quadrupole nature of gravitational waves,

http://www.gravity.uwa.edu.au/docs/review.pdf

http://books.google.com/books?id=IUdyO8TS2xAC&pg=PA64&lpg=PA64&dq=%22quadrupolar+gauge+field%22&source=bl&ots=hKiYfKXgLv&sig=WkZ4G_jYEoGe_RGqX18NiDqkDIM&hl=en&sa=X&ei=JpjVUYnDF8a_igK3tICwCQ&ved=0CC8Q6AEwAA#v=onepage&q=%22quadrupolar%20gauge%20field%22&f=false