1.2 What is Fundamental?

summary

String theory aims to be a fundamental theory of everything.

The theory has been pursued for over 45 years. It is the most promising candidate to unify Einstein’s theory of gravity with quantum mechanics.
The basic idea is that at small scales of about 10^-30 cm, elementary particles look like strings or other extended objects, like membranes.
All the physical interactions we observe can be described by string interactions, essentially unifying all fundamental forces. But it is too early to present the final formulation of string theory; it is still a work in progress.
Experiments, unfortunately, can’t help—at least not yet. The size of the string is about a trillion times too small to be experimentally measured at the energies that modern particle colliders can achieve. We don’t have the luxury of using experiments to help guide the theory. And there is no experimental evidence of string theory to date.
However, even the incomplete knowledge of string theory gives us lots of new principles of physics. It revolutionizes a lot of what we know about physics, even though we are far from a complete theory.

Many developments in physics hinge upon giving up what we thought to be fundamental.

To put things in perspective, it is helpful to recall historical lessons where scientists have changed their beliefs of what was fundamental. This has happened again and again in physics, and string theory continues in that tradition. We have to give up a number of principles we thought to be sacred.
Consider the Earth—it was once viewed as the center of the universe. Not only did ancient scientists have this belief, they used this idea to argue that the Earth wasn’t moving. If it were to move in any direction, they argued, it would break the natural symmetry of the world.
Of course, we know that the Earth is not the center of our universe, and it is moving, but this is one example of a clever use of the symmetry argument to try to convey a physical fact that was believed to be true.
Aristotle, while agreeing that the Earth was at the center of the universe, did not accept this argument because he believed symmetry could be spontaneously broken.

We know that time and the state of a system are not absolutes.

Before Einstein’s greatest discovery, we believed time to be an absolute notion, ticking everywhere at the same rate. Of course, Einstein showed us that time was relative to the observer, and there is no absolute notion of time. This was a fundamental principle we had to give up.
What about the absolute notion of the state of a system? That is, can you actually describe where things are in an absolute way? Quantum mechanics tells us no—there is an inherent fuzziness between an object’s position and momentum.
All of these principles, at the time, sounded correct. All of these, by now, have fallen down. They are not fundamental principles of physics anymore.
Similarly, string theory calls for radical changes in our understanding of the fundamentals of physics. It is surprising that even though we do not know what string theory is fully, some of its principles are already well understood.