To get a sense of the potential practical relevance for energy purposes of one aspect of as-yet-undiscovered physics, consider proton decay. Making the most general assumptions about the nature of its decay, the lifetime of the proton is currently bounded by τ_proton > 2.1 × 10²⁹ y. Assume that the lifetime is in fact 3 × 10²⁹ y, and that all of the proton’s rest energy emitted in the decay can be captured and put to use. Estimate the rate of useful power output from proton decay of a cubic kilometer of hydrogen gas at NTP.