The Aestivation hypothesis: popular outline and FAQ

Anders Sandberg & Milan Ćirković

Since putting up a preprint for our paper “That is not dead which can eternal lie: the aestivation hypothesis for resolving Fermi’s paradox” (Journal of the British Interplanetary Society, in press) we have heard some comments and confusion that suggest to us that it would be useful to try to outline and clarify what our idea is, what we think about it, and some of the implications.

Table of contents

The super-short version of the paper

Maybe we are not seeing alien civilizations because they are all rationally “sleeping” in the current early cosmological era, waiting for a remote future when it is more favourable to exploit the resources of the universe. We show that given current observations we can rule out a big chunk of possibilities like this, but not all.

A bit more unpacked explanation

Information processing requires physical resources, not just computers or brains, but energy to run them. There is a thermodynamic cost to perform information processing that is temperature dependent: in principle, running processing become 10 times more efficient if your computer is 10 times colder (measured in Kelvins). Right now the cosmic background radiation makes nearly everything in the universe hotter than 3 Kelvin, but as the universe expands this background temperature will decline exponentially. So if you want to do as much information processing as possible with the energy you have it makes sense to wait. It becomes exponentially better. Eventually the background temperature bottoms out because of horizon radiation in a few trillion years: at this point it no longer makes sense to wait with the computation.

Hence, an advanced civilization may have explored a big chunk of the universe, done what is doable with existing nature, and now mostly have internal “cultural” things to do. These things can be regarded as information processing. If they want to maximize processing they should not do it today but wait until the cold future when they will get tremendously more done (1030 times more!). They should hence aestivate, leaving their domain protected by some automation until they wake up.

If this is correct, there might be old and powerful civilizations around that are hard to observe, not because they are deliberately hiding but because they are inactive for the time being.

However, were this hypothesis true, they would not want to lose their stuff. We should expect to see fewer processes that reduce resources  that could be useful in the far future. In the paper we look at processes that look like they might waste resources: stars converting mass into energy that is lost, stars imploding into black holes, galactic winds losing gas into intergalactic space, galaxy collisions, and galaxy clusters getting separated by the expansion of the universe. Current observations do not seem to indicate anything preventing these processes (and most interventions would be very visible).

Hence, either:

  1. the answer to the Fermi question “where are they?!” is something else (like there being no aliens),
  2. advanced civilizations aestivate but do so with only modest hoards of resources rather than entire superclusters,
  3. they are mostly interested in spreading far and wide since this gives a lot of stuff with a much smaller effort than retaining it.

Necessary assumptions

The aestivation hypothesis makes the following assumptions:

  1. There are civilizations that mature much earlier than humanity. (not too implausible, given that Earth is somewhat late compared to other planets)
  2. These civilizations can expand over sizeable volumes, gaining power over their contents. (we have argued that this is doable)
  3. These civilizations have solved their coordination problems. (otherwise it would be hard to jointly aestivate; assumption likelihood hard to judge)
  4. A civilization can retain control over its volume against other civilizations. (otherwise it would need to actively defend its turf in the present era and cannot aestivate; likelihood hard to judge)
  5. The fraction of mature civilizations that aestivate is non-zero. (if it is rational at least some will try)
  6. Aestivation is largely invisible. (seems likely, since there would be nearly no energy release)

Have you solved the Fermi question?

We are not claiming we now know the answer to the Fermi question. Rather, we have a way of ruling out some possibilities and a few new possibilities worth looking for (like galaxies with inhibited heavy star formation).

Do you really believe in it?

I (Anders) personally think the likeliest reason we are not seeing aliens is not that they are aestivating, but just that they do not exist or are very far away.

We have an upcoming paper giving some reasons for this belief. The short of it is that we are very uncertain about the probability of life and intelligence given the current state of scientific knowledge. They could be exceedingly low, and this means we have to assign a fairly high credence to the empty universe hypothesis. If that hypothesis is not true, then aestivation is a pretty plausible answer in my personal opinion.

Why write about a hypothesis you do not think is the most likely one? Because we need to cover as much of possibility space as possible, and the aestivation hypothesis is neatly suggested by considerations of the thermodynamics of computation and physical eschatology. We have been looking at other unlikely Fermi hypotheses like the berserker hypothesis to see if we can give good constraints on them (in that case, our existence plus some ecological instability problems make berzerkers unlikely).

What is the point?

Understanding the potential and limits of intelligence in the universe tells us things about our own chances and potential future.

At the very least, this paper shows what a future advanced human-derived civilization may try to achieve, and some of the ultimate limits on far-future information processing. It gives some new numbers to feed into Nick Bostrom’s astronomical waste argument for working very hard on reducing existential risk in the present: the potential future is huge.

In regards to alien civilizations, the paper maps a part of possibility space, showing what is required for this Fermi paradox explanation to work as an explanation. It helps cut down on the possibilities a fair bit.

What about the Great Filter?

We know there has to be at least one the unlikely step between non-living matter and easily observable technological civilizations (“the Great Filter”), otherwise the sky would be full of them. If it is an early filter (life or intelligence is rare) we may be fairly alone but our future is open; were the filter a later step, we should expect to be doomed.

The aestivation hypothesis doesn’t tell us much about the filter. It allows explaining away the quiet sky as evidence for absence of aliens, so without knowing if it is true or not we do not learn anything from the silence. The lack of megascale engineering is evidence against certain kinds of alien goals and activities, but rather weak evidence.

Meaning of life

Depending on what you are trying to achieve, different long-term strategies make sense. This is another way SETI may tell us something interesting about the Big Questions by showing what advanced species are doing (or not):

If the ultimate value you aim for is local such as having as many happy minds as possible, then you want to spread very far and wide, even though the galaxy clusters you have settled will eventually drift apart and be forever separated. The total value doesn’t depend on all those happy minds talking to each other. Here the total amount of value is presumably proportional to the amount of stuff you have gathered times how long it can produce valuable thoughts. Aestivation makes sense, and you want to spread far and wide before doing it.

If the ultimate value you aim for is nonlocal, such as having your civilization produce the deepest possible philosophy, then all parts need to stay in touch with each other. This means that expanding outside a gravitationally bound supercluster is pointless: your expansion will halt at this point. We can be fairly certain there are no advanced civilizations trying to scrape together larger superclusters since it would be very visible.

If the ultimate value you aim for is finite, then at some point you may be done: you have made the perfect artwork or played all the possible chess games. Such a civilization only needs resources enough to achieve the goal, and then presumably will shut down. If the goal is small it might do this without aestivating, while if it is large it may aestivate with a finite hoard.

If the ultimate goal is modest, like enjoying your planetary utopia, then you will not affect the large-scale universe (although launching intergalactic colonization may still be good for security, leading to a nonlocal instrumental goal). Modest civilizations do not affect the overall fate of the universe.

Can we test it?

Yes! The obvious way is to carefully look for odd processes keeping the universe from losing potentially useful raw materials. The suggestions in the paper give some ideas, but there are doubtless other things to look for.

Also, aestivators would protect themselves from late-evolving species that could steal their stuff. If we were to start building self-replicating von Neumann probes in the future, if there are aestivations around they better stop us. This hypothesis test may of course be rather dangerous…

Isn’t there more to life than information processing?

Information is “a difference that makes a difference”: information processing is just going from one distinguishable state to another in a meaningful way. This covers not just computing with numbers and text, but having one brain state follow another, doing economic transactions, and creating art. Falling in love means that a mind goes from one state to another in a very complex way. Maybe the important subjective aspect is something very different from states of brain, but unless you think that it is possible to fall in love without having the brain change state there will be an information processing element to it. And that information processing is bound by the laws of thermodynamics.

Some theories of value place importance on how or that something is done rather than the consequences or intentions (which can be viewed as information states): maybe a perfect Zen action holds value on its own. If the start and end state are the same, then an infinite amount of such actions can be done and an equal amount of value achieved – yet there is no way of telling if they have ever happened, since there will not be a memory of them occurring.

In short, information processing is something we instrumentally need for the mental or practical activities that truly matter.


Like hibernate, but through summer (latin aestus=heat, aestivate=spend the summer). Hibernate (latin hibernus=wintry) is more common, but since this is about avoiding heat we choose the slightly rarer term.

Can’t you put your computer in a fridge?

Yes, it is possible to cool below 3 K. But you need to do work to achieve it, spending precious energy on the cooling. If you want your computing done *now* and do not care about the total amount of computing, this is fine. But if you want as much computing as possible, then fridges are going to waste some of your energy.

There are some cool (sorry) possibilities by using very large black holes as heat sinks, since their temperature would be lower than the background radiation. But this will only last for a few hundred billion years, then the background will be cooler.

Does computation costs have to be temperature dependent?

The short answer is no, but we do not think this matters for our conclusion.

The irreducible energy cost of computation is due to the Landauer limit (this limit or principle has also been ascribed to Brillouin, Shannon, von Neumann and many others): to erase one bit of information you need to pay an energy cost equal to kT\ln(2) or more. Otherwise you could cheat the second law of thermodynamics.

However, logically reversible computation can work without paying this by never erasing information. The problem is of course that eventually memory runs out, but Bennett showed that one can then “un-compute” the computation by running it backwards, removing the garbage. The problem is that reversible computation needs to run very close to the average energy of the system (taking a long time) and that error correction is irreversible and temperature dependent. Same thing is true for quantum computation.

If one has a pool of negentropy, that is, something ordered that can be randomized, then one can “pay” for bit erasure using this pool until it runs out. This is potentially temperature independent! One can imagine having access to a huge memory full of zero bits. By swapping your garbage bit for a zero, you can potentially run computations without paying an energy cost (if the swapping is free): it has essentially zero temperature.

If there are natural negentropy pools aestivation is pointless: advanced civilizations would be dumping their entropy there in the present. But as far as we know, there are no such pools. We can make them by ordering matter or energy, but that has a work cost that depends on temperature (or using yet another pool of negentropy).

Space-time as a resource?

Maybe the flatness of space-time is the ultimate negentropy pool, and by wrinkling it up we can get rid of entropy: this is in a sense how the universe has become so complex thanks to matter lumping together. The total entropy due to black holes dwarfs the entropy of normal matter by several orders of magnitude.

Were space-time lumpiness a useful resource we should expect advanced civilizations to dump matter into black holes on a vast scale; this does not seem to be going on.

Lovecraft, wasn’t he, you know… a bit racist?

Yup. Very racist. And fearful of essentially everything in the modern world: globalisation, large societies, changing traditions, technology, and how insights from science make humans look like a small part of the universe rather than the centre of creation. Part of what make his horror stories interesting is that they are horror stories about modernity and the modern world-view. From a modernist perspective these things are not evil in themselves.

His vision of a vast universe inhabited by incomprehensible alien entities far outside the range of current humanity does fit in with Dysonian SETI and transhumanism: we should not assume we are at the pinnacle of power and understanding, we can look for signs that there are far more advanced civilizations out there (and if there is, we better figure out how to relate to this fact), and we can aspire to become something like them – which of course would have horrified Lovecraft to no end. Poor man.

The end of the worlds

Nikkei existential riskGeorge Dvorsky has a piece on Io9 about ways we could wreck the solar system, where he cites me in a few places. This is mostly for fun, but I think it links to an important existential risk issue: what conceivable threats have big enough spatial reach to threaten a interplanetary or even star-faring civilization?

This matters, since most existential risks we worry about today (like nuclear war, bioweapons, global ecological/societal crashes) only affect one planet. But if existential risk is the answer to the Fermi question, then the peril has to strike reliably. If it is one of the local ones it has to strike early: a multi-planet civilization is largely immune to the local risks. It will not just be distributed, but it will almost by necessity have fairly self-sufficient habitats that could act as seeds for a new civilization if they survive. Since it is entirely conceivable that we could have invented rockets and spaceflight long before discovering anything odd about uranium or how genetics work it seems unlikely that any of these local risks are “it”. That means that the risks have to be spatially bigger (or, of course, that xrisk is not the answer to the Fermi question).

Of the risks mentioned by George physics disasters are intriguing, since they might irradiate solar systems efficiently. But the reliability of them being triggered before interstellar spread seems problematic. Stellar engineering, stellification and orbit manipulation may be issues, but they hardly happen early – lots of time to escape. Warp drives and wormholes are also likely late activities, and do not seem to be reliable as extinctors. These are all still relatively localized: while able to irradiate a largish volume, they are not fine-tuned to cause damage and does not follow fleeing people. Dangers from self-replicating or self-improving machines seems to be a plausible, spatially unbound risk that could pursue (but also problematic for the Fermi question since now the machines are the aliens). Attracting malevolent aliens may actually be a relevant risk: assuming von Neumann probes one can set up global warning systems or “police probes” that maintain whatever rules the original programmers desire, and it is not too hard to imagine ruthless or uncaring systems that could enforce the great silence. Since early civilizations have the chance to spread to enormous volumes given a certain level of technology, this might matter more than one might a priori believe.

So, in the end, it seems that anything releasing a dangerous energy effect will only affect a fixed volume. If it has energy E and one can survive it below a deposited energy e, if it just radiates in all directions the safe range is r = \sqrt{E/4 \pi e} \propto \sqrt{E} – one needs to get into supernova ranges to sterilize interstellar volumes. If it is directional the range goes up, but smaller volumes are affected: if a fraction f of the sky is affected, the range increases as \propto \sqrt{1/f} but the total volume affected scales as \propto f\sqrt{1/f}=\sqrt{f}.

Stable strangeletsSelf-sustaining effects are worse, but they need to cross space: if their space range is smaller than interplanetary distances they may destroy a planet but not anything more. For example, a black hole merely absorbs a planet or star (releasing a nasty energy blast) but does not continue sucking up stuff. Vacuum decay on the other hand has indefinite range in space and moves at lightspeed. Accidental self-replication is unlikely to be spaceworthy unless is starts among space-moving machinery; here deliberate design is a more serious problem.

The speed of threat spread also matters. If it is fast enough no escape is possible. However, many of the replicating threats will have sublight speed and could hence be escaped by sufficiently paranoid aliens. The issue here is if lightweight and hence faster replicators can always outrun larger aliens; given the accelerating expansion of the universe it might be possible to outrun them by being early enough, but our calculations do suggest that the margins look very slim.

The more information you have about a target, the better you can in general harm it. If you have no information, merely randomizing it with enough energy/entropy is the only option (and if you have no information of where it is, you need to radiate in all directions). As you learn more, you can focus resources to make more harm per unit expended, up to the extreme limits of solving the optimization problem of finding the informational/environmental inputs that cause desired harm (=hacking). This suggests that mindless threats will nearly always have shorter range and smaller harms than threats designed by (or constituted by) intelligent minds.

In the end, the most likely type of actual civilization-ending threat for an interplanetary civilization looks like it needs to be self-replicating/self-sustaining, able to spread through space, and have at least a tropism towards escaping entities. The smarter, the more effective it can be. This includes both nasty AI and replicators, but also predecessor civilizations that have infrastructure in place. Civilizations cannot be expected to reliably do foolish things with planetary orbits or risky physics.

[Addendum: Charles Stross has written an interesting essay on the risk of griefers as a threat explanation. ]

[Addendum II: Robin Hanson has a response to the rest of us, where he outlines another nasty scenario. ]