NOW, WHAT'S THE HYPE ABOUT INFRARED PHYSICS?

One primary way to probe nature's fundamental forces is to study Scattering processes. Here we want to understand the quantum gravitational scattering problems in the low energy regime. Soft theorems are the symmetry constraints of the scattering amplitudes in this regime. This is basically the low-energy sector of a scattering process. When we have a scattering process, there exist external particles of different energy scales. When we set the energy of one of the external particles to zero, we call that particle a soft particle. But the question is, what's the need to do that? In our observable universe, the asymptotic observer always sits at the infinities of the spacetime, which means at the asymptotic boundaries of the spacetime. Hence the information flow will only be possible via the messengers (which are photons in case of electromagnetic interaction of Electromagnetic Waves and gravitons in case of Gravitational interaction of Gravitational Waves). So the soft particles, in our case, are photons and gravitons. When we study one scattering process, we need to find the scattering amplitude. After taking one of the particles to be soft, we can write this higher point amplitude as a lower point amplitude multiplied by the universal soft factor. This is one incredible result from the Amplitude community because this theorem is independent of the theory we consider; the structure will be independent of the scattering pieces of information. Isn't this cool!! :)

The infinite symmetries of the four-dimensional asymptotically flat spacetime in both gauge and gravity theories lead us to the triangular relation between the memory effect, asymptotic symmetries, and soft theorems. Due to the passage of gravitational radiation, the difference in the initial and final spacetime geometries is related by a BMS supertranslation. The Ward identities of this invariance in Quantum Gravity (QG) are expressed as data representing gravitational radiation at null infinity. The recent uncovering of the relation between the on-shell physics of asymptotically flat theories and 2D CFT has proved to be a powerful tool. The boundary physics is captured by a 2D CFT (CCFT) of the conformal operators on the Celestial Sphere. Celestial amplitudes are of immense importance as an independent entity in itself. The infinite number of symmetries impose constraints on the celestial amplitude via Ward identities giving us the soft theorems. These constraints directly imply constraints on the bulk scattering amplitude. Hence a thorough understanding of celestial amplitudes can uncover new symmetries.