Most people would be surprised to discover that several 19th century scientists contemplated how we might communicate with possible Martians and Venusians.
These early proposals – which predate by 150 years the first intentional message aimed at extraterrestrials, beamed out from Earth in 1974 – were based on signals in visual light, as the invention of radio was still decades away.
In fact, as history shows, ideas for interplanetary communication have largely been driven by whatever the contemporary technology allowed – be it lamps, radios or lasers. “You go with what you know,” says Steven Dick, NASA’s Chief Historian.
Over two thousand years ago, the ancient Greeks argued over the existence of life on other planets, but the idea really took off after the Copernican revolution, says Dick. “Once it was realised that all the planets go around the Sun, it was not hard to imagine that the other planets could be like Earth.”
Galileo, Kepler and others considered the inhabitability of the planets, while being careful not to upset Church authority. Later “the idea blossomed in the 17th century into the ‘plurality of worlds’ debate, but it remained controversial,” adds Dick, who has written several books on the topic.
One of the most influential proponents for extraterrestrial life was Bernard le Bovier de Fontenelle, who wrote a book called Conversations on the Plurality of Worlds in 1686. Despite the interest, there was no recorded discussion of how we might locate or contact these potential aliens until more than a century later.
Florence Raulin-Cerceau of the Alexandre Koyré Centre in Paris, France, has documented the early attempts at communication with extraterrestrial intelligence (CETI), or what is now often called active SETI (search for extraterrestrial intelligence).
“As early as the 19th century, inventors imagined “sky telegraph” equipment to communicate with the supposed inhabitants of the Solar System’s planets,” Raulin-Cerceau recently wrote in the French magazine Pour la Science.
The first of these inventors was Carl Friedrich Gauss, a German mathematician. In the 1820s, he spoke of reflecting sunlight towards the planets with his land surveying invention, the heliotrope. He is also credited with the idea of cutting a giant triangle in the Siberian forest and planting wheat inside.
“The size and colour contrast should have made the object visible from the Moon or Mars, and the geometric figure could only be interpreted as an intentional construction,” says Raulin-Cerceau.
Twenty years later, the astronomer Joseph von Littrow came up with a similar idea to pour kerosene into a 30-kilometre-wide circular canal that would be lit at night to signal our presence. The second half of the 19th century saw more realistic proposals develop, however.
In a more modern fashion, French inventor and poet Charles Cros was one of the first to propose sending coded messages to other planets by flashing beams of light. In 1869, he imagined using a parabolic mirror to focus the light from electric lamps towards Mars or Venus. He figured the light could be flashed on and off to encode a message.
“Cros granted that the planets could be inhabited by beings not able to respond, but he was still persuaded that ‘the eternal isolation of the spheres [will be] vanquished,'” wrote Raulin-Cerceau.
A light-based "morse code" was also considered by the British statistician (and half-cousin of Charles Darwin) Francis Galton in 1896. He took care not to assume that Martians would have our same base-10 counting system, as they probably wouldn’t have 10 fingers.
Around the same time, A. Mercier, a member of the Astronomical Society of France, devised a plan to place several reflectors on the Eiffel Tower that could direct sunlight towards Mars. He also considered using the Moon as a giant screen on which to project light beams.
Could aliens have seen any of these light displays though? “It depends on how much money you think the Martians are spending on their telescopes,” quips astronomer Seth Shostak of the SETI Institute in Mountain View California.
Today, radio is thought of as a much more efficient means of extraterrestrial communication. Radio waves are less affected by cosmic dust than visible light, and there is less of a radio background to deal with in the sky.
With its 300-metre reflector dish, the Arecibo Observatory in Puerto Rico is the world’s largest single-dish radio telescope and most powerful radar. Frank Drake used it in 1974 to send the first CETI message.
Two of radio's pioneers showed interest in interplanetary radio communication. In 1901, American Nikola Tesla reported receiving a strange signal, we he thought might have come from Mars, on his giant transmitting tower in Colorado Springs. Nineteen years later, Guglielmo Marconi told reporters about his detection of radio emissions that appeared to come from outer space.
However, the switch to radio-based SETI didn’t happen immediately. As late as the 1920s, many people (including Albert Einstein) still considered visual-based communication more practical, since radio transmitters were not yet capable of focusing a beam at a distant planet.
What’s more, scientists gradually became convinced that Mars did not have the right conditions to support life, so any presumed extraterrestrials likely lived much, much further away. “It seemed hopeless to receive messages from other stellar systems, so people said ‘forget it’,” says Shostak.
It wasn’t until 1959 that radio-based SETI started to be taken seriously. In that year, Giuseppe Cocconi and Philip Morrison showed that radar transmitters of the time were already powerful enough to send signals many light years through space.
Then people started to think “If we can do it, then the aliens might be doing it,” Shostak says. In the year that followed, Frank Drake performed Project Ozma, the first radio sky survey to look for intelligent signals.
And then in 1974 – a century and half after Gauss – Drake transmitted the first actual SETI message using the Arecibo radio telescope. Scientists are still waiting for a response.
This is an edited version of a story that first appeared in NASA’s Astrobiology Magazine.