Physicists like to group concepts, but sometimes the theories don’t always get along. Nature’s laws can be currently explained by either general relativity or quantum physics. However, concepts from one group don’t necessarily agree with concepts from the other. Physicists have been trying to find the theory of everything for decades: a single or set of theories that could describe the whole universe and everything in it. One mathematical theory, which holds the possibility of becoming the theory of everything is string theory

According to string theory, everything in the universe is composed of tiny filaments of energy called strings. All the matter in the universe is made up of these strings, which exist in special extra dimensions we can’t see or detect. These strings are just like guitar strings — instead of producing a sound, each vibration is a different particle.  

Strings are incredibly small. The best estimate for a typical string length is 10-33 cm. This length is called the Planck scale. A thousand billion billion strings end to end would only just cover the width of a single atom. The partition strings are categorized into two subcategories. Open strings have two endpoints. These might be fixed like on a guitar or could be free to move as they please. Fixing the endpoints in particular ways gives rise to distinct vibrational patterns. Closed strings have no endpoints and so form a complete loop. Together they are called type-1 strings. As strings vibrate vigorously, they give rise to an infinite number of particles. Einstein’s famous equation E = mc2 tells us that energy and mass are equivalent: the more energetic the string, the heavier the corresponding particle is. String theory also predicts 10 to 11 dimensions – but aren’t we only familiar with 4 dimensions? According to the theory, the rest of the dimensions are curled up into incredibly small sizes through a process called compactification. The size of these compactified dimensions is similar to the length of a string.

Compactification of Orderly Arranged Closed Loops.

String theory also includes things called branes. These are multi-dimensional surfaces that move through 11 dimensions. Branes are places for fixed endpoints of open strings and strings attached to branes give rise to forces and particles. Branes are a central ingredient in modern research. They can be used to construct cosmological models within string theory. There are different types of branes like D-0, D-1, and D-2 depending on their characteristics.

Illustration of Open Strings connected to D-Branes. Image courtesy: Wikimedia

The process of developing string theory was not a clear one. Much of the essential work was done before physicists even realized of strings. The theory was kick-started in 1968 by an Italian physicist Gabriele Veneziano, a researcher at CERN, who was trying to explain the strong force. He realized that an equation, written by Leonard Euler several centuries earlier, seemed to do the job. However, he did not know why. Then in 1970, physicists Yoichiro Nambu, Holger Nielsen, and Leonard Susskind had all independently come to the same conclusion: the equation made sense if you thought of particles as tiny vibrating strings. In 1971 Pierre Ramond revised the theory to include fermions (matter particles). His superstring theory required 10 dimensions to work. String theory was now considered a possible theory of quantum gravity rather than a theory for strong force. For a brief period of time, it seemed that only one small push would be needed to obtain the final unified quantum theory of all the forces. However, the more physicists studied string theory, the more they realized the depth of the underlying structure. The result was not one, but five different consistent superstring theories. Then in 1995, Edward Witten showed that each theory is an alternative way of looking at the same world. He called it the M-theory which worked in 11 dimensions. From there, advancements in string theory took place and concepts such as supersymmetry and branes were introduced. All these developments bring forth a modified M-Theory that is used to study everything in the universe – from black holes to condensed matter.

There are four fundamental forces of nature — electromagnetism, weak force, strong force, and gravity — and all other forces can be built up as a product of their interaction. While the first three have been united by quantum field theory, gravity still remains to be combined to complete the theory of everything. Because gravity is the weakest of all forces, its effects are not detected and hence it cannot be combined. Many believe that gravity is carried by a messenger particle called the graviton, but it has not been detected yet. In fact, one of the most important tasks in modern research is to find the missing piece of the puzzle that reconciles gravity with quantum mechanics. One of string theory’s most celebrated results is that the vibrations of closed strings automatically provide a massless particle with all the properties of the elusive graviton. Thus, scientists believe that string theory might join gravity with the other three forces.

String theory helps physicists study black hole singularities which in time will aid them in studying the singularity present at the time of the Big Bang. Moreover, branes help in studying the inflation process of the universe. String theory has made important contributions to our understanding of entropy: it has offered impressive insights into the quark-gluon plasma, inspired new approaches to conventional calculations in quantum field theory and has revealed a profound connection between quantum field theory and string theory. In other words, string theory is more than merely an isolated body of research taking place in some obscure corner of physics. Instead, it has tentacles that have reached far and wide into more conventional areas. Above all, it is the closest we have come to the quantum theory of gravity and the theory of everything.

Despite all its merits, there is limited understanding and knowledge of string theory. In particular, physicists lack the defining mathematics for much of M-theory. It has many complex computations and messy calculations. Strings operate at energies far greater than colliders can detect. Scientists, after spending billions of dollars, have not been able to detect the extra dimensions. If extra dimensions exist, they could cause microscopic black holes to form. Until now string theory has produced no experimental evidence and to many scientists, the absence of evidence renders this theory invalid. 

Physicists believe that string theory, as the primary theory of quantum gravity, is the best prospect for the understanding universe as a whole. The mathematical beauty of the theory and its adaptability is seen as one of its virtues. Whether this resilience of string theory will prove testimonial of its correctness remains to be seen. However, for the people working on it, confidence is high.

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