  If a civilization wants to maximize computation it appears rational to
aestivate until the far future in order to exploit the low temperature
environment: this can produce a $10^{30}$ multiplier of achievable computation.
We hence suggest the "aestivation hypothesis": the reason we are not observing
manifestations of alien civilizations is that they are currently (mostly)
inactive, patiently waiting for future cosmic eras. This paper analyzes the
assumptions going into the hypothesis and how physical law and observational
evidence constrain the motivations of aliens compatible with the hypothesis.
