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What is Earth’s radio bubble?
The first intentional radio broadcast is credited to Reginald Aubrey Fessenden on December 24, 1906. Fessenden transmitted voice and music (a short speech, a violin performance of O Holy Night, and a Bible reading). Prior to this, radio signals were mostly Morse code. This was the first known broadcast meant to be heard by a general audience as sound. And it’s said that ship radio operators in the Atlantic, who heard it, were astonished to hear music and speech over their receivers!
And now, more than a century later, the radio waves that transmitted Fessenden’s broadcast – and many more radio waves since then – are still traveling outward into our Milky Way galaxy. But just how far have our radio waves reached? Radio waves are part of the electromagnetic spectrum, so they travel at the speed of light. And it’s been 119 years since that first radio broadcast.
So the radius of Earth’s radio bubble is 119 light-years, or a sphere with a diameter of 238 light-years across.
What is the extent of Earth’s radio bubble?
So the strains of Fessenden’s violin are still traveling through space, leading the way in Earth’s ever-expanding radio bubble. The distance of 238 light-years might sound like a lot, but it is just a tiny dot within the Milky Way galaxy. Our galaxy extends some 100,000 light-years across. So it would take Fesssenden’s message approximately 23,000 years to reach the galaxy’s nearest outer edge. And it would take his message about, say, 27,000 years – going in the opposite direction – to reach the galaxy’s center and maybe about 77,000 years for it to reach the far side of the galaxy.
So you might say that Earth’s radio bubble — with Fessenden’s 1906 broadcast on its leading edge — will take roughly 77,000 years to cross the entire Milky Way. One caveat, though. The “radio bubble” isn’t a sharp wall. It’s a thinning shell whose earliest signals (like Fessenden’s) mark the outermost edge and with later, stronger, and more numerous broadcasts following behind.
Meanwhile, the radio waves carrying Fessenden’s first broadcast have reached some of the closest stars. For example, they have reached the closest star, Proxima Centauri, at just 4.24 light-years away. Would any beings at Proxima Centauri be hearing our music and voices? No. While our music, news, and television signals have passed through their skies, the broadcasts are so faint and scattered that they dissolve into cosmic noise, leaving only a whisper that a technological civilization once existed. So, assuming they were using technology similar to that we know, they would know something “artificial” was there. But they wouldn’t hear the exact content.
And, the fact is we don’t know how many stars lie within 119 light-years of us. That’s because the majority of stars in the galaxy are dim red dwarf stars, which are difficult to detect. Lisa Kaltenegger and Jackie Faherty produced a study for Cornell University in 2019 to figure out which stars could see Earth transiting the sun. And as part of that study, they found 75 stars that Earth’s radio signals have washed over. These are just the stars that not only have received our radio broadcasts but also could look toward us and see that planets live here as they occasionally pass in front of and darken the surface of the sun.
So, even though our radio waves have only traversed a tiny fraction of the Milky Way, they’ve already encountered numerous stars.
Are we still broadcasting?
Interestingly, our evolving technology is becoming harder to detect. That’s because fiber optics and digital transmissions leak far less radio energy into space than the radio waves that carried Fessenden’s message.
So, while our radio bubble continues to expand, the signals near its center are becoming fewer and farther between.
Bottom line: Earth’s radio bubble is the extent to which our radio broadcasts have traveled. The first intentional radio broadcast was on December 24, 1906. And the radio waves have been traveling outward at the speed of light ever since.
Read more: 1st intentional signal to space sent by Arecibo 50 years ago
Read more: Voyager 1 spacecraft has sent a (partly) decipherable message