{"id":150,"date":"2006-04-21T01:30:02","date_gmt":"2006-04-21T05:30:02","guid":{"rendered":"http:\/\/blogs.law.harvard.edu\/jreyes\/2006\/04\/21\/a-joke\/"},"modified":"2006-04-27T16:11:33","modified_gmt":"2006-04-27T20:11:33","slug":"a-joke","status":"publish","type":"post","link":"https:\/\/archive.blogs.harvard.edu\/jreyes\/2006\/04\/21\/a-joke\/","title":{"rendered":"A Joke."},"content":{"rendered":"

<\/a><\/p>\n

Disclaimer:<\/strong> Okay, I admit it: despite the title of this post, no one, not even me, will think that this story is funny. It made me smirk at the time, but I sneak into this class. I happen to know that none of you do. I’d see you. There are only eleven of us, and it’s a small room. But if you feel brave, or are work wasting time, or are my mom (Hi, mom!), I invite you to read on. But know that what follows is in no way funny, so don’t take it out on me when you don’t laugh.<\/p>\n

In mathematics, as in physics, it is sometimes useful to treat the electric and magenetic fields separately. Historically, the electric field gets the symbolic designation E<\/strong>, while the magnetic field gets B<\/strong>. [I’m not especially sure why. Magnet in German is der Magnet<\/em>. And Maxwell, the guy who settled the theory way back when, was British. I’m not sure who fixed the notation, but it’s screwy. And to make matters worse, sometimes physicists use an alternate quantity, the magnetic strength, H<\/strong>, instead. But now I’m starting to confuse myself. I hated electrodynamics. The important player in this story is E<\/strong>, anyway.]<\/p>\n

I know, I know. Why would ever want to split up the E<\/strong> and B<\/strong> fields when we could combine them into a single, more manageable tensor? Well, it turns out to be useful when constructing the Ernst potential when studying the geometry of spinning black holes, and that’s exactly what we were doing in class on Wednesday.<\/p>\n

Yau had, as he always does, scrawled several chalkboards worth of equations for our benefit and understanding of a rather subtle and technically difficult proof of the uniqueness of a charged, stationary, axially symmetric black hole — the so-called Reissiner-Nordstrom-Kerr black hole. These equations made use of the aforementioned fields E<\/strong> and B<\/strong>. By this time he had introduced another important player, the guage potential — a sort of secret symmetry [It ammounts to the relabelling of space. Even old New York was once New Amsterdam, when they changed the name, however, the geography was pretty uneffected. Nature doesn’t care what you call it. That’s the whole point of guage symmetry.] — which he denoted by capital lambda. While this notation is conventional, it’s not universally accepted. Some people also use A<\/em> instead. Yau does. But the text he was lecturing from does not. As a result, he mixed A<\/em>s and uppercase lambdas freely. Eventually this bugged someone enough to ask about it.<\/p>\n

But Yau had already overloaded E<\/strong>, too, using it simulateously for the electric field and the Ernst potential, which [indirectly] depends on the the electric field! [I told you this wasn’t going to be funny. Stop rolling your eyes.] A little braver now that someone else had expressed his confusion, another man spoke up.<\/p>\n

“This E<\/strong> over there and this one over here aren’t the same E<\/strong>, are they?” he asked, pointing accordingly.<\/p>\n

Yau realized his abuse of notation and set to redress the error of his ways.<\/p>\n

“Oh. I’ll just erase this one, then,” he said. And then he did. The whole affair had a touch 1984<\/em> to it. Problem solved. Now there was only one, unambiguous E<\/strong>. Inconvenient history never happened. Not if it’s not recorded. The Founding Fathers knew this well. Orwell knew it. And apparently, Yau knows it, too.<\/p>\n

Even if it’s not funny, you can understand, perhaps, why I smiled, though.<\/p>\n","protected":false},"excerpt":{"rendered":"

Disclaimer: Okay, I admit it: despite the title of this post, no one, not even me, will think that this story is funny. It made me smirk at the time, but I sneak into this class. I happen to know … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":102,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[136,114],"tags":[],"class_list":["post-150","post","type-post","status-publish","format-standard","hentry","category-mathematics","category-personal"],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/posts\/150","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/users\/102"}],"replies":[{"embeddable":true,"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/comments?post=150"}],"version-history":[{"count":0,"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/posts\/150\/revisions"}],"wp:attachment":[{"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/media?parent=150"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/categories?post=150"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/archive.blogs.harvard.edu\/jreyes\/wp-json\/wp\/v2\/tags?post=150"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}