recoil: one construct, many systems

In ballistics, recoil is the backward momentum a firearm gains as it drives a projectile and propellant gases forward: because total momentum is conserved, the gun and shooter take up an impulse equal and opposite to that given the ejecta. Mann's The bullet's flight from powder to target studied how this reaction moves the barrel even before the bullet exits, affecting where the shot goes. The referent is the equal-and-opposite momentum of the weapon, measured as recoil momentum or felt energy, and it is the same conservation-of-momentum construct that reappears as a scattered photon's recoil electron and as the kick a struck nucleus receives.[1]

Unit

The recoil momentum (and energy) of the weapon, equal and opposite to the ejecta

Recovery means

Not applicable

Measured by

Computed from the mass and velocity of projectile and propellant gases, or measured by recoil velocity and free-recoil energy

In photon physics, recoil is the momentum an electron takes up when a photon scatters off it, the counterpart to the photon's lengthened wavelength. Treating light as carrying momentum, the same conservation law that kicks a gun backward requires the struck electron to recoil, and the geometry fixes the wavelength shift. Compton's 1923 A quantum theory of the scattering of X-rays by light elements derived the shift and the recoil electron, confirming the photon's momentum. The referent is the recoil of a charged particle in a scattering event, measured as recoil energy and angle, and it is the identical momentum-conservation construct as a gun's kick, realised at the scale of a single photon.[2]

Unit

The recoil energy and angle of the struck electron (with the photon's wavelength shift)

Recovery means

Not applicable

Measured by

Inferred from the scattered photon's wavelength shift and the recoil electron's energy and direction

In astroparticle physics, nuclear recoil is the small kick an atomic nucleus receives when an unseen particle such as a dark-matter candidate or a neutron scatters elastically from it, depositing a few keV of recoil energy that a sensitive detector tries to register. The same conservation of momentum that governs a gun or a Compton electron sets the recoil-energy spectrum expected from a given particle mass. Goodman and Witten's 1985 Detectability of certain dark-matter candidates showed that such nuclear recoils could be the signature of dark matter in a detector. The referent is the recoil of a target nucleus from an elastic collision, measured as keV-scale recoil energy, the same momentum-transfer construct as a firearm's kick at atomic scale.[3]

Unit

The recoil energy of the target nucleus, on the order of keV

Recovery means

Not applicable

Measured by

Detected as keV-scale recoil energy via ionisation, scintillation, or phonons in direct-detection experiments