The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 4: Psychology

Earth TV as Interstellar Ambassador

Dear Dr. SETI:
Say you're an alien watching our TV shows (it seems to me that decoding them would be fairly simple). First you get the Hitler rallies. Then Howdy Doody. Then Vietnam and Watergate. And now... the worst of all... Jerry Springer. What would you think?

You would indeed think Earth should remain in strictly enforced quarantine.

TC (who writes SF)

The Doctor Responds:
Although the thought of extraterrestrials watching Earth television makes for great speculation (and marvelous science fiction), in fact I consider it highly unlikely that any advanced technology can decode our television signals past a couple of light years distance (and the nearest star to our Sun is over 4 LY away). Thus, the content of our TV programming is not really an issue. The computations appear in the engineering literature, though the full analysis would fill pages. Thus, I will summarize it below.

How can I rule out ETI decoding our transmissions? Let's quantify that. The video information is contained in a portion of the signal called its sidebands. These are accompanied by a strong reference signal called the carrier. The peak carrier power is 10 dB stronger than the peaks of each sideband component (that's a power ratio of 10:1). The sideband information is spread across about 6 MHz of spectrum, while the carrier is concentrated in a 1 Hz bandwidth. That's a 6 million to one ratio. Multiplying the two together, the spectral density of the sidebands is thus 60 million times weaker than that of the carrier. Detection range varies with the square root of spectral density. Thus, the carrier can be detected at a range eight thousand times greater than that over which the TV channel can be decoded. So, say the carrier can be detected out to 30,000 LY (roughly the distance to the center of the Milky Way galaxy). Under identical conditions, the range for receiving the TV programming is under 4 LY (and the nearest neighboring star is more than 4 LY away). So, can aliens watch 'I Love Lucy'? I think not.

Note that we're talking here about decoding our TV signals, which is very different from detecting them. Over interstellar distances, it is relatively easy to detect the presence of our TV carriers, even though the information content is far below the noise threshold of alien radio telescopes. From the presence of signals, it is possible to deduce that there is technology on this planet, even if the specific information content is inaccessible.

As for interstellar quarantine, there are far better indicators than our TV programming that ours is a planet to shun. The chemical signature of our atmosphere (which is detectable over interstellar distances) most likely marks us as environmentally suicidal.

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