2 Responses to “Leave Comments”

1. Julian Moore Says:
   September 2nd, 2009 at 6:53 am

   Hi –

   I really wanted to brush up my tensors (which had lain rather neglected since what you would call 10th grade… I was reading Eddington’s “The Mathematical Theory of Relativity” at the time) in order to answer some wormhole questions… (actually whether the energy momentum tensor for a certain toy model that leads to a time machine is reasonable – but I digress, as I usually do).

   Anyway, re Tensors: the index gymnastics, covariant differentiation etc. etc. aren’t too bad (though I want to re-derive the Christoffel symbols from scratch, just for “fun”/practice) but I was getting really hung up on not being able to get a clear picture of covariant vs contravariant – and though it doesn’t make much difference in the end (as long as one knows which is which!) it was bugging me… so thank you for your Contra vs Covariant piece – very nicely done! Your other students have no doubt benefited greatly from your clarity of exposition.

   Best regards

   Julian

   PS The etymology of Idius was entertaining too…

2. idius Says:
   September 2nd, 2009 at 9:26 am

   Thanks for the comment. You reminded me that I need to work on writing quite a few more pages. The Christoffel symbols can be easily derived using the Euler-Lagrange equation. I’ll eventually write up the details here, but you wanted to do it yourself, so I guess there’s no rush. Have fun.

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