Page Content (OCR)
185 beacons or send powerful targeted signals. If they do search, they may pursue a listen-only strategy. Alternatively, there are no beacons now. One or more civilizations may have operated beacons in the past but are no longer doing so. Some of those signals still might be detectable if they are coming from a great dis- tance but have not yet reached us. Here we need to distinguish among beacons broadcast in all directions, beacons targeted at civilizations other than ours, and beacons aimed at us. The first two categories could exist even if the sending civilization did not know of our existence. There never have been interstellar point-to-point transmissions. Civiliza- tions capable of using them may have no need for such transmissions if they do not expand or do not detect others. Alternatively, there are no interstellar point-to-point transmissions now. No civilization is currently communicating with others, or, if it has expanded, with its colonies. If there are point-to-point transmissions now, we may not see them because we are rarely in their line of sight—or we may see them as flashes of energy that fade too quickly to be reacquired. More advanced civilizations use communication methods with little or no leakage. They may emit detectable signals only during a brief phase of their histories; our searches may not coincide with those eras. Other civilizations have chosen interstellar travel, by biological beings or machines, as their preferred means of exploration. They may find searching and communicating by technological artifacts more useful than sending signals. The science and technology of more advanced civilizations have evolved to a level that makes signaling irrelevant. Interstellar travel is impossible at the technological level achieved by any civilization, past or present. Explanations for the Lack of Artifacts No Long-Distance Calls No Local Calls They Prefer Travel Beyond Our Imaginations Explanations for the Lack of Artifacts Impossible