small momentum transfers. In the space-time picture this means that the process takes place at the large (macroscopic) distances. The longitudinal length (with respect to the direction of the initial momentum) of formation of the radiation usually is called the coherence (formation) length $l_f$. For emission of a photon with energy $\omega$ the coherence length is $l_f(\omega) \sim
\varepsilon(\varepsilon-\omega)/m^2\omega$, where $\varepsilon$ and $m$ is the energy and mass of the emitting particle ( here the system $\hbar=c=1$ is used). If the particle experiences some action in this length, the radiation pattern changes (in the case when the action is the multiple scattering of the emitting particle one observes the famous Landau-Pomeranchuk effect [@LP]).

A different situation exists in the bremsstrahlung process at the collision of electron and electron (positron) in colliding beams experiments. The point is that the external factors act differently on the radiating particle and on the recoil particle. For the radiating particle the criterion of influence of external factors is the same both at an electron scattering from a nucleus and at a collision of particles. For the recoil particle the effect turns out to be enhanced by the factor $\varepsilon^2/m^2$, which is due to the fact 