Both quantum physics and Einstein’s general theory of relativity work very well in their respective domain but are highly incompatible with one another. String theory speaks of a particle known as ‘graviton’ which is believed to be a quantum mechanical particle that mediates the force of gravity. Hence, string theory is considered to be a theory of quantum gravity and is a potential candidate for a theory of everything.
This article attempts to explain string theory using simple words. Lets begin!
What is a string?
String theory states that everything in our Universe is made up of tiny vibrating strings. These strings are one dimensional objects and are identical to one another. Every fundamental particle that we know such as electrons, quarks, photons, gluons, etc are made up of these strings.
The reason why a quark appears to be different than an electron is because both vibrate at different frequencies. These strings are very tiny. The length of a string is believed to be of the order of Planck length i.e. 10-35 meter or about 10-20 times the diameter of a proton. String theory works with two kinds of strings- closed strings and open strings.
What is a Brane?
As string theory progressed over the years, it was found that the theory required additional objects to work- known as branes. A brane, in string theory, is an object that generalizes the notion of a point particle to any number of dimensions. For example, a point is a brane of dimension zero, a string is a brane of dimension one, a plane is a brane of dimension two and so on. Thus a 0-brane is a point particle, 1-brane is a string and 2-brane is a plane.
Strings can attach to these branes at one ends or both the ends (as shown in figure below).
Why Higher Dimensions?
For the string theory to work, four dimensions (3 Space + 1 time) are not sufficient. Thus, string theory implies that our Universe must have additional ‘hidden’ dimensions. Where are these additional dimensions and why don’t we see them?
Physicists believe that these extra dimensions are curled up into a compact space and too tiny for us to observe or detect. Different types of string theories require different number of dimensions for their mathematical consistency. For example, spacetime is 26-dimensional in bosonic string theory (older version of string theory) whereas in superstring theory (new version of string theory) it is just 10-dimensional.
String theory also proposes the idea of parallel universes. The extra dimensions in string theory can adopt different possible shapes, each shape possibly corresponding to its own universe with its own sets of physical laws.
Types of Superstring Theory
Until 1995, there were five different versions or types of superstring theory each one of them being self-consistent. These five superstring theories were:
- Type I string theory,
- Type IIA string theory,
- Type IIB string theory,
- Heterotic SO(32) string theory and
- Heterotic E8×E8 string theory
In 1995, physicist Edward Witten insisted that all of these different versions can be unified into a single theory known as M-theory (where M can stand for mother, magic, mystery or membrane). M-theory proposes a Universe of 11 dimensions.
Although there is no experimental evidence for string theory, it still provides us a hope that we can unify Physics. The beauty of string theory lies in the idea that everything in our Universe (including gravity) can be tracked down into a fundamental identity known as the string. There are a number of proposed quantum gravity theories apart from string theory, the two top candidates being string theory and loop quantum gravity theory (LQG).
LQG attempts to unify gravity with quantum physics taking into consideration Einstein’s geometric formulation. It proposes that space and time are quantized (just like photons in quantum theory of electromagnetism). Both string theory and LQG theory have their own merits and demerits. Only time will tell which of the two theories survive if at all they do.
I would like to sum up this article on String Theory with a famous quote by Nikola Tesla:
“If you want to find the secrets of the universe, think in terms of energy, frequency and vibration.”