On computational aestivation

People often say to me “Jess, all this work you do on the foundations of quantum mechanics is fine as far as it goes, but it’s so conventional and safe. When are you finally going to do something unusual and take some career risks?” I’m now pleased to say I have a topic to bring up in such situations: the thermodynamic incentives of powerful civilizations in the far future who seek to perform massive computations. Anders Sandberg, Stuart Armstrong, and Milan M. Ćirković previously argued for a surprising connection between Landauer’s principle and the Fermi paradox, which Charles Bennett, Robin Hanson, and I have now critiqued. Our comment appeared today in the new issue of Foundations of Physics:

Comment on 'The aestivation hypothesis for resolving Fermi's paradox'
Charles H. Bennett, Robin Hanson, C. Jess Riedel
In their article [arXiv:1705.03394], 'That is not dead which can eternal lie: the aestivation hypothesis for resolving Fermi's paradox', Sandberg et al. try to explain the Fermi paradox (we see no aliens) by claiming that Landauer's principle implies that a civilization can in principle perform far more (~1030 times more) irreversible logical operations (e.g., error-correcting bit erasures) if it conserves its resources until the distant future when the cosmic background temperature is very low. So perhaps aliens are out there, but quietly waiting. Sandberg et al. implicitly assume, however, that computer-generated entropy can only be disposed of by transferring it to the cosmological background. In fact, while this assumption may apply in the distant future, our universe today contains vast reservoirs and other physical systems in non-maximal entropy states, and computer-generated entropy can be transferred to them at the adiabatic conversion rate of one bit of negentropy to erase one bit of error. This can be done at any time, and is not improved by waiting for a low cosmic background temperature. Thus aliens need not wait to be active. As Sandberg et al. do not provide a concrete model of the effect they assert, we construct one and show where their informal argument goes wrong.

Some innocent bystanders who have read our critique have responded like this: “OK, fine, maybe aestivation is out as an explanation for the Fermi paradox. But what about the basic claim that if you wait a really long time, the universe will get really cold, and then you’ll be able to perform a lot more computations than you would if you just went full-speed-ahead on doing computations? Is that valid or not?”

I think this claim is “probably approximately” false, but no one knows for sure. Our critique shows that, contra Sandberg et al., its validity can’t be assessed by appealing solely to basic facts about cosmology and the principles of the thermodynamics of computation. Rather, the claim depends in detail on whether the optimal devices that can be created in the universe have certain physical parameters (e.g., of all the ways of assembling atoms in a cubic light-year, the minimal thermal conductivity achievable is 10-15 Watts per meter-Kelvin). These are engineering/chemistry questions which their paper doesn’t seriously address. We demonstrated this dependence in Section 2 of our critique by constructing a toy model where an aestivation incentive exists when making a (contrived) assumption about the physical devices that are possible, but where that inventive disappears when the assumption is relaxed. The assumption is logically independent of the laws of thermodynamics.

The crux is then whether that assumption (or one with equivalent implications) is physically reasonable in our universe, which we address in Sections 3 and 4. We argue that

  1. The assumption is probably not reasonable.
  2. If the assumption holds, the correct course of action would not look much like animal aestivation/hibernation. Instead, it would look like taking control of the entire accessible universe and converting the matter into whatever the optimal insulating aestivation device is, which would definitely not involve, e.g., shining stars.

In personal correspondence responding to our critique, Sandberg et al. have advanced physical arguments for the assumption based on things like black holes and large asymmetries in effective insulation strengths. These arguments don’t smell right to me at all, but they are almost impossible to assess because I haven’t yet seen them presented thoroughly.

Bookmark the permalink.

Leave a Reply

Include [latexpage] in your comment to render LaTeX equations with $'s. (More info.) May not be rendered in the live preview.

Your email address will not be published. Required fields are marked with a *.