F***ing Strings, How Do They Work?
In the last 100 years, our understanding of the universe has increased more than at any previous point in human history. Einstein gave us relativity, which describes the nature of the universe at its largest scales. He described precisely the way in which the universe warps and bends at high speeds or around lots of stuff, by understanding that time itself is another dimension of reality (the 4th). Around the same time, physicists developed quantum mechanics: the characterization of the bizarre and ‘swervy’ behavior of the universe at the atomic level. Both of these theories have passed the most stringent experimental tests ever devised. However, there is a glaring problem staring physicists in the face: quantum mechanics and relativity are mutually incompatible.
The quest for a ‘unified theory’ has been taken up by the best and the brightest, including Einstein, who spend the last 20 years of his life unsuccessfully attempting to reconcile the two theories. The seeds of a new theory were planted by following Einstein’s example of rethinking the nature of reality. Just as Einstein came to understand gravity by mathematically including the 4th dimension, Kazula and Klein realized that they could understand gravity AND electromagnetism by modifying Einstein’s equations to include a 5th dimension. Unfortunately, the Kazula-Klein theory ultimately proved unsuccessful, but it was this radical notion of adding extra spatial dimensions beyond the familiar three dimensions (plus the fourth dimension of time) that allows us to understand a modern day contender for the unified theory: String theory.
The basic idea behind string theory is straightforward: the most fundamental thing in the universe is a string. These strings are so small that you could fit a billion billion strings into the width of one atom. The strings can be open or closed loops, but the important thing to know is that they vibrate, just like a guitar string. The specific ‘notes’ at which the string is vibrating correspond to more familiar bits: quarks, electrons, and the whole menagerie of the subatomic world. If the theory is correct, than that means that everything that exists is the product of the harmony of universal strings.
This idyllic view is a bit oversimplified though, because the math tells us that we have to do the same thing that Einstein did for everything to make sense. We have to add more dimensions. The bad news is that if string theory is correct, than our universe actually has 10 dimensions, not 3. To understand this idea, picture a cube. The corner of the cube has 3 lines (length, width and height) that make up the corner. A 10 dimensional universe would have 10 lines, all perpendicular to each other, making up the same corner! So, you might ask, where the hell are these 7 extra dimensions? String theory gets around this by making the other 7 dimensions really, really small (around the same size as the strings themselves) and curls them up in fiendishly complicated ways. You can picture these extra dimensions by imagining an ant walking on a cable. From far away, the cable looks like a line, and the ant can only go forward or backwards on the line. But if you ‘zoom in’ really close, you can see the cable has a thickness, so the ant can walk ‘around’ the cable without going forward or back. The extra 7 dimensions of string theory are so small and so curled up that we don’t even notice they are there!
There is more bad news: the specific way that those extra dimensions are curled up affects how the strings vibrate (and therefore the properties of the entire universe), but our mathematical understanding doesn’t tell us which exact configuration our universe has. In fact, string theory has five different kinds of over 10^500 possible solutions to this particular problem, with no way for us to tell which one uniquely describes our own universe. Unfortunately, due to the smallness of the strings and extra dimensions, there isn’t really any way we could possibly experimentally investigate them. For experimental physicists, this effectively means there is no way to directly investigate string theory, either to confirm or reject it.
As a scientific theory, it may not ever gain the experimental successes of quantum theory or relativity. Critics of string theory rightly point out that this lack of experimental verification makes string theory more of a mathematical construct than a proper scientific theory. These critics fear that the current research emphasis on string theory is excluding development of other possible unified theories. But currently, string theory is the best candidate for a theory of everything. Stephen Hawking has even used the theory to demonstrate that the universe could be essentially self-caused, answering long outstanding questions like “why are we here” with the elegant vibrations of untold strings drifting through the hidden dimensions of our wonderful and strange universe.