Since we’ve kicked around the idea of searching for SETI signals in the television bands (as noted in a previous story on Abraham Loeb and the Mileura Wide-Field Array), it’s interesting to note Seth Shostak’s thoughts on the subject. Because although planet Earth has been broadcasting TV signals for some time now, our transmissions are unlikely to be received at any great distance. And that makes a search for accidental TV-like emissions even from relatively nearby stars problematic.
Shostak imagines a civilization 55 light years away hoping to pick up I Love Lucy from Earth. He notes that the non-directional TV signal, assuming a million watts of transmitter power, will reach this distant world “…with a power density of about 0.3 million million million million millionths of a watt per square meter…” And because only a third of the transmission power is in the carrier signal — the most readily detected part of the transmission — even that number is too high.
It’s possible to run these numbers against a new facility, the Low-Frequency Array (LOFAR) now being built in Europe for radio astronomy work. At VHF television frequencies, LOFAR will have an effective collecting area similar to that of the Arecibo dish. Says Shostak:
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.
That’s bad news for our hopes of picking up extraneous signals from a technological civilization. It doesn’t disqualify these frequencies from SETI study, but does imply that if we were to find something interesting, it probably wouldn’t be an extraterrestrial sitcom. If any readers have references to other work on the strength of such signals at interstellar distances, please let me know. It’s a question that bears on how visible our own culture is at the distance of nearby stars. The answer may well be that despite I Love Lucy, we’re still all but undetectible.