The Four Fundamental Forces

Why does Earth stay in orbit around the Sun? How does light travel? What holds atoms and nuclei together?

The universe is governed by four fundamental forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Each force has distinct characteristics, ranges, and roles in shaping the structure and behavior of matter.

1.   Gravitational force

·       Gravitational force is the fundamental attractive force that exists between any two objects possessing mass (Classical Physics).

The formula for the Newton’s law of Gravitation

 

 

 

That means, the gravitation force is directly proportional to the product of the masses involved and inversely proportional to the square of the distance between them.

Its strength approximately increases with the masses of the two objects but decreases with the

square of the distance between them.

But according to Einstein’s General Relativity, gravity is the curvature of spacetime caused by mass-energy and momentum. It affects all particles and objects, whether they have mass (like protons and dark matter) or are massless (like photons). Both visible matter and dark matter are subject to gravitational effects.

 

 

2.   Electromagnetic Force

The electromagnetic force is a fundamental force that arises from the interaction between electrically charged particles, combining the forces of electricity and magnetism.

It can be both attractive (in case of opposite charges/polarity) or repulsive (in case of same charges/polarity).

·    When the charges are stationary the force acting is purely Electric Force. (described by

Coulomb’s Law)

·    When the charges start moving, they also produce a magnetic field in the space around them. (Right-Hand Thumb Rule by James Maxwell)

If you hold a current-carrying wire in your right hand with your thumb pointing in the direction

of current, then the curl of your fingers shows the direction of the magnetic field around the wire.

These oscillating or changing electric and magnetic field will generate electromagnetic radiations which will travel as waves (photons in quantum physics) at the speed of light (3 X 10⁸ m/s) in a direction perpendicular to both the electric and magnetic fields.

Gravity impacts photons, by bending their path and changing their energy. This occurs because massive objects bend the fabric of spacetime, and photons, like all other objects, follow the curved paths within this twisted spacetime. A key example is the gravitational lensing of starlight as it passes near a massive object like the sun.

 

 

3.   Strong Nuclear Force

The strong force is the most powerful fundamental force, acting to bind quarks into protons and neutrons and to hold atomic nuclei

together. It is the strongest force in nature, overcoming the electromagnetic repulsion between positively charged protons. This force operates over very short distances, with gluons as

its carriers, and its strength increases when its constituents are pulled apart.

This force is responsible for the immense energy released in nuclear fusion in stars and nuclear fission in power plants.

Its range is limited to extremely small distances (on the order of 1 to 3 femtometers (10⁻¹⁵ meters)), roughly the size of an atomic nucleus.

The strong nuclear force is primarily attractive, holding atomic nuclei together by binding protons and neutrons. However, it can become repulsive at very short distances (less than about 0.7 fm), which prevents the nucleons from collapsing into a point and gives the nucleus a stable structure.

The quantum particle gluon is massless and has no electric charge. But gluons carry color charge themselves, they are unique among force carriers (unlike the electrically neutral photon). This means gluons can interact with other gluons, a phenomenon called gluon self-interaction, which is responsible for the extreme strength and short-range nature of the strong force and the phenomenon of color confinement (the reason quarks and gluons can't be observed in isolation).

 

 

4.   Weak Nuclear Force

The weak nuclear force is one of nature's four fundamental forces and is responsible for beta decay and other radioactive processes, where subatomic particles transform into different types of particles. It operates at incredibly short, subatomic distances, changing a neutron into a proton (and emitting an electron and antineutrino) or a proton into a neutron (and emitting a positron and neutrino). Unlike the strong nuclear force, the weak force does not bind particles together; instead, it helps these particle transformations, playing a vital role

in processes like the Sun's energy production and the changing of elements.

The weak nuclear force has a very short effective range, on the order of 10^-17 to 10^-16 meters (or 0.1 to 0.01 femtometers). This extremely limited range is due to the high masses of the W and Z bosons, which are the particles that mediate the weak force.

·       The weak force's effective range is tiny, making it a "short-range" force.

·       The strength of the weak interaction decreases rapidly, exponentially, with increasing distance.

 

 

Conclusion: These four fundamental forces work together to shape our universe at every scale. Gravity governs the motion of planets, stars, and galaxies across vast cosmic distances.

Electromagnetism powers the interactions between atoms and molecules, giving rise to chemistry, light, and all electromagnetic phenomena we experience daily. The strong nuclear force binds quarks and nucleons, providing the stability of matter and fueling the stars. The weak nuclear force enables radioactive decay and particle transformations essential for stellar nucleosynthesis and the creation of heavier elements.

Understanding these forces has been one of humanity's greatest scientific achievements, yet mysteries remain. Physicists continue working toward a "Theory of Everything" that would unify all four forces into a single framework, particularly seeking to reconcile gravity with quantum mechanics. The quest to understand these fundamental forces deepens our knowledge of reality itself—from the smallest subatomic particles to the largest structures in the cosmos.

Aditya Kumar Ghosh (Batch 33)