In physics, force is a word that we have all heard about. Well, whenever you push or pull something, you exert a force on it. The fundamental forces in physics are also known as the Fundamental forces of nature i.e the most basic forces of the universe.
In other words, every single interaction taking place within the universe can be reduced to the natural forces of nature. Generally, there are four fundamental forces known to exist in nature.
In fact, each of these known forces of the universe can be described mathematically as a field. I mean, Without these fundamental forces, everything within this universe would fall apart.
The 4 fundamental forces of nature are as follow
- Gravitational Force
- Electromagnetic Force
- Weak Nuclear Force
- Strong Nuclear Force
In fact, depending on their range and strength; these universal forces can also be bifurcated into two groups.
- Gravitational Force and Electromagnetic force, both of them are significantly a long-range force whose strength can be exactly determined in our everyday life.
- Strong nuclear force and weak nuclear force; which acts as quantum forces; are significantly a short-range force whose strength cannot be exactly determined in our everyday life.
As a matter of fact, there is a consensus going on among some physicists for the existence of the fifth force. But, it is neither widely accepted, nor proven. Simply, because there is no practical evidence to support the claim.
What is gravity anyway? When two bodies attract each other by the virtue of their masses separated by a distance. A force of gravity is observed.
Gravitational force is the weakest fundamental interaction force among all the universal forces. On the other hand, it has the farthest reach among all forces of the universe.
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Not to mention, One of the most important information about gravity is that the force due to gravity is always attractive. Meaning, gravitational forces never repel. (And, frankly speaking, we still don’t know why).
Gravity is the only fundamental interaction that acts on all particles having energy, mass, and momentum. In fact, it keeps all planets in their orbit around the sun and the moon in orbit around the Earth.
Theory Of Gravity Before Scientific Revolution
Gravity is the first force among all the fundamental forces to be described mathematically. Classically speaking, Aristotle was the first individual to visualize the physics of gravity. According to Aristotelian physics, objects of different masses fall at different rates.
But during the scientific revolution, Galileo Galilei corrected Aristotle theory of motion. He experimentally demonstrated that all objects accelerate toward the Earth at the same rate. He did that by simply neglecting the frictional force due to air resistance.
Theory Of Gravity After Scientific Revolution
Inspired by Galileo, Issac Newton derived his Newton’s Laws Of Universal Gravitation which enabled mankind to reach the Moon. In fact, we can even send robotic probes to the outer space of the solar system and beyond.
Newton’s Laws Of Universal Gravitation dominated the scientific world for almost four centuries until Einstein’s General Theory Of Relativity completely replaced Newtonian View from the heart of all physicists.
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Theory Of Gravity After General Relativity
The sole reason for the demise of Newton’s laws of universal gravitation was that it was not able to explain the unusual orbit of Mercury. On the other hand, Einstein discovered that Newton’s laws of universal gravitation don’t apply to object at higher speed or higher gravity.
According to Einstein theory of gravity, the power of gravity is nothing but a distortion in space-time caused by mass. In Einsteinian relativity, Newton’s Laws Of Universal Gravitation were a special case of Relativity. Therefore, he even derived Newton’s laws of motion form his Theory Of Relativity.
When we are considering massive objects like stars, planets, or galaxies; gravitational force appears to be the most powerful among all the fundamental forces of the universe.
On the other hand, when we apply gravitational force to the sub-atomic level, it appears to be the weakest among all the universal forces.
Above and over the realm of Gravitational force, is an Electromagnetic Force. Well, what is electromagnetism?
Electricity + Magnetism = Electromagnetism
If we go deep down the valley of the scientific timeline (backward), electricity and magnetism were the two different aspects of science.
I mean, the definition of electricity was associated with the presence of an electric charge. On the other hand, the definition of Magnetism was mediated by the presence of a magnetic field.
Therefore, both phenomena were considered separate from each other until the arrival of Maxwell Equations. Maxwell Equations reconciles both different branches of physics into a single entity i.e Electromagnetism.
Electromagnetism Before Maxwell Equations
Not to mention, the electric phenomenon was known for ages. In the year 1785, French physicist Charles-Augustin de Coulomb was the first one to describe the electric effect or electrostatic force.
This law states that a force is directly proportional to the magnitude of the charged particles. And, inversely proportional to the square of the distance between them (same as gravitational force).
In today’s world, this law is commonly known as Coulomb Law. According to coulomb law, charged particles at rest interact through a force known as Electrostatic Force or Coulomb Force. Classically, an electric field is described as an electric potential and electric current.
After the discovery made by Charles-Augustin de Coulomb, in the year 1819, a Danish physicist Hans Christian Ørsted discovered that electric current and magnetic field are interrelated to each other. He demonstrated that both are different manifestations of the same phenomenon.
In other words, he discovered that an electric current creates magnetism. In fact, this was for the first time in the history of the scientific world, that a connection between electricity and magnetism had been observed.
Therefore, Ørsted’s experiment confirmed the relationship between electricity and magnetism.
After Hans Christian Ørsted in the year 1831, an English physicist Michael Faraday further progressed the results discovered by Ørsted. Faraday showed that magnetic fields are associated with electromagnetic induction. Therefore, magnetism could be used to generate electricity.
Electromagnetism After Maxwell Equations
And finally, a Scottish mathematical physicist James Clerk Maxwell summarized the work of Faraday and others to formulate the classical theory of electromagnetism under Maxwell’s equations.
In fact, for the first time, Maxwell’s equations bring together electricity and magnetism (mathematically). With that, maxwell equations explained light as a manifestation of a single phenomenon.
Maxwell Equations are also dubbed as the second great unification in physics. Because the first one was realized by the great Sir Issac Newton.
Strong Nuclear Force
Above and over electromagnetism and gravitational force, there is an altogether different force operating within the nucleus of an atom i.e Strong Nuclear Force.
Soon after the discovery of the atomic nucleus, the whole scientific world was uncertain that how the atomic nucleus is bounded together.
We know that nucleus of an atom is composed of protons and neutrons. As protons are positively charged particles and neutrons having no charge on it.
For example; from a helium atom, If both the electrons are removed, what we get is a bare nucleus of helium. A helium atom without electrons becomes an alpha particle.
An alpha particle is a stable particle. Once it is created, it will remain stable until it does not interact with other particles. An alpha particle contains two protons and two neutrons.
Strong Nuclear Force Before Quantum Physics
Coulomb law states that opposite charges attract each other. On the other hand, like charges repel each other, whether a proton-proton pair or electron-electron pair.
Therefore, as a consequence of classical understanding, protons should repel each other due to the Coulomb force. As a result, it should try to break the nucleus of an atom. However, this was never observed during experiments.
With this in mind, a consensus started to build within the scientific world for the era of new physics.
Strong Nuclear Force After Quantum Physics
Well, what is the strong nuclear force anyway? It is one of the forces in an atom that overcomes the Coulombic repulsion. Therefore, prevents the nucleus from breaking apart.
In the year 1935, a Japanese theoretical physicist Hideki (né Ogawa) Yukawa hypothesized the existence of this quantum force. He argued that protons and neutrons are made up of smaller particles called quarks.
In fact, for his hypothesis about strong nuclear force, he won a noble prize in physics. Therefore, became the first Japanese Nobel laureate.
The strong nuclear force held quarks together. And, in return, quarks held the nucleus together. These nuclear forces are generally attractive but are short-ranged.
On one hand, Strong nuclear forces are much weaker than coulomb force if the separation between the particles is more than 10−14 meters.
On the other hand, if the separation between the particles is less than say 10−15 meters as 1 femtometer (1 fm = 10−15 meters). Then, the nuclear force is 137 times stronger than the coulomb force and about 1038 times stronger than that of gravitational force.
Weak Nuclear Force
In the course of investigating the strong nuclear force, yet another kind of quantum force was encountered in reactions involving electrons, protons, and neutrons.
In beta emission, a neutron can change itself into protons by ejecting an electron and a particle called anti-neutrino. This type of beta emission is called beta negative decay.
In fact, there is another kind of beta emission where a proton can change itself into a neutron by ejecting a positron and a particle called a neutrino. This type of beta emission is called beta positive decay.
Weak Nuclear Force is sometimes also called Weak Force or Weak Interaction. In the study of particle physics, the weak force is a very powerful force that acts at a sub-atomic level and causes radioactivity decay.
The range of weak force or weak interaction is very small. I mean, even smaller than the diameter of a proton. The weak force is responsible for nuclear fusion in the Sun and all the stars. Because this type of quantum forces allows the hydrogen isotope (deuterium) to form as well as fuse to make stars shine.
How Forces Interact With Each Other
Well, according to the standard model of particle physics, each of the fundamental forces should interact with other forces via an elementary particle.
Not to mention, there is an exception for gravitational forces. Because gravitational force is thought to be conveyed via a hypothetical elementary particle called gravitons. But to date, physicists are unable to find it.
On the other hand, other fundamental interactions like the strong nuclear force or strong interaction interact with others via a particle called the gluon.
Gluons are responsible for binding the quarks together. So that quarks can prevent the nucleus of an atom from breaking apart.
W and Z Bosons
The weak interactions or Weak nuclear forces interact via particles called W and Z bosons. Weak forces play an essential role in a nuclear fission reaction.
The electromagnetic forces or electromagnetism interacts via a particle called photon which is responsible for the creation of electric as well as the magnetic field.
Photons also create electromagnetic waves including visible light. This is the reason that light is known as an electromagnetic wave.
Relative Strength Of Fundamental Forces
Well, If someone asks you which force is the strongest of them all, without even thinking your answer should be the strong nuclear force.
Although, Strong Nuclear forces are of short-range. Yet, they are largely attractive. Hence, the strongest among them all. Again, if someone asks you which force is the weakest among all, then go for the weak nuclear force.
So, now you can imagine how strong it should be. The range of a weak nuclear force is even smaller than strong interactions. In fact, much smaller than the size of a proton or a neutron.
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Now the very important one, the electromagnetic force or magnetism is 0.7 percent as strong as the strong nuclear forces.
In fact, Electromagnetic forces have an infinite range (but not more than gravitational interaction). Because according to Einstein’s special relativity; photons travel at the speed of light.
Finally, gravity; the gravitational forces are about 6 x 1029 times stronger that of the strong nuclear force. Although, the gravitational force is the weakest force among all. Yet, it has an infinite range.
Unifying The Fundamental Forces
There was a time when all the physicists thought that electricity and magnetism were separate entities. But, the work of Coulomb, Oersted, Faraday, Ampere, Maxwell and so many others showed that they are actually related to each other.
Therefore, nowadays many physicists believe to unify all the fundamental interactions into one single force or entity. Just like electricity and magnetism were unified into electromagnetism.
In fact, Electromagnetic interaction with weak interaction has already been unified into electroweak interaction by Sheldon Glashow, Abdus Salam, and Steven Weinberg for which they received the 1979 Nobel Prize in physics.
Now, physicists all over the world are trying to relate all fundamental interactions and subatomic particles into a single entity called a grand unified theory.
There is also an effort going on within the current quantum mechanical interpretation that all the particles responsible for fundamental forces do not interact directly. But, they rather manifest virtual particles, that plays the role of mediator to the actual interactions.
Currently, all the other forces have been consolidated into the standard model of particle physics except for gravity. Because the hypothetical elementary particle (graviton) has not been detected yet.
As a result, the effort to unify gravity with other three interaction in one unified theory named quantum gravity have not been successfully adopted or hypothesized.