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181 When intelligent beings consider which technological activities to engage in, from listening for signals to astroengineering, they are faced with choices. The crucial variable may be which decisions they make. Here the alleged paradox is divided into two questions: Why have we not detected electromagnetic signals suggesting the existence of alien tech- nology, and why have we not detected alien artifacts, which could be any- thing from an interstellar ark to an abandoned socket wrench found on an asteroid in our own solar system (one category of artifacts, astroengineer- ing, is addressed separately). Explanations are divided into three categories: those related to our nondetection of electromagnetic signals, those that apply to our nondetec- tion of artifacts or astroengineering, and those that are relevant to both. These explanations are not all mutually exclusive; more than one could be involved. There may be multiple factors influencing whether alien civilizations search for others, explore, or expand. The reader is invited to experiment with combinations. Hungarian astronomer Ivan Almar warned us that it would be rash to proclaim any of the proposed explanations of the paradox as final—or to reject them completely. Most explanations are likely to be wrong, yet the intellectual game is worth playing. As Sean Carroll said about string theo- ries in physics, “All these proposals are in the spirit of ‘unlikely to be right, but so extremely interesting if they are that they are well worth thinking about.” The explanation we ultimately find may not suit anyone’s There are no other technological civilizations now. We may be the first; or other technological civilizations existed in the past, but are now extinct. If there are natural cycles in which technological intelligent life evolves but is destroyed, we may be the first to emerge in the new cycle. We also may be the first to emerge if past technological civilizations destroyed themselves. The destruction of biological civilizations by intel- ligent machines they created would not necessarily mean the extinction of technological civilizations, if those machines could be considered a civili- zation themselves. Being the first in a new cycle does not exclude all forms of contact. A signal from an extinct civilization might reach us hundreds or thousands of years after it was sent. Artifacts from an extinct civilization could be much older. Explanations Common to Both convenience. Explanations Common to Both Uniqueness