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285 The radio search assumes that extraterrestrial civilizations are approxi- mately on the same technological level as terrestrial civilizations, using detection and communication systems similar to our own. Yet, Morrison acknowledged that there is no synchrony anywhere; we are either behind or ahead. We may be the youngest communicating civilization in the Galaxy, having only just arrived at a stage that allows us to build the powerful transmitters and receivers needed for contact at interstellar distances. However, no stage of communications technology is permanent. Many believe that our present modes of communicating will be short-lived— either because our technologies will change into something quite different, or because we will destroy ourselves.* Older technological civilizations alan nea also may move on to new communications technologies. White proposed that the impact of contact would increase as the differ- ences in the levels of development of the two civilizations increases.*° There may be limits to that paradigm. Civilizations very far advanced beyond our own may not bother with us, and we may not be able to detect or understand them. Assuming that 2 million intelligent societies have arisen in our Galaxy over the past 5 billion years, Ulmschneider derived an average interval of 2500 years between the births of such civilizations. In this formulation, the societies closest to our present state would be either 2500 years more advanced than we are (4500 A.D.) or 2500 years behind (500 B.C.). The probability that such a civilization is nearby is small. Initial radio “bursts” should be observable from, at most, 30 older societies; the number would be smaller if they spend only a brief time in the radio-emitting phase.*” These numbers are statistical artifacts resting on an assumption that intelligent societies have appeared at regular intervals over a 5 billion-year span. The statistics look more promising if there is a flowering of intelli- gence in our own era. Nonetheless, trying to imagine human society in 4500 A.D. shows us how difficult it is to picture a more advanced civilization. Mind-Stretcher. SETI conventional wisdom often assumes that older means proportionately more advanced in science and technology. Yet, as we have seen, continuous scientific and technological progress may not be inevitable. A million years older may not mean a million years ahead in knowledge or tools—or in the ability exert influence at a distance. SETI optimists often assume that more advanced societies will provide us with the information we need to reach their level, homogenizing the Galaxy’s technological civilizations. We cannot take it for granted that Parallelism and Synchrony Parallelism and Synchrony