Written by Yigit Berkehan Kose
Generally, 3 spatial dimensions are known: width, height, and depth. If we add to this the dimension of time, known as the 4th dimension, we get modern physics.
However, the biggest and most famous problem of physics is that there is an incompatibility between the Theory of Relativity, known as the physics of very large objects such as black holes, stars, and planets, and Quantum Physics, which is the physics of very small objects such as subatomic particles. Because we know that there is no interruption in the existence of the universe. In other words, subatomic particles and planets did not exist independently of each other. We can explain this wonderfully with Particle Physics and the Standard Model. Everything we see in the universe, from planets to stars and black holes, came into being by the coming together of these simple building blocks. Therefore, starting from the physics of the simplest particles, we should be able to reach the physics of the largest celestial bodies, but we cannot[1].
So, some physicists are working on a bridge that can connect these two sides of physics, sometimes known as the Theory of Everything. Numerous theories have been put forward that are claimed to be able to achieve this: Ring Theory of Quantum Gravity, Causal Fermion Systems Theory, and many more... However, String Theory is the most prominent and the most studied.
String Theory, in essence, is a candidate to be the Theory of Everything, which states that quarks are formed as a result of the vibrations of structures called strings in different ways and that all parts of the universe can be explained based on these quarks[2].
The strings mentioned in String Theory are enormously small: 10²⁰ times smaller than a proton; perhaps as small as the Planck distance, the smallest distance in the universe! The vibrations of these strings can produce all the building blocks of the universe as we know it, from protons to neutrons, from electrons to physical forces. This is not just a hypothetical narrative; it is a mathematically coherent expression; but unfortunately, experimental proof has not been done yet.
Since the main purpose of String Theory is to connect this Quantum Field Theory to the Theory of Relativity, to understand String Theory, we first need to talk a little bit about Quantum Field Theory. For this reason, we can imagine the fields as seas overlapping each other. Let's imagine that each of these seas is made up of different liquids and is vast.
The aim here is to address how these undulating surfaces interact with each other. When one of these liquids waves, if the wave is strong enough, it will affect other seas and cause fluctuations in those seas.
Each of these waves creates a particle such as an electron or a photon in its field. When these particles fluctuate in their field, they sometimes affect other fields and cause them to fluctuate as well. Since the particle and the wave are the same thing, the particle also causes a different particle to form in this other field. This is what we know as the interaction of particles. These interactions, expressed with Feynman diagrams, form the basis of everything. In short, we can say that what we call matter and every event that happens to it is the result of the interaction of energy trapped in certain force fields[3].
We have connected what we have said so far to energy, but if we do not explain what this energy is, then what we have been telling you will not make any sense. And to explain it, we can step into String Theory. First of all, let us emphasize this: String Theory still has no empirical evidence. Everything we have for this theory relies heavily on mathematics and some indirect inference. However, this does not mean that it is true, nor does it mean that it is an empty thesis.
Any energy emitted in space loses power as it moves away from a source, such as when a lamp is less bright depending on how far away it is. The main reason for this is the fact that a two-dimensional object has no volume. Two-dimensional objects only have space. Therefore, if you want to measure the intensity of the energy radiating from a certain center to any distance in a two-dimensional plane, what you need to do is to divide the energy we have at the beginning around an imaginary circle that will occur at that distance.
Let us now consider the energy radiating in all directions as a sphere. Here, the ratio should decrease proportionally to the surface area of the sphere; because the circumference of three-dimensional objects has expanded towards all three dimensions. The two-dimensional universe and the energy emanating from it are only a small part of this new three-dimensional universe. This situation, which takes place in the three-dimensional universe, is also known as the inverse square law: As you move away from the center, the effect decreases inversely with the square of the distance. So if you move 2 times away from the center, your force will decrease by 4 times. If you move 3 times away, the force decreases by 9 times[4].
A mathematician named Theodor Kaluza, while examining gravity in a 5-dimensional universe, saw that the gravity in this universe is very similar to that in our universe. Because if the universe is actually 5 dimensional, it is very understandable why gravity is weaker in our universe. Just as only a part of the energy emitted in a three-dimensional universe can be detected in a 2-dimensional universe, the gravity radiating in a 5-dimensional universe will appear weaker in 4 dimensions. This idea forms the basis of String Theory.
So, what are the upper dimensions in String Theory?
When you get into the mathematics of String Theory, you see that operations in 3 or 4 dimensions are not enough. So, problems arise in the absence of additional dimensions, especially in explaining other forces and creating a coherent physical system. So math is pushing us to think that there are more dimensions. This is the reason why it is not experimental but has a strong theoretical background.
Another reason is that when we try to solve some problems in particle physics with 3 spatial dimensions, we encounter very serious difficulties and problems, but the inclusion of these additional dimensions in mathematics makes these extremely difficult problems that have been tried to be solved since the 1970s suddenly become extremely simple[5].
Another requirement stems from our deficiencies in physics regarding the fundamental parameters of the universe. There are around 26 basic parameters in the universe, and these parameters define what the universe is and how it should work. Our explanations of the interrelationships of these parameters and why they exist the way they are, yield very difficult and complex results when trying to solve them with 3D physics. However, experts think that with the inclusion of the upper dimensions, these questions become simpler and we can explain some points that we could not explain before.
Studies on String Theory in its classical form show that 10 dimensions are sufficient to make String Theory coherent and functional. Using this number of dimensions, it seems like Quantum Theory and the Theory of Relativity can be brought together.
Although we see these parts separately now, at the time of the Big Bang, these parts were a whole. The structure that holds all these forces and parts together is called the super force. After the Big Bang, this super force disintegrated and began to work in separate pieces. However, according to String Theory, this super force did not exist in 3-dimensional space; It was a force that could exist in higher dimensions. Therefore, today, we cannot bring these pieces together in only 3 dimensions.
However, in calculations made with 10 dimensions, we can produce all the particles and forces we observe in our 3-dimensional space and 1-dimensional time universe, exactly in the forms we see in this dimension (in terms of behavior, power, etc.). Therefore, a 10-dimensional universe model offers a consistent and stable universe model, and the applications of this model in 4-dimensional space-time are compatible with our observational data[4,6].
To summarize, the experimental proof of String Theory, which acts as a bridge that can bring together the Theory of Relativity and Quantum Physics, has not yet been proven, but it is a consistent explanation in theory.
References:
String theory. (n.d.). New Scientist. https://www.newscientist.com/definition/string-theory/
Wood, C., & Stein, V. (2022, January 20). String theory: A brief overview. Space.com. https://www.space.com/17594-string-theory.html
String theory. (2022, September 5). Encyclopedia Britannica. https://www.britannica.com/science/string-theory
The roots and fruits of string theory – CERN courier. (2019, July 31). CERN Courier. https://cerncourier.com/a/the-roots-and-fruits-of-string-theory/
MasterClass. (2020, October 26). String theory explained: A basic guide to string theory. https://www.masterclass.com/articles/string-theory-explained
A Brief History of String Theory. (2014, March 26). Department of Mathematics at Columbia University - Welcome. https://www.math.columbia.edu/~woit/wordpress/?p=6806
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