When you consider that age of the Universe - 13.8 billion years by our most recent counts - and that which is "observable" to us measures about 27.6 billion light years in diameter, you begin to wonder why we haven't found signs of extra-terrestrial intelligence (ETI) beyond our Solar System. To paraphrase Enrico Fermi, the 20th century physicists who advanced the famous Fermi Paradox - "where the heck are all the aliens?"
Naturally, Fermi's Paradox has attracted a lot of theoretical explanations over the years - which include ETI being very rare, humanity being early to the Universe, and the aliens being extinct! But a new study by a team of scientists from the Future of Humanity Institute (FHI) offers a different take on this age-old paradox. According to their study, the key to answering this question is to consider the possibility that the aliens are engaged in "aestivation".
Essentially, aestivation is a prolonged state of torpor that organisms enter into during a particularly hot or dry period. Similar to what hibernating animals do during the winter, this state is designed to keep creatures alive until more favorable conditions emerge. And when applied to the cosmos, this concept could explain why one of the key things astronomers have been looking for - i.e. activity - has been lacking.
The study was led by Anders Sandberg, a research associate to the Oxford Uehiro Center for Practical Ethics, the Oxford Center for Neuroethics, and the James Martin Research Fellow at FHI. Cryptically titled, "That is Not Dead Which Can Eternal Lie: the Aestivation Hypothesis for Resolving Fermi's Paradox", their study considers the possibility that advanced alien civilizations might be difficult to find because they are sleeping right now.
This is not the first time Sandberg has addressed questions arising out of the Fermi Paradox. In a previous study, he and Stuart Armstrong (also a research associate with the EHI and one of the co-authors on this study) extended the Fermi Paradox to look beyond our own galaxy, addressing how more advanced civilizations would feasibly be able to launch colonization projects with relative ease (and even travel between galaxies without difficulty).
In the end, they concluded that civilizations from millions of galaxies should have been able to reach us by now, which only serves to bring the Fermi Paradox into greater focus. If these early civilizations are around, why are they not visible to us? The reason for this, they claim in this new study, has to do with the thermodynamics of computation.
According to this basic rule, the cost of a certain amount of computation is proportional to the temperature it generates. For some time, astronomers and cosmologists have been aware that the Universe is steadily cooling down over the time. Not only is star formation in galaxies slowly dying out over the course of billions of years, but even the cosmic background radiation is becoming colder.
Artist's conception of city lights on an alien planet. Credit: David A. Aguilar (CfA)
As such, it makes sense that ancient and advanced civilizations would want to wait for cooler conditions to prevail. Sandberg explained to Universe Today via email:
"The core idea is that if advanced civilizations mainly or solely care about computation, then it is rational for them to wait until the Universe is much older than now. The reason is that the energy cost (which will eventually limit how much computation you can do) is proportional to temperature, and this means that the far future is vastly more hospitable than the hot present. If this were true, we have a nice explanation for the apparent absence of big old civilizations. It would also lead to observable consequences: a reduction in processes that waste resources they would want in the late eras."
Timing is a key feature to this hypothesis. Much like the theory that humanity may have arrived early to the Universe, this theory states that the lack of detection has to do with species being in different places in their biological/technological evolution. In this case, the aestivation period of early civilizations has coincided with the subsequent rise of humanity as an space-faring and technologically-adept species.
Herein lies another reason why ancient civilizations might want to take a cosmic nap. Given how long life needs in order to emerge - humanity took roughly 4.5 billion years to get to where it is today - then it stands to reason that ancient civilizations might want to skip ahead a few eons in order to let new races emerge.
Ever since it was first announced in 2015, there has been speculation as to what could account for the dimming of KIC 8462852. Credit: SentientDevelopments.com
"There is an entropy cost to irreversible logical operations, including error correction," said Sanders. "So unless there is some magical energy source or entropy sink, if you want to do as much computation as possible you should wait until the cosmic background radiation levels off. In addition, civilizations may want to go to the future if they want to meet other, independently evolved civilizations. If intelligence is rare in time and space but aestivates to the far future, then it will meet there."
Of course, the aestiation hypothesis (much like the Drake Equation and the Fermi Paradox) is based on a few assumptions about what ETI would be capable of. These include:
- There are civilizations that mature much earlier than humanity.
- These civilizations can expand over sizeable volumes, gaining power over their contents.
- These civilizations have solved their coordination problems.
- A civilization can retain control over its volume against other civilizations.
- The fraction of mature civilizations that aestivate is non-zero
- Aestivation is largely invisible.
In other words, the hypothesis assumes the existence of civilizations that are more advanced than humanity which is based on the notion that they have had billions of years to develop elsewhere in the Universe. These civilizations would be higher on the Kardashev Scale (between Level II and III) by now, meaning that they had evolved to the point where they could harness the energy of entire star systems and perhaps even galaxies.
Also, it assumes that these civilizations would have become space-faring races that had expanded to occupy parts of the cosmos that lie well beyond their own star systems. Ultimately, those civilizations that have chosen to become dormant would therefore be invisible to us since they are not currently traveling between stars and galaxies, smashing up planets to create megastructures, or consuming entire stars for fuel.
You know, the kind of stuff we think mega-civilizations would do. Which naturally raises the question, how might we be able to detect such civilizations at rest? To this, Sandberg has a few possible suggestions, ones which ETI-hunters may want to heed:
"Look for galaxies that either move out of the way of galaxy collisions or towards big clusters by ejecting mass or energy in one direction, or have an unusually low number of heavy blue-white stars, or otherwise avoid losing gas to interstellar space. Or, try launching a self-replicating space probe to pave the universe and see if somebody stops you."
As with all things having to do with aliens and ETI, a measure of guess-work is required here. And some would naturally argue that it is also possible that advanced civilizations are not subject to the same limitations we humans are, which would limit our ability to speculate here. In the end, we humans are required to theorize about what we don't know based on what we do - aka. the "low-hanging fruit" approach.
The findings reported in the study were also the subject of a talk that took place at the second annual meeting of the UK SETI Research Network (UKSRN), which took place on September 11th and 12th, 2014, at Birkbeck College in London.
Further Reading: arXiv
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