We’ve now had humans in space for 25 continuous years, a feat that made the news last week and one that must have caused a few toasts to be made aboard the International Space Station. This is a marker of sorts, and we’ll have to see how long it will continue, but the notion of a human presence in orbit will gradually seem to be as normal as a permanent presence in, say, Antarctica. But what a short time 25 years is when weighed against our larger ambitions, which now take in Mars and will continue to expand as our technologies evolve.

We’ve yet to claim even a century of space exploration, what with Gagarin’s flight occurring only 65 years ago, and all of this calls to mind how cautiously we should frame our assumptions about civilizations that may be far older than ourselves. We don’t know how such species would develop, but it’s chastening to realize that when SETI began, it was utterly natural to look for radio signals, given how fast they travel and how ubiquitous they were on Earth.

Today, though, things have changed significantly since Frank Drake’s pioneering work at Green Bank. We’re putting out a lot less energy in the radio frequency bands, as technology gradually shifted toward cable television and Internet connectivity. The discovery paradigm needs to grow lest we become anthropocentric in our searches, and the hunt for technosignatures reflects the realization that we may not know what to expect from alien technologies, but if we see one in action, we may at least be able to realize that it is artificial.

And if we receive a message, what then? We’ve spent a lot of time working on how information in a SETI signal could be decoded, and have coded messages of our own, as for example the famous Hercules message of 1974. Sent from Arecibo, the message targeted the Hercules cluster some 25,000 light years away, and was obviously intended as a demonstration of what might later develop with nearby stars if we ever tried to communicate with them.

But whether we’re looking at data from radio telescopes, optical surveys of entire galaxies or even old photographic plates, that question of anthropocentrism still holds. Digging into it in a provocative way is a new paper from Cameron Brooks and Sara Walker (Arizona State) and colleagues. In a world awash with papers on SETI and Fermi and our failure to detect traces of ETI, it’s a bit of fresh air. Here the question becomes one of recognition, and whether or not we would identify a signal as alien if we saw it, putting aside the question of deciphering it. Interested in structure and syntax in non-human communication, the authors start here on Earth with the common firefly.

If that seems an odd choice, consider that this is a non-human entity that uses its own methods to communicate with its fellow creatures. The well studied firefly is known to produce its characteristic flashes in ways that depend upon its specific species. This turns out to be useful in mating season when there are two imperatives: 1) to find a mate of the same species in an environment containing other firefly species, and 2) to minimize the possibility of being identified by a predator. All this is necessary because according to one recent source, there are over 2600 species in the world, with more still being discovered. The need is to communicate against a very noisy background.

Image: Can the study of non-human communication help us design new SETI strategies? In this image, taken in the Great Smoky Mountains National Park, we see the flash pattern of Photinus carolinus, a sequence of five to eight distinct flashes, followed by an eight-second pause of darkness, before the cycle repeats. Initially, the flashing may appear random, but as more males join in, their rhythms align, creating a breathtaking display of pulsating light throughout the forest. Credit: National Park Service.

Fireflies use a form of signaling, one that is a recognized field of study within entomology, well analyzed and considered as a mode of communications between insects that enhances species reproduction as well as security. The evolution of these firefly flash sequences has been simulated over multiple generations. If fireflies can communicate against their local background using optical flashes, how would that communication be altered with an astrophysical background, and what can this tell us about structure and detectability?

Inspired by the example of the firefly, what Brooks and Walker are asking is whether we can identify structural properties within such signals without recourse to semantic content, mathematical symbols or other helpfully human triggers for comprehension. In the realm of optical SETI, for example, how much would an optical signal have to contrast with the background stars in its direction so that it becomes distinguishable as artificial?

This is a question for optical SETI, but the principles the authors probe are translatable to other contexts where discovery is made against various backgrounds. The paper constructs a model of an evolved signal that stands out against the background of the natural signals generated by pulsars. Pulsars are a useful baseline because they look so artifical. Their 1967 discovery was met with a flurry of interest because they resembled nothing we had seen in nature up to that time. Pulsars produce a bright signal that is easy to detect at interstellar distances.

If pulsars are known to be natural phenomena, what might have told us if they were not? Looking for the structure of communications is highly theoretical work, but no more so than the countless papers discussing the Fermi question or explaining why SETI has found no sign of ETI. The authors pose the issue this way:

…this evolutionary problem faced by fireflies in densely packed swarming environments provides an opportunity to study how an intelligent species might evolve signals to identify its presence against a visually noisy astrophysical environment, using a non-human species as the model system of interest.

The paper is put together using data from 3734 pulsars from the Australia National Telescope Facility (ATNF). The pulse profiles of these pulsars are the on-off states similar to the firefly flashes. The goal is to produce a series of optical flashes that is optimized to communicate against background sources, taking into account similarity to natural phenomena and trade-offs in energy cost.

Thus we have a thought experiment in ‘structure-driven’ principles. More from the paper:

Our aim is to motivate approaches that reduce anthropocentric bias by drawing on different communicative strategies observed within Earth’s biosphere. Such perspectives broaden the range of ETI forms we can consider and leverage a more comprehensive understanding of life on Earth to better conceptualize the possible modes of extraterrestrial communication… Broadening the foundations of our communication model, by drawing systematically from diverse taxa and modalities, would yield a more faithful representation of Earth’s biocommunication and increase the likelihood of success, with less anthropocentric searches, and more insights into deeper universalities of communication between species.

The authors filter the initial dataset down to a subset of pulsars within 5 kpc of Earth and compute mean period and duty cycle for each. In other words, they incorporate the rotation of the pulsar and the fraction in which each pulse is visible. They compute a ‘cost function’ analyzing similarity cost – how similar is the artificial signal to the background – and an energy cost, meaning the less frequent the pulses, the less energy expended. The terms are a bit confusing, but similarity cost refers to how much an artificial signal resembles a background pulsar signal, while energy cost refers to how long the signal is ‘on.’

So if you’re an ETI trying to stand out against a background field of pulsars, the calculations here produce a signal background period of 24.704 seconds and a duty cycle of ~0.004 (meaning that the signal is ‘on’ for 0.4 percent of the period). Such signals appear at the edge of the pulsar distribution – they would be signals that stand out by being relatively rare and also brief in contrast to the rest of the pulsar population. They would, in other words, serve as the optimal beacon for ETI attempting to communicate.

I spare you the math, which in any case is beyond my pay grade. But the point is this: A civilization trying to get our attention while broadcasting from a pulsar background could do so with a signal that has a long pulsar period (tens of seconds) and a low duty cycle. This would be sufficient to produce a signal that becomes conspicuous to observers. Now we can think about generalizing all this. The pulsar background is one of many out of which a possible signal could be detected, and the principles can be extended beyond the optical into other forms of SETI. The broad picture is identifying a signal against a background, proceeding by identifying the factors specific to each background studied.

Any time we are trying to distinguish an intentional signal, then, we need to optimize – in any signaling medium – the traits leading to detectability. Signals can be identified by their structural properties without any conception of their content as long as they rise above the noise of the background. Back to the fireflies: The paper is pointing out that non-human signaling can operate solely on a structure designed to stand out against background noise, with no semantic content. An effective signal need not resemble human thought.

Remember, this is more or less a thought experiment, but it is one that suggests that cross-disciplinary research may yield interesting ways of interpreting astrophysical data in search of signs of artificiality. On the broader level, the concept reminds us how to isolate a signal from whatever background we are studying and identify it as artificial through factors like duty cycle and period. The choice of background varies with the type of SETI being practiced. Ponder infrared searches for waste heat against various stellar backgrounds or more ‘traditional’ searches needing to distinguish various kinds of RF phenomena.

It will be interesting to see how the study of non-human species on Earth contributes to future detectability methods. Are there characteristics of dolphin communication that can be mined for insights? Examples in the song of birds?

The paper is Brooks et al., “A Firefly-inspired Model for Deciphering the Alien,” available as a preprint.