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46 We now have the means to detect ultrahigh-energy neutrinos that were not even known to exist in 1990 (scientists had proposed using neutrinos for interstellar communication at least 11 years earlier). Astronomers have produced the first sky survey based on detecting positrons, the antimatter equivalent of electrons.” As our potential means of detection expand, the universe becomes more transparent to us. Like spectroscopy in the nineteenth century, new methods may evolve unexpectedly from conjunctions of technological developments and scientific insights. Such new capabilities will be focused on conven- tional astronomy rather than on finding evidence of extraterrestrial tech- nology. However, serendipity might apply. A Warning. Windows opened can become windows closed. Orbiting tele- scopes have finite lifetimes, often only a few years. If they are not replaced, the portion of the electromagnetic spectrum that they study may once again become invisible to us. Body is the last thing we are likely to know of them. Of their mind as embodied in their works, we may learn much more. nn... TT .---.11 1AN061 Science fiction author Olaf Stapledon suggested in the 1930s that we might discover alien civilizations by searching for signs of astroengineer- ing. More advanced civilizations may be able to transform natural objects and natural energies to suit their purposes. Although these works would not be intended as means of communication, they might be detectable by their effects. Searching for radio signals is a fine idea, Dyson observed, but it only works if you have some cooperation at the other end. He suggested that we look for passive signals from uncooperative targets—evidence of intel- ligent activity without anything in the nature of a message. As any high technology must radiate away waste heat, we could search in the infrared, which does not assume anything special about the nature of extraterrestrial intelligence except that it be technological and carrying out activities on a large scale. The largest-scale activities will be the ones most likely to be found.® Mind-Stretcher. Computer scientist Marvin Minsky questioned the assumption that advanced technological civilizations would radiate more infrared emission. Because radiation at any temperature above the cosmic background level is wasteful, the higher the civilization, the lower the infrared emission.” This implies that more advanced technological civi- lizations may be undetectable. Searching for Intelligence Looking for the Astroengineers —Percival Lowell, 1908°!