String Theory & Parallel Universe

by Chai Yik Xin

Introduction

When we talk about Theoretical Physics, people consider it overly complicated to understand as abstract concepts are difficult to imagine. The general public’s exposure to physics theories, especially those that have remained not proven, is diminutive. Here is a bizarre yet beautiful theory, string theory. It all started from a hypothesis that states a string makes every other subatomic particle. Theorists claim this structure is tinier than quarks and gluons, subatomic structures that form protons and neutrons. They are the smallest and the most basic structure of everything.

For instance, if we see a cable line from a distance, it merely looks like a long cylinder. But if we zoom in on the cable line, we can see it constitutes many other tinier cable lines grouped together. The string exists in a particle similar to these cables invisibly. This invisible string would vibrate in different patterns and produce a number of distinct particles such as protons, electrons, and quarks; not unlike guitar strings which look alike but produce different musical notes. Due to this invisibility, it is arduous to prove the existence of the string-like structures.

Importance

But why do we need string theory? Why do physicists in quest of this abstract theory for their whole lifetime? The answers lie in General Relativity and Quantum Physics. General Relativity is a theory by Albert Einstein which explains the macro-perspectives, such as how the sun, black holes, and galaxies work. Quantum Physics explains the natural behaviour of all things at atomic and subatomic levels, which is in the micro-perspective. These two theories work perfectly well separately. As we can see, General Relativity has helped in the massive development of our understanding of black holes and the Big Bang, as well as helping us develop the atomic bomb. However, when we look closer, these two theories contradict each other. Therefore, an ultimate explanation is required to solve this issue. In other words, we need a theory to unify these two theories. However, string theory was not founded for this very reason. It was discovered accidentally in mathematical calculation by Leonard Susskind, due to his curiosity about the “strong force” binding protons and neutrons in atoms in 1969.

Einstein, too, tried to find the Theory of Everything which can unify all the physics equations in merely one ‘simple’ equation. Alas, he did not finish his work on the Theory of Everything during his lifetime. After a very long time of research and tons of hypotheses, scientists found out that string theory is the most suitable and possible “backup theory” or solution to the Theory of Everything.

Extra Dimensions

Over the years, theorists came up with various versions of string theory such as bosonic string theory, five different versions of superstring theory, and lastly, M-theory conjectured by Edward Witten which unifies all versions of superstring theory. Superstring theory claims that string theory works in 10 dimensions whereas M-theory claims that string theory works in 11 dimensions (7D hyperspace + 4D).

So what are dimensions, and what is hyperspace? Dimensions are the “2D”, and “3D” that we mention in our daily lives. For instance, a drawing of a rectangle which is made up of width and length is 2D whereas an actual object that we can hold as a handphone that has width, length, and height is 3D. For hyperspaces instead, they are spaces that are more than three dimensions.

Theorists claim that dimensions more than six are so small that we cannot see or observe them. Therefore, a hypothesis was presented, that higher dimensions curl up or undergo “compactification” and appear as a Calabi-Yau manifold.

Calabi-Yau manifold. (Credit: https://plus.maths.org/content/hidden-dimensions)

There are no shreds of evidence of the existence of extra dimensions. The dimensions mentioned in string theory are for the mathematical calculations to work and be reasonable. It is merely a prediction of a particular model that would work for a specific dimension. In quest of this, the Large Hadron Collider (LHC) was built in Geneva, Switzerland. It is the largest and highest-energy particle collider which involves the collaboration of over 100 countries, 10000 scientists, and hundreds of universities and laboratories. Other than that, the LHC is also in quest of many other unsolved problems in physics. For instance, in studying why the behaviour of the fourth fundamental physics force (gravity) is much weaker than the other three fundamental forces (electromagnetism, strong nuclear force, and weak nuclear force).

Multiverse Hypothesis

Have you ever imagined how a parallel world would look? What would the other version of you living in the parallel world be? String theory could make the multiverse not merely a renowned topic in novels and dramas but explain its plausibility. Leonard Susskind, a professor at Stanford University, linked string theory and parallel universes together. As different vibrations of the string can make up various particles and internal spaces — he figured that it could also make up a variety of universes with different properties. He proposed a hypothesis that our universe is merely one of the boundless variety of multiverses, just like an island among an archipelago.

If we investigate glasses, the world is finely tuned for us, humans, and other living things on Earth. For instance, gravity is neither too weak nor too strong, the mass of a proton is almost the same as a neutron, and the Earth is placed conveniently at an exact position that is suitable for the survival of living organisms. They are like a series of perfect coincidences and a delicate God’s creation. But if a boundless multiverse exists, some with suitable living conditions for living things and some not, then this becomes sensible in a more physical sense.

However, string theory reached an impasse after it was proposed. As string theory had 10500 solutions, this means the theory could be “The Theory of Nothing” instead of “The Theory of Everything”.

Contributions of the String Theory

Although string theory has not been proven, it has already made massive contributions to mathematical physics, which helped solve many problems in black hole physics, nuclear physics, early universe cosmology, as well as condensed matter physics. For instance, string theory helps explain the nature of black holes, which are small in terms of length but big in terms of mass, perfectly.

Conclusion

String theory is a strange yet fascinating theory that can take years or even a lifetime in the quest of searching for the answer. Stanford physicists helped bring string theory and cosmic inflation, the two most enduring ideas in modern physics that circled each other together, into a String Theory Landscape that helps to enhance the explanation of the Big Bang Theory. However, none of these theories is proven though String Theory Landscape has been proposed for nearly two decades.

Voices saying researching such abstract topics is a waste of time and resources might be heard frequently. Some research might not look useful or make any contributions at the time, but it can change human history or even save lives. For instance, when the concept of Young’s Modulus was developed in the 17th century and finally proven in the 19th century, no one, even the one who developed, nor the one who proved this equation, expected this equation can be used to make a bulletproof shirt that has saved countless of lives today.

References

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