J L F Barbón, "Black holes, information and holography" J. Phys.: Conf. Ser. 171 01 (2009) http://iopscience.iop.org/1742-6596/171/1/012009 p.1: "The most
important departure from conventional thinking in recent years, the holographic principle…provides a definition of quantum gravity… [and] guarantees that the whole process is unitary. "