How would we talk to aliens? What we've already sent, and what linguists say

Almost nothing is guaranteed. Aliens may have no voice, no eyes and nothing we would recognize as language. But anyone able to send a radio signal from another star must know physics: they built a transmitter, they understand electromagnetism, they can count. That is why nearly every attempt to write a message for aliens starts with numbers.

The Dutch mathematician Hans Freudenthal put it plainly in 1960: "Mathematics is the most abstract subject we know, and at the same time a subject that may be supposed to be universally known to humanlike intelligent beings." Alongside numbers, messages almost always use hydrogen, the most common element in the universe. When a hydrogen atom flips state it emits a radio wave about 21 centimeters long, at about 1420 MHz, which any physicist in any galaxy can measure. It serves as a universal ruler and clock: a length of 21 cm and a period of about 0.7 nanoseconds.

Pioneer 10 (1972) and Pioneer 11 (1973) each carry a gold-anodized plaque designed by astronomers Carl Sagan and Frank Drake and drawn by the artist Linda Salzman Sagan. It shows a man and a woman, the solar system, and the Sun's position relative to 14 pulsars, dead stars that blink at steady rates. The unit of length in the drawings is that same 21 cm hydrogen wave.

On November 16, 1974, at a ceremony marking the upgrade of the Arecibo radio telescope in Puerto Rico, humanity sent its first deliberate message meant for aliens. Drake and colleagues at Cornell wrote the content, with help from Sagan. It was 1,679 bits, about three minutes of transmission at 2380 MHz, aimed at the star cluster M13, some 25,000 light-years away. The number 1,679 was chosen on purpose: it is the product of just two primes, 23 and 73. Arrange the bits in a 23-by-73 grid and a picture appears.

The picture shows the numbers 1 to 10, the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus, the formulas of DNA's building blocks and its double helix, a human figure with our height and population, the solar system and the telescope itself. The message will reach its target in about 25,000 years, and any reply would arrive in about 50,000. The telescope won't be waiting: it collapsed on December 1, 2020.

Both Voyager spacecraft, launched in 1977, carry a 30-centimeter gold-plated copper record. A committee chaired by Sagan chose 115 images, sounds of Earth, about 90 minutes of music and greetings in 55 languages, including Hebrew ("Shalom," peace). The cover is engraved with instructions in the language of physics: how to play the record, the Pioneer pulsar map, and a tiny sample of uranium-238 whose decay lets a finder work out when it was sent.

Later messages got more ambitious. In 1999 and 2003 a 70-meter radio dish at Yevpatoria in Crimea transmitted Cosmic Call, including a 23-page primer by the Canadian physicists Yvan Dutil and Stéphane Dumas that begins with mathematics, moves on to physics, and is built to survive noise along the way. In 2017 and 2018 METI International sent a message from Tromsø, Norway, toward a planet of Luyten's Star, 12.4 light-years away. It arrives in 2030, so the earliest possible reply would come around 2042.

NASA's Europa Clipper, launched toward Jupiter's moon on October 14, 2024, carries a plate with the word "water" in 103 languages, drawn as sound waves, and a poem by U.S. Poet Laureate Ada Limón. It isn't addressed to aliens as such, but the idea is the same: say something that makes sense without a dictionary.

Freudenthal went beyond pictures. In Lincos: Design of a Language for Cosmic Intercourse (1960) he built a whole language that could be learned from scratch, with no shared words. It starts with radio pulses standing for numbers, moves to arithmetic and logic, and only later to ideas such as time and behavior. The planned second volume was never written, and Dutil, who borrowed its ideas, called it "the most boring book in the world."

The approach lives on. In 2022 a team led by Jonathan Jiang of NASA's Jet Propulsion Laboratory proposed "A Beacon in the Galaxy," an updated binary message that could be sent from telescopes such as China's FAST. It starts with math and physics, moves to biochemistry and Earth's position in the galaxy, and ends with an invitation to reply. So far it is a proposal, not a transmission.

The linguist Jessica Coon of McGill University, who consulted on the film Arrival, urges caution. Alien languages, she says, "likely are very, very different from human languages," and "all bets are off." All human languages share core principles, but there is no reason an alien one would. It might not be spoken or written at all. Still, she argues, the communication of an advanced civilization should have patterns we could eventually decipher.

Sheri Wells-Jensen of Bowling Green State University, a director of METI International, adds that language grows out of the body: "Alien languages just get harder and harder to understand as the forms of the body diverge."

Meanwhile, researchers practice at home. Laurance Doyle of the SETI Institute and colleagues analyzed dolphin whistles with information theory as early as 1999. In August 2021 researchers from Alaska and California, Doyle among them, held a 20-minute "conversation" with a humpback whale named Twain: they played her 38 calls and she answered with 36. Project CETI is trying to decode sperm whale clicks, and papers from 2024 to 2026 found structure in them that resembles features of human speech. Yet no one knows what the whales are saying, and they share our planet and our mammal ancestors.

On May 24, 2023, the European ExoMars Trace Gas Orbiter, circling Mars, beamed an encoded message to Earth as if it came from aliens. The project, A Sign in Space, was led by the artist Daniela de Paulis of the SETI Institute, and three radio observatories picked up the signal. Thousands of people tried to crack it online.

It took more than a year. Only on June 7, 2024, did a father and daughter, Ken and Keli Chaffin, send in the answer: a diagram of five amino acids. What does it mean? According to the European Space Agency, the interpretation "remains open." The lesson is plain: a message written by humans for humans took a year to decode, and its meaning is still uncertain.

No signal moves faster than light. The nearest star, Proxima Centauri, is about 4.25 light-years away, so one question and answer would take at least eight and a half years. Most stars are tens or hundreds of times farther. Even close to home it adds up: Voyager 1 reaches a full light-day from Earth on November 18, 2026, so a message to our own spacecraft takes a day each way.

So talking with aliens wouldn't look like talking. It would look more like trading encyclopedias: each side sends as much as it can at once, without waiting for questions. And if a signal reached us, the first job wouldn't be replying but decoding, possibly for years. What would happen after a detection, who would announce it and whether anyone would answer is in our first contact protocol guide.

In Denis Villeneuve's Arrival (2016), based on Ted Chiang's 1998 novella "Story of Your Life," the aliens land and a linguist learns their writing face to face. In reality the likely route to contact is a signal from a distant star: no face, no window, no chance to ask "what did you mean?" The film also leans on an extreme version of the Sapir-Whorf hypothesis, the idea that language determines thought, to the point of changing how time is perceived. Coon says that idea "has been pretty clearly shown to not be correct" for humans, and that without it "it's not the same movie anymore."

Sagan's Contact, the 1985 novel and 1997 film, is closer to what scientists expect: a signal that opens with prime numbers, because no natural process produces them. And Star Trek's universal translator has no real-world counterpart. As Coon puts it, "We don't already have some kind of magical universal translator in place."

The International Academy of Astronautics' declaration, updated in June 2026, says SETI researchers should take part in international consultations, through the UN and other broadly representative bodies, on whether to answer a confirmed signal and what to say, and that until then "no reply should be sent." It doesn't cover messages sent before any detection, and no law stops anyone from transmitting.

Whether to transmit at all is a debate of its own, and some of the worry draws on the dark forest theory. Even the Breakthrough Initiatives, which in 2015 announced $1 million in prizes for designing a message to aliens, pledged not to transmit any message until there has been a wide-ranging debate.