Earth's radio signals have traveled approximately 119 light-years in every direction, forming a "radio bubble" with a diameter of about 238 light-years. This expanding sphere, originating from early 20th-century broadcasts, has now encountered at least 75 star systems that could potentially observe Earth transiting the Sun.^[1][2][3] ### Humanity's Expanding Radio Sphere The journey of Earth's radio signals into space began over a century ago. Reginald Aubrey Fessenden made the first known public radio broadcast on December 24, 1906.^[1][4][2][5] This transmission, which included a violin performance and a Bible reading, marked a revolutionary moment in communication.^[4][2][5] These radio waves, like all electromagnetic waves, travel at the speed of light, which is about 186,000 miles per second (299,792,458 meters per second) in a vacuum.^{[6][7][8][9][10]} Since that initial broadcast, countless other signals have followed, steadily expanding Earth's radio bubble.^[1][4][2][5] While this bubble has grown significantly, it remains a tiny dot within the vastness of the Milky Way galaxy, which stretches about 100,000 light-years across.^{[1][2][11][12][13]} The earliest signals, though incredibly weak, continue to define the outermost edge of this expanding sphere.^[1][2] ### Detecting Distant Signals Scientists in the Search for Extraterrestrial Intelligence (SETI) actively look for "technosignatures," or signs of technology from other civilizations.^{[14][15][16][17]} Earth's own powerful radio emissions, such as planetary radar signals from observatories like the now-collapsed Arecibo telescope, are considered our most detectable technosignatures.^{[14][15][18][19][17]} These types of signals could theoretically be detected by an alien civilization with similar technology from as far as 12,000 light-years away.^{[14][15][18][19][17]} However, the detectability of everyday "leakage" signals, like those from cell towers and television broadcasts, is much more limited.^[2][18] These signals rapidly weaken over vast distances and become buried in cosmic background noise.^{[2][11][20][21]} For an alien civilization to detect such signals, they would need extremely sensitive receivers or be relatively close to Earth.^[6][20][21] ### Intentional Messages and Future Challenges Beyond unintentional leakage, humanity has also sent intentional messages into space. The Arecibo Message, broadcast in 1974 towards the M13 globular star cluster, was a particularly powerful transmission.^[22][23] This message, designed by scientists like Frank Drake and Carl Sagan, contained information about human biology and Earth's place in the solar system.^[22][23] While the Arecibo signal has traveled about 50 light-years since its transmission, M13 is 25,000 light-years away, meaning the message still has a long journey ahead.^[22][19] NASA's Deep Space Network (DSN) also sends and receives powerful radio signals to communicate with spacecraft exploring our solar system and beyond.^{[24][18][25][26][27][28]} These targeted signals are much stronger than terrestrial broadcasts.^[18] Scientists estimate that DSN-like signals could be detectable by Earth-level alien technology out to about 60 light-years.^[18][19] As technology evolves, our methods of communication are also changing. Fiber optics and digital transmissions leak less radio energy into space compared to older broadcast methods.^[1] This means that while our radio bubble continues to expand, the signals emanating from Earth are becoming fewer and weaker near its center.^[1] The ongoing search for extraterrestrial intelligence continues to refine its methods, using Earth's own technosignatures as a guide for what to look for in the cosmos.^[14][15][17]

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