Radio Bubble: alien couch potatoes? It's possible, thanks to escaping TV and radio signals

Natural History, Nov, 2001 by Neil deGrasse Tyson

In the opening scene of the 1997 film Contact, a virtual camera executes a controlled, slow pullout from Earth to the outer reaches of the universe. For this journey, you can decode Earth-based television and radio broadcasts that have escaped into space. Initially you hear a cacophonous mix of loud rock music, newscasts, and noisy static, as though you were listening to dozens of radio stations simultaneously. As the journey progresses out into space, and as you overtake earlier broadcasts that have traveled farther, the discordance lessens and the signals report historical events that span the broadcast era of modern civilization. Amid the noise, you hear--in reverse sequence--sound bites from the space shuttle Challenger disaster of January 1986; the Moon landing of July 1969; Martin Luther King's "I Have a Dream" speech, delivered in August 1963; President John F. Kennedy's January 1961 inaugural address; President Franklin D. Roosevelt's December 1941 address to Congress, asking for a declaration of war; and Adolf Hitler's speech given at the opening ceremonies of the 1936 Olympics. Eventually the human contribution to the signal disappears entirely, leaving a din of radio noise emanating from the cosmos itself.

Poignant. But in fact, this scroll of acoustic landmarks would not unfurl exactly as you hear it. If you somehow managed to violate an important law of physics by traveling fast enough to overtake a radio wave, few words would be intelligible, because you'd hear everything played backward. Furthermore, we hear King's famous speech as we pass the planet Jupiter--an audio-video moment implying that Jupiter is as far as the broadcast has traveled. In fact, King's speech passed Jupiter thirty-five minutes after he delivered it.

Contact's opening scene was nonetheless poetical and powerful, as it marked the extent to which we have presented our modern lives to the rest of the Milky Way Galaxy. The radio bubble, as it has come to be called, centers on Earth and continues to expand at the speed of light in every direction, while its middle gets continuously refilled by modern broadcasts. Our bubble now extends nearly a hundred light-years into space. Its leading edge corresponds to the first artificial radio signals that ever escaped from Earth, and its volume now contains about a thousand stars, including Alpha Centauri (4.3 light-years away), the nearest star system to the Sun; Sirius (10 light-years away), the brightest star in the nighttime sky; and every sunlike star around which a planet has thus far been discovered.

Not all radio signals escape our atmosphere, however. The plasma properties of the ionosphere, beginning at more than fifty miles up, enable it to reflect back to Earth all radio-wave frequencies of less than about twenty megahertz. This allows some forms of radio communication, such as the well-known shortwave frequencies of ham radio operators and the BBC World Service, to reach thousands of miles beyond their transmitters' horizons. All the broadcast frequencies of AM radio are also reflected back to Earth, accounting for the extended range these stations enjoy.

But if you broadcast at a frequency that isn't reflected by Earth's ionosphere (or if Earth didn't have an ionosphere), your radio signals would reach only those receivers in its line of sight. Tall buildings give a significant advantage to radio transmitters mounted on their roofs. While the horizon for a 5'8" person is just three miles away, the horizon seen by King Kong as he climbed atop New York City's Empire State Building was more than fifty. After the filming of that 1933 classic, a broadcast antenna was installed there. An equally high receiving antenna could, in principle, be located fifty miles farther still, enabling the signal to graze the treetops at the fifty-mile horizon, thereby extending the signal's reach to a hundred miles.

The ionosphere reflects neither FM radio nor broadcast television, itself a part of the radio spectrum. Each of these travels no farther on Earth than the most distant receiver it can see, allowing cities that are relatively near each other to command separate audiences for their own television programs. For this reason, locally broadcast, unsyndicated TV and FM radio cannot possibly be as influential as AM radio, which may account for AM'S preponderance of politically acerbic talk shows. But the real influence of FM and TV may not be terrestrial. While their signals are purposely broadcast parallel to the ground, part of every signal leaks straight up, crosses the ionosphere and travels through the depths of space. For these signals, the sky is not the limit. And unlike some other bands in the electromagnetic spectrum, radio waves have excellent penetration through the gas and dust clouds of interstellar space. For them, the stars are not the limit either.

If you add up all the factors that contribute to Earth's radio signature--such as military radar; the total number of shortwave, AM, FM, and TV stations; the distribution of stations across the planet's surface; the energy output of each station; and the bandwidth over which the energy is broadcast--you find that television accounts for the largest sustained flux of radio signals emanating from Earth. One part of a TV signal's anatomy is skinny; the other is wide. The skinny, narrowband part is the video carrier, through which more than half the total energy is broadcast. A mere 0.1 hertz wide in frequency, the signal establishes the station's location on the dial (the familiar channels 2 through 13) as well as the existence of the signal in the first place. A low-intensity, broadband signal 5 million hertz wide surrounds the carrier at higher and lower frequencies and is imbued with modulations that contain all the program information.