Aliens Lose in Switch to Digital TV
The United States is finally ditching analog television broadcasting, and the rest of the world is doing the same. Unless you've got a converter, the government has just morphed your trusty analog boob tube into an inert piece of furniture.
Mind you, this is a good thing. Digital TV (DTV) offers better picture quality. For example, the ghost images caused by signal reflections off that high-rise office building down the block will be a thing of the past. In addition, you should sleep better knowing that DTV makes improved use of the broadcast spectrum – primarily because modern digital processing can more compactly encode the picture and sound. This will offer you increased access to other essentials of your neoteric lifestyle, such as high-def, interactive television, as well as wireless internet. DTV sounds like a winner.
But there may be losers, zillions of viewers who might not have a converter box or a digital-ready TV – namely, the aliens.
That's right: extraterrestrials who might be picking up our analog broadcasts could miss out. Ever since the Second World War, television signals (as well as FM radio and radar) have served as Homo sapiens' emissaries into deep space. High-frequency, high-power broadcasts have filled an Earth-centered bubble more than 60 light-years in radius with signals. If there are any aliens nearby, they would have been hard-pressed to find trilobites, dinosaurs, or even the Greeks and Romans. But, thanks to "I Love Lucy," they could find you – or at least your parents.
Unfortunately, the switch to digital might leave the aliens with nothing but snow on their wall-size plasmas.
Now, a lot of people believe there's no way extraterrestrials could pick up our television, either analog or digital. They argue that these broadcasts would fade to black long before they bridged the distance to even the nearest star – that they would be hopelessly buried in the natural static of the cosmos.
That may seem reasonable. After all, it's a fact universally acknowledged that radio signals become weaker with distance. Consider how your local easy-listening station comes in like gangbusters when you're cruising the neighborhood in your car. But head out of town, and you'll lose that station within an hour or three.
Those who remember high school physics will recognize this diminution with distance as the inverse-square law, and it applies even to focused radio or light waves (radars and lasers, for example). Every doubling of distance causes a four-fold reduction in intensity.
Your favorite TV station might be pumping 50 thousand watts into your ‘hood, and yet it has a reach of less than a hundred miles. So is there any chance that aliens – who might be tens (or more) light-years away could pick up our television? After all, that's a trillion times farther, which (according to the inverse-square law) means the signal intensity would drop by trillions of trillions!
That sounds weaker than a bee's knees.
But radio technology is incredibly sensitive, and – given sufficiently large antennas – we can detect faint radio static from the distant corners of the universe. That's what radio astronomers do, after all. You can always pick a signal out of the cosmic background static with a large enough antenna.
So how feasible would it be for ET to tune in our sitcoms? If you look at the spectrum of one of our (now antiquated) analog television signals, you'll note that it is very spiky. In particular, in that part of the band where the so-called luminance carrier is located, a lot of energy is concentrated in a small range of frequencies. Roughly 10 or 20 percent of the total transmitter power for a given broadcast might be huddled in a 100 kilohertz band centered on that carrier. If the aliens could only find this emission spike, they wouldn't have TV's picture and sound, but they'd know we were on the air.
OK, this is rapidly becoming a stultifying technical discussion, but I'll cut to the chase. At 50 light-years distance, a nosy alien could find that signal peak in a hundred-hour search using an antenna the size of Texas. Note that the antenna needn't be a giant, expensive parabolic dish like the one at Arecibo, Puerto Rico, or even thousands of small backyard dishes. It can just be acres and acres of cheap rabbit ears or rooftop yagis.
Of course this outsized collector is much larger than any we've built, but note that a new radio telescope being planned for completion in two decade's time will have an area of more than 250 acres. The first radio telescope, constructed in the 1930s, had a dish the size of your backyard patio. That's a mammoth improvement in less than a century. Imagine what a society a few thousand years beyond ours could build.
Lamentably, alien audiences may be frustrated by the switch to digital television. That's because the transmitter power for DTV is fairly evenly spread across the spectrum. The spikiness is gone, and from afar, the attention-grabbing squeals of analog television's carriers have been replaced by DTV's smooth, low hiss. The mountains peaks are no more, supplanted by hard-to-find, endless mesas. It's not impossible to pick up our DTV broadcasts from your favorite M-class planet, but I reckon it would require antennas at least five times larger than demanded for good, old analog TV. ET may balk at the additional cost.
Of course, this tale becomes more woeful in a few decades, when high-powered TV broadcasting is replaced entirely – when your favorite reality show enters the living room via cable, optical fiber, or pipsqueak-power wi-fi.
However, while extraterrestrial audiences may be destined to miss out on our quality network programming, that isn't to say they won't pick up anything from Earth. The strongest signals leaking off our planet are radar transmissions, not television or radio. The most powerful radars, such as the one mounted on the Arecibo telescope (used to study the ionosphere and map asteroids) could be detected with a similarly sized antenna at a distance of nearly 1,000 light-years.
Television is ephemeral, a fact that some will find reassuring. But earthlings will continue to pump the kilowatts into the ether. And eventually, when those signals have washed over a few hundred thousand star systems, someone may notice.
Seth Shostak is host of the Are We Alone radio program.
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