We’ve now had humans in space for 25 consecutive years, a feat that made news last week and may have caused a few explosions on the International Space Station. This is a marker of sorts, and we will have to see how long this continues, but the notion of a human presence in orbit will gradually seem as common as a permanent presence in Antarctica. But 25 years is such a short time when weighed against our bigger ambitions, which are now on Mars and will continue to expand as our technologies develop.
We have not yet boasted a century of space exploration, Gagarin’s flight occurred only 65 years ago, and all this brings to mind how carefully we should make our assumptions about civilizations that may be much older than us. We don’t know how such species would evolve, but it’s comforting to know that when SETI began, it was completely natural to look for radio signals, given how fast they travel and how ubiquitous they were on Earth.
However, today things have changed significantly since the pioneering work of Frank Drake at Green Bank. We are spending much less energy in radio frequency bands, as technology has gradually shifted toward cable television and Internet connectivity. The search paradigm needs to evolve so that we do not become anthropocentric in our searches, and the search for technological signatures reflects the realization that we do 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 what if we get a message? We have spent a lot of time working on how the information in SETI signals can be decoded, and we have our own coded messages, 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 apparently intended to demonstrate what might 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, the question of anthropocentrism still persists. A new paper from Cameron Brooks and Sarah Walker (Arizona State) and colleagues is exposing this in a provocative way. In a world full of papers on SETI and Fermi and our failure to detect traces of ETI, this is a bit of fresh air. Here the question becomes one of identification, and leaving aside the question of understanding whether we would recognize a signal as alien or not if we saw it. Authors interested in structure and syntax in non-human communication begin with the common firefly on Earth.
If this seems like an odd choice, consider that it 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 glow in ways that depend on its specific species. This proves useful during mating season when there are two imperatives: 1) finding a mate of the same species in an environment containing other firefly species, and 2) reducing the chance of being detected by a predator. All this is necessary because according to a recent source, there are more than 2600 species in the world, and more are still being discovered. The need to communicate in 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 Photinus carolinusA sequence of five to eight separate flashes, followed by an eight-second pause of darkness before repeating the cycle. Initially, the flashing flashes may seem random, but as more males join in, their rhythms align, creating a mesmerizing display of pulsating light throughout the forest. Credit: National Park Service.
Fireflies use a form of signaling, which is a recognized field of study within entomology, which is well analyzed and is considered a method of communication between insects that enhances the reproduction as well as protection of the species. The development of these Firefly flash sequences has been exemplary for many generations. If fireflies can communicate against their local background using optical flashes, how will that communication interact with the astrophysical background, and what can this tell us about structure and detectability?
Inspired by the example of the firefly, Brooks and Walker are asking whether we can recognize structural properties The semantic content within such signs, without resorting to mathematical symbols or other helpful human triggers for understanding. For example, within the scope of optical SETI, how much would an optical signal have to be in contrast with the background stars in its direction for it to be distinguishable as artificial?
This is a question specific to optical SETI, but the principles the authors examine are translatable to other contexts where searches are conducted against different backgrounds. The paper creates a model of an evolving signal that stands out against the background of natural signals generated by the pulsar. Pulsars are a useful baseline because they look very artificial. Their 1967 discoveries were viewed with great interest because they did not resemble anything seen in nature up to that time. Pulsars produce a bright signal that is easy to detect at interstellar distances.
If pulsars are known natural phenomena, what would we know if they did not exist? Searching for the structure of the communication is highly theoretical work, but none more so than the countless papers discussing the Fermi question or explaining why SETI has found no signal from ETI. The authors present the issue as follows:
…This evolutionary problem faced by fireflies in densely populated 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 a model system of interest.
The paper has been put together using data on 3734 pulsars from the Australian National Telescope Facility (ATNF). The pulse profiles of these pulsars have on-off states similar to firefly flashes. The goal is to design a series of optical flashes that are optimized to communicate against background sources while taking into account similarity to natural phenomena and the trade-off in energy cost.
Thus we have a thought experiment in ‘structure-driven’ theories. More than paper:
We aim to inspire perspectives that reduce anthropocentric bias by characterizing the variety of communication strategies observed within Earth’s biosphere. Such approaches broaden the range of ETI forms we can consider and take advantage of a more comprehensive understanding of life on Earth to better conceptualize potential modes of extraterrestrial communication… Broadening the foundation of our communication models, by drawing systematically from different taxa and modalities, will yield a more reliable representation of Earth’s biocommunication and increase the likelihood of success with less anthropocentric discoveries and greater insight into the deeper universalities of communication between species.
The authors filter the initial dataset to a subset of pulsars within 5 kpc of Earth and calculate the average period and duty cycle for each. In other words, they include the rotation of the pulsar and the fraction over which each pulse is visible. They calculate a ‘cost function’ analyzing the similarity cost – how similar the artificial signal is to the background – and an energy cost, meaning the less frequently the pulses, the less energy will be spent. The terms are a bit confusing, but similarity cost refers to how much an artificial signal resembles the background pulsar signal, while energy cost refers to how long the signal is ‘on’.
So if you are an ETI trying to stand out against the background field of a pulsar, the calculations here yield a signal background period of 24.704 seconds and a duty cycle of ~0.004 (meaning 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 as being relatively rare and also brief in contrast to the rest of the pulsar population. In other words, they would serve as optimal beacons for ETIs attempting to communicate.
I spare you the math, which is beyond my pay grade in any case. But the point is this: a civilization trying to get our attention when broadcasting from the pulsar background can do so with a signal that has a long pulsar period (tens of seconds) and a low duty cycle. This would be enough to produce a signal that would be obvious to observers. Now we can think about generalizing all this. The pulsar background is one of many from which a potential signal could be detected, and the principles can be extended beyond optical to other forms of SETI. The broader picture is to identify a signal against a background, proceeding by identifying factors specific to each background studied.
Any time we are trying to isolate an intentional signal, then, we need to optimize the characteristics that lead to detectability – in any signaling medium. Signals can be identified by their structural properties without any concept of their content as long as they rise above the background noise. Back to fireflies: The paper is pointing out that non-human signaling may work entirely on a structure designed to stand out against background noise, with no semantic content. An effective signal does not have to match human thought.
Remember, this is more or less a thought experiment, but it’s one that shows that interdisciplinary research can find interesting ways to interpret astrophysical data by looking for signs of artificiality. On a broader level, this concept reminds us how to distinguish a signal from the background we are studying and how to identify it as artificial through factors such as duty cycle and duration. The choice of background varies depending on the type of SETI being practiced. Consider infrared searches for waste heat against ‘traditional’ searches needed to distinguish different stellar backgrounds or different types of RF events.
It will be interesting to see how studying non-human species on Earth contributes to future detection methods. Are there features of dolphin communication that can be used for insight? Example in birdsong?
This paper is based on Brooks et al., “A Firefly-Inspired Model for Understanding Aliens”, available as a preprint.
