What is science? Why do we study it? These are seemingly simple questions that we rarely ponder upon, yet when we do, we find ourselves at a loss. After all, why do we spend millions of dollars on laboratories and facilities, searching for unseen and unknown particles (which may not even exist). If ignorance truly is bliss, then why are we so restless and intrigued by the world we live in and what lies beyond? Why aren’t we content with the crude, rudimentary lifestyle of our antecedents? Whatever our reason may be, one cannot deny the fact that it is this very insatiable curiosity of ours that has driven our innovation and advancement as a species. Our curiosity has allowed us to evolve from stone tools to automatic machines, from horse carts to spaceships, from mono-syllabic grunts to flowery and beautiful languages. And today, it has allowed us to reach a stage where we are close to developing a theory, that could explain everything about our universe. A theory, that could bring order to chaos, that could arrange all the pieces to finally complete the jigsaw puzzle that is this universe (as far as physics is concerned); a theory of everything, one might say. One of the most controversial, daring and of course, complicated theories of physics – the String Theory.
String theory is essentially a theory that aims to solve one of the most difficult problems of physics, namely, how to merge quantum mechanics with the special theory of relativity, thereby, ultimately creating a theory of everything (ToE).
There are four fundamental forces or interactions in physics, namely, gravitational force, electromagnetic force, strong force (the force that binds the nucleus of atoms together) and weak force (a force responsible for a type of radioactivity called beta decay).
Quantum mechanics, in particular, the Standard Model, provides an explanation for all the fundamental interactions barring gravitational force. The Standard Model classifies all the known elementary particles, including quarks, leptons and bosons. According to the Standard Model, a force is described as an exchange of bosons between the objects affected. Those bosons are called force carriers or messenger particles. As a result, electromagnetic force is now known to be caused by a transfer of photons, strong force by a transfer of gluons and the weak force by the ‘W and Z’ bosons. However, the Standard Model and thus, quantum mechanics, fails to provide an explanation for gravitational force.
The current explanation for gravitational force is given by Einstein’s general theory of relativity. It explains that what we perceive as the force of gravity in fact arises from depressions created in the space-time continuum. Einstein proposed that objects such as the sun and the Earth change this geometry due to their mass. Imagine setting a large body in the centre of a trampoline. The body would press down into the fabric, causing it to dimple. A marble rolled around the edge would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space. So although Earth appears to be pulled towards the sun by gravity, there is no such force. It is simply the geometry of space-time around the sun telling Earth how to move.
Thus, gravitational force, as described by Einstein, is not caused by the exchange of particles as stated by the Standard Model. Rather, it is basically a theory of geometry; the geometry of space-time.
Scientists attempted to explain gravitational force by introducing a new particle called the graviton, which would act as the force carrier for gravitational force. However, no one managed to make it mathematically compatible in the Standard Model.
Thus, the quantum theory is used to describe the micro world, and the general theory of relativity is used to describe the macro world. But, they are difficult to fit into a single framework.
Enter String Theory…
String Theory says that all elementary particles (i.e., quarks, leptons, bosons, etc.) are composed of tiny, vibrating strands or strings of energy. This debunks the conventional notion that subatomic particles are 0-dimensional point particles and instead, models them as one-dimensional ‘strings’ of energy. These strings are continuously vibrating and just as different vibrations of the strings of a cello or guitar produce different notes, the different vibrations of these strings produce different particles with their different properties such as quarks and leptons. Thus, ‘strings’ create every particle and force in the universe.
Moreover, one of the vibrational states of a string, in this theory, corresponds to the graviton, and the mathematics checks out. Hence, string theory allows the existence of the graviton, thereby successfully unifying quantum mechanics with the theory of relativity. Thus, string theory is what scientists call a unified field theory or a theory of everything (as it manages to describe all the fundamental particles and forces known in physics).
The strings of string theory are unimaginably small. The average string, if it exists, is about 10-33 centimetres long. Interestingly, if an atom were magnified to the size of the solar system, a string would be the size of a tree.
But string theory comes with some strings attached (Pun intended)…
High-Strung mathematics (Pun intended)
The drawback of string theory is that its mathematics does not work in a world with just 3 spatial dimensions. The 3 spatial dimensions visible to us are length, breadth and height. Einstein added a 4th dimension, time, to this list. Thus, we see the world around us has 4 dimensions. However, for string theory’s maths to work, we have to assume that the world has not 4, not 5 but 10 dimensions! Therefore, string theory implies that our world has more than the 3 dimensions we can see. The questions that arise are, where are these other dimensions and why can’t we see them?
Following a suggestion made in the 1920s by Theodor Kaluza of Germany and Oskar Klein of Sweden, string theorists envisioned that dimensions come in two distinct varieties. Like the unfurled length of a long garden hose, dimensions can be big and easy to see. But like the shorter, circular girth of the garden hose, dimensions can also be far smaller and more difficult to detect. This becomes more apparent by imagining that the circular cross section of the garden hose is shrunk ever smaller, below what can be seen with the naked eye, misleading a casual observer into thinking the garden hose has only one dimension, its length. Similarly, according to string theory, the three dimensions of common experience are large and manifest, while the other six dimensions are crumpled so small that they have so far evaded detection.
There is not only one unique string theory; physicists have built 6 different string theories depending on various factors, such as whether the strings are open or closed. For example, the bosonic string theory considers only bosons (particles that transmit forces) and not fermions and requires 26 dimensions whereas the other 5 string theories require only 10 space-time dimensions.
Since string theory is considered to be a theory of everything, it can have only one unique version, giving rise to yet another problem. However…
In 1995, Edward Witten proposed a unified version of the 5 different string theories (not the bosonic string theory) called the M-theory. Witten argued that each of the 5 different theories was, in fact, a manifestation of a single underlying theory. Witten and others identified several forms of duality between the theories which, together with certain assumptions about the nature of the universe, could allow for them to all be one single theory: M-Theory. However, this required adding yet another dimension to the existing 10, bringing the total number of dimensions required for string theory to 11.
Interestingly, Witten himself has said that the ‘M’ could stand for anything and can be selected for taste. Some possibilities include Membrane, Master, Magic or Mystery. One of the most exciting possibilities is also the Matrix Theory, inspired by the Matrix trilogy.
It truly is awe-inspiring how far mankind has come, from hunting with sticks and stones to being on the verge of unlocking the secrets of the universe. Yet, there will always be more to see, more to discover and more to know. No one can predict where our curiosity will take us, what secrets we may unravel or what the future holds in store for us, but one thing is for certain, our thirst for knowledge, our insatiable curiosity should and shall never be quenched.
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.” – Isaac Newton
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