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Ask Dr. SETI ®

Chapter 6: Technology

The Day the Earth Stood Out

Dear Dr. SETI:
The producers of the remake of "The Day The Earth Stood Still", as a publicity stunt, transmitted the film towards the nearby star Alpha Centauri. If there's anyone there, do they have any chance at all of actually watching the film?

Larry, Boston

The Doctor Responds:
If we start with the assumption that the Alpha Centaurans are infinitely advanced, and possess technology limited only by the laws of physics, then the easy answer is, "of course they could." I prefer not to place limits on ETI's technological prowess, budget, or resources. But, from a practical standpoint, I think there's little chance of advanced extraterrestrials watching this particular transmission. Here's why:

It's axiomatic in telecommunications circles that there is no substitute for capture area. Given an antenna large enough, there's no limit to the sensitivity of a receiving system, or to the distance over which a weak signal can be recovered. To paraphrase Clarke's Law, "any sufficiently advanced antenna is indistinguishable from being there."

But, make no mistake, the signal in question is incredibly weak, beamed from a tiny antenna driven by a very low-power transmitter. For the system used in this transmission is no planetary radar, but rather an off-the-shelf commercial satellite uplink terminal, not very different from the equipment many of our members use in their ham radio pursuits. Thus, the antenna required to intercept it over interstellar distances is incredibly daunting. I'll save you the math here (the entire link analysis will appear in a paper presented to the International Academy of Astronautics) and cut to the chase. My calculations show that the Centauran receive antenna would have to be more than 3,000 km in diameter.

Consider that Arecibo, the largest radio telescope currently extant on Earth, is a mere 305 meters across, and that the most grandiose array even contemplated by humans, the planned Square Kilometer Array or SKA, will have about ten million times less capture area than the antenna I have tasked the Centaurans to build. Even if they succeeded in constructing a suitable array, can you imagine the challenges of aiming it within a millionth of a degree, from a moving planet, toward a moving target? By my rough calculations, it would have to be re-aimed about 500,000 times per second in order to keep our transmission within its beamwidth. And even that assumes that they know, a priori, that a transmission is being beamed toward them, and at what time, and on what frequency, and from what direction, and what its modulation characteristics are, and...

No, I think it safe to say that even the most technologically advanced of extraterrestrials will be spared from watching this particular Hollywood blockbuster.

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