# BMS blocks Just like [[0031 Conformal block|conformal blocks]] for [[0036 Conformal bootstrap|conformal bootstrap]], **BMS blocks** are an important ingredient in [[0123 BMS bootstrap|BMS bootstrap]]. For systems with [[0064 BMS group|BMS symmetry]], four point functions can be expanded in a basis of BMS invariant quantities called BMS blocks. ## Refs - [[2016#Bagchi, Gary, Zodinmawia]]: derives BMS global blocks (by taking the large central charge limit). - [[2018#Hijano]] ## BMS blocks v.s. [[0038 Celestial conformal blocks|celestial conformal blocks]] The BMS blocks will include the contributions by BMS descendants automatically, just like how [[0032 Virasoro algebra|Virasoro]] blocks incorporate contributions by Virasoro descendants automatically. On the other hand, conformal blocks will view BMS descendants as separate [[0029 Primary operator|conformal primaries]] and there’ll be a separate block for each descendant. <!-- - The problem is that we are in 4d so we really need bms4 blocks which no one has constructed yet. - The bms blocks will indeed contain the conformal blocks within themselves. In the celestial case tho, we can’t even use the standard conformal blocks but are needing to create our own. It might be possible that the bms4 blocks already contain the ccft conformal blocks one would want, but explicitly constructing the bms blocks is an intractable problem in general. Even Virasoro blocks are super hard to compute, and Virasoro is a subsector of our extended bms symmetry! --> <!-- #todo - decide which basis we want to calc. the blocks in - momentum basis? or conformal basis? or the one with extra parameter (u) in [[BanerjeeGhoshGonzo2020]] ## Tasks - take flat limit like Hijano but for BMS4. - we already know BMS4 2- and 3-pt. functions of BMS primaries [[BagchiBasuKakkarMehra2016]] (same as a 2d relativistic CFT) -->