in the comments on this blog. The only bad news is with respect to Charsets. This blog uses ISO-8859-1. That handles Western Europeen languages just fine, but doesn’t know anything about non-Europeen languages. So if you enter
<span dir="rtl" xml:lang="he">הבנתי</span>
into the Comment Form and click “PREVIEW”, your browser will convert the text to numeric entities
<span dir="rtl" xml:lang="he">הבנתי</span>
which will display correctly, but which is not exactly the easiest thing to edit.
If I converted to UTF-8, presumably, this problem would be solved. Unfortunately, the last time I tried it, the interaction between UTF-8 and MT’s Comment Form was such a horror story that I’m loath to try it again.
April 23, 2004
Erdős Number
It transpires that Bill Tozier (Erdős #4) is auctioning on eBay the opportunity co-author a paper with him, and thereby secure the successful bidder an Erdős number of 5. John Quiggin quickly jumped in to offer willing co-authors a free Erdős number of 4.
This got me to thinking. What’s my Erdős number? Can I profit from the opportunities currently flying about the 'net?
After checking the Erdős Number Project, I now know that I have an Erdős number of 4, thanks to three co-authors (Greene, Hubsch and Vafa) I have in common with Shing-Tung Yau (Erdős #2).
No help to be had from Tozier and Quiggin. If I am to improve my Erdős number, I’ll have to do it the old-fashioned way.
April 18, 2004
Never Again!
Tomorrow is Yom Hashoah, Holocaust Remembrance Day.
I wish I could say that the events of 60 years ago have little relevance today. But the bloody history of the intervening decade shows that the ability of Man to turn, savagely annihilationist, on his fellows is hardly diminished. As we learned in Bosnia, a little over a decade ago (to pick but one example) the veneer of civilization can sometimes be astonishingly thin.
My father survived the war by dint of luck, and good engineering skills. On this day, it’s worth taking some time to listen to