Roberto posted his solution as "b" because Laura already had a solution posted up on her website.

For reference, you can find Laura's solution here:

http://camoo.freeshell.org/19.16.pdfHowever, I'd like to point out that there's a significant error in it:

Laura says

Quote:

"the volume acceleration is the Lie derivative of the 3-volume element wrt the acceleration vector

."

Ignoring the fact that

is not defined anywhere, you simply can't get an acceleration by taking a first derivative (Lie or otherwise). You need to take the derivative twice!

...take a look at my solution ([19.16] c) for what I

think Penrose had in mind here.