What String Theory Attempts to Accomplish
To many, the goal of string theory is to be a “theory of everything” — that is, to be the single physical theory that, at the most fundamental level, describes all of physical reality. If successful, string theory could explain many of the fundamental questions about our universe.
Explaining matter and mass
One of the major goals of current string theory research is to construct a solution of string theory that contains the particles that actually exist in our universe.
String theory started out as a theory to explain particles, such as hadrons, as the different higher vibrational modes of a string. In most current formulations of string theory, the matter observed in our universe comes from the lowest-energy vibrations of strings and branes. (The higher-energy vibrations represent more energetic particles that don’t currently exist in our universe.)
The mass of these fundamental particles comes from the ways that these string and branes are wrapped in the extra dimensions that are compactified within the theory, in ways that are rather messy and detailed.
For an example, consider a simplified case where the extra dimensions are curled up in the shape of a donut (called a torus by mathematicians and physicists), as in this figure.
A string has two ways to wrap once around this shape:
A short loop around the tube, through the middle of the donut
A long loop wrapping around the entire length of the donut (like a string wraps around a yo-yo)
The short loop would be a lighter particle, while the long loop is a heavier particle. As you wrap strings around the torus-shaped compactified dimensions, you get new particles with different masses.
One of the major reasons that string theory has caught on is that this idea — that length translates into mass — is so straightforward and elegant. The compactified dimensions in string theory are much more elaborate than a simple torus, but they work the same way in principle.
It’s even possible (though harder to visualize) for a string to wrap in both directions simultaneously — which would, again, give yet another particle with yet another mass. Branes can also wrap around extra dimensions, creating even more possibilities.
Defining space and time
In many versions of string theory, the extra dimensions of space are compactified into a very tiny size, so they’re unobservable to our current technology. Trying to look at space smaller than this compactified size would provide results that don’t match our understanding of space-time. One of string theory’s major obstacles is attempting to figure out how space-time can emerge from the theory.
As a rule, though, string theory is built upon Einstein’s notion of space-time (three space dimensions and one time dimension). String theory predicts a few more space dimensions but doesn’t change the fundamental rules of the game all that much, at least at low energies.
At present, it’s unclear whether string theory can make sense of the fundamental nature of space and time any more than Einstein did. In string theory, it’s almost as if the space and time dimensions of the universe are a backdrop to the interactions of strings, with no real meaning on their own.
Some proposals have been developed for how this might be addressed, mainly focusing on space-time as an emergent phenomenon — that is, the space-time comes out of the sum total of all the string interactions in a way that hasn’t yet been completely worked out within the theory.
However, these approaches don’t meet some physicists’ definition, leading to criticism of the theory. String theory’s largest competitor, loop quantum gravity, uses the quantization of space and time as the starting point of its own theory. Some believe that this will ultimately be another approach to the same basic theory.
The major accomplishment of string theory, if it’s successful, will be to show that it’s a quantum theory of gravity. The current theory of gravity, general relativity, doesn’t allow for the results of quantum physics. Because quantum physics places limitations on the behavior of small objects, it creates major inconsistencies when trying to examine the universe at extremely small scales.
Currently, four fundamental forces (more precisely called “interactions” among physicists) are known to physics: gravity, electromagnetic force, weak nuclear force, and strong nuclear force. String theory creates a framework in which all four of these interactions were once a part of the same unified force of the universe.
Under this theory, as the early universe cooled off after the big bang, this unified force began to break apart into the different forces we experience today. Experiments at high energies may someday allow us to detect the unification of these forces, although such experiments are well outside of our current realm of technology.