The strange behavior of sound through solids
Two sound waves (1 and 2) are produced, scattering into two other sound waves (3 and 4), which are then detected. The probability for this process is described by a 4-point scattering amplitude. Credit: Angelo Esposito

Not everything needs to be seen to be believed, like when a train approaches a station. The researchers discovered a new property of scattering when they studied sound waves through solid matter.

Physicists look at the likelihood of particle interactions in terms of probability curves or scattering amplitudes. When the energy of one of the scattered particles goes to zero, scattering amplitudes should always scale with the power of momentum. The research team was able to show that the amplitude can be proportional to the fractional power.

Why is this important? It's possible to improve upon current foundations of fundamental physics with the help of quantum field theories. When a new behavior is demonstrated, scientists have the chance to revisit or revise their theories.

The Institute for Advanced Study, Tom Brauner, and Riccardo Penco collaborated on the project. Picture a block of wood with speakers on both sides. The sound waves that meet each other and scatter are similar to the sound waves that collide in a particle collider. The result can be proportional to a fractional power if one speaker is adjusted to a limit. The team believes that this scaling behavior can be used to study scattering amplitudes in many different contexts.

The properties of scattering amplitudes have recently been studied with great interest. To build new foundations of quantum field theory, the goal of this broad program is to make some of our computations more efficient and ambitious.

Particle physicists have used fronman diagrams for a long time, but they have some limitations. To describe particle interactions, tens-of-thousands of Feynman diagrams are required. Researchers may be able to more easily determine particle behavior if they gain a better understanding of scattering amplitudes.

The present work reveals a twist in the story, showing that Condensed Matter physics displays much richerphenomenological of scattering amplitudes than what was previously seen in fundamental, relativistic physics. There is more work to be done on scattering amplitudes of collective oscillations of matter.

More information: Tomáš Brauner et al, Fractional Soft Limits of Scattering Amplitudes, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.231601 Journal information: Physical Review Letters