In 1968, a young Italian theoretical physicist named Gabriele Veneziano was searching for equations that would explain the strong nuclear force, which can be defined as the extremely powerful force that binds the neutrons and protons together in the nucleus of atoms. As sometimes the story is told, Gabriele happened upon an old mathematics books where he discovered the Euler beta function, first studied by Leonhard Euler and Adrien-Marie Legendre.
He noticed that when the function was interpreted as a scattering amplitude (In quantum physics, the scattering amplitude is the amplitude of the outgoing spherical wave relative to the incoming plane wave in the stationary-state scattering process), it contained many of the physical properties needed to describe strongly interacting particles.
This amplitude, later named the Veneziano amplitude, is interpreted as the scattering amplitude for four open string tachyons (which will be simply defined here as a hypothetical subatomic particle that moves faster than light).
However, what is more likely is that the discovery of the equations was not accidental but in fact the outcome of intense research and the correlation to string theory was an accidental discovery. But however it was discovered, Veneziano's work led to a model that explained the strong force by a field theory of strings. This was the birth of string theory.
The Euler beta function was passed from colleague to colleague, until it came across the American physicist Leonard Susskind. He understood that this formula explained the strong force mathematically but he figured out that beneath these symbols was something simple, abstract and elegant. The story goes that " a young physicist got stuck in an elevator with Murray Gell-Mann, one of physics' top theoreticians, who asked him what he was working on. Susskind said he was working on a theory that represented particles 'as a kind of elastic string, like a rubber band.' Gell-Mann responded with loud, derisive laughter." He noted that this formula seemed to explain particles with an internal structure, which could stretch and compress and even oscillate. However, as revolutionary as it sounded, it was mainly rejected by the scientific community. It appeared that string theory was dead.
Particle physics was embracing the idea that particles were indeed points and that microscopic particles smashed together at high speeds would lead to release of multiple even smaller particles. But what is revolutionary here is that physicists were not only discovering the constituents of matter, but they realized that the forces of nature could also be described in terms of particles. The way to imagine this is to imagine a game of catch where particles of matter are throwing a particle of force (called a messenger particle) back and forth. The more messenger particles that are exchanged between the particles of matter, the stronger the force, and this is what we feel as force. Experiments by physicists confirmed these predictions by the discovery of messenger particles for electromagnetism, the strong force, and weak force. Using this model, scientists believed that we were once step closer to unifying the forces. To understand this further, imagine we were to rewind time to moments just until after the big bang, when the universe was very much smaller and very much hotter, the messenger particles for electromagnetism and the weak force would have been indistinguishable and would have been united as the ‘electroweak force’. And theoretical physicists believe that if we went further back in time, the electroweak would be united with the strong force. Although that has yet to be proven, quantum mechanics does explain how these three forces operate on the subatomic level. And now we have a consistent model of elementary particle physics that allowed the explanation of all of the interactions – the strong, weak and electromagnetic forces – in the same context known as the ‘standard model’. However, the standard model has a major missing piece. It does not include an explanation for the force that governs the universe, gravity.
Eclipsed by the standard model, string theory started to become very indiscernible by the scientific community. And although, the early pioneers of string theory believed in the model and continued to explore string theory, they found several problems. Among other things, early string theory predicts a particle that can travel faster than the speed of light, an unnatural and hypothetical particle called a tachyon. Furthermore, the theory also predicts 10 dimensions, which at the time was impossible to believe. And to make complications worse, many of the predictions were not testable experimentally classifying this theory as more of a mathematical framework for building models than a physical theory, and to say that string theory is a theory of everything is a failure. Jim Holt, a journalist, was noted as saying “…dozens of string-theory conferences have been held, hundreds of new Ph.D.s have been minted, and thousands of papers have been written. Yet, for all this activity, not a single new testable prediction has been made, not a single theoretical puzzle has been solved. In fact, there is no theory so far—just a set of hunches and calculations suggesting that a theory might exist. And, even if it does, this theory will come in such a bewildering number of versions that it will be of no practical use: a Theory of Nothing.”
By the early 1970s, only a few physicists were still struggling with the concepts and ambiguous equations of string theory. John Henry Schwarz, an American theoretical physicist, was working on some of these inconsistencies, one of them specifically the prediction of a mass-less particle never seen in nature. For several years, he struggled with the equations, but nothing worked, the equations would always seem unruly and messy. Just as Schwarz was thinking about abandoning the theory, he had an idea. What if his equations were describing gravity? That would mean rethinking the size of these ‘strings’ of energy.
By supposing that he was studying the theory of gravity, he had to tremendously change the view of how big these strings were. By suggesting that these strings were actually hundred billion billion times smaller than an atom, the mass-less particle could now be viewed as a ‘graviton’ – the particle believed to transmit gravity at the quantum level – and string theory had described the missing piece of the standard model. Schwarz submitted his results for publication; however, there was very little reaction from physicists. However, Schwarz was not discouraged from his bold predictions. He envisioned that if strings could describe gravity at the quantum level, they must be the key to unifying the four forces – electromagnetic, strong force, weak force, and gravity.
Regardless of this incredible idea, the equations of string theory still contained major mathematical anomalies, and the only way to make string theory viable was to get rid of these anomalies. Schwarz and Michael Green, a British physicist, decided to calculate and it all came down to one equation. The story goes that on one side of the blackboard they got 496. And if they managed to get 496 on the other side, this would prove that string theory was free of anomalies. This was termed the Green-Schwarz anomaly cancellation mechanism and indeed, the two numbers did match and this meant that string theory was free of anomalies and it had the mathematical depth to encompass all fours forces, and the possibility of unification of forces, a dream that Einstein had expressed. This time the reaction of the scientific community was enormous and interest in string theory skyrocketed.
This updated version of string theory seemed capable of explaining all the components of nature. If one imagines the components of an atom: electrons orbiting a nucleus containing neutrons and protons, which are made up of even smaller parts called quarks. String theory explains that the particles making up everything in the universe are made of even unimaginably small strands of energy that look like strings. Furthermore, just as strings of musical instruments have unique vibrational patterns or frequencies that create what we hear as musical notes, the different way that these strings oscillate give particles their unique properties like mass or charge. So in essence, the only difference in particles those make up matter and the particles that make up the forces is the way these tiny strings resonate. And it’s this elegant idea of string theory that one can view the universe as one big musical cosmic symphony.
And it’s this idea that bridges the gap between the unpredictability of the universe on the subatomic level as depicted by quantum mechanics and the smooth picture of the universe on the large scale as depicted by Einstein’s General Theory of Relativity. In essence, string theory ‘spreads out’ the unpredictable nature of a point particle into a string, thereby bringing an ordered fluctuation of quantum mechanics that allows it to stitch together with general relativity.
Regardless of the great advances made so far in string theory, there is still one major criticism, and this leads us back to the Heisenberg uncertainty principle which states that, In quantum mechanics, certain pairs of physical properties, such as position and momentum, cannot be simultaneously known to arbitrarily high precision.
Simply put, the more precisely one property is measured, the less precisely the other can be measured. Therefore, at the distances being studied, experiments cannot be used to validate or refute this theory. And thus, whether string theory can be considered a theory of physics or a theory of philosophy is still highly debated.
Another concept that makes string theory even harder to prove is that one must also consider that this theory predicts extra dimensions of space. Most people consider that the universe has only 3 dimensions and one more dimension of time. We know that Einstein showed that gravity can be visualized simply as warped space-time. Theodor Kaluza, a German mathematician, suggested that electromagnetism might also work the same way. He proposed a hidden dimension where space is warped to facilitate the electromagnetic force. But how can we visualize this extra dimension. Oskar Benjamin Klein, a Swedish theoretical physicist, proposed an unusual answer. Imagine a line that extends between two points, such as a rope tied between two poles. From a distance, we cannot see that the rope has a thickness, and it is simply a line with a single dimension. However, if we were very close to the rope, we would see the presence of a second dimension that wraps around the rope, a dimension that we can travel around the rope in a clockwise (or anti-clockwise) motion. Therefore, dimensions come in two forms, either long like the length of the rope, but can also be very tiny, like the circular direction that wraps around the rope. Kaluza and Klein suggest that the universe may be just like this rope, with both big and extended dimensions, like the ones we know about, but also with small curled up dimensions that are smaller than atoms, dimensions we cannot possible see. Kaluza and Klein theorize that if we were able to decrease our size to the size smaller than an atom, we’d find one extra circular dimension located at every point in space. The concept that extra dimensions exits all around us lies at the heart of string theory, as string theory demands 6 extra dimensions curled up into different shapes at the subatomic level. But how do these extra dimensions of space curled up into unique shapes have any impact on our everyday world? Recall that strings can resonate at specific frequencies, and that this in turn dictates the properties of the particle. When you change the shape of the space around the string, you also change the frequency at which the strings vibrate.
