Can someone explain string theory in simple language ?

Answer 1

There are currently four known fundamental forces in the universe: electromagnetic, weak nuclear force, strong interactions, and gravity.

At present the standard model of particle physics does a pretty accurate job of describing the behaviors of the first three at the quantum scale, but fundamentally fails to describe gravity. We have attempted to theorize an additional subatomic particle, the graviton, in order to explain gravity at the quantum level, but so far to no avail. (Relativity already describes it quite well at larger scales).

A potential solution to this was the idea of string theory. The concept that the elementary particles such as quarks are not the smallest objects in the universe, and that they themselves are made up of incredibly tiny strings, the vibrations of which determine the kind of elementary particle that is created.

This theory was quite popular for some time because it allowed us to describe gravity as a quantum field and looked to potentially allow one general unified theory that could merge quantum mechanics with relativity. However, it came with many additional complications such as requiring there to be many more spatial dimensions than there currently appear to be in order to maintain mathematical consistency. The number of additional dimensions vary based on the particular brand of string theory.

Although it is an intriguing field, since its inception there has been no actual evidence that these tiny strings actually exist and we're far far away from resolving its many issues and the various theories have begun to decline in popularity despite some of their mathematical elegance.