Along with sentience, dexterous hand, sharp brain, what comes with a human body is an inquisitive and overly curious mind which keeps figuring out the ideas behind everything around it. The amazingly coordinated universe of ours is a source of awe for us and a fitting subject to study.

We have come a long way from Aristotle’s fundamental element hypothesis to the prevalent Standard Model in our quest to unravel the secrets of the Universe. But the line of thoughts has been pretty linear all this time.

The String Theory challenges the basic approach and presents us with a mind-blowing new perspective to see the reality. But first, let's see what makes String Theory revolutionary and so interesting.

Physicists were having a good time explaining the observable universe with Galilean laws of motion. Newton added to their arsenal gravitational force and things started making more sense.

Then came electric and magnetic forces, which were explained by Coulomb’s and Ampere’s laws. A major breakthrough was made by Maxwell when he unified the electric and magnetic forces into electromagnetism.

He even explained the carriers of the force that are photons. But we were still clueless about the gravitational force. What brings it into being?

Einstein was the first to make advancement in this direction with his theory of special and general relativity. He tried to explain gravity solely in terms of geometry and it went pretty well.

But the world of science was completely turned upside-down by Heisenberg and others when they added the quintessential chapter of quantum physics. Till this point, the classical model and electromagnetism were doing a great job, explaining the phenomenon and interactions at the macroscopic level.

Quantum Physics now enabled physicists to explain the microscopic world as well. Later on, weak and strong nuclear forces were discovered and we got our four fundamental forces.

Along the way, we discovered a lot of really small particles which seemed to make up the Universe. Molecules hold the place for the ‘fundamental particle’ for a long time.

Then it was atom’s turn. Later on, electrons, protons, and neutron grabbed that spot.

As of now, we consider bosons (like gluons, Higgs) and fermions (like quarks, leptons) to be the elementary particles. These fundamental particles and their interaction with each other illustrated the nature of reality quite precisely with gravity being the only exception.

This 2010 video by CERN before the discovery of Higgs boson nicely explains the Standard Model of Particle Physics:

## So, how do we come to contemplate the string theory?

Now, we do not want to delve deep into the mathematics behind these discoveries, but you must know that our knowledge about them comes from mere speculations and calculation. Elementary particles are so small, of the order of a Planck dimension that is 10^{-33}, that we cannot experiment or even observe them.

So, how do we play with them? Physicists solved this enigma by considering these particles as ‘point’ in our 3-dimensional world. When coupled with the fourth dimension of time, they trace a ‘worldline’.

Moreover, these points have quantum states which we call mass, charge, etc. But you cannot do much with these point particles.

This proved to be a major hurdle while formulating the interaction between different particles.

In the 50’s and 60’s, particle accelerators were producing all-new composite particles. Gabriele Veneziano came up with the String Theory to describe the pattern of mass and spins of ‘hadrons’.

However, his theoretical model of strings failed for hadrons, but it was later revived to describe all the elementary particles.

## String theory simplified

Remember our little problem with point particles being unable to account for interactions? Well, the String Theory proposed to generalize the idea of the fundamental constituent entity to 1-dimension.

What it means is that String Theory suggests that little strings of the size of the Planck length are the elementary objects, not the elementary particles. All the elementary particles can be described as strings with different quantum states.

There is a rather famous analogy of these theoretical strings to the strings of a violin. As different vibrations of a string on a violin produce different notes, in a similar way, different quantum states of a string give rise to all the possible elementary particles, be it an electron or a quark or a gluon.

On a larger scale, you do not see strings, they look like the elementary particles, which we are well familiar with.

You may ask what these strings are made up of. Well, they are not made of anything but they themselves make everything.

For example, in a Standard Model, a chair is made up of wood, which in turn comprise different molecules, which themselves are made of different atoms comprising of electrons, protons (made of quarks) and neutrons (also formed by quarks).

But what are these quarks and electrons have in their ultimate breakdown? Well, nothing; because they are the ultimate breakdown product.

We have just replaced the zero-dimensional elementary objects with a 1-dimensional elementary constituent called string.

So, how is 1-dimensional elementary object an advancement over point particles? You see, other than their quantum states, these strings also have a length.

So, these strings can have several arrangements by themselves, like they can be ‘open’ strings or ‘closed’ strings. The open strings can join one end and form a new open string, or two open strings can come together and form a closed string.

These string interactions give rise to five different string theories, which we will learn as different versions of superstring theory.

Intuitively, these strings must have some ‘tension’. This tension gives rise to different modes of vibration and brings forth all forms of elementary particles.

The most mind-boggling feature of String Theory is its extra-dimensions. We live in a 4-dimensional world with three spatial and one temporal dimension.

However, the mathematics of String Theory begins to fall apart with just 4 dimensions. It stabilizes only with a total of 10 dimensions.

Before we delve into understanding these extra dimensions, let’s look at the history of this revolutionary theory to have more clarity on the need of 10 dimensions.

## History of string theory

Let's probe into the origins of the String Theory. But first make sure you understand that unlike many popular theories which start with certain postulates and describe a system or phenomenon, String Theory is very complex.

It is a collection of several mathematical frameworks and theoretical models which though fundamental principles differ from each other drastically. The story of String Theory starts with Kaluza-Klein hypothesis which tried to unify gravitation and electromagnetism.

Kaluza-Klein, in 1921, proposed that there are four, not three, spatial dimensions. One of these dimensions is not infinitely extended but curled up on itself, a phenomenon called compactification.

Further, there is only gravitation in this 4-dimensional world and no electromagnetism. This proposition with the help of some excruciating calculations revealed that graviton (the force-carrier of gravitation), a spin-2 particle splits into a spin-1 particle in the three-dimensional world.

Here’s a beautiful explanatory video on spin which is an intrinsic property of the particles:

The proposition thus accounted for the unification of electromagnetism and gravitation. If you are uneasy with the idea of 4 spatial dimensions, think of a wire or stick which has been made so small that you cannot perceive its width.

You will just experience one dimension and others will be hidden or compactified.

Kaluza-Klein theory didn’t go viral because the concept of quantum gravity that it adopted seemed too wacky to the scientific community.

The 70’s, however, was the decade of the String Theory. Geoffrey Chew, Leonard Susskind, C. Schmid and others worked out the String Theory to explain ‘hadrons’. Gabriele Veneziano was another big contributor, who is credited with developing String Theory as we know it today.

The prevalent notions of the theory got a big push with “Yang-Mills” hypothesis. This new theory tried to unify all the fundamental forces except gravitation. This attempt was aptly termed as ‘grand unification’.

## Superstring theory

We know Superstring Theory is a fancy name and you may be thinking what new ‘beast mode’ string theory is into. Superstring Theory stands for Supersymmetric String Theory.

When you combine the idea of supersymmetry with string theory, you get a better theory, the superstring theory.

So, what’s supersymmetry?

We know that there are two kinds of fundamental particles: bosons and fermions. Bosons are integer spin particles which mediate fundamental forces and fermions are half-integer spin particles which make up the matter.

Physicists expected the bosons and fermions to be connected in some way, but the mathematics suggested otherwise. It was then that the notion of supersymmetry was brought to the scene.

Watch the video below, where Dr. Don Lincoln from Fermilab explains about the concept of supersymmetry in the easiest possible way.

Supersymmetry says that all bosons have a fermionic ‘super-partner’ and vice versa with a spin that differs by half of a unit. Along with their differences in spin, they also differ in collective properties.

Fermions like to remain in the different state, while bosons prefer to be in the same state. This is how supersymmetry brought the two types together with their differences and formed the basis of Superstring Theory.

The new theory predicted most of the known particles and a new spin-2 particle ‘graviton’, which is a candidate for gravitational force-carrier.

Over time, physicists came up with five different versions of Superstring Theory, namely Type I, Type IIA, Type IIB, Heterotic, and Heterotic with E(8) x E(8) gauge symmetry.

First superstring revolution in 1984 attracted a lot of activity in this field. In 1995, Edward Witten unified all these versions in one 11-dimensional theory, popularly called ‘M-theory’.

## Extra-dimensions of string theory

We left our discussion on extra-dimensions incomplete while talking about Superstring Theory. Let’s continue it.

The String Theory supports 10 dimensions, 3 of which extend indefinitely and are observable to us. So, where are the other 6 spatial dimensions?

They are right here but curled upon themselves, i.e. they are compactified.

The extra-dimensions are inherent in the mathematical notion of ‘manifold’. Take, for example, a relatively large sphere and place an ant over it.

The surface of the sphere will look flat to the ant. So, we have a ‘local’ shape (flat surface) and a ‘global’ shape (the sphere) here. The sphere, in this case, is an example of a 2-dimensional manifold.

It is the same with our world.

We live in a 3-dimensional manifold. Even if we do not consider the postulates of String Theory, general relativity suggests a fourth dimension, gravity, and the universal phenomenon is described with the help of curvature of this additional dimension.

Thus, we can easily conclude that a manifold may have curvature and other non-trivial properties.

Calabi-Yau manifold is a class of 6-dimensional manifold and is a subject of study in String Theory. They wonderfully predict several realistic theories in a 4-dimensional spacetime.

We have drawn heavily from the earlier discussed Kaluza-Klein idea of compactification. The extra-dimensions are just compact manifold which is too small to be detected by us.

Furthermore, M-theory extends upon string theory’s 10 dimensions and works with a total of 11 dimensions.

## Is string theory the theory of everything?

We know that the true knowledge of a field lies in the very essence of the subject of study. You won’t want a gazillion of seemingly different theories and parameters to elaborate on the nature of reality but a unified fundamental approach.

The Standard Model, while pretty successful, doesn’t hold up to this expectation. It only makes sense with scores of particles and hundreds of not-so-well defined parameters.

Most importantly, we have quantum mechanics, general relativity, quantum field theory and many more models to gain insight on the same subject that is the Universe.

Can’t they all be connected in some way? Shouldn’t quantum mechanics and gravitation come under a common framework? Yes, they should.

The Theory of Everything is our attempt to unify different theoretical models to probe the Nature. String Theory came out as the strongest candidate for such a unified approach.

It has unexpectedly predicted the quantization of gravity with gravitons. So, why isn’t it the ultimate discovery?

String Theory has been successful in explaining many complex phenomena, most importantly black holes. Black holes are very small objects with very large mass and it requires general relativity as well as quantum states to study them.

String Theory has offered new insights into quark-gluon plasma and a relationship between quantum field theory and string theory has been established, called AdS/CFT correspondence.

String Theory has produced numerous results, some of which may seem absurd or incomprehensible. For example, it predicts the existence of 10^{500} universes or a massive multiverse.

Due to this fact, String Theory has faced many setbacks in the past. But what startles the scientific community is the recurring comeback of this controversial theory.

Its solutions and results keep popping up in all sorts of the field of study. Also, String Theory has inspired many new ideas like supersymmetry which are well-established now and are used on their own.

String Theory has been the object of interest in the media and popular science as much as in the scientific community for its counter-intuitive solutions. The biggest problem with String Theory, however, is that most of its predictions and notions can’t be tested with experiments as they require very high energy, which is currently not possible with probes we have.

But String Theory is not just a theory but a revolution in the world of physics and it’ll always be a part of mainstream academia in one way or another. Theoretical physicists and popular faces of physics Brian Greene and Michio Kaku attest to this.

So, here is String Theory simplified.