Our understandings of civilizations on Earth inevitably influence speculation about extraterrestrial civilizations. Specifically, our assumptions about the development of Earth’s societies continually shape our conjectures about alien societies, on topics including the probability of their existence, their possible characteristics, appropriate search strategies, and the wisdom of seeking contac...

An estimate of the number of advanced technical civilizations on planets of other stars depends on our knowledge of the rate of star formation; the frequency of favorably situated planets; the probabilities of the origins of life, of intelligence and of technical civilization ; and the iifetimes of technicai civilizations. These parameters are poorly known. The estimates of the present paper le...

Early state societies (civilizations) developed in areas of the world where intensive agricultural systems could be developed to produce food on the scale necessary to support relatively large and dense populations. The centralized structure of these states contributed to the process of intensification and agricultural investment, though in most cases direct state involvement appears not to hav...

We speculate about possible cosmic exploration by extraterrestrial civilizations using interstellar smart probes. We call attention to some emerging technologies that may be used for interstellar exploration. We estimate that advanced civilizations may already have these technologies and may be using them to contact their galactic neighbors. We explore some observational consequences of these c...