Intuitively, we are all familiar with the concept of force due to our experiences in everyday life. There are many kinds of forces seen in the macroscopic domain, such as gravitational force, friction, viscous force, elastic force, and much more. Similarly, in the microscopic domain, such as electric force, magnetic force, nuclear force, and so on.
In the twentieth century, it was discovered that many of these forces arise from only a small number of fundamental forces in nature.
For example, the net attraction or repulsion between neighboring atoms in a spring when it is compressed or elongated gives rise to the elastic spring force. The aforementioned attraction or repulsion takes place because of the unbalanced sum of electric forces between the charged constituents of the atom.
As you can see, this implies that the laws for derived forces are dependent on the laws of fundamental forces in nature.
The fundamental forces (or fundamental interactions) of physics are the ways that individual particles interact with each other. It turns out that for every single interaction that we’ve observed take place in the universe, they can be broken down to be described by only four (well, generally four – more on that later) types of interactions:
- Gravity/Gravitational Force
- Weak Interaction (or Weak Nuclear Force)
- Strong Interaction (or Strong Nuclear Force)
As of now, we know of four fundamental forces in nature, which have been discussed in detail below.
This is a universal force of mutual attraction between any two objects by virtue of their masses.
Each and every object experiences this force due to every other object in the universe. For example, all objects on earth experience the force of gravity due to the earth.
Gravity is a long-range force and does not need any intervening medium. It is the weakest force in nature compared to other fundamental forces.
Gravity also plays an important role in the motion of satellites around the earth, the motion of the planets around the sun, and the formation of and evolution of galaxies, stars, and galactic clusters.
Electromagnetic force is defined as the force acting between charged particles.
When charged particles are at rest, the force between them is known as static electric force and is given by Coulomb’s law (attractive for unlike charges and repulsive for like charges).
Charges in motion produce magnetic effects, and the magnetic field gives rise to a force on the moving charge.
Since the combined effect of the electric and magnetic fields is generally not separable, the combined effect of this force is called electromagnetic force.
Similar to the gravitational force, the electromagnetic force is a long-range force and does not need any intervening medium.
Electromagnetic force is much stronger than the gravitational force. For example, the electric force between two stationary protons is 1036 times the gravitational force between them, for any fixed distance. Thus, it dominates all phenomena at atomic and molecular scales.
Strong nuclear force
You already know that an atom’s nucleus is comprised of protons and neutrons. Protons are positively charged particles, whereas neutrons don’t have any charge.
You can clearly foresee that in the absence of a strong attractive force, the nucleus will be highly unstable because of the electric repulsion between its protons.
We’ve already discussed that gravity is negligible compared to the electric force; thus, the attractive force in question cannot be gravitational.
Strong nuclear force is a third different fundamental force that binds protons and neutrons in an atom’s nucleus.
It is around 100 times stronger than the electromagnetic force and, being restricted to the nucleus, is a short-range (10-15 m) force. It is the strongest of all fundamental forces
It is independent of charge and acts equally between two protons, two neutrons, and a neutron and a proton.
Electrons, being outside the nucleus, do not experience this force.
It is now known that protons and neutrons are comprised of even more elementary constituents known as quarks.
Weak nuclear force
The weak nuclear force is only seen in a handful of nuclear processes like the β-decay of a radioactive nucleus. In this phenomenon, the nucleus emits an electron and an uncharged particle known as the neutrino.
The interaction of a neutrino with other particles gives rise to a weak nuclear force.
The weak nuclear force is stronger than the gravitational force but much weaker than the electromagnetic and strong nuclear forces.
Its range is extremely small, of the order of 10-16 m.
Unifying the Fundamental Forces
Unification is one of the basic quests in physics. For several years, physicists have been wondering whether all the fundamental forces we’ve discussed earlier arise from a single force.
Eminent scientists like Newton, Maxwell, Einstein, Faraday, and Oersted have contributed to this venture.
Many physicists believe that all four of the fundamental forces are, in fact, the manifestations of a single underlying (or unified) force which has yet to be discovered. Just as electricity, magnetism, and the weak force were unified into the electroweak interaction, they work to unify all of the fundamental forces.
The current quantum mechanical interpretation of these forces is that the particles do not interact directly, but rather manifest virtual particles that mediate the actual interactions. All of the forces except for gravity have been consolidated into this “Standard Model” of interaction.
The effort to unify gravity with the other three fundamental forces is called quantum gravity. It postulates the existence of a virtual particle called the graviton, which would be the mediating element in gravity interactions. To date, gravitons have not been detected and no theories of quantum gravity have been successful or universally adopted.