Could SETI efforts be simpler? While today's radio searches rely on highly sophisticated antennas and receivers, our fledgling optical search systems are far less intricate. Experiments to look for fleeting flashes of light from other star systems have been conducted using only a modest telescope and about 20 thousand dollars' worth of electronics; simple and cheap, like onion dip.

But there's a problem: these searches for momentary pulses of light check only one part of the sky at a time. Therefore, unless some alien society is incessantly aiming their lasers in our direction, there's a distressingly good chance that we'll be looking the wrong way when our solar system gets pinged.

This is known in the SETI trade as the "synchronicity problem." I was recently thinking about it over a fourth-rate burger, when it reminded of a passing remark once made by California radio astronomer Jack Welch, offered at a SETI meeting in Paris two dozen years ago. Welch wondered whether any gregarious extraterrestrials might not make a signal so strong and obvious that "even cows could see it."

Despite the important fact that cows seldom write for refereed journals, Welch's point is worth ruminating over: why couldn't an alien civilization aim a signal at Earth that we could detect simply by pivoting our eyeballs to the night sky?

Indeed, this type of signal might make sense as part of a two-component transmitting strategy, something I've broached in these electronic pages before. One component is an intermittent, powerful "ping" designed to mark a spot on the sky and get our attention. Having found it, we would surely expend the time and currency to build whatever instrumentation was necessary to detect the second component, the (far weaker, but always on) "message."

Historically, intermittent lights in the sky have frequently grabbed our attention. Supernovae are an example. Probably the brightest in recorded history (as seen from Earth) occurred in the year 1006, and was a favorite subject at elite cocktail parties for months. Then five centuries later Tycho Brahe, the celebrated Danish astronomer with the metal nose, saw a "guest star" in the constellation of Cassiopeia. It shone as bright as Venus, and eventually gained notoriety as Tycho's Supernova.

True, your average Renaissance man-in-the-street probably didn't remark on Tycho's find. But it was enough that a few people did. They made it known to everyone, and forever.

So imagine there was an occasional, very bright light in the night sky-appearing at least as frequently as once a year-something that we could see easily, even without a telescope and 20 thousand dollars' worth of electronics. A few tens of billions of people have trod our planet since Homo sapiens emerged, and it seems pretty certain that this intermittent flash would not only have been noticed many times - it would be written into our folklore, and as familiar as the evening star.

Yet we don't seem to have witnessed such a thing. Is this because it's too hard for the aliens to arrange? Does it require too much energy for them to light up our sky this way?

Let's look at the numbers. Virtually everyone (including cows) has observed the strobe lights affixed to airplane wing tips. You can easily see these at any altitude, but when a jetliner is only a mile high, the strobes are much brighter than the brightest star. They flash about once a second, at an average power of roughly 50 watts.

So what would it take to make a similarly noticeable flash once a year from, say a thousand light-years away - a reasonable maximum distance for the nearest aliens?

For those readers who love the requisite arithmetic, I note that a thousand light-years is about 6 thousand trillion miles. Ergo, the average power necessary for a once-a-year flasher that, at Earth, would be as bright as a mile-high jetliner strobe, is 5 1025 watts. That's trillions of times more power than all humans use; trillions of times the sum total of every light bulb, factory, SUV, and Wii. In fact, it's 10 percent of the power output of the Sun.

But perhaps I should say, it's only 10 percent of the power output of the Sun. For some advanced aliens, that might not be very consequential. In 1964, Russian astrophysicist Nikolai Kardashev theorized that the universe could be host to civilizations on three levels: Type I societies that use the energy resources of their planet; Type II that use most of the output of their home star; and Type III societies that command the energy output of an entire galaxy. Civilizations falling into either of the latter two categories would have the resources to make our theoretical galactic flasher.

So does the fact that naked-eye SETI has failed to find such a beacon mean something? Are we on the horns of a dilemma? It sounds as if we've somehow missed seeing such a signal, or perhaps there's no nearby civilization with the technical talents to construct one. The latter would suggest that there's nothing more accomplished than a Type I society in our neck of the woods, despite the fact that the Milky Way is 10 billion years older than Earth.

That's possible. Then again, maybe the truly sophisticated aliens have better means of telling us where to find information. Or perhaps they have sound reasons to be at least slightly cryptic.

Still, it strikes me as paradoxical, given the vastness of the cosmos, that such a simple signal has not been recognized, a signal that even a cow could see. No lasers for our grazers. Call it the cow paradox.

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