Non-relativistic limit
From DispersiveWiki
The non-relativistic limit of a relativistic equation (which thus involves the speed of light 'c') denotes the limit when 
. It is the opposite of the vanishing dispersion limit.
Non-relativistic limit of NLKG
By inserting a parameter c (the speed of light), one can rewrite the nonlinear Klein-Gordon equation as
One can then ask for what happens in the non-relativistic limit 
(keeping the initial position fixed, and dealing with the initial velocity appropriately). In Fourier space, u should be localized near the double hyperboloid
.In the non-relativistic limit this becomes two paraboloids
and so one expects u to resolve as
where v + , v − solve some suitable NLS.
A special case arises if one assumes (ut − ic2u) to be small at time zero (say o(c) in some Sobolev norm). Then one expects v − to vanish and to get a scalar NLS. Many results of this nature exist, see Mac-p, Nj1990, Ts1984, MacNaOz-p, Na-p. In more general situations one expects v + and v − to evolve by a coupled NLS; see MasNa2002.
Heuristically, the frequency 
portion of the evolution should evolve in a Schrodinger-type manner, while the frequency 
portion of the evolution should evolve in a wave-type manner. (This is consistent with physical intuition, since frequency is proportional to momentum, and hence (in the nonrelativistic regime) to velocity).
A similar non-relativistic limit result holds for the Maxwell-Klein-Gordon system (in the Coulomb gauge), where the limiting equation is a coupled Schrodinger-Poisson system under reasonable H1 hypotheses on the initial data BecMauSb-p. The asymptotic relation between the MKG-CG fields f , A, A0 and the Schrodinger-Poisson fields u, v^+, v^- are
where 
(a variant of c2).
