Previous Entry Share Next Entry
Am I misreading this?
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?

  • 1
There's also the question of *what* an advanced civilization located more than a hundred light years away would receive from us . . .

Well, there's also the detail that "a few hundred light years" is a lot larger than the radius of our transmissions.

Anyone setting up a Square 'Chicago-Sized' Array 300 years out would have to be prepared to wait a long time to detect us...

And now I'm wondering what year Earth's radio output began to out-shine Jupiter's.

(Actually, I'm wondering if it does so today.)

That's consistent with what I've heard from SETI researchers. The way they usually put it is, they're looking for civilizations much more developed than ours, which are trying to be seen.

(Deleted comment)
I am 90+% sure Australia still uses over-the-horizon radar. [googles] Yup.

Let's check Benford's numbers. The Square Kilometer Array has an estimated cost of E1.5 billion. The city of Chicago is 606 square kilometers. Using these numbers, the Honorable Richard J. Daley Array would have an estimated cost of E909 billion, or $1.24 trillion at today's exchange rate.

The current GNP of the United States is $14 trillion.

It's possible Gilster's writeup might be missing a step from Shostak to Benford. It's also possible that Benford gets his grasp on reality from his bro.

Building an Extremely Large Array probably doesn't scale linearly in terms of price per square kilometre. Integrating the data collection system is going to be pricey with computer systems capable of dealing with exabytes/second of raw data from the antennas and receivers. There's also the power requirements which are external costs in a limited array -- a Daley Array would probably need a dozen dedicated 1GW nuclear reactors to power it at about 10 billion dollars each as capital costs, never mind the operating costs.

"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.

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?

Hasn't this been obvious since the dawn of SETI?

Thus contributing to galactic chibification.

(Deleted comment)
(no subject) (Anonymous) Expand

eavesdropping and SETI science

I'm James Benford, whose work you're discussion. The paper Paul Gilster described covers much more than eavesdropping. You can read the paper at:
The cost of a Chicago-sized antenna is on pg. 11. My motivation in all my recent SETI-related work is to make it more of a science: quantify beacons, expose Lucy & Desi observation claims, i.e., the video can't be detected past Pluto with our technology, no unquantified claims we've been 'heard across the galaxy', and no contradictory statements.

Speaking of which, here's the section where I quantify the Chicago-sized antenna:

"Recently others have agreed with the above, i.e., that our leakage radiation is not detectable by ETI: ‘To assume that leakage automatically constitutes a ‘ ‘reply from Earth’ ’ to any SETI signal we might receive is unrealistic.’ (Shostak 2011), as well as taking the opposite position: ‘If there are any aliens within a few hundred light-years, these clues to our existence could be found with an antenna the size of Chicago.’ (Shostak 2010).
Certainly, with ever-larger antenna area, at ever greater cost, advanced ETI can detect us. From the above analysis, we calculate that at 50-ly range, the antenna area must be ~1 km2. To assess Shostak’ s claim, note that Chicago’s area is 24,800 km2 = 2.48 1010 m2. At the present value of SKA antennas, 2.4 k$/m2, the cost is 60 T$, comparable to Earth’s GNP of 70 T$. So if comparable to us, ETI would have to devote their entire science budget for a time perhaps of order a century to build Shostak’ s antenna, a sobering prospect."

Re: eavesdropping and SETI science

If you've read this discussion, you'll know that's the size of Chicago's metropolitan statistical area. Only 20 percent of that is classified as urban. The MSA includes a fair amount of farmland, including such vital urban hubs as Newton County, Indiana (population 14K) and Grundy County, Illinois (population 38K).

Chicago's MSA has undergone substantial redefinition over the last few decades. In 1950, it was only Cook, DuPage, Kane, Lake and Will Counties in Illinois, and Lake County in Indiana. The MSA is not an administrative unit, but a unit of convenience devised by the federal Office of Management and Budget.

It's not the area of the city of Chicago, which is a well-defined political unit with rather stable borders -- the last major annexation was for the O'Hare airport complex in the 1950s. Perhaps you could ask Shostak which he meant.

  • 1

Log in

No account? Create an account