Quantum physics is weird. As soon as you think you understand it, you find five more reasons you don't. Take this, for example: the Standard Model of quantum physics says that everything is made up of elementary particles, including things like quarks, electrons, and protons. But string theory says nope, not quite.
All of these different particles? They're just strings, each vibrating in their own way. That might sound like it adds unnecessary complexity, but it could solve a problem that has plagued quantum physics for decades.
As you can see in the chart below, the Standard Model says that there are 12 basic building blocks to the universe: six quarks, and six leptons. Bundled up with this are the four fundamental forces: gravity, electromagnetism, and the weak and strong nuclear forces. Surprisingly, it's the most familiar of these—gravity—that has scientists the most puzzled. Why?
Well, the three other forces each arise from the exchange of an elementary particle: photons impart the attraction of electromagnetism, gluons are the "glue" that binds the strong nuclear force, and the W and Z bosons whip the weak nuclear force together.
Scientists have proposed a particle that could be responsible for gravity, called the graviton, but we're not sure if it exists.
That's where string theory comes in. String theory proposes that each elementary particle is just a different version of a very tiny loop of string. Just like a guitar string vibrates at different frequencies to create an A or an F#, these strings oscillate in certain ways to create the different kinds of particles. Oscillate in one way, and we see an electron; oscillate in some other way, and we may see a photon or a strange quark. Once you accept that everything is just oscillating strings, it's pretty simple to accept that there's a kind of oscillation that creates a graviton. Boom: the mystery of gravitation is solved, and the four forces of nature are united at last.
Okay, well, maybe string theory isn't all that simple. For one thing, it requires the universe to have at least 10 dimensions to work (and some versions require as many as 26). So how is it we only perceive four dimensions: up-down, right-left, forward-backward, and time? There are a few explanations.
One is compactification, or the idea that the other dimensions are folded down in a way that keeps us from perceiving them. It's kind of like looking at a piece of paper directly from the side makes you see just a line, even though that paper has a lot of surface area you aren't seeing. Other theories invoke the existence of higher-dimensional objects called branes. Unfortunately, all these possibilities and explanations have yet to come up with that big shiny unifying theory that physicists are chasing. Still, that won't keep them from trying.