The Sciences

Relatively Pleasant

Cosmic VarianceBy cjohnsonApr 27, 2006 1:10 AM

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It is that time of year again. Because I am insane, I am teaching -by choice- two classes this semester. One of them is quite small, having 12 students, the other is not. The small class is my electricity and magnetism class. With a class this small, it is nice sometimes to have a change of scenery, and -if the material can handle it- I show up one sunny Spring day (after the hard slog through rainy Winter days) and declare that we are going outside to sit under a tree. There are several lovely spaces on the USC campus perfectly suited to this, and so we go outside. Last year, when I first had the idea for doing this, eventually I thought to buy a little portable whiteboard in case I need to write or draw something. (I discovered that the board fits perfectly into my Brompton's front carrier bag, sticking pleasantly out of the top as I cycle along!) It's just a perfect setup actually, since this time of year usually coincides with the last part of the class where we are doing Special Relativity. I can think of few physics topics which are better suited to sitting outside and discussing under a tree -with a few diagrams and equations- than the classic thought experiments of Special Relativity. Here's everyone in the class (except Chris Cantwell, who unusually could not make it this time), and also the whiteboard, my coffee cup, and the Brompton (of course):

(L to R: Michael Crampon, Jeff Pennington, Michael Johnson, Douglas Mason, Omar Hussein, Aleksandr Rodin, Amanda Fournier, Christopher Winterowd, Christopher Palmer, Justin Seymour, Matthew Recker.) You can see, if you look closely, the last remnants of the moment of confusion that appeared on their faces from just being told that a vector can have zero squared length while not itself being zero. (That's a "null four-vector", we were talking about, for the uninitiated. You need them for describing things that move at the speed of light, for example.) I have to design them a final exam very soon. Time for a trip to the Cat and Fiddle then, some evening soon. I'm going to see if I can get in a "fun" question at the end again, like last year. During the exam last year, one student (Lauren Schenkman) raised her hand to ask a question. I thought there was a typo or something on the exam. No, there wasn't. Her question - asked cautiously and respectfully, was, "Are you serious?". I love that. Here was the question:

Consider the following lyrics to the song, "'39", written by Brian May and performed by the rock band Queen back before you were born.

Offer a relevant and reasonably selfâ€"consistent interpretation of these words, with illustrative equations.

(Sigh. I loved this song so much when I was a teenager.....) Some ex-Mathematical Physics students of mine from Durham may also recall me using this question one year. Shameless recycling. There, they had to write a little essay over Christmas holidays about it. I still have them. I kept meaning to mail them all to Brian May. Maybe I will, one of these days. Some of them were rather good. Anyway.....Well? What's your explanation? What would you have turned in to be graded? -cvj

In the year of '39 assembled here the Volunteers In the days when lands were few Here the ship sailed out into the blue and sunny morn The sweetest sight ever seen. And the night followed day And the story tellers say That the score brave souls inside For many a lonely day sailed across the milky seas Ne'er looked back, never feared, never cried. Don't you hear my call though you're many years away Don't you hear me calling you Write your letters in the sand For the day I take your hand In the land that our grandchildren knew. In the year of '39 came a ship in from the blue The volunteers came home that day And they bring good news of a world so newly born Though their hearts so heavily weigh For the earth is old and grey, little darlin' we'll away But my love this cannot be For so many years have gone though I'm older but a year Your mother's eyes from your eyes cry to me. Don't you hear my call though you're many years away Don't you hear me calling you All the letters in the sand cannot heal me like your hand For my life Still ahead Pity Me.

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