Listening for ET’s Television

The first episode of "I Love Lucy" was broadcast sometime on October 15, 1951. About 0.0002 seconds later, the signal glided over the rooftops of the farthest city suburbs, and headed into space.

It's still going. Every day, that first installment passes through an additional 4 thousand trillion trillion trillion cubic kilometers of the cosmos.

Given that stars in our galactic neighborhood are separated by about 4 light-years, it's easy to figure that roughly 10 thousand star systems have been exposed to "I Love Lucy" in the past five decades. That may suggest a high Nielson rating, but the chance that extraterrestrials are now hooked on 1950s television is low. Look at it this way: Carl Sagan, who was singularly optimistic about such things, figured that the number of technically competent societies in our Galaxy was a million or more. That's a lot. But even so, it would imply that only one in every few hundred thousand star systems would actually boast such a society. Consequently, there's little probability that hairless gray guys are puzzling over the domestic difficulties of Lucy and Ricky, a fact that no doubt will disappoint the advertisers.

The widespread use of television on Earth is a phenomenon of the last half-century. But the cosmos is three times as old as our planet. So there could be galactic civilizations that have been churning out sitcoms for thousands of years or more - time enough for the signals to reach our world.

This possibility was evidently on the mind of Abraham Loeb at Harvard University, who recently noted in the New Scientist that a radio telescope being built to study distant galaxies might also be able to pick up ET's TV. The so-called Low-Frequency Array (LOFAR), a telescope consisting of 25 thousand tent-shaped antennas spread across Holland and Germany, can be tuned to frequencies under 250 megahertz. This is a spectral range far below what's usually searched by SETI, but it's the band in which much of your local television is broadcast. And maybe theirs, as well.

So how realistic is this? Could LOFAR really pick up "I Love Zork"?

To answer that question requires doing some numbers (you can read through this quickly if quantitative arguments cause your brain to idle).

First, we reverse the situation, just to see if tuning in remote TV makes sense. Imagine that there are alien couch potatoes 55 light-years away who, bored with their own Fall lineup, have constructed a LOFAR-style antenna in hopes of picking up "I Love Lucy's" debut. Hunky TV transmitters on Earth belch out a few hundred thousand watts of power. That energy is not beamed in all directions equally; most of it is aimed around the horizon (which, of course, is where the audience is). Because of this slight beaming, the effective transmitter power is a bit more: let's say a million watts, to keep the math simple.

OK, how strong is that signal by the time it reaches our putative alien audience at 55 light-years distance? Not very. The megawatt broadcast washes over ET's world with a power density of about 0.3 million million million million millionths of a watt per square meter, which is not exactly a scorching signal. Actually, only about a third of that transmission power is in the "carrier" - the part of the broadcast that's very narrow in frequency and easily detected. So knock that piddling power density down by another factor of three if you want to know the strength of the easily detectable part of the transmission. (Of course, if they only find the carrier, they won't get the picture and sound. But Lucy's jokes might not appeal to aliens anyway.)

Could their LOFAR-style antenna find that carrier, thereby indicating that a program was on the air? Well, engineers have computed that at the frequency of VHF television, LOFAR will have an effective collecting area similar to that of the 305-meter diameter Arecibo antenna in Puerto Rico.

That's big. That's brawny. But not brawny enough. In our SETI experiments at Arecibo, we could find a signal if it were about 0.1 million million million millionths of a watt per square meter. That number, you will notice if you count up the words, is a million times bigger than the "I Love Lucy" carrier at 55 light-years. The aliens' LOFAR would be inadequate to detect the broadcast by a factor of a million, a not entirely negligible amount. Simply stated: LOFAR couldn't hear it.

So here's the bottom line: LOFAR would only be able to find TV signals comparable to ours from a distance of much less than one light-year! Turning this around, the mother of all rabbit ears couldn't pick up the Alien Broadcasting Network at the distance of even the nearest star.

Disappointing, but you might argue that the extraterrestrials will have much, much more powerful TV transmitters than we do. In fact, their broadcasts would have to be millions of times more powerful to even produce a blip on LOFAR, which seems a bit silly and likely to set alien roofs on fire.

Now Loeb points out that LOFAR and other large telescopes now being planned can stare at the same spot on the sky for months or years. That allows a signal to build up, making even weak transmissions visible. After a year of staring, LOFAR's sensitivity will be several hundred times better than our work at Arecibo. That's the good news. The bad news is that it's still inadequate to hear TV transmitters similar to our own.

But there's another point, and one that's possibly of greater significance. The whole idea of television broadcasting may be passe for the aliens. Consider: how do you pick up your daily dose of boob-tube fodder? With a rooftop antenna strapped to your chimney? Probably not. You most likely get your TV via a cable, fiber optic, or direct broadcast satellite dish. The powerful television transmitters on the hills outside town are going to vanish in the next few decades. They likely vanished long ago on ET's world.

So while it's certainly an excellent idea to look for cosmic signals at low frequencies, it's unlikely that any will resemble the type of entertainment we still loft into the skies here on Earth. And some would say that's reassuring.

• All About SETI
• Shadows and Silhouettes: Looking for Transits
• Images: Venus Transit Gallery
• Detecting Other Worlds: The Transit or 'Wink' Method
• Way-Out World: New Technique Finds Most Distant Planet Ever
• Mercury Transits Sun, Images on Web