SETI quite naturally started with the assumption that we should look in the realm of photons for signals from other stars. After all, radio or optical wavelengths were things we understood, and the interest in radio and attendant theorizing about ‘waterhole’ frequencies and interstellar beacons continues to be worth examining. But a truly advanced civilization might be using methods we haven’t yet managed to exploit. Of these, a singularly interesting choice is communication by neutrino.
John Learned (University of Hawaii) and colleagues take on this issue in a new paper just posted to the arXiv site, looking at the advantages of the notoriously elusive neutrino. A major plus is that the signal to noise problem is tricky for radio and optical methods, especially in the galactic plane, whereas neutrinos, depending on their energy levels, can offer an essentially noise-free band. We also run into severe problems with photons as we look at line of sight communications anywhere near the galactic center, intervening materials causing signals to be attenuated.
But neutrinos show up with little attenuation from almost any direction, and are free of photon scattering that introduces jitter in arrival time and direction. The paper looks at the neutrino energies best suited for galactic communication, noting that low energy neutrinos are a problem because natural sources (like supernovae) produce emissions that can obscure a signal. The paper runs through the factors considered in choosing a high energy level near 6.3 PeV “…such that it would be clear at once that it is an artifical source such as ETI and not some random background.”
Given the distances and times involved, the question of when an extraterrestrial civilization might choose to send a message becomes intricate. Although it’s a digression from the neutrino beam technology considered in the rest of the paper, the discussion is provocative:
We presume that the ETIs, though in our galaxy, are remote. Even if an ETI has been observing us, it may be a long while (timescale of thousands of years) before they would send us an introductory message. So if they want to send a message in advance, saying hello and welcome to the galactic network, they are going to have to speculate about when to bother to transmit. From the jittering of advances in speciation, with the great die offs, it seems clear that evolution is a stochastic process, with ﬂuctuations on a timescale of many millions of years. The evolution of technology may ﬂuctuate over a timescale of thousands of years, as exempliﬁed by the long periods of lack of technological progress in post-Roman Europe, China and India. One must reason that no useful prediction could be made as to when the industrial revolution would take off and high technology would arise. Thus the ETI would have to be transmitting speculatively over a long period.
The possibility of two stages of communication arises, the first stage being an ‘attention-getter’ signal, the second the sending of information. Artifacts could be more efficient than transmitting data, and could be sent to promising star systems with the assumption of later discovery by the inhabitants there. So a message from the stars might simply be short and to the point, a set of instructions telling us where to find the alien object.
Producing the needed neutrino beam goes beyond our current technology, but making neutrino beams in this energy range may well be feasible for a sufficiently advanced culture:
…we do not know the methods that may be available to advanced civilizations to make a neutrino (or any other) beam. We have direct evidence in the 1020 eV cosmic rays, the gamma ray bursts (GRBs), the micro-Quasars, and the amazingly collimated jets from active galactic nuclei (AGN), so that we might suspect that we do not yet understand some fundamental issues on particle acceleration. For example, how does one get an earth mass accelerated to a gamma of 1000 in a distance of a few light seconds, as has been inferred for gamma ray burst jets or “cannonballs”? So, for present purposes, we shall assume that an ETI would ﬁnd it affordable and worthwhile to expend such resources to communicate with our TES [Technically Emergent Society].
The beauty of directed beams of neutrinos at the energy levels considered here is that their signal would clearly signal the presence of an extraterrestrial civilization, there being no known natural mechanism for making neutrinos in only this energy range. The authors estimate that properly encoded data could accumulate at a rate of roughly 1000 pages per year. If any civilizations have taken this course and are actively transmitting to us, we can sit back and wait for the result, for the neutrino detectors coming online should soon discover their signatures.
The paper is Learned, Pakvasa and Zee, “Galactic Neutrino Communication,” available online.