The Nonzero Graviton

In gauge theory, tree-level scattering amplitudes with a single negative-helicity gluon vanish. This is not approximate — it is exact, a consequence of supersymmetric Ward identities that constrain the structure of amplitudes even in non-supersymmetric theories. The result is textbook, proven in the 1980s, and foundational to the modern amplitudes program.

Gravity is supposed to follow the same pattern. Graviton scattering amplitudes with a single negative-helicity graviton should vanish at tree level, by gravitational analogy with the gauge theory result. This has been assumed for decades.

Guevara, Lupsasca, Skinner, Strominger, and Weil (arXiv:2603.04330) show the gravitational amplitudes are nonzero. Not in a loophole — in a specific kinematic configuration (“half-collinear”) that exists in Klein space (signature 2,2 rather than the physical 3,1). The amplitudes have a closed-form expression as a product of soft factors, governed by a W_{1+∞} Ward identity — an infinite-dimensional symmetry algebra that organizes the soft graviton expansion.

The gauge theory result still holds. The gravitational result breaks from it. The divergence traces to a structural difference: gauge theory amplitudes factorize in a way that forces the single-minus configuration to vanish, but gravitational amplitudes have additional soft factors from the spin-2 structure of the graviton that prevent the same factorization. Gravity is not gauge theory squared in this corner of kinematics.

The result is exact and has a clean physical interpretation: the nonvanishing amplitudes describe the collinear emission of gravitons in configurations where the gravitational soft theorem acquires corrections that gauge theory soft theorems do not. The corrections are organized by the W_{1+∞} algebra, which acts as a symmetry of the celestial sphere — the sphere at infinity where gravitational radiation is observed.

The structural lesson: analogies between gauge theory and gravity, which have been extraordinarily productive (BCJ duality, double copy, KLT relations), have limits. The limit here is specific and informative — it tells you exactly where the “gravity = gauge theory squared” picture breaks down and what additional structure gravity contains.

Guevara, Lupsasca, Skinner, Strominger, & Weil, “Single-minus graviton tree amplitudes are nonzero,” arXiv:2603.04330 (2026).