L'invraisemblable défi de la communication extraterrestre

Attempting to communicate with extraterrestrials is akin to shouting across the universe with a sign written in sign language. While other forms of intelligence might exist on other planets, we've so far been met with a great silence. The lack of messages from alien civilizations, despite potentially hundreds of millions of habitable planets in our galaxy alone, could simply be due to the immense vastness of space, where no one can hear you scream. The challenge of interstellar communication is enormous. Regardless of whether the message contains instructions for a fondue machine or blueprints for a particle accelerator, it must overcome staggering distances to reach its target. The idea of making contact isn't new; throughout the modern era, brilliant minds have conceived ingenious ways to bridge this immensity. In the early 19th century, when intelligent life on the Moon, Mars, or Venus was still considered possible before telescopes revealed their hostile environments, some thinkers suggested creating intense light signals to attract attention on a grand scale. In 1919, a serious proposal was made to dig a series of trenches in the Sahara, hundreds of meters wide, forming geometric shapes thirty kilometers across. These trenches would then be filled with water and kerosene, which, when ignited nightly, would send a signal from the desert, transformed into a giant blackboard where one could "write with fire." A year later, in 1920, mathematician Carl Friedrich Gauss proposed planting lines of trees several kilometers wide in the Siberian tundra, encircling vast fields of rye and wheat to form a geometric figure demonstrating the Pythagorean theorem, large enough to be seen by hypothetical inhabitants of the Moon. None of these large-scale surface patterns were ever created. With the advent of electricity, new methods were imagined. In 1874, engineer Charles Cross, convinced of Martians' existence, unsuccessfully sought funds to build a massive parabolic mirror to reflect and concentrate sunlight, drawing figures on the red planet's surface. The following year, in 1875, Finnish mathematician Edward Zelberg Novius envisioned a device with 22,500 electric lamps designed to wave to Martians. Nearly 150 years later, despite giant leaps in technology, the difficulties of communicating across space have paradoxically grown. We now know that no planets in our solar system harbor intelligent life. To be noticed by hypothetical extraterrestrials, we must look to distant planets orbiting other stars. The surrounding space is so vast that communicating with classical light signals, like visible light, is currently inconceivable. Over phenomenal distances, apparent luminosities quickly vanish. In fact, space is so immense that only the most powerful stars are visible from afar. Even Proxima Centauri, our closest star at just 4.2 light-years, is not visible to the naked eye and was only discovered in the early 20th century, despite being nearly 1000 times less luminous than the Sun, yet still emitting far more energy than humanity has consumed since the industrial revolution. Aware of these limitations, other avenues are being explored, some quite outlandish, like communicating with nuclear bombs. In 1971, during the Cold War, at the first conference on identifying and contacting aliens, one participant proposed gathering all US and Soviet nuclear warheads in space and detonating them simultaneously. This would produce a phenomenal amount of X-rays, detectable by advanced civilizations. The idea was to halt the nuclear arms race while simultaneously signaling extraterrestrials. Following this, Andrey Sakharov, a prominent Soviet physicist and nuclear disarmament advocate, suggested positioning all existing bombs at specific locations in the solar system and detonating them in a particular rhythm, using sporadic flashes to send a simple message, such as sequences of prime numbers. These complex and extremely dangerous proposals were never taken seriously. In the years that followed, four probes were sent to explore the outer planets of our solar system, carrying engraved gold plaques and discs intended for other civilizations. However, these probes, drifting in the cold immensity of space, are not aimed at any particular star system. They are so small and slow relative to their surroundings that there is virtually no chance of them ever being intercepted. To illustrate the vastness of our solar system, consider that light takes just over a second to travel from Earth to the Moon, 8 minutes and 20 seconds from the Sun to Earth, and nearly 4 hours and 10 minutes to Neptune. To reach the edge of our star's gravitational influence and exit the solar system, light takes a year and a half. The closest star is 4.2 light-years away. Voyager 1, carrying one of the engraved gold discs and currently the fastest man-made object, travels at a staggering 60,000 km/h. Yet, this speed is nearly 18,000 times slower than light. To cover the distance to our nearest star, Voyager 1 would take almost 70,000 years. Therefore, serious attempts at cosmic communication always turn to light, the fastest thing in the universe. The challenge is that creating a light source powerful enough to be visible for a few weeks from the nearest star would require several thousand times the energy consumed by humanity in a year, which is impractical. A potential solution could be intense laser pulses, concentrating light into narrow beams to minimize signal loss in space. However, even with this, the energy required for such a beam to be detectable without advanced technology up to a few dozen light-years is considerable, equivalent to the output of a nuclear power plant. Beyond that, the gradual dispersion and loss of intensity make the beam unrecognizable. Interstellar gas and dust subtly perturb light propagation, and over great distances, these small perturbations accumulate, distorting or erasing the message content. Moreover, building such powerful and precise lasers is a colossal and costly technological challenge. So, how can a message—whether instructions for a morning alarm clock or a question about the meaning of life—reach its destination without being erased? Fortunately, when transmitting a message at light speed, we are not limited to the visible spectrum. The entire electromagnetic spectrum, from very low-frequency radio waves to high-energy gamma rays, is available. Each part of this spectrum has unique properties influencing wave propagation, allowing us to seek the optimal channel for extraterrestrial communication. Among all types of light, radio waves, at the lower end of the electromagnetic spectrum, offer a significant advantage: they can traverse the interstellar medium with minimal deterioration from gas and dust clouds, while requiring relatively little energy to emit. This seems perfect for sending coded greetings without them being erased before arrival. However, even with radio waves, signals must be powerful enough not to be drowned out by the ambient background noise of space. The interstellar medium is continuously bathed in electromagnetic radiation from solar flares, exotic natural objects like pulsars and quasars, and cosmic microwave background radiation—faint light from the early universe. Every body above absolute zero continuously emits small amounts of radiation; it's estimated that only four out of 100 light particles in space are from stars. All these emissions create a background hum that must be overcome. For instance, radio waves from Earth's most powerful FM antennas become indistinguishable from cosmic background noise at just a few light-years, even for our most sensitive radiotelescopes. Since signal intensity rapidly dilutes across vast distances, decreasing proportionally to the square of the distance traveled, even economical radio waves cannot blanket the entire cosmos with greetings. This loss of intensity means that after a few hundred light-years, even our most powerful signals become difficult to detect. To illustrate this staggering dilution, detecting a radio message 100 light-years away in all directions with equivalent technology would require 7,000 times more energy than all European power plants combined, plus a gigantic network of radiotelescopes. This is why signals are almost always concentrated towards a specific point in the sky. This leads to another difficulty: aiming. Stars are not stationary relative to each other; they move through the cosmos. To avoid missing, one must target moving objects dozens, hundreds, or even thousands of light-years away, all while transmitting from a planet orbiting a moving star. This precision challenge is partly why the iconic Arecibo message, sent in 1974, was directed towards the Hercules globular cluster, a region of the sky concentrating nearly 100,000 stars within 150 light-years. It's ironic that this message, the most powerful ever transmitted into space using a 305-meter antenna—concentrating the equivalent of several thousand conventional radio stations—will likely arrive as a degraded, almost dead signal. The Arecibo message will arrive in tatters because it was not repeated and contained no redundancy to protect against the "erasures" of space over such vast distances. Aimed at a star cluster 25,000 light-years away, parts of it will be erased and destroyed by noise along the way. The loss of even a few bits of information or pixels of content can render the entire final image meaningless. Finally, there's the critical choice of radio frequency. The radio wave band for transmitting signals is enormous. The question is whether a specific frequency stands out as the most logical for listening and transmitting. In other words, what radio station do extraterrestrials likely tune into? Most researchers agree that extraterrestrials likely listen to "Radio Water Hole" (a non-official term). This frequency band, between 1 and 3 GHz, is unanimously considered a potential meeting point. Firstly, interstellar gas and dust clouds are largely transparent to these wavelengths. Secondly, and crucially, it encompasses the "water hole"—a region including the spectral signatures of water's constituent elements, notably hydrogen, the most abundant and simplest atomic element in the universe. The underlying idea is that this frequency choice could be universally recognized as a meeting point by other advanced extraterrestrial civilizations also seeking to broadcast or listen. In total, since we gained the ability to send messages into the cosmos, only 12 radio emissions powerful enough to reach their targets have been sent, hoping to be intercepted and deciphered. Most were sent somewhat blindly towards star systems about which we know almost nothing, sometimes not even if planets exist there. This is like trying to send a letter across the world without knowing if the address exists or if anyone lives there. And even if someone does, for them to reply, they must be listening at the precise moment the message arrives, have the desire to open it, read it, and understand its meaning, decades after it was sent, without certainty that its content hasn't been partially erased by the journey. The immense challenges in ensuring our messages survive in the cosmos might explain why the search for extraterrestrial intelligence has so far been met with deafening radio silence. Perhaps they exist but our messages are too weak, or they've captured them but mistake them for noise, or are simply perplexed. If one of our messages ever received a reply, we would have to contend with the enormous time lag imposed by the vast distances between stars to engage in a discussion. Such a discussion would be surrealistically slow, with decades separating questions and answers, allowing ample time for thoughts to mature. Interstellar communication can only be a long-term project, with discussions spanning centuries and generations. Grandparents might pose a question, and their great-grandchildren might receive the answer. A final thought: to the question, "Are we the only advanced intelligent life form in the universe?" there are only two possible answers: either we are alone, or we are not. The implications of either answer are staggering. If intelligent life exists only on Earth, we bear an enormous responsibility for it. If it exists elsewhere, discovering it would be a terribly difficult narcissistic wound for many contemporaries who believe themselves to be children of a god. Until a response might one day be received, our planet hosts 100% of all known life forms in the universe, as we have yet to find any elsewhere.