[LINK] "The ‘I Love Lucy’ Signal"

[rfmcdonald]

This Centauri Dreams post explores the possibilities of extraterrestrial civilizations picking up our radio leakage--old television transmissions from the 1950s, that sort of thing. Briefly summarized, it amounts to "thank God, we're safe".

Let’s talk about [. . .] the actual status of those interesting signals from the dawn of television. Here I’m drawing on James Benford’s presentation to the Royal Society meeting “Towards a Scientific and Societal Agenda on Extraterrestrial Life,” which convened last October in Britain and included a debate on extraterrestrial messaging that was sent to me in DVD form by Astronomy Now editor Keith Cooper. Benford looks at what an extraterrestrial civilization would be able to detect from Earth.

Remember, now, we’re talking about accidental signals, so-called ‘leakage’ radiation that was never intended as a directed signal. Benford goes to work on the math to ask whether installations like those we have on Earth would be able, if located around a nearby star, to pick up what we have been sending. The answer is no. A typical large radio telescope like the Parkes instrument in Australia could not, from a vantage near Alpha Centauri, see video footage from Earth. [. . . H]ere’s his conclusion:

Picking up signals from commercial radio and television broadcasts is difficult. Because they are not intended to broadcast into space; broadcast antennas aim most of their transmitted power toward the surface. Most signal information is transmitted in bands on each side of the central frequency. What little detectable power reaches space is from many sources, not at the exact same frequencies, but in bands constrained by regulation by governments. Therefore, they are not coherent, so phase differences cause them to cancel each other out at great range.

What about over-the-horizon radars built during the Cold War? Much of their power was indeed radiated into space, but they have been replaced by frequency-hopping spread spectrum broadband radars that would likewise be undetectable by any technology like ours. The highest power emissions, it turns out, are those from interplanetary radars used for asteroid searches. But these signals are not directed at nearby stars, and Benford quantifies the issue using the specs of the Arecibo radar telescope. Again, I will hold off on the math, but the conclusion is that ‘there is a negligible chance of ETI noticing our asteroid search radars.’

So what would it take for an extraterrestrial civilization to notice us? Seth Shostak is on the record as saying that within a few hundred light years, clues to our existence could be picked up with an antenna the size of Chicago. Benford’s analysis shows that building such an antenna, given what we know of the present value of building an installation like the Square Kilometer Array, would run up a cost comparable to the entire GNP of planet Earth. If ETI were at our level of development, then, its entire science budget would be consumed by the project.

And even directed messages, Gilster observes, like the ones from Ukraine's Evpatoria radio telescope facility, won't make it 19 light years before they become indecipherable to civilizations at our levels. That facility's "A Message From Earth" was sent to Gliese 571c, 20 light years away. The challenges are daunting:

Detectability, Benford notes, depends on the bandwidth of the transmission. Low data rates can show that the signal is artificial but also carry little information, while high data rates require high bandwidth and suffer greatly from noise.

To detect a low-bit-rate signal, a number of additional factors must swing into play, including a predisposition to be looking at our system in the first place so that ETI would concentrate resources on that small patch of sky where our Sun is located. ETI would also have to guess the bit rate of the message, and would have to figure out that the message used binary frequency-shift keying instead of any other modulation method.

Go, read, be reassured.