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String Theory

Last updated date: 09th Apr 2024
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What is String Theory?

String theory is a particle physics theory that attempts to merge the general theory of relativity of Einstein with quantum mechanics. The name comes from the modeling of the subatomic particles in a manner that shows them as small one-dimensional entities that are ‘stringlike’, instead of the usual approach where they are zero-dimensional. This is a theory that envisions a string undergoing a specific vibration that corresponds with a particle that has properties that are definitive like charge and mass. 

Brief on String Theory 

Here is a more simplified version of the theory that is also known as the ‘theory of everything’ and it includes concepts like extra dimensions, superstrings, and branes. Scientists believe that this is a theory that will explain one of the biggest mysteries of how the universe works, which is how quantum physics and gravity fit together. 

Since string theory is a work in progress it can be a little difficult to understand what it's fundamentals are, but below are some of its key features.

  • All the objects that exist in the universe are made of membranes (branes) of energy and vibrating filaments (strings).

  • This is a theory that attempts to merge quantum physics and general relativity.

  • Several dimensions to the universe exist but are yet unobservable. 

The String Theory PDFs on the site are freely available if you want to do some in-depth research and gain a scientific understanding of how this theory actually works and what it does.

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Who Invented the String Theory?

Physicists in the 1980s realized that string theory could possibly incorporate four forces of nature, that is weak force, strong force, electromagnetism, and gravity, and all types of matter in one quantum mechanical framework, and this could perhaps be the unified field theory. This is an area of research that is developing rapidly, but it is mostly a mathematical construct without having made any experimental observations. 

It has a long history since then with numerous physicists that have worked on this theory. However, Leonard Susskind is said to be one of the fathers of string theory who introduced the idea that particles could be vibrating filaments along with Holger Bech Nielsen and Yoichiro Nambu. He was also the first to bring the string theory landscape in the year 2003.

Albert Einstein spoke of a ‘God equation’ that could explain how the universe works. However, this string theory equation isn’t something that has been found yet, even though people like Michio Kaku have been working on it for decades. Kaku, in fact, is responsible for the standard model of this theory and says that string theory is something that could be the equation that Einstein was looking for.

String Theory Dimensions

In string theory, there are vibrating filaments or strings (fundamental objects of reality) that appear and manifest as various particles (quarks, electrons, etc.) and also as nature’s force-carriers (gravitons, gluons, photons, etc.). This is how they vibrate, and each string is said to be so tiny that they look like point-like particles, but they vibrate with different modes. This is explained in the same way that a guitar string can give you different notes. 

Each mode of vibration is said to relate to a different sort of particle, so strings that vibrate a certain way will look like photons, others as electrons, and so on. In the view of this theory, particle collisions are said to be strings that merge and split apart.

If the mathematics for this has to make sense, there need to be more than four dimensions in the universe.

Our known space-time or our four dimensions do not give these strings enough room to vibrate. And only if they have space to vibrate, they can express themselves as different particles like photos or electrons. 

The current version of the theory states ten dimensions as needed, but there is a hypothetical M-theory theory that needs eleven. The reason why this seems far-fetched and why this theory is still in progress is because we only see the three spatial dimensions along with the one of time and no more than those four.

Loop Quantum Gravity Vs String Theory

The biggest competitor that string theory has is the loop quantum gravity. The loop quantum theory focuses on the quantum theory of gravity making the scope far less narrow than the string theory that is known as the ‘theory of everything.’ This is precisely why this is a far less popular theory and what it does is that it tries to quantise space, meaning that it treats spaces as if it comes in tiny pieces. The flaw here is that there has been no success in showing that you can extract time-space out of the quantised space. Not to say that string theory has been anything more than theoretical.

On the other hand, string theory begins with methods of particle physics and attempts to explain all of particle physics, which includes a method of creating a quantum theory of gravity, and basically unifying it with all other forces, not to forget its predictions of extra dimensions.

The key insight from general relativity states that space-time is not a fixed framework and instead is something dynamic. String theory is a theory that is built on a fixed framework, so it does not account for how general relativity states that space-time is dynamic.

LQG researchers also say that the theory of quantum gravity must not be a theory that is put into a space-time stage that is already existing but should be background-independent. 


For years, physicists believed and hoped that the string theory would do all that it said it would. There was a dream of a singular theory, but in the early 90s, people seemed to give up on connecting the theory to the real world. In the last two decades, there has been an extension of theoretical tools, but there is very minimal progress in understanding what is in the universe and how it works.

It was said that people realized that the bar had been set too high, but now this theory has become increasingly complicated theoretically, and there seems to be a challenge even in understanding what string theory is and what it is not. The main issue lies in how the theory lies in theoretical physics, but there is no understanding of how it connects as a theory of gravity to nature. 

FAQs on String Theory

1. What are the Five Superstring Theories?

The five consistent superstring theories are as follows:

  • Type I String Theory: This is the only theory based upon the unoriented open and close strings while all other theories are based upon the oriented closed strings. The type I string consists of only one supersymmetry in the ten-dimensional sense.

  • Type II A String Theory: These theories have two supersymmetries in the ten-dimensional space based upon oriented close strings. It is non-chiral i.e. parity-observing.

  • Type II B String Theory: Type IIB also has two supersymmetries in its ten-dimensional space based upon oriented close strings. This theory is chiral i.e. parity-violating.

  • Heterotic String Groups: This theory arises out of a hybrid between type I and bosonic string. The two types of Heterotic string groups HO(E8 x E8) and HE( SO(32)) differ in their ten-dimensional gauge groups. 

2. What Do You Mean By Supersymmetry?

Supersymmetry is one of the most essential features of string theory. Supersymmetry is defined as a mathematical property that requires every known particle species to have a partner particle species. This partner species is called a superpartner. 

This property is often referred to as superstring theory for string theory. Superstring string theory, also called supersymmetry string theory, is an attempt to define all the particles and forces of nature in one theory by modeling them as a one-dimensional object called strings. As of now, a superpartner is yet to be discovered but according to scientists, this is due to their weights. If we succeed in finding a superpartner, it would provide evidence that the unification approach of string theory is on the right track. 

3. What is string theory?

The string theory, in simple words, tries to merge quantum physics with the relativity theory given by Einstein. It states that the universe is made up of vibrating filaments or strings that vibrate in a particular way. When certain strings vibrate a certain way, they make up photons, other electrons, and so on. Even particle collisions are explained as strings coming together and splitting apart. 

And these strings need space to vibrate, it is not possible in the four known dimensions, and therefore the theory states that there are ten or eleven that exist. It is a purely theoretical concept and has seen many physicists work on it to expand it over the decades. 

4. Is string theory a failure?

String theory is said to be an intellectual dead end. It is a theory that is based purely on theoretical physics, and there has been considerable progress on this end over the last few decades. But no progress has been made in the sense of the theorized dimensions being observable or any understanding of how quantum physics and the relativity theory actually fit together. Some critics consider it a failure due to this, yet others have hope in the unexplored potential that this theory has and that someday a physicist will be able to find the equation that explains the way the universe works.

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