by James and Gregory Benford

Interstellar beacons continue to draw discussion, and the Benford brothers now return with further thoughts on the matter in response to reader comments here. How to distinguish a beacon from a natural source, and why consider it in terms of cost? The answer is below, as is an interesting twist on the Fermi paradox.

We proposed making cost a useful, perhaps universal, standard because it is a quantitative constraint. Our aim is to help observers look for plausible beacons that may exist. Using transient events seen by observers is an economic way to ask these next, exploratory questions. Speculations always yield to data, and at its 50th anniversary SETI needs a vital data point: first detection.

In our latest work, we point out that researchers should be aware of the likely properties of beacons. In particular, beacons may mimic pulsars in repetition rate. But they would distinguish themselves in some way, such as amplitude modulation, varying pulse interval, frequency modulation, etc. And they may provide ‘enough’ pulses on a given target area for their artificial origin to be noticed.

While discussions of many social issues, particularly motivations, do provoke – they seldom illuminate. Are aliens beyond economic arguments? We call this the Altruistic Alien argument — that aliens of great ability, near-infinite resources and benign intent will transmit to us without taking any consideration to the cost (which would be very high in our terms, especially for long range). They will make everything easy for us. And an omni-directional Beacon, radiating at the entire galactic plane, for example, would have to be enormously powerful and so very expensive.

On this site, several comments speak of robot manufacture, future economic miracles, etc. – but forget that the idea of exponentially falling costs in future technologies has an observable test: If vastly rich aliens can easily afford beacons of the sort we seek – microwave or IR or even visible – then where are they? If there are Altruistic Aliens, where is the evidence? They certainly haven’t made beacons that are apparent in our sky. So, the rich aliens argument fails the test.

This Fermi question limits useful speculations. Whether alien motivations will resemble ours we’ve dealt with, but think secondary – all motivations meet constraints, and biological evolutionary theory suggests that cost (economic cost, environmental, etc.) is a universal constraint.

We think 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 in the form of automata? With a finite number of automata, i.e., slaves, 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. One might think that might be if aliens 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 be a near-universal driver for alien attempts at interstellar communication.