by James & Gregory Benford

Talk of interstellar beacons invariably heats up the discussion, and I was fascinated to read not only Bob Krekorian’s take on the concept, but the follow-up comments of James and Gregory Benford, whose work on beacons has been examined previously in these pages. See A Beacon-Oriented Strategy for SETI, as well as Jim Benford’s Regarding METI and SETI Motives and Jon Lomberg’s Interstellar Beacons: A Silence in Heaven? for our treatment of this topic. Meanwhile, what about putting some constraints on how an interstellar beacon would operate? Here are the Benford brothers with a look at one way to proceed.

Bob Krekorian’s ideas invite comments. He takes a simple model which has the virtues of minimizing Doppler shifts in SETI beacons: an outward-facing array in an AU scale orbit around a star, fed by solar panels and radiating outward. The concept isn’t fleshed out quantatively, so can’t be compared to approaches such as ours. And of course he couldn’t say much in a short editorial. Perhaps he should write a detailed version of his idea, so comparisons can be made.

But this concept has the limitations of previous attempts at describing beacons: insufficient constraints, leading to little quantifying. There’s no condition placed on a beacon to estimate its principal features. That’s why we introduced cost as a driving factor (for a detailed discussion of Earth-based beacon costs, see James Benford, Gregory Benford & Dominic Benford, Messaging with Cost Optimized Interstellar Beacons; for ET beacons, see Gregory Benford, James Benford & Dominic Benford, Searching for Cost Optimized Interstellar Beacons.

This quantifying approach is sobering, as it forces tradeoffs on otherwise open-ended speculations. But it also advances the subject, which many beacon speculations do not do. It’s simply much clearer to pick a major organizing principle – economics – than generalize from a special design (“stellar orbital beacon hypothesis”). As we’ve argued, minimizing cost and effort are far more general traits, and we’ve given reasons for expecting this among any evolved species. See our ‘Searching’ paper for references to others who previously pointed this out.

Other specific comments on Bob Krekorian’s ideas:

  • 1. The concept seems to assume that the galactic plane and the orbital plane of the beacon solar system are the same. That’s highly unlikely. For example, the angle between the plane the Galaxy and the ecliptic of our solar system is slightly more than 60 degrees. Almost all star ecliptic planes will be angled with respect to the galactic plane, with few exceptions: this beacon concept could be used only if the angle were near zero, an unlikely event.
  • 2. Bob gets the number of stars vs. distance a bit wrong. In the near region, the distribution is uniform, so star numbers increase as the cube of the beacon range, but only out to about 650 ly, where the disk thins to half the density of the plane. Total disk depth is~1300 light years, so the stellar distribution is not spherical beyond that distance. Then it goes over to a number increasing as the square of distance. Bob’s numbers don’t fit either of these domains. He could get better numbers from Project Cyclops, pg. 54.
  • 3. Of course, narrower bandwidth beaming is more efficient. But, quite generally, high powers radiators are not narrow band. Broadband emitters (~MHz) eliminate the need for all Doppler adjustments anyway (See appendix in ‘Messaging’).
  • 4. It’s an old idea that the acquisition signal will be in the form of a pointer that directs us to the primary communications channel.
  • 5. Of course, placing it very close to the star will rule out laser beacons.

A comment on John Hunt, who appears to believe that beacons are cheap. Our analysis says otherwise. With the very lowest price technology we have today, beacons cost $200,000 per light year. A more likely cost is about ten times that. So a 1,000 ly beacon will be 200 M$-2 B$. Making many of these ‘200 billion beacons’ would cost at least 40 T$.

To quote ourselves:

“We assume that if they are social beings interested in a SETI conversation or passing on their heritage, they will know about tradeoffs between social goods, and thus, in whatever guise it takes, cost. But what if we suppose, for example, that aliens have very low cost labor, i.e., slaves or automata? With a finite number of automata, you can use them to do a finite number of tasks. And so you pick and choose by assigning value to the tasks, balancing the equivalent value of the labor used to prosecute those tasks. So choices are still made on the basis of available labor. The only case where labor has no value is where labor has no limit. That might be if aliens may live forever or have limitless armies of self-replicating automata, but such labor costs something, because resources, materials and energy, are not free.

“Our point is that all SETI search strategies must assume something about the Beacon builder, and that cost may drive some alien attempts at interstellar communication.”