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The “instant” and “light front” forms of relativistic dynamics postulated by Dirac refer to physical conditions on flat spacelike surfaces in the Minkowski space. We consider another flat surface in the Minkowski space that upon parametric rotation interpolates between “instant” and “light front” surfaces. A projection of the covariant perturbation theory on the new surface is then considered. One limit of the theory reproduces the usual instant dynamics, whereas a different limit gives a light front dynamics. Neither infinite momemtum frame limit nor infinite momenta are involved. Although the interpolating surface is the “instant” surface in the moving system of reference, we argue why our procedure should not be associated with a Lorentz transformation.

We calculate the differential cross sections at various angles for reactions 3 He(p, pp)d and 3 He(p, pd)p at 35 MeV and 45 MeV incident proton laboratory energy, solving the integral equations of four-body theory with separable, S-wave, spin-dependent two body forces.

Generalized separable potential model of Alt, Grassberger and Sandhas is applied to 4-N system with the use of Bateman method and unitary pole approximation (UPA). Our principal aim is the evaluation of cross sections for processes like p 3He → p 3He and dd → pt. Bound states for 2-, 3-, and 4-body systems turned out to be very close to the experimental data either by input or by accident. However, the good fit at the negative energy, combined with the UPA method gave us a very good agreement for scattering processes at laboratory energies 9.75, 13.8, 25.3 and 51.5 MeV.