Here: http://en.wikipedia.org/wiki/Holographic_principle I read: "Entropy, if considered as information (see information entropy), is measured in bits. The total quantity of bits is related to the total degrees of freedom of matter/energy" Bold is mine. Here is my question. I believe that gravitational waves propagating thru empty space also carry information (do you agree?). Is Bekenstein bound also applicable to that sort of matter-less "storage"?