Part two of our series on the Theory of Everything, click here for the first. String theory is one of the leading candidates for a theory of everything, but what actually is it? And, what is the answer to the age old question, how long is a piece of string?
We have talked about particle physics before. In the post we talked about the particles in the standard model, one of our most advanced particle theories at the moment. I described these particles as “fundamental”in the sense that they can not be broken down any further. Conventionally they are thought of points. And as a point they have no width, length or depth. These points can move about through space-time, and have properties like charge and spin, but that is all. At its most basic level, String Theory states that these particles are not just points, but instead strings. These strings have a length, 10 x 10-35m, which is also know as the planck length, but no width. Because they are no longer points, these strings can do more than just move through space-time, they can oscillate. However, because they are so small, these strings still appear to be point particles. The different properties of the string’s oscillation changes what properties the particle has, and therefore, which particle it is.
So what is the importance of this? Well firstly, lets look a little into a bit of quantum mechanics. When electromagnetism and gravity were first described by physicists, both equations were similar in one way. Both the gravitational force and electromagnetic force were inversely proportional to the distance between the two objects. However, when physicists started to consider these forces at a very small scale, with point particles, they had to deal with distances of zero between the particles, and therefore infinite forces. Quantum mechanics lead to a solution to this for electromagnetism, based on the uncertainty principle, in that we can never know where a particle is exactly and so there isn’t a proper singularity when the distance is 0. However, this couldn’t work for gravity, as gravity was no longer described with the Newtonian equation, but instead by Einstein’s theory of relativity. Gravity could not be adapted to fit in with this, as we have talked about in the first part of this series, but this is where String Theory becomes relevant. Unlike in quantum field theory, String Theory requires a force of gravity. String theory appears to be able to describe almost everything very well. So how do these come about? Well lets continue with our quantum mechanics. When we describe interactions between particles, we use Feynman diagrams.
|A Feynman Diagram showing the decay of a proton to a neutron through the weak force. One of the down quarks (D) decays to an up quark (U) by emitting a W- boson (W-) which then becomes a antielectron neutrino (Anti-Ve) and a electron (e-).|
Feynman diagrams show each particle as a line. At the part were two lines intersect, there is some sort of interaction, and the wavy lines are the bosons that carry these interactions. All the particles move along a axis of time. The other axis is that of space. Quantum theory leads to problems, there are infinities that arise when you try to think about interactions in this way, and it is this that leads to what we saw before with the inability to integrate gravity. However, when using String Theory, we can replace these lines with cylinders:
|Two Particles moving through space. On the left we see it as a string, as apposed to a point, like on the right.|
We see now that when strings interact, they merge and break in such a way that would lead their oscillation to change, and therefore can change the particle that they are. More interesting, is that now that there is always a distance between the different parts of the strings, there are none of the infinities that occur with point particles.
There is more to String Theory than that. There are five different types of String Theory, Type I SO(32),Type IIA, Type IIB, SO(32) Heterotic and E8 x E8 Heterotic. They all differ in individual ways, but have one thing in common. They require 10 space-time dimensions, instead of the 4 that we are used too. Before this, we have to discuss the idea of Dp-branes. A brane is, in essence, a group of dimensions that open strings can be attached to either at one end, or both. Only open strings attach to these branes, whilst closed ones, the main one being the graviton, are not attached. The p shows the number of dimensions. Here is an example of a D2-brane:
However, even closed strings can interact with these branes:
The last, very distinctive, difference between String Theory and the standard model is its predictions for “supersymmetry”. Each particle will have a related superpartner, these particles are very similar to their superpartners, but one is a boson, and one is a fermion.
String theory is one of the most advanced candidates for a theory of everything we have. It seems, though, that it will be a long time until it is either verified or disproven. As the strings are supposed to be so small, they cannot be detected, and the energy required to create a state of quantum gravity is so heigh that it is unlikely to be reached for decades. The theory is incredibly complicated, we are only scratching the surface and if you want to find out more, please check out these sources that we used: 1 2 3. And here is a fantastic video on the subject: