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Quantum mechanics, quantum electrodynamics and quantum chromodynamics deal with point particles. In quantum theory of fields the point particles are causes of grave difficulties. Quantum field theory based on special theory of relativity should satisfy three principles. It should:

I.  be Lorentz-invariant

II. be gauge-invariant

III. give finite results, which should agree with experimental data.

These three principles cannot be simultaneously satisfied. There were and there are made attempts to improve quantum field theories. In most cases of improvement of the theory two principles were not changed: Lorentz invariance and gauge-invariance. There are two main ways to improve the theory in the third point.

1. the renormalization without changing the point-like character of the particles,
2. the passage to extended particles by introducing form factors.