String Theory — True nature of reality… Maybe?
When asked “What do physicists worship?” Stephen Hawking replied, “A single theory or equation that can describe every single behavior and movement of large stellar galaxies to small and strange quantum substances, a single theory that can unify all the physical laws in our universe: A theory of Everything.”
Physicists like to reduce the description of the mechanics of our reality down to the simplest possible form. They expect this simplest form to have minimal parameters or moving parts. Our best theory in physics, called the Standard Model of Particle Physics, can describe most things with stunning accuracy, but it is still considered incomplete. This is because some parts of this theory require us to tune many mathematical parameters to get answers and then there is gravity that does not fit into this at all. These problems caused physicists to go in search of a deeper theory that can bring all observable phenomena into a single mechanical framework.
String Theory is one of the candidates for the Theory of Everything. It is a theoretical framework in which the point-like particles in the Standard Model of Particle Physics are replaced by a one-dimensional object called the string. It also describes how the strings propagate and interact with each other. The vibrational state of these strings are what determine the properties of the elementary particles that we observe. For example, a string vibrating at a certain frequency with a certain amount of energy gives rise to a particle called the electron.
From our perspective these miniscule strings appear as though they are ordinary particles, exactly like when we view a cable line from a sufficient distance it looks as though it has only one dimension, its length. However, when one approaches the line, a second dimensional appears, its circumference.
The idea of string theory had started in the 1960s when physicists were trying to understand Hadrons. Hadrons are subatomic composite particles made of two or more quarks that are held together by the strong nuclear force, in a similar way as molecules are held together by the electromagnetic force. Protons, Neutrons and Mesons come under the category of Hadrons. Mesons are a combination of a quark and an antiquark. While studying the strange peculiarities of the interaction between pairs of mesons and their relationship between angular momenta and masses, suggested that the quarks inside the mesons are connected by string. These strings can be described as vibrating tubes of elastic bands made of Gluons (a type of elementary particles responsible for exchange of strong nuclear force between quarks). Physicists started working on a quantum theory describing the strong nuclear force, but was soon replaced by Quantum Chromodynamics because the string theory of the strong nuclear force described a vibrational modes of something that was unexpected and unwanted. Vibrational modes in a quantum field are known as particles. One of the unexpected modes described a hypothetical particle called the Graviton, the quanta of the gravitational field. Physicists realised that there was no reason for the graviton to appear in the investigation of Hadronic string and abandoned this theory.
Later in the 1970s, physicists learned that the math of this string theory can be used to describe quantum gravity. The final piece needed to complete the picture of the theory of everything. After much heavy and confusing mathematics and a few tweaks to the hadronic string theory, scientists had worked it out. On paper it unified all of the physical laws in our universe, but in order to do so it had to add 22 more dimensions on top of our 3 spatial and 1 time dimensions. This was called the Bosonic String Theory. Physicists thought, if this theory could explain force carrying bosons why not the fermions (electrons and quarks that comprise matter) too. Soon they proposed the idea of Supersymmetry in order to bring fermions and bosons in the same theoretical framework. Fermions are the matter particles and Bosons are the force carrying particles. They combined the principles of Supersymmetry and String Theory, the result was Superstring Theory. Supersymmetry states that for every boson there is a fermion superpartner and vice versa. This theory shaved off the total of 26 dimensions to 10 dimensions, required to explain the underlying true nature of our reality. This was the First Superstring Revolution, it presented us with five theories where one-dimensional quantum strings moved around in ten dimensions of spacetime.
Sometimes different mathematical theories describe the same physics, this is called Duality. Superstring Theory is awash with dualites. Superstring theory had five different versions or rather five different types of superstrings. In 1995, Physicist Ed Witten had composed a way to fit the many forms of Superstring Theory into a single framework called M-Theory. This only required an extra dimension bringing the total to 11 dimensions.
Now there isn’t a string theory in 11 dimensions, but there is a supersymmetric theory of gravity, Supergravity. With Supergravity comes a theory of Supermembranes, for M-theory to work in 11 dimensions, it must include surfaces called membranes. Supergravity says that the one-dimensional quantum strings can stretch throughout different dimensions into a kind of membrane. These membranes are spread throughout our universe and describe a hyperspace within which the membranes exist with universes attached to them. These ‘brane universes’ make up the multiverse. One of these branes is our universe.
It should be noted that M-theory is an attempt to unify the Theory of General Relativity, Supersymmetry and String Theory. Witten explained M-Theory to be the skeleton, while the five superstring theories to be individual bones. Our understanding of M-theory is by no means complete. Even after identifying dualites in the strings and hints of supergravity, the middle of the web remains impenetrable.
Even with our current knowledge of this theory, we can predict or explain multiple phenomena that could not be explained using our best theories in physics. String theory explains why gravity is weaker than the other three fundamental forces. It shows that gravity permeates through different brane universes, it seeps through our brane universe into other brane universes and higher dimensions. The second thing it attempts to explain is how our universe came to be. Our best theories break down at the moment of the big bang, string theory gives us its own unique explanation of how universes are born. Universes are born out of brane collisions or brane separations. This is the promise of String Theory.
Unfortunately, there is no experimental evidence found that could prove String Theory. String theory got its fame because of the maths surrounding it. Physicists have yet to derive any concrete evidence or experimental procedure of the required extra dimensions. Part of the problem is the fact that these quantum strings are so small. They are thought to be as small as 10 to the power of -33 cm in length. In order to see the strings we would need a particle collider that is a million times more powerful than the LHC (Large Hadron Collider). This collider would be the size of our galaxy. This means it’s pretty impossible to see the strings directly.
Another factor that makes string theory nearly impossible to prove is the mathematical predictions that it makes. Physicists imagine that the compactified additional dimensions form a shape called the Calabi-Yau shape. The behavior of these quantum strings in hyper dimensional surfaces is only understood in some idealized cases. There are countless possible Calbi-Yau manifolds to choose from. The minimum number given is 10 to the power of 500 different topologies. Each compactified version of these dimensions implies a different set of properties for vibrating strings. This means that there could be a different set of elementary particles with different properties and different sets of physical laws to go with them. It seems an impossible task to find out which set corresponds to our universe. This is the impasse.
Nevertheless, our biggest hope with string theory is the proof of supersymmetry. Finding the superpartners of the elementary particles would perhaps solve numerous problems with the standard model. It might be possible to detect the extra dimensions predicted by string theory. The conjecture that gravity is weaker compared to the other forces is because it works in many more dimensions. The force carrying particle of gravity, the graviton, should be able to move between these dimensions. If a graviton were to be produced at the LHC, then move to another dimension, it would seem like energy has gone missing at the end of the experiment.
At present, the most certain experimental procedure for string theory lies at the point that is inaccessible with our current technologies. In the meantime, string theory continues to offer delightful insights to our impossible problems. Maybe the story of string theory might not be the theory of everything, but it is a useful one. As wrong or incomplete our current String Theory may be, it may also be the inevitable step to point us in the right direction.