1.4 Fundamental forces in nature

What are the fundamental forces of Nature?

There are four fundamental forces in nature that govern the diverse phenomena of the macroscopic and the microscopic world. These are the ‘gravitational force’, the ‘electromagnetic force’, the ‘strong nuclear force’, and the ‘weak nuclear force’. The unification of different forces/domains in nature is a basic quest in physics.

Gravitational Force: The gravitational force is the force of mutual attraction between any two objects by virtue of their masses. It is a universal force. Every object experiences this force due to every other object in the universe. All objects on the earth, for example, experience the force of gravity due to the earth. In particular, gravity governs the motion of the moon and artificial satellites around the earth, the motion of the earth, planets around the sun, and, of course, the motion of bodies falling to the earth. It plays a key role in the large-scale phenomena of the universe, such as the formation and evolution of stars, galaxies, and galactic clusters.

Electromagnetic Force: Electromagnetic force is the force between charged particles. In the simpler case when charges are at rest, the force is given by Coulomb’s law: attractive for unlike charges and repulsive for like charges. Charges in motion produce magnetic effects and a magnetic field gives rise to a force on a moving charge. Electric and magnetic effects are, in general, inseparable – hence the name electromagnetic force. Like the gravitational force, electromagnetic force acts over large distances and does not need any intervening medium. It is enormously strong compared to gravity. The electric force between two protons, for example, is \(10^{36}\) times the gravitational force between them, for any fixed distance.

Strong Nuclear Force: The strong nuclear force binds protons and neutrons in a nucleus. It is evident that without some attractive force, a nucleus will be unstable due to the electric repulsion between its protons. This attractive force cannot be gravitational since the force of gravity is negligible compared to the electric force. A new basic force must, therefore, be invoked. The strong nuclear force is the strongest of all fundamental forces, about 100 times the electromagnetic force in strength. It is charge-independent and acts equally between a proton and a proton, a neutron and a neutron, and a proton and a neutron. Its range is, however, extremely small, of about nuclear dimensions \(\left(10^{-15} \mathrm{~m}\right)\). It is responsible for the stability of nuclei. The electron, it must be noted, does not experience this force.

Weak Nuclear Force: The weak nuclear force appears only in certain nuclear processes such as the \(\beta\)-decay of a nucleus. In \(\beta\)-decay, the nucleus emits an electron and an uncharged particle called the neutrino. The weak nuclear force is not as weak as the gravitational force, but much weaker than the strong nuclear and electromagnetic forces. The range of weak nuclear force is exceedingly small, of the order of \(10^{-16} \mathrm{~m}\).

Table 1: Fundamental forces of nature

What is Unification of Forces: Unification is a basic quest in physics. Great advances in physics often amount to the unification of different theories and domains. Newton unified terrestrial and celestial domains under common law of gravitation. The experimental discoveries of Oersted and Faraday showed that electric and magnetic phenomena are in general inseparable. Maxwell unified electromagnetism and optics with the discovery that light is an electromagnetic wave.