In nature, there are four different types of fundamental interactions or forces: gravitational, electromagnetic, weak and strong interactions. Gravitational interaction is the weakest of all the fundamental interactions and acts between all bodies having mass and is described by the long range inverse square type Newtonian law of gravitation. This interaction is believed to be mediated through the quantum of interaction -graviton – which is yet to be discovered and provides a large attractive force between the planets and other heavenly bodies. The force that binds our Milky Way galaxy and any other galaxy together is the same as that which holds Venus (or any other planet) in orbit and you to earth – gravitational force. This force is also responsible for the formation of the black hole where not even light (or photon) can escape. The riddle of how black holes and neutron stars are formed, supernovae explosions and many such other problems may be solved easily if we can completely ‘identify’ gravitational interaction.

Electromagnetic interaction is the unification of electric and magnetic fields. It is much stronger (1036 times) than gravitational interaction and is described by long-range inverse square type law: Coulomb’s law. This force arises due to charges of particles and their motion. Electromagnetic force is mediated through the exchange of photons which are massless particles. This field is described by quantum electrodynamics (QED) and is responsible for atomic structure, chemical reactions and other electromagnetic phenomena. This force may be attractive or repulsive.

The third fundamental interaction is the weak nuclear interaction and the strength of weak interaction is 10-10 times that of the electromagnetic interaction. The weak nuclear force is the second weakest force, after gravity. The beta decay of radioactive nuclei, decays of strange particles and nuclear fusion in the sun are some examples of weak interaction. Unlike the previous two interactions, weak interaction is a very short range force and is mediated through massive W± and Z0 bosons. In fact, weak interaction takes place only at very small, subatomic distances. Weak force is not symmetrical under parity transformations. The theory that describes the weak interaction is called quantum flavourdynamics (QFD).

Strong interaction is the strongest interaction in nature and occurs between a neutron and a neutron or a neutron and a proton or a proton and a proton. This is a short range, charge independent and attractive force. This force is mediated through the exchange of gluons which are massless. In true sense, quark-quark (quarks are the ultimate building blocks of matter) interaction is believed to be mediated through the exchange of gluons. This force is responsible for binding together the fundamental particles (quarks) of matter to form larger particles (neutron or proton). Strong interaction is described by quantum chromodynamics (QCD) and so strong force is also called color force.

Physicists hope that a “grand unified theory” (GUT) will unify the strong, weak, and electromagnetic interactions. There have been several proposed unified theories (Standard Model, String Theory, SO (10) etc.) but we need data to pick which, if any, of these theories describes nature correctly. The discovery of Higgs boson and other such recent discoveries will get physicists closer to knowing which GUT is correct.

The above three forces are responsible for all of the pushes and pulls in the universe. If gravity is also combined with these three forces, then GUT will become the proposed “theory of everything” (TOE). GUT is thus an intermediate step towards TOE. If a grand unification of all the interactions is possible, then all the interactions we observe are all different aspects of the same unified interaction. Current data and theory suggest that these varied forces merge into one single force when the particles are at ultra high energy (1016 GeV, 1GeV=109 eV; 1eV = energy needed to bring an electron through a potential difference of 1 Volt). This energy/temperature prevailed only at the time of Big Bang explosion. The problem is that the unification energy is so high that we cannot conceive how to build a particle accelerator to achieve this energy.

The standard model of particle physics is the theory describing three of the four known fundamental forces (electromagnetic, weak and strong interactions except gravitational force) in the universe, as well as classifies all known elementary particles into two groups – fermions and bosons. Standard model was developed in stages throughout the latter half of the 20th century, through the works of many scientists around the world, with the current formulation being finalised in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, confirmation of top quark (1995), the tau neutrino (2000) and the Higgs boson (2012) have added further credence to the standard model. Although the standard model is believed to be theoretically self-consistent and has demonstrated huge successes in providing experimental predictions, it leaves some phenomena unexplained and falls short of being a complete theory of fundamental interactions. It does not incorporate the theory of gravitation as described by Albert Einstein’s general relativity or account for the accelerating expansion of the universe as described by dark energy.

The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It does not incorporate neutrino oscillations and their non-zero masses. It is very interesting to note that 96% of our universe is missing or invisible (72% is dark energy and 24% is dark matter). The theory of super symmetry (SUSY) suggests that all known particles have, as yet detected, ‘super partners’ and these super symmetric particles may help explain one mystery of the universe – missing matter and energy.

According to some physicists the problem of grand unification may be tackled with string theory which basically replaces all the elementary point like particles that form matter and interactions with a single extended object of vanishing length. The length scale should be comparable to the Plank length (10-15 metre). Accordingly, every known elementary particle, such as an electron, photon, neutrino, quark etc corresponds to a particular vibration mode of the string. Since string theory incorporates all of the fundamental interactions, including gravity, many physicists hope that it may fully describe our universe, making it a theory of everything In fact, string theory is a promising candidate for quantum gravity also. The current quest for a unified field theory is mainly focused on superstring theory.

Many physicists believe that the universe has more dimensions than the four (three space and one time) we are aware of. The introduction of extra dimension may probably solve the problem of unification, but how many dimensions would be needed to unify modern particle physics with gravity is the current topic of modern research for particle physicists. Large Hadron Collider (LHC) experiment may allow us to see evidence of these extra dimensions. One explanation for gravitational force to be so many orders of magnitude weaker than the other three forces may be that our universe is part of a multidimensional reality and that gravity can leak into other dimensions, making it appear weaker.

For over a century, unified field theory remains one of the major unsolved problems in physics. However, physicists want to find a single theory that describes the entire universe, but to do so they have to solve some of the hardest problems in physics. For decades, confident physicists have said that unification is just around the corner. The discovery of extra dimensions would herald the first change in our view of spacetime since Einstein’s theory of relativity.

It may also solve some longstanding problems in physics and astrophysics. Solving these problems would be a fitting tribute to Einstein, the first superstar in physics who actually started this revolution.

(The writer is associate professor and head of the department of physics, Netaji Mahavidyalaya, Hooghly)