To what extent does String theory present a possibility of a Unified Theory of Quantum Gravity.
The report discusses the theoretical background of String Theory and makes a case for its resurgence in recent times to be a potential candidate for the Theory of Quantum gravity by addressing its former shortcomings and experimental methods.
The objective of science has always been to make sense of the nature of reality. Scientists do this by following the Scientific Method. A fundamental part of the scientific method involves “observing” the phenomenon. This produces no issues as long as thing are easily seeable, but when science reached the borders of particle physics, this caused a big problem. Elementary particles are so small that they are “un-seeable”. Visible light just passes between particles. To solve this issue, scientists tried using light with higher energies and wavelengths per unit phase. However, such high energy interactions change the outcome of the result and hence provide no empirical understanding of how particles behave. (Roberston)
What is a particle? What does it look like? How do we record its behavior? To work their way around this problem scientists hypothesized the idea of the point particle. Scientists began hypothetically treating particles as points in 3-dimensional space. Comparing this particle to the vague results they would get from observing particles allowed the development of Quantum Field Theory which is the basis of all particle physics today and describes particle behavior fairly accurately. The principle that allowed scientists to make these calculations is called the Heisenberg Uncertainty Principle which claims that a particles position in space is inversely proportional to its momentum. (Roberston). In simpler words, it allowed us to get a general idea of particle interactions without having to accurately observe their behavior.
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However, since there is no way to measure things precisely in the Quantum world, the theory is not compatible with the theory of Gravity and General Relativity. GR requires accurate geometric measurements of matter in space time to work. When scientists tried to invent a carrier particle for gravity to make it work with QFT, the mathematics did not work. For some reason the point particle provided adequate accuracy to describe elementary particles but not for a carrier particle of gravity.
To unify the two theories, scientists would have to develop a mechanics more complex than the point particle (Kurzgesagt – In a Nutshell). Hence, they hypothesized the existence of strings the size of plank scale, since topologically speaking, that is the second most complex structure following a point (Roberston). The hypothesis claimed that these strings served as the fundamental building blocks of reality instead of inherently differently behaving particles. It meant that everything was made up of very small strings of energy of specific frequency that would create these particles. Much like a vibration in a guitar string produces a sound, a vibration in an elementary string produces a particle. But most importantly, one of these vibrations would account for the particle carrier of Gravity, the graviton. This was revolutionary since string theory could describe interactions and behaviors of matter at both the quantum and cosmological scale. (Greene)
String theory immediately became a potential candidate for the unified Theory of Quantum Gravity which even Einstein could not put his finger on. However, as developments began, there was an obvious flaw in its workings.
String theory only works mathematically if we allow there to be 10 spatial dimensions. This is a huge problem since our universe only has 3 spatial dimensions. String Theorists tried to make calculations in 10 dimensions and then predict them in 3 dimensions, but the math was inconsistent.
This was essentially going to be the end of string theory even though it would accurately describe nature in a 10-dimensional universe. However, more recently, strides have been made in the field which might result in the extra 7 dimensions being a possible solution to the issue of string theory. This is called “Super-string Theory”
Respected scientists such as IAS’ Dr. Edward Witten and Columbia University’s Dr. Briane Greene are devoted proponents of string theory.
The new model of String Theory embraces the 10 dimensions instead of trying to get rid of them. String Theory claims that there might actually be 10 dimensions in the world and that 7 of them are just extremely small to the point that they cannot be experienced by us. Take for example a wire. Its width might not be apparent to anyone but a very small ant on the wire. Similarly, it is entirely possible that we live in 10-dimensional space of which 7 are just not as apparent.
While this might just sound like an easy escape out of a situation, the 7 dimensions have actually much more to them.
String Theory Hypothesizes that the extra 6 spatial dimensions are not directional like the 3 we experience but extremely small dimensions that fold over each other at the smallest levels. And that it is this folding and the different possible structures it creates that holds the key to understand some basic axioms of the model of Newtonian Physics. (Greene)
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To make these calculations however, we need to know the shapes of those intertwined dimensions which we have no knowledge of. However by making calculations around a hypothetical set of potential shapes, if we manage to find those Newtonian axioms (Simon), it will be evidence of the theory being accurate since coincidences like that do not happen.
How was can we test this? One of the way to make calculations in particle physics is to use particle accelerators to collide two particles of known physical particles into each other at very high speeds. The debris and changes in energy pre and post collision allow us to make educated statements about the process.
If we can do the same experiment at very high speeds and can find that a certain amount of energy that breaks away from the collision corresponds to any of those Newtonian Axioms it will be evidence for not only the presence of the extra 6 dimensions but it will also predict their shape.
This will affirm String Theory in Solid Contention for a Theory of Quantum gravity if we can repeat the process for the properties of a Graviton, the carrier particle of Gravity.
- Greene, Dr. Briane. “String Theory: Briane Greene.” TED-Ed, 9 August 2013. Video.
- Kurzgesagt – In a Nutshell. “String Theory Explained- What is the true nature of reality.” Youtube, 1 March 2018. Video.
- Roberston, O’Connor J J and E F. “A history of Quantum Mechanics.” 1966. Quantum Mechanics History. Document.
- Simon, Herbert A. “Axioms of Newtonian Mechanics.” The London, Edinburgh and Dublin Philosophical (2010): 887-905. Document.
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