@article{Abramowicz:2009,
author
=
"M.Abramowicz and S.Bajtlik and W.Kluźniak",
title
=
"Twin paradox on the photon sphere",
journal
=
"Physical Review A",
volume
=
"Vol. 75",
number
=
"no 4",
pages
=
"044101-1--044101-2",
year
=
"2007",
}