Thinking that we can understand the motivations of an extraterrestrial civilization seems like a fool’s gambit, but we have to try. The reason is obvious: We have exactly one technological society to work with — we’re all we have — and if we want to look for SETI signals, we have to interpolate as best we can. An alien culture, it is assumed, will do the same. This was the procedure outlined by Giuseppe Cocconi and Philip Morrison in their classic 1959 paper “Searching for Interstellar Communications,” that began the modern era of SETI.

If there are civilizations around stars like the Sun, the paper reasons, then some will be motivated to reach out elsewhere. From the paper:

To the beings of such a society, our Sun must appear as a likely site for the evolution of a new society. It is highly probable that for a long time they will have been expecting the development of science near the Sun. We shall assume that long ago they established a channel of communication that would one day become known to us, and that they look forward patiently to the answering signals from the Sun which would make known to them that a now society has entered the community of intelligence. What sort of a channel would it be?

What results is an explanation of the factors needed to overcome signal attenuation and the frequencies most likely to be used. Thus we get what Cocconi and Morrison called “a unique, objective standard of frequency, which must be known to every observer in the universe.” This is 1420 MHz, the 21 centimeter wavelength of neutral hydrogen. Thus we know where to look assuming a civilization is trying to make contact with us. The authors conclude “… the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now at hand.”

The Hunt for SETI Observables

We’ve looked frequently in these pages at how SETI has changed since the Cocconi and Morrison days, with so-called ‘Dysonian SETI’ invoked as a way of looking for observable evidence in our astronomical data. The unusual star KIC 8462852 has elevated the method to wider attention because of the possibility that some kind of astroengineering could explain its unusual light curves (search the archives here for numerous articles on the star).

In any case, Dysonian SETI assumes, contra Morrison and Cocconi, no intent to contact anyone. We are simply looking for activity, albeit on a colossal scale. We must be the ones to figure out what that activity might be.


Which brings me to a new paper from James Benford and son Dominic (NASA GSFC). “Power Beaming Leakage as a SETI Observable,” submitted to The Astrophysical Journal, asks whether we might detect the use of power beaming to transfer energy and accelerate spacecraft within a distant solar system and, perhaps, beyond it if the technology is being used to accelerate starships. KIC 8462852 is also considered here because there have been two attempts to study it, one optical and one at radio frequencies, in the context of a SETI search.

As the paper notes:

The most observable leakage from an advanced civilization may well be from the use of power beaming to transfer energy and accelerate spacecraft, both within and beyond the star system where the civilization is located. In future, such applications may make the Earth’s radiation in the microwave, millimeter and visible/near-IR parts of the electromagnetic spectrum be very intense…. The power levels are high, focused, and transient and could easily dwarf any of our previous leakage to space. These are not SETI signals so much as leakage, a detectable aspect of advanced civilizations.

The kind of SETI observable the Benfords are examining is essentially leakage, but of a different kind than the widely repeated trope of picking up alien TV signals, just as extraterrestrial cultures are presumably now enjoying “I Love Lucy” from our signals. As the Benfords argue, the leakage of radio and television signals is essentially undetectable between stars not only due to the weakness of the signal but to its lack of coherence. Planetary radars are much more likely to be detectable, but signals like these are also extremely transient.

Intense beams of radiation being used to move power about in a distant solar system or accelerate spacecraft on long journeys should be more readily detected. James Benford argued as much in 2008, and more recently James Guillochon and Abraham Loeb have quantified the leakage to be expected from space propulsion-related beaming, showing that a beam being used to drive a large sail would be observable. The duo write: “…for a five-year survey with ~10 conjunctions per system, about 10 multiply-transiting, inhabited systems would need to be tracked to guarantee a detection” using our existing radio telescope infrastructure.

Fig-1-Benford (1)

Image: Taken from the Guillochon and Loeb paper, this diagram shows a Mars mission using microwave beaming for propulsion. In the schematic, the dashed line represents the sailcraft. You can see the prospects for signal leakage here. Notice the inset showing the beam profile as it overlaps the sail of diameter Ds. Credit: Guillochon & Loeb.

But remember the notion we started with: If we are trying to understand what an extraterrestrial civilization might do, we have to look at the uses we ourselves would make of these technologies. We try to be as flexible as possible while acknowledging the fundamental gap in our knowledge of an alien culture, but we have to start somewhere. And this kind of reasoning takes us down an interesting path, as the Benfords note in their paper. For possessed of powerful beaming technologies, such a culture should be aware of SETI implications:

It has previously been noted that such leakage from other civilizations could be observable (Benford 2008). Guillochon & Loeb (2015) have quantified leakage from beaming for interplanetary space propulsion, its observables, and implications for SETI. Extraterrestrial Intelligence (ETI), having done the same thinking, could realize that they could be observed. Hence there may be a message on the power beam, delivered by modulating it in frequency, amplitude, polarization, phase, etc., and broadcast it for our receipt at little additional energy or cost. By observing leakage from power beams we may well find a message embedded on the beam.

That’s a fascinating notion in its own right. It’s a kind of METI signal sent out by an alien civilization that is, like a beacon, broadly targeted rather than aimed at a specific solar system. And it operates on the assumption that another culture might in the course of its SETI investigations notice high-powered, focused and transient beams and analyze them. Or put another way, if our own civilization has figured this out, our extraterrestrial counterparts working the SETI various concepts must surely have come up with the same implications.

Uses of the Beam

Detecting a message embedded in a power beam would, of course, be a breakthrough of historic proportions, but so would the simple detection of power-beaming itself — no message attached — which would signal the presence of an advanced civilization. One of the things recently looked at by the Allen Telescope Array was whether the intriguing star KIC 8462852 showed any signs of activity, and it turns out that the observations, while finding nothing, do allow us to set some limits on power-beaming in that system. KIC 8462852 became, in other words, a useful exercise even though the observations were short-lived. More tomorrow on this, and also see the Benfords’ Quantifying KIC 8462852 Power Beaming in these pages.

And just what might a technological civilization do with power-beaming methods that we could observe? One answer is obvious: We’ve been talking about beamed sails for well over ten years on Centauri Dreams, keying off Robert Forward’s fascination with the subject as it applied to interstellar missions. Here the requirements are enormous, with a space-based solar power station beaming to a 1000-kilometer sail in some of Forward’s missions, although subsequent work found ways to trim the sail down to the 1-10 kilometer range.

The idea of a power station orbiting in the inner part of the Solar System where the photon-flow is maximized has dual use, as using microwave beams to transport power to a planet’s surface is a major driver. As a SETI observable, power beaming to a planet is not a likely target. The paper notes that the beam would have to be tightly controlled, with side lobes maximally reduced. In contrast, power being beamed to a spacecraft should show increasing leakage as the craft is accelerated — the beam increasingly leaks around the edges of the accelerating vehicle — making transportation applications a realistic SETI observable.

The Benfords’ paper quantifies the various kinds of power beaming missions and applications and their observable parameters, looking at twenty concepts in terms of power radiated, duration and likely time for the radiation to repeat. More on this tomorrow as we continue looking at the paper and the question of how an advanced technology could use power beaming.

The Benfords paper is “Power Beaming Leakage Radiation as a SETI Observable,” submitted to The Astrophysical Journal and available as a preprint. The Guillochon and Loeb paper is “SETI via Leakage From Light Sails in Exoplanetary Systems,” The Astrophysical Journal 811, No. 2 (23 September 2015), with abstract here. Jim Benford also discusses the Guillochon and Loeb paper in Seeing Alien Power Beaming.