Standard-Model Extension (SME) is an effective field theory that contains the Standard Model, general relativity, and all possible operators that break Lorentz symmetry.
[10][11][12] In 1989, Alan Kostelecký and Stuart Samuel proved that interactions in string theories could lead to the spontaneous breaking of Lorentz symmetry.
[13] Later studies have indicated that loop-quantum gravity, non-commutative field theories, brane-world scenarios, and random dynamics models also involve the breakdown of Lorentz invariance.
[14] Interest in Lorentz violation has grown rapidly in the last decades because it can arise in these and other candidate theories for quantum gravity.
In the early 1990s, it was shown in the context of bosonic superstrings that string interactions can also spontaneously break CPT symmetry.
This work[15] suggested that experiments with kaon interferometry would be promising for seeking possible signals of CPT violation due to their high sensitivity.
The first case considered the mixing of neutral mesons, because their interferometric nature makes them highly sensitive to suppressed effects.
In 1997 and 1998, two papers by Don Colladay and Alan Kostelecký gave birth to the minimal SME in flat spacetime.
This invariance is implemented in field theories by writing a scalar lagrangian, with properly contracted spacetime indices.
Particle Lorentz breaking enters if the theory includes fixed SME background fields filling the universe.
Typical experiments seek couplings between the background fields and various particle properties such as spin, or propagation direction.
One of the key signals of Lorentz violation arises because experiments on Earth are unavoidably rotating and revolving relative to the Sun-centered frame.