Theory of fundamental interactions

In this particular article Theory of Fundamental interactions, we are going to discuss about different types of interactions. The interactions between elementary particles are classified into four fundamental interactions. They are:

  1. Strong interaction
  2. Electromagnetic interaction
  3. Weak interaction
  4. Gravitational interaction

Strong interaction

The interaction takes place in between nucleons of the nucleus. It is the strongest interaction among all the four fundamentals, but its range is very short of the order of 10⁻¹⁵ m i.e. within the nucleus. For explaining the strength and short-range properties of strong interaction Yukawa in 1935 proposed a concept that strong interaction between nucleons is generated due to the exchange of meson particles. According to Yukawa in 1935 proposed a concept that strong interaction between nucleons is generated due to the exchange of meson particles. According to Yukawa a fundamental coupling acts between two nucleons is due to the exchange od meson particles (π-meson).

Using uncertainty the principle the mass of meson particle exchanging interaction for a short range 1.4× 10⁻¹⁵ m comes out to be about 140 MeV/c² or 274 me.

According to the latest knowledge, neither nucleon nor meson are elementary particles. They are made up of quark particles, more elementary than these particles. Thus the interaction is not fundamental. Fundamental strong interaction is produced by the exchange of gluon particle between the quarks. Thus nuclear force is generated by the fundamental quark-gluon interaction.

Theory of Fundamental interactions
The strength of strong interaction is measured by the strong coupling constant f≈√h/2πc. From pion-nucleon scattering experiment, the coupling constant is found it be f Comparing this coupling constant with the coupling constants of other interactions, it is found that it is the strongest force among all other fundamental forces. Its strength is 100 times more than that of electromagnetic interaction, about 10⁵ times greater than that of weak interaction and about 10³⁹ times greater than that of gravitational interaction.
Strong interaction takes place in hadrons (baryons and mesons particles).

Electromagnet interaction:

Electromagnet interaction takes place in between charged elementary particles such as leptons (e⁻,μ⁻,τ⁻ ), mesons (π⁺,π⁻), and baryons (p,∑⁺,∑⁻). This interaction is produced due to the exchange of the photon between the charged particles. The fundamental electromagnetic coupling between electron (e⁻) and photon (γ) is shown. Electromagnetic force acts between the electrons due to the exchange of the photon. Since the range of electromagnetic force is infinite, therefore the mass of photon would be zero.


The strength of the electromagnetic force is described in terms of electromagnetic coupling constant α=ke²2π/hc = 1/137. Electromagnetic interaction is weaker in comparison to strong interaction and stronger in comparison with weak and gravitational interactions.

Weak interaction:

The weak interaction is produced due to the exchange of gauge bosons (W⁺, W⁻, Z⁰). They are the carrier quanta of weak interaction. The example of weak interaction is -decay in which neutron decays as n→ p+ e⁻+v is fundamentally the following decay.
d →u+ e⁻ +v


This decay is shown in fig. where W⁻ is the carrier quantum of weak interaction. Three carrier quanta W⁺, W⁻, Z⁰ of weak interactions is also called intermediate vector bosons. The masses of gauge boson W⁺, W⁻, Z⁰ are about 100 times heavier than the mass of the proton. It means that the range of action of weak interaction is very short i.e. of the order of 10⁻¹⁸ m.
The coupling constant g of weak interaction comes out to be about 1.4×10⁻⁶²  j-m³ by experiment. Thus Weak interaction is weaker than the strong and electromagnetic interactions. The weak interaction is found in lepton, meson and baryon decay.

Gravitational interaction:

Gravitational interaction takes place on all the particles having matter. The range of gravitational interaction is infinite and it is weakest among all the fundamental interactions, even then it is effective in massive objects such as stars, planets, etc but it is negligibly small in elementary particles. Due to this reason for interaction among elementary particles, it is not taken into account.
If we consider this force is due to the exchange of quanta, then its mass must be considered zero due to its infinite range. This carrier of gravitational force is called graviton. Its spin is s =2. Experimentally the existence of graviton has not been verified up till now.

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