Recently, at a mass in Vatican City, Pope Francis said that, if given the chance, he would baptize aliens. (“Who are we to close doors?” he asked.) Unfortunately, judging by “Archaeology, Anthropology, and Interstellar Communication,” a new book, about the complexities of communicating with extraterrestrials, released last month by NASA, it won’t be that simple. For a long time, the people most interested in searching for extraterrestrial intelligence came from “hard science” disciplines like astronomy or physics; to them, the main obstacles seemed technical (building radio telescopes, processing signal data). But, in recent years, the field has broadened to include people who already study other civilizations here on Earth. In these essays, they report that their jobs are hard enough as it is. Archaeologists struggled to decipher ancient Greek; deciphering a transmission from another world will be even more difficult. Even if we do manage to detect a signal, they write, fully understanding what it means may be impossible.

The challenges described by the contributors are daunting (and, at least to me, surprising). On Earth, they write, we were able to use the Rosetta Stone to figure out Egyptian hieroglyphics. (It contained the same text written in glyphs, Demotic script, and ancient Greek.) But there will be no Rosetta Stone for our communication with extraterrestrials, and the distances involved make conversation unlikely—which may mean that our comprehension of their message will be confined to math and numbers, never able to make the jump to broader concepts or less abstract words. (How do you describe a lake, or a tree, with math?) The speed of the message presents another problem: here on Earth, human language happens at a speed somewhere between birdsong and whalesong, so how fast should our message be, and on what scale should we be listening? And then there are all the difficulties created by the nature of our interlocutors. What if they’re so different from us that our messages are mutually incomprehensible? What if the message is sent by some sort of automated system—a voicemail from a long-dead civilization?

Douglas Vakoch, the editor of “Archaeology, Anthropology, and Interstellar Communication,” is the director of interstellar message composition at the SETI Institute, in Mountain View, California. (The Institute’s name refers to the “search for extraterrestrial intelligence,” an umbrella term for a number of projects that began in the sixties, some funded by NASA.) Vakoch has degrees in comparative religion, the history and philosophy of science, and clinical psychology (“I expected to become an astronomer, but discovered that I was more interested in people than in stars,” he told me). At SETI, Vakoch is responsible for designing the messages that we might send to extraterrestrials; he is also a member of the International Institute of Space Law, where he works on the policy issues surrounding the messages’ composition. (There are currently no laws about sending signals into space; in theory, anyone with a powerful enough antenna could be talking to the cosmos right now.) “We used to think we would get an Encyclopedia Galactica,” Vakoch said. One of his primary goals, in editing the book, was to give air time to the less optimistic views of social scientists, and to start thinking about what an incomplete or indecipherable message from space might mean to humankind.

Vakoch spoke to me by phone, from his home in California. This interview has been edited and condensed.

“Stargate” notwithstanding, when I think of space, archaeologists and anthropologists don’t come to mind. What do they contribute to the SETI effort?

Anthropologists are very familiar with encountering those who are radically other; archaeologists are very good at saying, We have only a fragment of a past civilization, and we’re trying to reconstruct it. Both of those situations will be at the center of communicating with extraterrestrials. When we take seriously the distance between us and any other civilization, we see that we have no easy way to ask a question and get a reply. We don’t even know what language they’re going to be using.

And if we’re thinking of replying, and of getting a reply back, then we have to ask: How do we make sure that, one thousand years from now, future humans will be able to understand that message? Because it’s only at those time scales that it becomes plausible that we’ll begin to understand something.

Your job is to help design the messages we might send to extraterrestrials. What’s the starting point for that work? How do you begin?

There is no obvious starting point, but there are some approaches that are easier than others—for example, mathematics. So then the question is, What are the most fundamental parts of mathematics? Maybe it’s counting. Maybe mathematics as a whole isn’t universal, but if we can start with something fundamental, we can build up to communicating, step by step, our way of bringing order into the universe.

It’s also useful to step back and look at communication more generally. In the early days of SETI there was this idea of “send them information, and make it redundant, and the patterns will be self-evident.” Nowadays, that seems ridiculous. So what’s the alternative? If something like language—whether a natural language like English or Mandarin, or a language like mathematics—if those aren’t universal, maybe we can step back and look at signs in a more general sense. That’s one of the ways in which semiotics has been helpful. Maybe a big accomplishment in communicating with an extraterrestrial is just to convey that there’s something on this end who’s intending to send something. Even if it’s something as rudimentary as sending an index—a message that points toward an astronomical object—or an icon, something that looks like, say, the radiation pattern of hydrogen. Just so they say, “Oh, these are sign-bearing, sign-using creatures—there’s hope.”

Has SETI, or has anyone, actively started broadcasting in a way designed to attract notice?

There have been a few transmissions. There hasn’t been anything sustained, though, and I think there would be real advantages to starting a sustained transmission. That would increase the sense of this being an intergenerational project. It signals our own hopes for humanity—that we hope to be around in one thousand years to get a reply back. We recognize that what we do today is just one step in contact.

One of the most inhibiting aspects of interstellar communications is the sense that, somehow, we need to get everything absolutely right and completely comprehensible the first time out. That’s not how we communicate on Earth.

You’re in favor of transmitting sooner rather than later?

If it turns out that we actually have little hope of understanding a message, how does that affect our search strategies? Maybe it makes sense for us to start transmitting. The way it’s typically been cast in the past is, They’re smarter, and they’ll be better at sending an intelligible message. But the flip side is, if they’re smart, they’ll also be better at decoding an ambiguous message. Maybe they’ll be able to detect, from the form of the message, “This seems to be something we’ve seen from visually oriented species, or auditory-oriented species.” That might be more important.