String Theory

What if our universe is made of vibrating strings? What if these invisible strings vibrate to create all sorts of matter and energy in this universe?

String theory is a concept in physics that states the universe is constructed by tiny vibrating strings, smaller than the smallest subatomic particles.

As these fundamental strings twist, fold and vibrate, they create matter, energy and all sorts of phenomena like electromagnetism, gravity, etc.

What is String Theory?

String theory is a theoretical framework that tries to tackle the problems of how and why the universe is like what it is now. In string theory, point-like particles are replaced by one-dimensional entities called strings. String theory explains how these infinitesimal strings travel and interact with each other. In normal classical distance scales (larger than the string scale), a string is very similar to a normal particle with its charge, mass and other characteristics. The characteristics of strings are directly controlled by their vibrational states.

One of the vibrational conditions of the strings matches the states of the graviton. It is a quantum particle that regulates gravitational force. In this regard, string theory is also called the theory of quantum gravity.

String Theory as Theory of Everything

The unification of quantum physics and relativity has been the biggest hurdle since the dawn of modern physics. Many famous physicists regarded the String theory as the Theory of Everything. It is considered the ultimate framework that could merge general relativity and quantum physics. We cannot go beyond without a unified theory, because both mechanics control almost everything modern physics deals with.

Quantum mechanics regulates all phenomena at the subatomic level, and general relativity controls gigantic interstellar activities in the universe. The weird part is both of them do not merge well together. Flag bearers of the string theory propose that string theory could solve the disparities between the two physical realms.

String Theory Origin

In general relativity, gravity is a force that bends and warps space-time around supermassive bodies. Even though gravity is one of the four fundamental forces in nature, it is very weak compared to the other three forces (electromagnetism, weak force and strong force). So it can’t be observed or identified on the scale of subatomic particles. However, gravity is very dominant in long-distance scenarios. It controls the structure of the macro universe (galaxies, planets, stars, moons).

As far as quantum mechanics is concerned, gravity doesn’t have much effect. The probable nature of the quantum realm also poses a significant challenge for the induction of gravity in the quantum realm.

Generally, gravity does not act as a particle as its own. Even if a hypothetical model is introduced to explain the particle nature of a gravity particle, it violates fundamental energy laws.

In the 1970s, theorists tried to discard the self-destructive idea of point-like gravity particles. Instead of point particles, strings were introduced. Even if strings collide, there will be no infinite energy problem. Strings can smoothly smash and rebound without implying any physically nonsense infinities.

Video about the history of gravity

Bosonic string theory was one of the earliest versions of string theory. It only included fundamental particles called bosons. It was then evolved into superstring theory. It predominantly introduced a relation called supersymmetry between bosons and fermions. Later, physicists developed five concrete versions.

Strings in String Theory

String theory was developed from the idea of remodelling point particles to one-dimensional entities called strings. The interplay of strings can simply be stated by generalising the theory of perturbation from quantum field theory.

Since string theory has no full non-perturbative explanation, most hypothetical and theoretical questions are still out of their domain.

A string can be open or closed. An open string has two open ends, and a closed string forms a closed loop. Strings have a tiny dimension, approximately the order of planck length

In classical or general relativity scales, strings are not very different from zero-dimensional point particles. The vibrational nature of strings determines the characteristics of the particle.

Different Versions of String Theory

Even though bosonic string theory was the first concrete version of string theory, it was only concerned about bosons (a group of particles that transfer forces between fermions or matter). In order to negotiate such limits, bosonic string theory was replaced by a class of theories called superstring theories. These theories explain both fermions and bosons. So the whole standard model came under the string theory hypothesis. The most striking development was the introduction of supersymmetry. It allows each boson to have a counterpart fermion.

There are many versions of superstring theory.

• Type I
• Type IIA
• Type IIB
• Heterotic string theory (SO(32)
• Heterotic string theory (E8×E8)

The various versions have different sets of strings. The particles that form at low energy levels show varied symmetries.

Heterotic, TypeIIB and TypeIIA theory consists of only closed strings. However, Type I consists of both closed and open strings.

M-Theory

In 1995 Edward Witten stated that all the string theory versions are just different limiting cases of one super theory.

It is called the M theory. It is based on eleven dimensions. Edward formulated this conclusion, based on the findings of Michael Duff, Chris Hull, Ashok Sen and Paul Townsend.

Multiple Dimensions in String Theory

We, as humans, can only perceive and control three dimensions (length, height and width). With the inception of general relativity, time is considered the fourth dimension. The only difference is that we can control the first three spatial dimensions, but we cannot handle the time dimension.

In general relativity, space and time are not separate; space-time is one entity.

Gravity is considered as the byproduct of space-time geometry.

String theory must incorporate far more space-time dimensions than standard 3d or 4d space-time for a solid mathematical base.

There are twenty-six and ten dimensions in bosonic string and superstring theory, respectively.

However, M-theory points to space-time with 11-dimensions.

Branes in String Theory

Branes are objects which can travel through space-time fabric, as per the constraints of quantum mechanics.

Branes possess mass and can have properties like charge.

Brane is an entity that is extended in multiple dimensions. The brane concept arises in string and universal theories like general relativity and quantum mechanics.

Here are some of the technical sides of different branes.

• A 0-brane is a zero-dimensional entity.
• Point: 1-brane is a one-dimensional entity.
• String: 2-brane is a two-dimensional entity.
• Membrane: p-brane is a p-dimensional entity. As few types of string theory have nine spatial dimensions, p-branes may have dimension values of p up to 9.

Limitations of String Theory

• The major challenge of string theory is that the entire theory does not have a consistent definition in all scenarios.
• Even though string theory is often hailed as the theory of everything, it still struggles to combine quantum physics and general relativity into a single universal theory.
• Another hurdle string theory puts forth is the prediction of multiple universes. The landscape of numerous universes has tangled the efforts to create particle physics theories under string theory.
• In quantum mechanics, the principle of observer-dependence is a fundamental characteristic. String theory is still not able to integrate and resolve this quantum nature.
• String theory has not been able to explain dark energy and dark matter. They constitute almost 95% of the known universe.
• String theory cannot state whether the forces and particles forces in quantum physics can be merged or unified.
• String theory does not have a stable explanation of how the constants are created by nature in the standard model (particle physics).

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