# String Theory II: A Brief History Lesson

### Origins of String Theory

String theory started out as an explanation for some of the mysterious properties of the Strong Nuclear force. In 1968, before String theory, Italian Physicist, Gabriele Veneziano made a breakthrough. He was one of the people trying to provide an understanding of the Strong nuclear force. He found that a 200-year-old formula which was provided by the great mathematician Leonhard Euler could explain everything. The formula is called the Euler Beta Function. After Veneziano’s realisation, the Euler Beta Function was used to solve problems at particle accelerators around the globe. Interestingly, even though the Euler Beta function seemed to have solved the problem, physicists did not know how it worked.

To explain the Strong nuclear force, few physicists such as Leonard Susskind, Yoichiro Nambu and Holger Nielsen put forward that when elementary particles were replaced by 1 dimensional strings, the properties of strong interactions using the Euler Beta function could be explained. Though it seemed promising at first, later works showed that the String description was flawed. It was not consistent with observations. Physicists almost abandoned String theory.

### New insights into String Theory

Though most physicists had abandoned String Theory before it was even explored, few researchers still worked on it. These few physicists found that String Theory was not only a description for the strong force but gravity as well. They found that some vibrations of String Theory produced a particle that had properties similar to that of the Graviton (the hypothetical particle of gravity). This showed that String theory described a Quantum Theory for Gravity. Gravity and Quantum Mechanics can be unified using the String description of nature.

These results were widely ignored by the physics community due to String Theory’s early experimental failure when trying to describe the Strong force. Later work in 1970s and 1980s also showed that String Theory was conflicting with Quantum Mechanics. There seemed no hope for the theory, until 1984…

### The Rebirth and Death of String Theory

In 1984, Physicists Michael Green and Henry Schwarz showed that the conflicts with quantum mechanics can be resolved. They also showed that String Theory is capable of uniting all the 4 fundamental forces. This result sparked interest and lead to intense research by physicists around the globe. The period between 1984 to 1986 is called the “First Superstring Revolution”. Thousands of papers were published in these 3 years.

But, there was a problem with String Theory. The equations were extremely complex. So complex that not only the results but even the equations were approximate. This led to frustration as the theory was not moving forward. The theory was abandoned by most Physicists again.

### The Re-rebirth of String Theory

After a few years of work, 5 variants of String theory were developed. these were the Type I, Type IIA, Type IIB, Heterotic-O and Heterotic-E. In 1995, at a conference at the University of Southern California, legendary physicist Edward Witten showed that the 5 variants of string Theory were just parts or extensions of 1 grand theory. It is now known as the Membrane theory or M-theory. This marked the beginning of the “Second Superstring Revolution”. M-theory is now the central problem of 21st century theoretical physics.