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Ask Dr. SETI ®

Chapter 6: Technology

Do Signals Degrade to Thermal Noise?

Dear Dr. SETI:
I have heard that some folks had done an analysis that showed that all the radio energy that we are currently pumping into space is actually thermalizing to noise within a few light-years. What are your thoughts about this?

Gerry (via email)

The Doctor Responds:
I disagree, Gerry. Over interstellar distances, there is indeed some spectral dispersion in the ISM. Frank Drake and George Helou showed decades ago that free electrons in interstellar space have a Doppler effect on radio signals passing through them, placing a lower limit on the narrowness of the bandwidth of any interstellar signals. This is known as the Drake-Helou Limit. For a coherent (CW) signal at L-band (say, the hydrogen line, 1420 MHz), it calculates to about 30 mHz of dispersion. (That's milliHertz, not MegaHertz). So, the signal is still clearly artificial and coherent. I know of no recent theory that credibly refutes these calculations.

There have been some good studies that show that our UHF TV broadcasts spread sufficiently that the video cannot be decoded over interstellar distances. But, since most of the energy is in the video carrier, and it's essentially CW, and the Drake-Helou limit still applies, I conclude that the hallmark of artificiality is still evident, even if the programming content (which I argue is not a manifestation of intelligence in any case) is lost in the journey.

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