String Theory
String Theory is one of the most famous attempts at a theory of everything, a theory that combines quantum mechanics and general relativity. It tries to do this by searching for more fundamental aspects of reality than either theory.
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This search for what is most fundamental is one of the driving forces of physics. We want to reduce the universe to its simplest form. This is also why the standard model is considered incomplete; it has too many moving parts and parameters that must be fixed using measurements.
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String theory started in the 60s when scientists noticed the flux tubes in mesons. A Quantum theory was created using the flux tubes with the physics of strings, but it ran into problems and got replaced by QCD. One of the main reasons it got stuck was because it predicted unexpected vibrational modes in the gluon field of the "strings". You may recall that a vibrational mode of a quantum field is just a particle. In other words, it is the field vibrating in a discrete value (Go to the QFT page for a review).
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One of the particles that it predicted looked like a massless particle with a spin of two, which fits the description of the graviton, the theoretical particle that carries the gravitational force. However, nothing like the graviton should appear in this kind of string.
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This led to the idea that the math of strings could be used to describe a theory of quantum gravity. Furthermore, it could be used to describe all force-carrying particles. This version of String theory was called bosonic string theory.
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In order to do this, the math changed to describe strings that were 20 times smaller and added 22 extra dimensions. When we talk about dimensions, we are talking about length, width, height, and time. String theory predicts that there are more dimensions to reality that we are unable to perceive.
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By combining this with supersymmetry, which is another theory that we will discuss, Superstring Theory was created, and the number of dimensions is reduced to 10. Later in 1995, Ed Witten would create M-theory out of many forms of Superstring Theory.
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How do Strings, and String Theory, work? It comes from the idea that strings can carry waves, which can loop and cause constructive and destructive interference. Imagine something like a jump rope or exercise rope.
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Constructive interference only happens when the wavelength​ fits the string length a certain number of times. And so, with certain energy and length values, the strings vibrate like this.
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The strings that don't vibrate cleanly eventually die out and the ones that vibrate cleanly stay. This discreteness is a behavior that you might have seen many times in quantum mechanics.
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Tweaked correctly, these strings could match the properties of all known particles. Particle mass could come out of String length and tension. String length also describes which vibrational modes are possible, which then defines other particle properties. In this way, the number of fundamental parts is reduced and we get closer to a fundamental theory. By defining one value, all the other values are defined as well.
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These strings can vibrate (hold energy), stretch, merge, and split apart. This lets them decay and interact. This structure of the string gets past many of the problems we encounter in the math for a theory of quantum gravity, making it a huge part of string theory.
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In order to describe all the properties of these particles, the strings vibrate in a bunch of extra dimensions. So where are they?
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Imagine we lived in a flat, 2d universe with length and width. The extra dimensions are stored in an extremely tiny height dimension, so small that only the strings could access it. Likewise, we live in a universe in 3 spacial dimensions and 6 tiny dimensions.
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It is important to know that these strings are very real. That begs the question: what are the strings made of? There are many answers but what theorists usually say is that it is most fundamental and it isn't made of anything.
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While string theory is an interesting theory that answers a lot of questions we have about the universe, there are problems. It has no confirmed predictions, and some could say that it has no predictions that can be tested. However, because of how elegant and promising it seems, it may be the correct path.
