radiation stress: one construct, many systems

Acoustic radiation pressure is the steady force a sound wave exerts on a surface or an object, over and above the oscillating pressure, because the wave carries a net flux of momentum. Rayleigh's 1902 On the pressure of vibrations gave the theory of this time-averaged pressure in a vibrating fluid. The referent is the mean momentum flux of an acoustic field, measured as a force per unit area: the same construct, the momentum carried by a wave, that reappears as the pressure of light and as the radiation stress of ocean waves.[1]

Unit

The time-averaged force per unit area exerted by a sound field

Recovery means

Not applicable

Measured by

Measured as the steady force on an absorbing or reflecting target in a sound field, e.g. with a radiation-pressure balance

Radiation pressure is the force electromagnetic waves exert on a surface that absorbs or reflects them, a consequence of light carrying momentum as well as energy. Nichols and Hull's 1903 The pressure due to radiation measured this tiny force directly, confirming Maxwell's prediction. The referent is the momentum flux of an electromagnetic field expressed as a pressure, the identical wave-momentum construct that acoustics calls acoustic radiation pressure and that coastal science calls radiation stress.[2]

Unit

The force per unit area exerted by electromagnetic radiation

Recovery means

Not applicable

Measured by

Measured as the force on an absorbing or reflecting surface exposed to radiation, scaling with the incident energy flux

In water waves, the radiation stress is the excess flux of momentum carried by the waves, and its gradients drive set-up, set-down, and longshore currents as waves shoal and break. Longuet-Higgins and Stewart's 1964 Radiation stresses in water waves; a physical discussion, with applications introduced the term by explicit analogy with the radiation pressure of light and sound, applying the same wave-momentum idea to the sea surface. It is the same construct as acoustic and electromagnetic radiation pressure, here a tensor describing momentum flux in a surface-wave field and read off through its coastal effects.[3]

Unit

The excess momentum flux (a stress tensor) carried by a surface-wave field

Recovery means

Not applicable

Measured by

Inferred from wave height and direction via the radiation-stress tensor, and from the set-up and longshore currents its gradients produce