fundamental  interactions  There are four types of fundamental interactions or forces in   nature,  namely  gravitational, electromagnetic (em),  weak  and strong. The last two are short range and  only  manifest  between  nuclear and sub nuclear particles at very  short  distances.  In  table FI, the characteristics of the fundamental interactions have been summarized.

Out  of  these four fundamental interactions, the electromagnetic interaction  which occurs between the charged particles is the most well understood.  Quantum electrodynamics (QED) which accurately describes e.m. interactions, envisages the photon as the mediator of  the  force  between two charged particles. To illustrate the working of this theory we take the electron magnetic moment which is predicted  by Dirac  to  be eh /2me. However measured values are slightly   higher (~ 0.001%). QED explains  this difference  using radiative  corrections  which  arise  from  interaction  of the charged particle with its own electromagnetic field. The electron emits a virtual photon to be re-absorbed later (see fig. f6b). In the next order  the emitted photon may be materialized into e± pair which annihilates  creating the virtual photon, to be reabsorbed by the electron  (fig.  f6c).  These  processes will contribute  to  the correction,  the  magnitude of which decreases as we go to higher orders.  The  theory which  has been worked by R.P.Feynman gives predictions   which   are   in excellent agreement  with  the experimental results.

Strong  forces that binds the nucleons inside a nucleus has a  very short range  and are charge independent. It is the same force  that  operates between the  hadrons* (mesons and baryons).  Since hadrons are made of quarks* which are "colored" it  is believed  that  color  plays the same role as charge. The theory   which   is being  worked   out   is   called  quantum chromodynamics.  In this theory the mediators are called gluons, which  are eight  in  number,  and  they themselves carry color charges unlike the photon.

Table FI

Fundamental interactions. The strength of e.m. interaction is taken 1 arbitrarily.

The  weak  interactions is believed to be mediated by spin 1 particles called intermediate vector bosons (0 spin and negative parity),  W+, W-  and Z₀ which are massive. The weak interaction responsible  for  b  decay,  and many  other  processes  is well described by V-A theory in which two forms of vector interactions (V  and  A)  participate. The mixing of V and A causes the parity violation,  or  the  lack  of invariance under mirror reflection, which is characteristic of weak interactions.

The  gravitational  interaction  is  the  weakest of all and least understood.  It is speculated that gravitons are mediators of this force.

An  unified theory of all interactions has eluded scientists for  many years. In  1967  a  major success was achieved by Abdus Salam, Steven Weinberg and Sheldon Glashow who described the weak and  the electromagnetic  interactions  under  one frame work of electro-weak  force described  by a family of four particles-the photon, two charged W± bosons and a neutral boson Z₀ .