While many explanations have been put forward to explain Fermi's Paradox, given our current knowledge, it is reasonable to assume that intelligent life can exist on other star systems, and interstellar travel does not violate the laws of physics and can be assumed to be practicable.
Since there is no way to reliably predict the capabilities and motivations of alien civilizations, it cannot be excluded that they exist yet do not behave the way we would. One possible explanation for Fermi's Paradox is the Zoo Hypothesis, first proposed by John Ball (1973). The Zoo Hypothesis states that one or more extraterrestrial civilizations know of our existence and can reach us, but remain committed not to disturb us or even make their existence known to us. Many authors have debated the Zoo Hypothesis and its related Interdict Hypothesis. The rationale behind these hypotheses is that extraterrestrial civilizations, perhaps in agreement as part of a “Galactic Club”, will only contact us when we reach one or more technological or social milestones. In this context, since the 1970's that many have argued that extraterrestrial intelligences monitoring us might wait for us to initiate contact and thus that we should attempt to communicate with them, but no practical way of doing this has been put forward.
We propose here a proactive test of the Zoo Hypothesis. Specifically, we propose to send a message to any extraterrestrial civilization(s) that might be listening inviting them to respond. Our aim is to attempt to bring forward the communication with extraterrestrial civilizations by stating that we are ready to engage with them at a high level. The rationale is that, assuming the Zoo Hypothesis is true, extraterrestrial civilizations must be observing human civilization, which must involve monitoring our radio leakage as this is readily detectable at long distances (i.e., from outside the solar system). Our assumption is also that it is possible to influence the decision-making process of extraterrestrial civilization(s), by initiating contact or perhaps by painting a more favorable picture of human civilization. In spite of the unlikeliness of the many assumptions underlying our proposal (starting with the Zoo Hypothesis itself), we think this is a worthwhile endeavor since it can be achieved with very modest resources (e.g., by using existing radio and television broadcasts). Besides, the prospect of being successful, no matter how unlikely this is, is tantalizing since establishing contact with extraterrestrial intelligences would change humanity forever.
Opposition to direct communication efforts is mostly based on concerns related to costs and potential dangers of revealing ourselves. If extraterrestrial civilizations are already aware of us and eavesdropping on us, however, then attempting to communicate with them will not put us in any danger, at least not in any more danger than we are already. On the contrary, if advanced extraterrestrial civilizations intend to help us at some point then communicating with them sooner rather than later will benefit our species. Therefore, and unlike traditional direct communication efforts aimed at other systems, which have been a source of controversy, there is little risk in our proposed endeavor.
Perhaps simply issuing an invitation is the necessary milestone for extraterrestrial intelligences to respond. On the other hand, there are many reasons for an extraterrestrial civilization not to appreciate human civilization and we would do well to try to convince extraterrestrial intelligences to engage with the human species. One major open question, thus, concerns the content of our message. We believe this invitation message should: 1) acknowledge the Zoo Hypothesis and express our wish to communicate with extraterrestrial intelligences as soon as possible; 2) with limited resources on Earth, a growing population and the capacity for self-destruction (e.g., due to nuclear weapons), our civilization is in more danger than ever; therefore, we may well need help to survive and this is why we would like to learn from older, more advanced civilizations; and 3) suggest an easy way for extraterrestrial civilizations to respond to us. We aim to start a rational, scientific discussion on the context of the message to be transmitted in due course. Please send us any suggestions.
Even though we accept our proposal is unlikely to be successful in the sense of resulting in a response from extraterrestrial intelligences, the possibility that extraterrestrial civilizations are monitoring us cannot be dismissed and our proposal is consistent with current scientific knowledge. Besides, issuing an invitation is technically feasible, cheap, safe and with tremendous potential benefits since few would deny the profound importance of establishing contact with one or more extraterrestrial intelligences. Communicating with more advanced extraterrestrial civilizations would forever change humankind.
Further details on our proposal are available in our paper published in Space Policy (download PDF). Any thoughts, comments and suggestions are welcomed. Our goal is for this to be an international effort representative of the scientific community and of humankind. Please feel free to contact us.