Seth Shostak, Ph.D., is the Senior Astronomer and Director of the Center for SETI Research at the SETI Institute, as well as host of the weekly radio program Big Picture Science. He earned his undergraduate degree in physics from Princeton University and his Ph.D. in astronomy from the California Institute of Technology. His books include Sharing the Universe: Perspectives on Extraterrestrial Life; Cosmic Company: The Search for Life in the Universe; and Confessions of an Alien Hunter: A Scientist’s Search for Extraterrestrial Intelligence.
Dr. Shostak is a Fellow of the Committee for Skeptical Inquiry, and he has served as chair of the SETI Committee of the International Academy of Astronautics. In 2004 he won the Astronomical Society of the Pacific’s Klumpke–Roberts Award in recognition of his contributions to the public’s understanding and appreciation of astronomy.
Despite the fact that major efforts have been expended on passive searches for extraterrestrial signals, few deliberate “transmissions” to potential alien recipients have occurred. These have generally taken the form of simple graphics depicting such things as our appearance, location, and biological construction. In this paper, we consider (a) the fundamental technical and astronomical limitations to interstellar messaging—in other words, how many “bits” could any society reasonably send, and (b) what might be a likely transmission strategy. These considerations suggest approaches for SETI programs, as well as giving insight into the types of messages we might construct for eventual replies to received signals.