A satellite beacon would not be tamperproof. And space tends to be controlled by The Man. A simpler version is to establish a retroreflector N light years away, and send some secret on its way. After 2N years you get your bits back, and no one could have outraced them (cf Michaelson-Morley). Problem is transmit power :-) You'd also worry about adversaries placing their own retroreflecting 'taps' along the beampath before you sent. As to whether there are un-parallizable crypto methods, I believe that even Shamir's wafer-scale optoelectronic RSA cracker has a necessarily sequential step. True, the future will surprise us, but there is no proof that such algorithms are impossible. And yes, most social 'scientists' are most charitably thought of as incubators for organs we may need later.
