Asymptotic symmetries govern the allowed transformations of fields at the boundaries of spacetime and therefore shape the long-distance, low-energy behavior of gravitational interactions. Asymptotic symmetries such as the BMS group discovered in the 1960s act at null infinity and are larger than the Poincaré group, adding angle-dependent supertranslations and possibly superrotations. These symmetry transformations are not pure gauge at infinity; they generate physically distinct states and enforce conservation laws that control the infrared regime of quantum gravity.
Mechanisms linking symmetries and infrared behavior
The connection between large-distance symmetries and soft particle emissions was first given concrete form by the soft graviton theorem of Steven Weinberg at University of Texas at Austin which shows that any scattering process emitting a graviton of vanishing energy is governed by a universal factor. Building on that, Andrew Strominger at Harvard University demonstrated that these soft theorems are equivalent to Ward identities for the BMS group, making the infrared structure of gravity a consequence of asymptotic symmetry. The same web links the memory effect, a permanent displacement of detectors after a burst of gravitational radiation, to soft emissions and asymptotic charges. These identifications are exact in perturbative frameworks that treat null infinity as the arena for conserved charges.
Consequences for quantum gravity and black holes
Because asymptotic symmetries impose infinite families of conservation laws, they restrict the allowed transitions in the gravitational S-matrix and organize infrared divergences into symmetry-protected sectors. This reframing has practical consequences: it explains why soft radiation must accompany hard scattering and suggests how to define finite, physical observables after accounting for soft contributions. Stephen Hawking at University of Cambridge together with Malcolm J. Perry and Andrew Strominger proposed that related asymptotic charges, dubbed soft hair, could encode information about black hole microstates and affect discussions of black hole evaporation and information loss. Whether soft hair suffices to resolve the information paradox remains an open, debated question that mixes technical quantum-field arguments with conceptual issues about locality and quantum gravity.
These ideas reshape how physicists view long-range gravitational correlations across astrophysical and cosmological settings and motivate international, interdisciplinary work combining mathematical analysis, particle-scattering experiments, and gravitational-wave observations to test the interplay of symmetry and infrared physics.