re: They're looking for radio signals.
Interesting ideas re:starships. Not enough of a physicist to propose any specific experimental look-sees, but someone should run the numbers and get a published paper. The starship exhaust search concept has been mentioned before but as far as I know numerical anaylsis showing viability of any experimental search is still lacking.
I'm not one who thinks that the search for directly beamed radio or optical signals has much of a chance to succeed. Sorry, Paul Allen.
