Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)
SECTION 4. THE MEANING OF LIFE
Chapter 27. OUR RADIO BUBBLE
For the opening scene to the 1997 film Contact, a virtual camera executes a controlled, three-minute zoom from Earth to the outer reaches of the universe. For this journey, you happen to be equipped with receivers that enable you to decode Earth-based television and radio broadcasts that have escaped into space. Initially, you hear a cacophonous mixture of loud rock music, news broadcasts, 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 signals become less cacophonous and distinctly older as they report historical events that span the broadcast era of modern civilization. Amid the noise, you hear sound bytes in reverse sequence that include: the Challenger shuttle disaster of January 1986; the Moon landing of July 20, 1969; Martin Luther King’s famous “I Have a Dream” speech, delivered in August 28, 1963; President Kennedy’s January 20, 1961, inaugural address; President Roosevelt’s December 8, 1941, address to Congress, where he asked for a declaration of war; and a 1936 address by Adolf Hitler during his rise to power in Nazi Germany. Eventually, the human contribution to the signal disappears entirely, leaving a din of radio noise emanating from the cosmos itself.
Poignant. But this scroll of acoustic landmarks would not unfurl exactly as shown. If you somehow managed to violate several laws of physics and travel fast enough to overtake a radio wave, then few words would be intelligible because you’d hear everything replayed backward. Furthermore, we hear King’s famous speech as we pass the planet Jupiter, implying Jupiter is as far as the broadcast has traveled. In fact, King’s speech passed Jupiter 39 minutes after he delivered it.
Ignoring these facts that would render the zoom impossible, Contact’s opening scene was poetic and powerful, as it indelibly marked the extent to which we have presented our civilized selves to the rest of the Milky Way galaxy. This 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 getting its center continuously refilled by modern broadcasts. Our bubble now extends nearly 100 light-years into space, with a leading edge that corresponds to the first artificial radio signals ever generated by Earthlings. The bubble’s 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 star around which a planet has thus far been discovered.
NOT ALL RADIO signals escape our atmosphere. The plasma properties of the ionosphere, more than 50 miles up, enable it to reflect back to Earth all radio-wave frequencies less than 20 megahertz, allowing some forms of radio communication, such as the well-known “short wave” frequencies of HAM radio operators, to reach thousands of miles beyond your horizon. All the broadcast frequencies of AM radio are also reflected back to Earth, accounting for the extended range that these stations enjoy.
If you broadcast at a frequency that does not correspond to those reflected by Earth’s ionosphere, or if Earth had no ionosphere, your radio signals would reach only those receivers that fell in its line of “sight.” Tall buildings give significant advantage to radio transmitters mounted on their roofs. While the horizon for a 5'8" person is just 3 miles away, the horizon seen by King Kong, while climbing atop New York City’s Empire State Building, is more than 50. After the filming of that 1933 classic, a broadcast antenna was installed. An equally tall receiving antenna could, in principle, be located 50 miles farther still, enabling the signal to cross their mutual 50-mile horizon, thereby extending the signal’s reach to 100 miles.
The ionosphere reflects neither FM radio nor broadcast television, itself a subset of the radio spectrum. As prescribed, they each travel no farther on Earth than the farthest receiver they can see, which allows cities that are relatively near each other to broadcast their own television programs. For this reason, television’s local broadcasts and FM radio cannot possibly be as influential as AM radio, which may account for its preponderance of politically acerbic talk shows. But the real influence of FM and TV may not be terrestrial. While most of the signal’s strength is purposefully broadcast horizontal to the ground, some of it leaks straight up, crossing the ionosphere and traveling through the depths of space. For them, 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, so the stars are not the limit either.
If you add up all factors that contribute to the strength of Earth’s radio signature, such as the total number of stations, the distribution of stations across Earth’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 detectable from Earth. The anatomy of a broadcast signal displays a skinny and a wide part. The skinny, narrow-band part is the video carrier signal, through which more than half the total energy is broadcast. At a mere .10 hertz wide in frequency, it 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.
AS YOU MIGHT guess, the United States is the most significant contributor among all nations to Earth’s global television profile. An eavesdropping alien civilization would first detect our strong carrier signals. If it continued to pay attention, it would notice periodic Doppler shifts in these signals (alternating from lower frequency to higher frequency) every 24 hours. It would then notice the signal get stronger and weaker over the same time interval. The aliens might first conclude that a mysterious, although naturally occurring, radio loud spot was rotating into and out of view. But if the aliens managed to decode the modulations within the surrounding broadband signal they would gain immediate access to elements of our culture.
Electromagnetic waves, including visible-light as well as radio waves, do not require a medium though which to travel. Indeed, they are happiest moving through the vacuum of space. So the time-honored flashing red sign in broadcast studios that says “On the Air” could justifiably read “Through Space,” a phrase that applies especially to the escaping TV and FM frequencies.
As the signals move out into space they get weaker and weaker, becoming diluted by the growing volume of space through which it travels. Eventually, the signals get hopelessly buried by the ambient radio noise of the universe, generated by radio-emitting galaxies, the microwave background, radio-rich regions of star formation in the Milky Way, and cosmic rays. These factors, above all, will limit the likelihood of a distant civilization decoding our way of life.
At current broadcast strengths from Earth, aliens 100 light-years away would require a radio receiver that was fifteen times the collecting area of the 300-meter Arecibo telescope (the world’s largest) to detect a television station’s carrier signal. If they want to decode our programming information and hence our culture, they will need to compensate for the Doppler shifts caused by Earth’s rotation on its axis and by its revolution around the Sun (enabling them to lock onto a particular TV station) and they must increase their detection capacity by another factor of 10,000 above that which would detect the carrier signal. In radio telescope terms, this amounts to a dish about four hundred times Arecibo’s diameter, or about 20 miles across.
If technologically proficient aliens are indeed intercepting our signals (with a suitably large and sensitive telescope), and if they are managing to decode the modulations, then the basics of our culture would surely befuddle alien anthropologists. As they watch us become a radio-transmitting planet, their attention might first be flagged by early episodes of the Howdy Doody show. Once they knew to listen, they would then learn how typical human males and females interact with each other from episodes of Jackie Gleason’s Honeymooners and from Lucy and Ricky in I Love Lucy. They might then assess our intelligence from episodes of Gomer Pyle, The Beverly Hillbillies, and then, perhaps, from Hee Haw. If the aliens didn’t just give up at this point, and if they chose to wait a few more years, they would learn a little more about human interactions from Archie Bunker in All in the Family, then from George Jefferson in The Jeffersons. After a few more years of study, their knowledge would be further enriched from the odd characters in Seinfeld and, of course, the prime-time cartoon The Simpsons. (They would be spared the wisdom of the hit show Beavis and Butthead because it existed only as a nonbroadcast cable program on MTV.) These were among the most popular shows of our times, each sustaining cross-generational exposure in the form of reruns.
Mixed in among our cherished sitcoms is the extensive, decade-long news footage of bloodshed during the Vietnam war, the Gulf wars, and other military hot spots around the planet. After 50 years of television, there’s no other conclusion the aliens could draw, but that most humans are neurotic, death-hungry, dysfunctional idiots.
IN THIS ERA of cable television, even broadcast signals that might have otherwise escaped the atmosphere are now delivered via wires directly to your home. There may come a time when television is no longer a broadcast medium, leaving our tube-watching aliens to wonder whether our species went extinct.
For better or for worse, television might not be the only signals from Earth decoded by aliens. Any time we communicate with our astronauts or our space probes, all signals that do not intersect the craft’s receiver are lost in space forever. The efficiency of this communication is greatly improved by modern methods of signal compression. In the digital era, it’s all about bytes per second. If you devised a clever algorithm that compressed your signal by a factor of 10, you could communicate ten times more efficiently, provided the person or machine on the other side of the signal knew how to undo your compressed signal. Modern examples of compression utilities include those that create MP acoustic recordings, JPEG images, and MPEG movies for your computer, enabling you to swiftly transfer files and to reduce the clutter on your hard drive.
The only radio signal that cannot be compressed is one that contains completely random information, leaving it indistinguishable from radio static. In a related fact, the more you compress a signal, the more random it looks to someone who intercepts it. A perfectly compressed signal will, in fact, be indistinguishable from static to everyone but the person who has the preordained knowledge and resources to decode it. What does it all mean? If a culture is sufficiently advanced and efficient, then their signals (even without the influence of cable transmissions) might just disappear completely from the cosmic highways of gossip.
Ever since the invention and widespread use of electric bulbs, human culture has also created a bubble in the form of visible light. This, our nighttime signature, has slowly changed from tungsten incandescence to neon from billboards and sodium from the now-widespread use of sodium vapor lamps for streetlights. But apart from the Morse code flashed by shuttered lamps from the decks of ships, we typically do not send visible light through the air to carry signals, so our visual bubble is not interesting. It’s also hopelessly lost in the visible-light glare of our Sun.
RATHER THAN LET aliens listen to our embarrassing TV shows, why not send them a signal of our own choosing, demonstrating how intelligent and peace loving we are? This was first done in the form of gold-etched plaques affixed to the sides of the four unmanned planetary probes Pioneer 10 and 11 and Voyager 1 and 2. Each plaque contains pictograms conveying our base of scientific knowledge and our location in the Milky Way galaxy while the Voyagerplaques also contain audio information about the kindness of our species. At 50,000 miles per hour—a speed in excess of the solar system’s escape velocity—these spacecraft are traveling through interplanetary space at quite a clip. But they move ridiculously slow compared with the speed of light and won’t get to the nearby stars for another 100,000 years. They represent our “spacecraft” bubble. Don’t wait up for them.
A better way to communicate is to send a high-intensity radio signal to a busy place in the galaxy, like a star cluster. This was first done in 1976, when the Arecibo telescope was used in reverse, as a transmitter rather than a receiver, to send the first radio-wave signal of our own choosing out to space. That message, at the time of this writing, is now 30 light-years from Earth, headed in the direction of the spectacular globular star cluster known as M13, in the constellation Hercules. The message contains in digital form some of what appeared on the Pioneer and Voyager spacecraft. Two problems, however: The globular cluster is so chock full of stars (at least a half-million) and so tightly packed, that planetary orbits tend to be unstable as their gravitational allegiance to their host star is challenged for every pass through the cluster’s center. Furthermore, the cluster has such a meager quantity of heavy elements (out of which planets are made) that planets are probably rare in the first place. These scientific points were not well known or understood at the time the signal was sent.
In any case, the leading edge of our “on-purpose” radio signals (forming a directed radio cone, instead of a bubble) is 30 light-years away and, if intercepted, may mend the aliens’ image of us based on the radio bubble of our television shows. But this will happen only if the aliens can somehow determine which type of signal comes closer to the truth of who we are, and what our cosmic identity deserves to be.