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Am I misreading this?
james_nicoll
Centauri Dreams reports on discussions about whether ETI can notice us:


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.


So basically anyone around our level of development, too poor and backward to present a threat, probably won't see us, meaning that anyone who does see us will likely be far more developed and with much deeper pockets than humanity circa 2011?

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"Eavesdropping: The Radio Signature of the Earth," a Golden Oldie

James Benford is considering the same problem as W. T. Sullivan III, S. Brown, and C. Weatherill did in their classic 1978 paper "Eavesdropping: The Radio Signature of the Earth." (PDF)

They wrote: "the most intense repeatable and nondirective sources on the earth, namely UHF television stations of 5 MW effective radiated power and BMEWS-type radars, would be detectable at distances of ~25 and ~250 light years, respectively, by an observer with our present technology." [By this the authors mean with an Arecibo-class instrument.] "We argue that the observer able to detect only television carrier signals would still recognize them as artificial-- despite the fact that ~104 times more sensitivity would be required to obtain program material." [In other words, it's easy to detect the carrier wave, but hard to see the program that's riding on it.]

These conclusions have been assimilated into the folklore of SETI. They already seem to say that "clues to our existence" are available to an Arecibo dish at 250 light years, so Seth Shostak seems pessimistic.

Check on whether Seth is correct:

Presuming the scaleup in the TV-watching case is only a matter of area, scaling the 305-meter Arecibo bowl up 10,000 times would mean a circle 3050 meters in diameter, if you were 25 light years away and wanted to watch TV. This would cover a small city, but not Chicago.

250 light years away, you'd need a circle 30 kilometers in diameter, which would cover Chicago and most of its its suburbs. So Seth Shostak seems to be in the right ballpark for watching TV shows, but not for merely detecting an artificial signal (and getting Doppler shifts, etc.).

But there's another wrinkle. At a meeting in Green Bank last fall, Frank Drake showed a spectrum of a modern missile-warning radar. Apparently, if you drive around Alaska with a spectrum analyzer on the front seat of your car, nobody stops you. Anyway, thanks to frequency-hopping, there is much less power at any one wavelength than there was in 1978.

Woody Sullivan mused after Prof. Drake's presentation that maybe it was time to update the old estimates. Movies are not the only things that get remakes.

Re: "Eavesdropping: The Radio Signature of the Earth," a Golden Oldie

Apparently, if you drive around Alaska with a spectrum analyzer on the front seat of your car, nobody stops you.

This is awesome.

Re: "Eavesdropping: The Radio Signature of the Earth," a Golden Oldie

I was going to reply that in most of Alaska there's nobody there TO stop you, but then I realized you can't drive your car around in nearly all of Alaska either, so I suspect that part cancels out.

--Dave

Re: "Eavesdropping: The Radio Signature of the Earth," a Golden Oldie

Actually, what Sullivan et al. say (p. 384) is that Arecibo could detect TV carrier signals at a distance of only 1.8 light years. They go on say, "... something similar to the proposed Cyclops array of 1000 100-m dishes would allow detection of video carriers out to a distance of ~25 light years (which includes ~300 stars) and of BMEWS-type radars out to ~250 light years." So what they meant by "our present technology" was the hypothetical Cyclops array.

Cyclops would have a collecting area of ~ 8 square kilometers (an order of magnitude larger than the SKA, but much, much smaller than Chicago). If you want to detect the TV carrier signal at 250 light years, you need to scale the area up by a factor of 100, which gets you to 800 square kilometers, just a little bit larger than the City of Chicago. That might be where the "antenna the size of Chicago" bit in Glister's blog post comes from.


For what it's worth, if you want to watch the TV shows, then the 10^4 scaleup in receiving area that Sullivan et al. call for implies an antenna area of 80,000 km^2 for a distance of 25 light years (considerably bigger than even the "metropolitan area" of Chicago). Using an (optimistic) SKA rate of $1000 per m^2, the total cost would be $80 trillion. So that's one way you could get to an "entire GNP of planet Earth" claim.

(Since we're trying to divine the calculations via blog post by someone -- a person keen to avoid giving us any mathematical details -- who watched a DVD of an oral presentation... figuring who is or was wrong at this remove is a mug's game.)

Re: "Eavesdropping: The Radio Signature of the Earth," a Golden Oldie

So what they meant by "our present technology" was the hypothetical Cyclops array.

Aha! In my haste to knock out a blog comment, I literally jumped to (the) conclusion. After all, I read this paper around 1981, so why should I read it again?

(Since we're trying to divine the calculations via blog post by someone -- a person keen to avoid giving us any mathematical details -- who watched a DVD of an oral presentation... figuring who is or was wrong at this remove is a mug's game.)

And I fear I have only added to the confusion. My apologies; I should have known better. Thanks for clarifying.

Mr. Centauri Gilster says he has been given a bootleg DVD of the Royal Society's October meeting on SETI, and he's working from that. I remain hopeful that the Society will eventually post audio or video of these sessions online, as they have with their SETI conference of last January. Especially since we now know a DVD exists.

Re: "Eavesdropping: The Radio Signature of the Earth," a Golden Oldie

Thanks for the link -- I think. (As in: Oh, I really don't need some intriguing new set of mp3's to take up my time, darn it ;-)

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