In 1977, NASA launched two little Voyager probes within 15 days of each other. They were built to run for about four years. More than 45 years later, they're still chugging deeper into space and sending data home every single day, and Voyager 1 is predicted to outlive Earth by at least a trillion years. What they pick up out there isn't silence, exactly, but it's a quiet so vast you find yourself straining to hear anything at all.
That quiet raises the oldest question a stargazer can ask. If the universe is full of life, where is everybody?
Whistling behind the moon
In 1969, Apollo 10 swung around the far side of the moon, where the crew had no contact with NASA. The astronauts heard a whistling. One of them said it sounded like "outer space type music." The tapes sat in the archives for four decades, and when they surfaced on Good Morning America, the hosts laughed the whole thing off. One NASA technician had a sensible explanation: the radios on two different spacecraft interfering with each other. The astronauts weren't so sure, and they admitted they had been very frightened.
The idea of music in space is centuries old. Johannes Kepler had his hands crippled and his eyesight damaged by childhood smallpox, which ended his dream of becoming an observational astronomer. So he looked at the sky through math instead, and it hit him that geometry might be the foundation of the universe. He worked out the geometry of the planets' orbits and turned it into sheet music. In The Harmony of the World, published in 1619, he proposed that the planets could sing together in perfect harmony. It was rare, he said, and it had first happened at the precise moment the universe was created.
The payphone that kept ringing
Years ago, before anyone carried a phone in their pocket, a teenage Sheri Lynch worked nights and biked home well after 11 p.m. She took the long way around St. Mary's Cemetery rather than cut through it after dark. One damp spring night, she heard a payphone ringing in an empty junior high parking lot. It rang and rang, with no cars and no dog walkers anywhere. Her heart started racing, and she swore she could still hear it long after she'd passed the point where the sound should have faded. "It was like the whole world was one huge, empty house."
Listening on 1420 megahertz
Back when serious scientists wouldn't go near UFO research, astronomer Frank Drake had a simple idea: listen for radio waves. In 1960, he aimed a radio telescope at two stars about as bright as our sun, each about 11 light years away. He tuned in to 1420 megahertz, a frequency he figured any reasonably advanced civilization would know. He also used off-the-rack equipment so no one could accuse him of wasting money. He found nothing. A year later, he introduced the Drake equation, which estimates how many alien civilizations we could detect using seven variables. Six of those variables were unknown at the time, and most still are.
Then, in August 1977, Ohio State's Big Ear radio telescope caught the legendary Wow signal. About 40 years later, two astronomers broke the bad news: it wasn't aliens. It came from huge clouds of hydrogen gas around a pair of comets. Hydrogen naturally gives off radio waves at 1420 megahertz, the very frequency Drake had picked out as the universe's hailing channel.
By then, the Great Silence already had a name. Back in 1950, Enrico Fermi, the Nobel Prize winner who fled Mussolini's Italy and built the first nuclear reactor, raised it over lunch. The galaxy has hundreds of billions of stars, and many of them must have planets like ours. Some of those worlds would have a head start of billions of years. So why was there no artifact and no stray radio transmission? That question became the Fermi paradox, and skeptics have leaned on it ever since.
The Great Silence may not mean we're alone. It may mean nobody has gotten our call yet: Earth has been broadcasting for barely a century, so no alien civilization more than 100 light years away could know we're here.
The call hasn't gone through
In December 2022, Hebrew University astrophysicist Dr. Amri Wandel published a polite rebuttal in the Astrophysical Journal. For billions of years, Earth was an ordinary biotic planet, meaning one that supports life, circling a yellow dwarf star. Nothing about it stood out. That changed on May 13th, 1897, when Guglielmo Marconi sent the first radio transmission. On Christmas Eve 1906, ship radio operators off New England heard a man's voice cut into their Morse code, reading the Christmas story from the Gospel of Luke. KDKA in Pittsburgh made the first commercial broadcast in 1920. By the 1930s, shortwave signals were strong enough to break through the ionosphere and escape into space.
Wandel calls this growing bubble of broadcasts Earth's radiosphere. It is our technosignature, the sign that a technological civilization lives here. His point is that it hasn't traveled past 100 light years yet. Even if aliens 50 light years away caught our signal, their reply would need another 50 years to get back. Earth has only just entered what Wandel calls the contact era. Two less cheerful points follow. A planet can support life for eons while a technological civilization lasts a much shorter time. And advanced species may go radio-quiet, the way fiber optics already are here. Maybe the Great Silence doesn't mean we're alone. Maybe everyone is just very far apart.
Eight signals from next door
In early 2023, University of Toronto student Peter Ma built an AI algorithm to sift through the noise. Scientists at Breakthrough Listen, a SETI program using the Green Bank Telescope in West Virginia, ran it on data they'd already collected from stars 30 to 90 light years away. It found eight intriguing signals. They were narrowband, which natural sources rarely produce. They had slope, a drift that means they couldn't have come from Earth. And they were on source: the signals showed up only when the telescope pointed at the star and disappeared when it looked away, which rules out human interference. Space that everyone had called empty turned out not to be.
As Carl Sagan warned, "absence of evidence is not evidence of absence." Someday, someone may finally pick up that ringing phone.
