by James Benford
We recently looked at a paper by Duncan Forgan and Robert Nichol on the question of detecting extraneous emissions from an extraterrestrial civilization using technology like the Square Kilometer Array. James Benford (Microwave Sciences) has some thoughts on the issue growing out of his own work with brother Gregory on interstellar beacons and SETI reception in general. No one has put the question of interstellar beacons to tighter scrutiny than the Benfords, with particular regard to bringing the SETI discussion, as Jim puts it, “onto a quantitative basis, as opposed to rampant speculation, as is typical of the playing-tennis-without-a-net approach taken previously.” The Benfords’ work on interstellar beacons appears this month in Astrobiology. I give full citations at the end of this post.
The Forgan & Nichol paper on detection of leakage radiation does neglect our continuing use of microwave beams not only for radar, but also for likely future beaming of power for space purposes, such as power transfer. This has been noted by several others in their comments to this website.
Understanding Leakage Radiation
I feel that leakage radiation should include both what we have generated, such as TV and radar, but should also include future likely radiation for power beaming.
Examples of applications that have been studied are transferring energy from Earth-to-space, space-to-Earth, and space-to-space using high power microwave beams. Such beams have been quantified for spacecraft launch to orbit, orbit raising to GEO, launch from orbit into interplanetary and interstellar space, and deployment of large space structures (see “Space Applications of High Power Microwaves”, James Benford, IEEE Trans. on Plasma Sci., 36, pg. 569, 2008). For example, even a single microwave beam powered launcher of Kevin Parkin would produce an increase of the effective isotropic radiated power (EIRP) of Earth by five orders of magnitude beyond that of Arecibo, our present highest EIRP radar. I feel that in the future we will be radiating in the microwave in these ways, not just as leakage from communication channels.
Strategies for Detection
Should we observe such activities by ETI, the signals would appear to us, Thomas Hair points out, as transient events. But such quick, powerful bursts would be verifiable by a ‘staring’ strategy, with smaller dishes looking continuously at the skies, most profitably at the galactic plane. Once such a burst appears, watching time can be focused on such possible sites, perhaps with some dishes linked so their effect could be coherent, raising the detection capacity of the network.
Project Argus, led by the SETI League, is the right direction to go. They’ll need short integration times to spot such transients. I would expect the transients would recur as ETI’s launches, transfers and such repeat. But even a frequent launcher aims at different parts of the sky, as planets rotate. So, we must be patient.
Distance, Receptivity and Cost
I also found that the distances Forgan & Nichol give, which are taken from the Leob & Zaldarriaga paper, although portrayed as discouraging for detecting leakage from ETI, are in fact overly optimistic for conventional TV and radar. Their work assumes that receivers are going to be integrating over hours-days-months. That doesn’t work when the TV station you are observing is transmitting in your direction only for a time typically ~hour, before it disappears around the limb of the Earth, as shown by Sullivan in his seminal paper. There is also a confusion in both papers over the distinction between transmitter power and EIRP. Forgan & Nichol misunderstand the EIRP quantity, saying on their page 1 that radars produce ‘isotropic radiation with billons of watts’. Although EIRP, the product of radiated power and the antenna gain, is that high, the radiated power is of order megawatts, since the gain is ~1000. The problem of observing leakage due to TV stations is identical to that of detecting a single strong station, as shown by Sullivan, so their estimates of detectability of leakage in these papers are systematically high.
One important implication of this is that the case made by advocates of sending messages to the stars (METI), who argue the ‘we’ve already announced ourselves’ is wrong. We have not been noticed at interstellar range by societies of our order of magnitude of technical and economic capability. One might argue that aliens will be far richer, so will have vastly larger antennas. But if so, why are they not radiating at oxygen-obvious planets like ours?
Michael Simmons, who suggests a phased array radiating rapidly cycling, short, high-powered transmissions directed at millions of nearby stars, might look at my cost equations to estimate what the expense will be (see “Messaging with Cost Optimized Interstellar Beacons,” referenced below). Hint: they won’t be cheap.
To follow the latest Benford thoughts on SETI matters, see James Benford, Gregory Benford and Dominic Benford, “Messaging with Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5, pp. 475-490 (abstract / preprint), and the same authors’ “Searching for Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5, pp. 491-498 (abstract / preprint).