The Aestivation Hypothesis: Are Aliens Waiting For The Next Era Of The Universe?

Proposed answers to the paradox range from the optimistic to the downright frightening. It could be that we simply haven’t been looking long enough to find aliens, nor emitted our own traceable signatures for long enough for aliens to find us yet. Or it could be that no aliens will ever make it to the point where they are able to make contact with other species, destroying themselves long before they get to the kind of tech required to do so.

A lot of suggested solutions are tech-optimistic, and assume that civilizations do achieve technological maturity before wiping themselves out, or facing some other civilization-ending threat. One of these suggests that there may be plenty of advanced civilizations out there, but that they may be in a state of hibernation, emitting little or no thermal signature, until the universe is more suited to their needs.

Known as the aestivation hypothesis, the idea rests heavily on Landauer’s principle. This is the principle, proposed by physicist Rolf Landauer, that all irreversible changes in information stored on a computer dissipate a certain amount of heat to the environment. If you’re a civilization to whom computation is important, say that you live your days inside simulations, then you are going to produce a lot of waste heat, and require a method of cooling. While we might improve computer efficiency and speed, there’s no getting around Landauer’s limit. 

Or is there? In a 2017 paper proposing the idea, posted to preprint server arXiv, a team suggests that a civilization that has got its survival sorted may wish to get around Landauer’s limit by simply waiting until the universe itself has cooled down. By waiting around 270 billion years (and assuming that you can do so by e.g. slowing down your computational processes) the universe’s background temperature would decrease so that you get a lot more bang for your computational buck.

“A comparison of current computational resources to late era computational resources hence suggest a potential multiplier of 10³⁰!” the team writes in their paper, which was not peer-reviewed. “Even if only the resources available in a galactic supercluster are exploited, later-era exploitation produces a payoff far greater than any attempt to colonize the rest of the accessible universe and use the resources early. In fact, the mass-energy of just the Earth itself (5.9 x 10²⁴ kg) would be more than enough to power more computations than could currently be done by burning the present.”

According to the team, it’s plausible that such a civilization, perhaps born long before our time, could choose to aestivate (hibernate during hotter times, rather than like bears in winter) given how much further stored energy will go in the far future.

“As the universe cools down, one Joule of energy is worth proportionally more,” the team write. “Hence a civilization desiring to maximize the amount of computation will want to use its energy endowment as late as possible: using it now means far less total computation can be done. Hence an early civilization, after expanding to gain access to enough raw materials, will settle down and wait until it becomes rational to use the resources. We are not observing any aliens since the initial expansion phase is brief and intermittent and the aestivating civilization and its infrastructure is also largely passive and compact.”

While a fun idea, it is far from the final word on the Fermi paradox. Even if civilizations do survive to technological and societal maturity where aestivation is an option, it’s difficult to picture all civilizations taking it. According to the team, expansionist civilizations could still exist and potentially an aestivating civilization would have to defend themselves against potential invasions of their space. But we don’t see signs, so far, of expansionist civilizations either. Also, like Dyson spheres, our ideas may be limited by the technology of our times.

“More fundamentally there is the uncertainty inherent in analysing extreme future scenarios or future technology: even when we base the arguments on well-understood and well-tested physics, there might exist unexpected ways of circumventing this,” the team adds. “Unfortunately there is little that can be done about this.”