What is the true nature of the universe? To answer this question, humans have come up with stories to describe the world. We test our stories and learn what to keep and what to throw away. But the more we learn, the more complicated and weird our stories become. Some of them, so much so, that it’s really hard to know what they’re actually about… like string theory, a famous, controversial, and often misunderstood, story about the nature of everything. Why did we come up with it and is it correct? Or is it just an idea that we should chuck…? To understand the true nature of reality, we looked at things, up close, and were amazed at the wonderous landscapes in the dust, the zoos of bizarre creatures, the complex protein robots, et al. — all of them made from structures of molecules consisting of countless, even smaller, things: atoms. We thought they were the final layer of reality, until we smashed them together, really hard, and discovered things that couldn’t be divided anymore: elementary particles. But now, we had a problem: They were so small that we could no longer look at them. Think about it: what is seeing? To see something, we need light, an electromagnetic wave.
This wave hits the surface of the thing and gets reflected back from it into our eyes. The wave carries information from the object that our brains use to create images. So one can’t see something, without somehow interacting with it. Seeing is touching, an active process, not a passive one.
This is not a problem, with most things. But particles are so small that the electromagnetic waves, that we used to see, are too big to be touched. Visible light just passes over them. We can try to solve this by creating electromagnetic waves with more, and much smaller, wave lengths. But, more wave lengths also means more energy. So, when we touch a particle with a wave that has a lot of energy, it is altered. By looking at a particle, we change it.
So, we can’t measure elementary particles, precisely. This fact is so important that it has a special name: The Heisenberg Uncertainty Principle – The basis of all quantum physics. So, what does a particle look like, then? What is its nature? We don’t know…
If we look really hard, we can see a blurry sphere of influence, but not the particles, themselves. We just know that they exist. But if that’s the case, how can we do any science with them? We did what humans do and invented a new story: A mathematical fiction – The story of the point particle.
We decided that we would pretend that a particle is a point in space. Any electron is a point with a certain electric charge and a certain mass, all indistinguishable from each other. This way, physicists could define them, and calculate all of their interactions…
This is called Quantum Field Theory, and it solved a lot of problems. All of the standard models of particle physics are built on it, and it predicts lots of things, very well. Some quantum properties of the electron, for example, have been tested and are accurate up to a small, infinitesimal percentage. So, while particles are not really points, by treating them as if they were, we get a pretty good picture of the universe…
Not only did this idea advance science, it also led to a lot of real-world technology that we use everyday. But there’s a huge problem: gravity. In quantum mechanics, all physical forces are carried by certain particles. But according to Einstein’s general relativity, gravity is not a force, like the others in the universe.
If the universe is a play, particles are the actors, and gravity is the stage. To put it simply, gravity is a theory of geometry, the geometry of space-time, itself, which we need to describe with absolute precision.
But, since there is no way to precisely measure things in the quantum world, our story of gravity doesn’t work with our story of quantum physics. When physicists tried to add gravity to the story by inventing a new particle, their mathematics broke down, and this was a big problem. If we could marry gravity to quantum physics and the standard model, we would have the theory of everything. So, very smart people came up with a new story.
They asked: What is more complex than a point? A line or a string. Thus, String Theory was born. What makes string theory so elegant, is that it describes many different elementary particles as different modes of vibration of the string. Just as a violin string, vibrating differently, can give you a lot of different notes, a string can give you different particles. Most importantly, this includes gravity.
String theory promised to unify all fundamental forces of the universe. This caused enormous excitement and hype. String theory quickly graduated to a possible theory of everything. Unfortunately, string theory comes with a lot of “strings attached”. Much of the mathematics involving a consistent string theory does not work in our universe, with its three spatial dimensions plus one temporal dimension.
String theory requires ten dimensions to work out. So, string theorists do calculations in model universes. And then they try to get rid of the six additional dimensions, to describe our own universe. But so far, nobody has succeeded, and no prediction of string theory has been proven in an experiment. So, string theory did not reveal the nature of our universe. One could argue that, in this case, string theory really isn’t useful, at all.
Since science is all about experiments and predictions, if we can’t do those, why should we bother with strings? It really is all about how we use the theory. Physics is based on mathematics. This is true, no matter how we feel about it. And the mathematics in string theory does work out. That’s why string theory is still useful.