2021/05/20 by Toby S. Cubitt, Cubitt, Toby S.
Computer Science · Mathematics · #FOS: Physical sciences #Mathematical Analysis and Transform Methods #Matrix Theory and Algorithms #Quantum Physics (quant-ph) #Spectral Theory in Mathematical Physics
paper · pdf · doi:10.48550/arxiv.2105.09854
openalex publication_date 2021/05/20 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Pick a formal system. Any formal system. Whatever your favourite formal system is, as long as it's capable of reasoning about elementary arithmetic. The First Spectral Gap Incompleteness Theorem of [CPGW15] proved that there exist Hamiltonians whose spectral gap is independent of that system; your formal system is incapable of proving that the Hamiltonian is gapped, and equally incapable of proving that it's gapless. In this note, I prove a Second Spectral Gap Incompleteness Theorem: I show how to explicitly construct, within the formal system, a concrete example of a Hamiltonian whose spectral gap is independent of that system. Just to be sure, I prove this result three times. Once with Gödel's help. Once with Zermelo and Fraenkel's help. And finally, doing away with these high-powered friends, I give a simple, direct argument which reveals the inherent self-referential structure at the heart of these results, by asking the Hamiltonian about its own spectral gap.