In the last article, we have understood the concept of quantum physics, and how it explains the laws of the universe. In this article let us learn the theory of relativity which brought a massive revolution in modern physics.

Before that we need to know how and who came with this idea. Albert Einstein came up with the ideas of theory of relativity. One day when he was working as a patent clerk at Bern, finished his work and boarded a tramp car on his way home. He used to finish his work as soon possible so that he can contemplate the truths of the the universe. In his free time, he used to think of different experiments for the better understanding of the universe.

That day, he devised one experiment on that tramp car which changed modern physics forever. While travelling in the tramp car, Einstein saw a clock tower, and he is moving away from it. He imagined what would happen if the tramp car would move with the speed of light? The speed of light is 186,000 miles per second; he realised if he travelled at that speed, the clock’s hands would appear to be completely frozen. At the same time, he knew that back the clock tower the hands would tick along with normal pace. Time has slowed for Einstein.

This thought blew his mind; he concluded that the faster you move in space, the slower you move through time. His work was heavily influenced by two of the great physicists of all time. First, the “laws of motion by” “Sir Isaac Newton” and second by the “laws of electromagnetism” by “James Clerk Maxwell.” According to the laws of motion, velocities are always relative. For example, imagine a train travelling at 50 kilometres per hour. The speed of the train is 50 Kilometers per hour for someone at rest. It is only 30 kilometres per hour if another train is travelling with respect to this train and in same direction with a speed of 20 kilometers per hour.

This is true for the earth, sun, Milky Way galaxy and other galaxies. According to Maxwell, the speed of an electromagnetic wave(in this case light) is constant, regardless of the observer. If we take Newton’s laws into this, there is a clash, according to newton the velocity and speed are relative but the speed of light is constant regardless of the observer. Thus Einstein proposed his “Special Relativity theory” in 1905.

Special Relativity Theory: There are two postulates of special relativity.

1. The laws of physics take the same form in all inertial frames of reference.
2. As measured in any inertial frame of reference, light is always propagated in space with a definite velocity “c” that is independent of the state of motion of the emitting body. Or: the speed of light in free space has the same value “c” in all inertial frames of reference.

Pictorial representation of Special relativity.


The first postulate says that the laws of physics are the same for the person who is in moving in constant velocity or at rest. If we apply any laws of physics the results will be the same. The second postulate is very explanatory.

Mass–energy equivalence: Mass-energy equivalence states that the total mass of a system may change, although the total energy and momentum remain constant. In simple words,  mass can be converted into energy and visa-versa.

E=mc2

Where E = Energy.
M = Mass.
C = Speed of Light.

Applications of special relativity: There are many applications of some of the important ones are GPS (Global Positioning System), Cathode-ray tubes (Old Television screens), modern computer chips and most important of all, by the concept of “Energy – Mass equivalence” was essential for the development of “nuclear fission” and “nuclear fusion” which are used in all nuclear plants.

So we understood the concept of special relativity, now let us understand the concept of general relativity. Before that, we need to understand the back story of the same.

Again back to Einstein and his brilliant imagination. One day he was observing a window washer on a ladder from his office. He imagined what would happen if the washer fell, for most of us it is a horror story which will never end will. But Einstein thought it differently, he imagined himself in the place of the washer while falling. He didn’t imagine what would happen if he hits the ground but imagined what he would experience while falling.

He realised that gravity was the only force acting on him while he was falling and accelerating towards the ground. Without the wind resistance, he would be free in fall, which is no different from being weightless as the ground is not coming up. This is nothing, but he is falling in space where there is zero gravity. He wanted to connect gravity to his theory of relativity through acceleration.

Einstein imagined a room without windows and it has a weighing scale and checked his weight, it would be constant at any point on the earth. Now he imagined the room is in a spaceship, which is moving upward with a speed of 9.8 meters per second which the speed of acceleration due to gravity on earth. Now, what would happen if he stepped on the scale? The weight should be the same as the space ship is accelerating upwards with the same speed that of acceleration due to gravity but this time it is upwards.

Einstein thought about telling the difference between acceleration and gravity, now he imagined shooting a light or laser beam from one point of the room to the other while the spaceship is moving upwards. He saw that the light is bent while moving from one point to another. The size of the light would be lower compared to the projection point. This is because the room is moving upwards with a speed of 9.8 meters per second. The light beam would appear a bit curved downward.

However if you are on earth, you wont see this phenomena. The projection point and the projected points height would be the same. Einstein thought why is this different as it would the equivalence of gravity. The force is constant while it is upwards or downwards. He thought light must bend under the influence of a gravitational field, but how could this be possible as the light always takes the shortest path. May be the shortest point between two points is a curve?

Imagine earth which is spherical in shape, the surface if the earth is never a straight line, it is always a curve. So may be the gravitation causes the curvature of space somehow. He hypothesized may be the straight line is not the shortest path in space, with the presence of mass and energy space somehow becomes curved. This is again different from Newton’s hypothesis in which space and time are fixed. In Newton’s model, gravity acted within space and time.

Einstein’s theory was gravity emerges when there is an interaction between space and time. This is General Relativity in a nutshell.

General Relativity: The central idea of general relativity is that space and time are two aspects of spacetime. Spacetime is curved when there is matter, energy, and momentum resulting in what we perceive as gravity.

Pictorial representation of general relativity.

Spacetime: Space-time is a mathematical model that joins space and time into a single idea called a continuum.

Pictorial representation of spacetime.

Applications of general relativity: It can explain why planets revolve around in their orbits, why light bends at blackholes, correcting high precision clocks on satellites, gravitational lensing in astronomy, and a large variety of corrected astrophysics calculations.

In the last article, we have learnt the closest theory to the theory of everything, which is “String Theory.” In this article, let us try to understand Quantum physics.

Quantum Physics: Quantum physics is the study of matter and energy at its most fundamental level. The basic story of everything around us and including us are elementary particles. So in simple terms, quantum physics is the study of everything in the universe. It explains how everything works in the universe.

Pictorial representation of quantum physics.

Quantum physics and quantum mechanics are similar; the term “mechanics” indicates doing calculations. So how did the idea of quantum mechanics come to existence? To understand that we should know the concepts of electromagnetism. 

Electromagnetism: At the subatomic level, electromagnetism is defined as the force between electrically charged particles. It is the fundamental reason electrons bound to the nucleus and responsible for the complete structure of the atom.

Electromagnetic force: It is is a type of physical interaction that occurs between electrically charged particles. It acts between charged particles and is the combination of all magnetic and electrical forces. It can be attractive or repulsive.

Electromagnetic waves: These are the waves generated when an electric field comes in contact with the magnetic field. These are the combination of oscillating electric and magnetic fields.

In 1900, a group of bulb manufacturers came to Max Planck to know how to make their bulbs more efficient and take less electricity. He conducted a few experiments and was shocked by the results. According to classical laws of physics, the energy emitted by a body is continuous, but when he observed the energy emitted by the light bulbs, it was discrete. So the basis of all quantum physics, the “Planck’s Law” was born.

Black Body:   A blackbody is an object that absorbs all the radiation falling on it. An object that absorbs all the radiation can also emit all radiation. Therefore, a blackbody will radiate maximum energy when heated to a given temperature. The energy was not continuous, so Planck came up with the “Planks Quantum Theory” Or “Planck’s Law.”

Black body graph.

Planck’s Quantum Theory: Planks Quantum Theory says that energy of electromagnetic radiation is transferred in terms of energy bundles, these bundles are called “Quantum” or “Quanta” can also be called as “Photons.” The energy of each photon is directly proportional to the frequency of electromagnetic radiation.

It was a revolution in the field of physics, Planck got Nobel prize for the same in 1918. However, this theory was limited to the black body. In 1913 “Niels Bohr” came up with his atomic model which paved the way for quantum theory to the atomic level. According to his atomic model, energy is transferred only in specific, well-defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr’s theory could explain why atoms emitted light in fixed wavelengths. The modern shape of quantum physics was given by “Albert Einstein” with his “Photo-Electric Effect.”

Photo Electric Effect:  It is a phenomenon in which, electrons are emitted from a metal surface when light is incident on it. In physics terms, electrically charged particles are discharged from or within a material when it absorbs electromagnetic radiation. In this, Einstein used the principle of packet energy called quanta and proved the quantum physics existence on materials absorbing light.

Pictorial representation of photo electric effect.

The phenomena of Quantum Physics:

Wave-Particle Duality: Imagine subatomic particles as balls bouncing around, but sometimes we have to imagine them as spread-out waves and now consider them doing both, yes it is a bit hard to imagine. Try painting this picture. You took a subatomic particle and increased its sizes to that of a stone. You have thrown that into a small water body. What do we observe? Ripples on the surface, which are nothing but waves. Now Imagine one of the waves has hit a stick on the way, and all the waves disappear. Now your stone sized subatomic particle pops up from that the point of the stick. It is strange, but yeah that is how subatomic particles exhibit the properties of a wave and a particle. 

Pictorial representation of wave-particle duality.

Quantum Tunneling: Now imagine you have a football and kicking it against the wall, it keeps coming back, and you continue this for some time. Suddenly, the football without smashing or interacting with the wall passed through the wall and came out on the other side and moving away. Yes, this crazy! But this is what happens at the subatomic level. Quantum tunnelling is an important reason why we exist. Inside the sun as we know emits light by nuclear fusion in which two hydrogen atoms come together, and the protons in their nucleus will bounce off each other. If there were no quantum tunnelling, even the nuclei and the hydrogen atoms would bounce of each other, and there would be nuclear fusion. For fusion to happen, they quantum tunnel through each other, giving us the sunlight and without sunlight, there is no existence of life form. 

Pictorial representation of quantum tunneling.

Superposition: Imagine a ballet dancer doing a pirouette, yeah it is lovely same is quantum physics, now imagine a ballet dancer doing an anti-clock pirouette, yes it is hard now comes the fun part. Imagine a ballet dancer doing the clockwise and anti-clockwise pirouettes at the same time. Yes, it is tough to imagine, but that is the concept of superposition. When we got to get a scan under the MRI machine, the machine turns the hydrogen atoms in our body in both ways so that the doctors can see inside the body. 

Pictorial Representation of Super position.

It is tough to imagine these like two spins at the same time, tunnelling etc., But we can describe it properly using mathematics. The applications of quantum physics are very vast and very useful. As said above the MRI machine is one of them. By understanding the properties of silicon, we designed silicon chips which are the building blocks of computers and its technology. Not only these but they helped us build x rays, lasers, holography, the modern-day periodic table, fluorescence materials and many more important technologies that we see today. 

In short, without the understanding and usage of quantum physics, we would never be living this lifestyle, and we would be backward by at least 100 years. 

We all are curious in one way or the other in understanding the mysteries of the universe. All have different curiosities from feeling the beauty of the universe, getting intrigued by the complexity of it to loving the elegance of it. Let us try to understand the same in simple terms.

If we need to understand something, we need to go to the base level of it and come up by understanding all the necessary information. For example, to understand and read a sentence in any language, we need to learn the alphabets, their sounds, punctuations so and so forth.

In the realm of science, the branch that deals with matter, its motion and behaviour through space and time is “physics.” The main goal of physics is to understand how the universe works. So, we will take help of physics and come up from the base level to understand the universe.

Everything around us consists of “matter”, and the definition of matter is anything that occupies space and has mass. From small sand grain to a complex structured human being everything is made of matter. Matter consists of tiny particles called “atoms”. As we all know, atoms have protons, neutrons, electrons and unstable nucleus.

It is the definition of “Subatomic particles” or “elemantary particles.” These are the basic building blocks of atoms and in turn, building blocks of everything. The particles are so small that light passes through them. For something to be visible to an eye, the light must touch them and reflect on our eye.

Elementary particles representation.

We can decrease the wavelength of the light and try to see these particles. However, as per the wave theory, when the wavelength decreases, the energy of the wave increases. So when the more energy wave touches these particles, it alters them. So we cant measure them precisely.

This fact or phenomenon is called “Heisenberg’s uncertainty principle” this is bases of all “Quantum Physics.” In rudimentary terms the Heisenberg’s uncertainty principle states, we cannot determine the position and the momentum ( in other words, speed) at the same time. If we calculate momentum, we cannot calculate the position and vice versa.

Quantum physics is the representation of the smallest things in the universe, and it describes how elementary particles work and interact with light. Quantum physics helps us with the fundamental rules of how the universe works. We do not know the nature of the subatomic particles, but we know they exist.

So how can we do science with them if we cannot see them? The physicists took the mathematical approach on them. It is the story of the “point particle.” Physicists decided that they would pretend a particle is a point in space. For example, an electron is a point with a specific electric charge, and a certain mass and all are indistinguishable with each other.

This way, physicists were able to define and calculate all their interactions mathematically. In other words, this is the definition of “Quantum field theory.” It solved many problems, and all of the standard models of particle physics stands on it. When the calculations of a few quantum properties of electrons are done, the results are accurate up to 0.000000000000.2%. They are not points, but by treating them as points, we get a better understanding of the universe.

Till now, the story is good. We got the right approach, but there is one problem “Gravity.” In quantum mechanics, all physical forces are carried by some particles. However, according to the Einsteins theory of general relativity, the gravity is not like other forces of the universe. In simple words, gravity is the geometry of spacetime itself of distances which we need to describe with absolute precision.

However, as the point articles are free in space, there is no way to measure things with exact precision in the quantum world. When the physicists tried to add gravity to this story with mathematics, the math broke down, and the calculations were very different and inaccurate. It is a massive problem as the particles should obey the laws of gravity and give the same results under gravity as well.

When we combine quantum physics with the standard model, we got the perfect theory of everything. Everything can be proven mathematically with precision. Some of the genius physicists came with a new story, they questioned what is more complicated than a point? A group or set of points or a line or a string. It is how “string theory” was born, which helps us understand everything in this universe.

String theory states that the fundamental elements of the universe are one-dimensional “strings” rather than point-like particles what we observe as particles are vibrations in loops of string, each with its characteristic frequency. In string theory, one of the vibrational states of the string gives rise to the graviton, a quantum mechanical particle that carries gravitational force. By this, we combine the quantum physics and standard model.

Pictorial Representation Of String Theory.

It is like a guitar string when pulled instead of making sounds each vibration is a different particle. There are currently ten dimensions in string theory. When we express the particles in all the ten dimensions, Einstein’s relativity equations pop out which are the theoretical proofs of gravity. So this solves the Gravity conundrum of quantum physics.

Thus string theory helps us understand the universe effortlessly and straightforwardly. String Theory contributed to many mathematical approaches which apply to a variety of problems in black hole physics, early universe cosmology, nuclear physics, and condensed matter physics, and it has stimulated several significant developments in pure mathematics.

The drawback of the string theory is that we cannot prove it experimentally as we have learnt that we cannot see the elementary particles. By mathematics, this is the closest theory to the “Theory Of Everything.” It helps us understand the complex universe with an elegant approach.

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