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EarthSky\u2019s 2026 lunar calendar is available now. Get yours today! Makes a great gift.<\/p>\n
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\u2019s said that ship radio operators in the Atlantic, who heard it, were astonished to hear music and speech over their receivers!<\/p>\n
And now, more than a century later, the radio waves that transmitted Fessenden\u2019s broadcast \u2013 and many more radio waves since then \u2013 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\u2019s been 119 years since that first radio broadcast. <\/p>\n
So the radius of Earth\u2019s radio bubble is 119 light-years, or a sphere with a diameter of 238 light-years across.<\/p>\n
So the strains of Fessenden\u2019s violin are still traveling through space, leading the way in Earth\u2019s 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\u2019s message approximately 23,000 years to reach the galaxy\u2019s nearest outer edge. And it would take his message about, say, 27,000 years \u2013 going in the opposite direction \u2013 to reach the galaxy\u2019s center and maybe about 77,000 years for it to reach the far side of the galaxy. <\/p>\n
So you might say that Earth\u2019s radio bubble \u2014 with Fessenden\u2019s 1906 broadcast on its leading edge \u2014 will take roughly 77,000 years to cross the entire Milky Way. One caveat, though. The \u201cradio bubble\u201d isn\u2019t a sharp wall. It\u2019s a thinning shell whose earliest signals (like Fessenden\u2019s) mark the outermost edge and with later, stronger, and more numerous broadcasts following behind.<\/p>\n
Meanwhile, the radio waves carrying Fessenden\u2019s 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 \u201cartificial\u201d was there. But they wouldn\u2019t hear the exact content.<\/p>\n
And, the fact is we don\u2019t know<\/em> how many stars lie within 119 light-years of us. That\u2019s 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\u2019s 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.<\/p>\n So, even though our radio waves have only traversed a tiny fraction of the Milky Way, they\u2019ve already encountered numerous stars.<\/p>\n Interestingly, our evolving technology is becoming harder to detect. That\u2019s because fiber optics and digital transmissions leak far less radio energy into space than the radio waves that carried Fessenden\u2019s message. <\/p>\n So, while our radio bubble continues to expand, the signals near its center are becoming fewer and farther between.<\/p>\n Bottom line: Earth\u2019s 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.<\/p>\n Read more: 1st intentional signal to space sent by Arecibo 50 years ago<\/p>\n Read more: Voyager 1 spacecraft has sent a (partly) decipherable message<\/p>\n <\/div>\nAre we still broadcasting?<\/h3>\n