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S1 announcing our existence to the universe. “If we did not,” he argued, “surely someone by now would have pointed out the dangers of continually emitting large amounts of electromagnetic radiation.” In fact, several people have pointed out potential dangers from transmit- ting powerful signals. British Astronomer Royal Sir Martin Ryle, who believed that calling a more advanced civilization’s attention to our exis- tence could be dangerous, reacted to the Arecibo signal by asking the International Astronomical Union to ban any further transmissions to the stars unless they were internationally agreed upon.® Sagan responded defensively to Ryle’s critique. The Arecibo message was clearly not intended as a serious attempt at interstellar communication, he claimed, but rather as an indication of the remarkable advances in terrestrial technology. The staff of the Arecibo Observatory saw the signal as a demonstration that radio telescopes were entirely adequate for interstellar communication over immense distances. Nonetheless, .o4 4 1 aod aa ee) ° such demonstrations might prove to be the most likely cause of our being detected—if another civilization is listening at radio wavelengths. Sagan later seemed to have reconsidered his views, favoring listen- ing instead of sending.’ The NASA workshop report of 1977 established a conventional wisdom. There is an immediate payoff if we receive a signal; transmitting requires that we wait out the round-trip light time before we can hope for any results. Transmission should be considered only in response to a received signal or after a prolonged listening program has failed to detect any signals. Twenty-five years later, a group of SETI experts reached the same conclusion."° Deliberate transmission does not yet make sense; the Earth already is quite bright with radio leakage. Transmitting enough to improve this sig- nificantly is expensive, and a transmitting strategy cannot pay back for many years. While our own technology is still leaking distinctive signals, passive lis- tening remains the most cost-effective strategy for discovering extrater- restrials. “Sending on our own remains a delayed option,” Morrison wrote in the prologue to the SETI 2020 report, “perhaps to be considered at the close of a century of search.”"! Bernard Oliver, noting that there are people who are fearful about announcing our presence, said “You have to convince these people or enough of the populace that it is a good idea before you go ahead and do it in a democratic society. On the other hand, listening has no such hazards in anybody’s mind.” If we are worried about calling attention to ourselves, others argue, we can choose not to respond to a signal from extraterrestri- als. There is no way for the transmitting civilization to determine if its message was received and understood on Earth.” Despite his central role in the Arecibo blast, Drake stated that “we will not send signals until we have received them, because we do not know in The Arecibo Blast