Imagine an extraterrestrial civilization that manages to colonize the entire galaxy. Then imagine the colonizing civilization collapsing so definitively that no trace of its existence has yet been detected, at least from our planet. We can call it, as Jacob Haqq-Misra and Seth Baum (Pennsylvania State University) do in a recently released paper, a ‘graveyard civilization,’ one whose remains might still be accessible provided we know where and how to look.

Pushing the Limits of Growth

What could bring down such a civilization? The idea here is that we can explain the Fermi paradox (‘Where are they?’) by assuming that exponential growth is not a sustainable development pattern for intelligent civilizations. The authors draw on human experience in analyzing this possibility. Here’s the gist of it:

The consequences of unsustainable development are often dire. In many documented cases, resource depletion caused by human activities has led to the permanent collapse of human populations, and resource depletion and environmental degradation can also cause or exacerbate violent conflict. Note that collapsed human populations do not necessarily disappear —they may persist in diminished numbers. This is particularly evident in the case of Easter Island, where resource depletion is believed to have caused or significantly contributed to a major population decline. Some analysts are concerned that the unsustainable practices of human civilization could lead to a global-scale collapse. Should such a collapse occur, human civilization would not be able to colonize the galaxy.

Fermi’s question, of course, was built around the assumption that alien civilizations would do more or less what we would do if we had their technology, which is to explore and colonize the galaxy the way we have done these things on our own world. Thus one answer to Fermi is that the reason we do not see evidence of extraterrestrials is that exponential growth patterns throughout the galaxy are not feasible. This does not rule out the existence of ETI, but does establish some constraints:

…the Paradox can only conclude that other intelligent civilizations have not sustained exponential growth patterns throughout the galaxy. It is still possible that slower-growth ETI civilizations exist but have not expanded rapidly enough to be easily detectable by the searches humans have yet made.

Such a slower growth pattern resonates in an era when ‘sustainable development’ in everything from technology to agriculture and environmental protection is much in the air. Civilizations moving beyond their growth limits, under this scenario, might find themselves in a state of retreat:

It is also possible that faster-growth ETI civilizations previously expanded throughout the galaxy but could not sustain this state, collapsing in a way that whatever artifacts they might have left have also remained undetected. Both of these growth patterns can be observed in human civilization, suggesting that they may be possible for ETI civilizations as well.

Recalibrating the SETI Search

You can see the effect of this on SETI strategy. Haqq-Misra and Baum suggest that SETI be recalibrated to focus on slow-growth civilizations, and those that have already endured their collapse. After all, a society that has chosen a sustainable, slow growth path may still broadcast a signal detectable by SETI. So, too, might a post-collapse culture, which might beam the story of its downfall out to the broader universe, or perhaps use automated systems to send out a galactic requiem.

But the authors’ first choice for SETI is to follow up on the possibility that slow-growth cultures might still be able to send small, long-duration star probes, in the absence of true colonizing forays into the universe. If so, we might consider complementing the SETI search with SETA (Search for Extraterrestrial Artifacts), which has been considered at both visible and radio wavelengths. The paper goes so far as to suggest that “…a survey of the solar vicinity may be more pragmatic than an all-sky search for encoded messages.”

What we don’t know is how representative we are. Nor do we know the limits of exponential growth, for they may lie not at the planetary but the solar system level, assuming they’re not fully surmountable in the first place (by some future civilization if no one has done it in the past). A success at finding some kind of artifact here in our own system would at least tell us that an interstellar crossing is not out of the question, but how much further do we want to take these conclusions? The authors raise the question themselves, and point out that “…we cannot rule out the possibility that ETI civilization may follow a development pattern sufficiently different that we wouldn’t recognize it even if we detected its signal.”

The paper is Haqq-Misra and Baum, “The Sustainability Solution to the Fermi Paradox,” Journal of the British Interplanetary Society 62 (2009), pp. 47-51 (available online).