Solving 1D crossing and QFT$_2$/CFT$_1$

We provide an effective solution of the 1D crossing equation. We begin by arguing that crossing constraints can be recast in terms of bases of sum rules associated to special sets of CFT data -- extremal solutions -- which solve these constraints in a minimal way and naturally saturate positivity bounds on the space of CFTs. We conjecture, argue and check extensively that any extremal solution behaves as a generalized free field in the UV. This allows us to reconstruct the entirety of their CFT data using a rapidly convergent ``hybrid bootstrap'' method, which combines numerics and analytics. Strikingly, as we approach special corners in the space of extremal solutions we find that their CFT data can present non-trivial structure up to arbitrarily large energies. We interpret these corners as flat space limits of QFTs in AdS$_2$, which extremal solutions naturally describe. This picture allows us to bootstrap their CFT data in these limits in terms of 2d S-matrices, and conversely provide a microscopic CFT construction of the latter. Further evidence for this QFT in AdS description of extremal solutions comes from an explicit construction of bulk QFT operators solving an AdS locality problem. Concretly we show that it is possible to canonically associate one or more such operators to any extremal solution by explicitly solving for their BOE data. In the special case where this operator is the bulk stress-tensor we combine crossing and bulk locality constraints to derive stronger bounds on the OPE and BOE data, including an exact bootstrap lower bound on the central charge $C_T\geq 1/2$.