Just for fun, let's talk about an engaging field in theoretical physics.

String theory is an attempt to model fundamental particles as one-dimensional extended objects (aka strings), as opposed to point-like particles. Secondary goals include developing a quantum theory of gravity, and a unified theory which can describe the four basic forces of nature (gravity, electromagnetic, and the weak and the strong nuclear forces) with a single set of self-consistent equations.

What is the problem? Quantum mechanics describes the behavior of the electromagnetic force and the strong and weak forces, but has not yet been merged successfully with general relativity, the theory of gravitation. Quantum theory visualizes particle fields embedded within a flat space-time, while general relativity characterizes gravity as a variable curvature in space-time itself. General relativity has been very successful in describing and predicting the large-scale structure of the Universe (the motion of planets, stars, galaxies, and clusters of galaxies). In contrast, the tests of the electromagnetic and nuclear forces have been made on the microscopic scale of individual of molecules, atoms, and particles.

We can understand string theory by analogy to the behavior of real-world strings, such as those used in musical instruments. Imagine a single cello string, one which is stretched and held at both ends under tension. By varying the amount of tension along the string, one can create musical tones of different wavelengths (energies). These tones are equivalent to a set of excitation modes, for each string. By analogy, elementary particles, the building blocks of nature, could be modeled as the excitation modes of elementary strings. When strings split and combine together, we perceive the process as particles which are emitting and absorbing other particles. Unlike cello strings, elementary strings drift freely within our spacetime. However, they still have the property of varying tension.

If strings exist, however, they are expected to have sizes of order 10^-35 meters, far, far smaller than the smallest objects that we can detect with existing (or projected) particle physics technology.

String theory predicts the existence of high-dimensional stringlike objects, called branes, and therefore the existence of higher dimensions of spacetime than we perceive (roughly ten eleven, or twenty-six, rather than our three spatial and one timelike).

Certain string theories include particles called fermions (quarks and leptons, particles which may make up the basic building blocks of matter), as well as bosons (photons, gravitons, gluons, particles which can transmit forces). (Note that the protons, electrons, and neutrons which we have been studying in atoms are built from sets of these two types of fundamental particles.) This subset of string theory requires a condition called supersymmetry, under which every boson has a corresponding fermion, unequivocally connecting matter to the fundamental forces. Supersymmetric partner particles to known particles have not yet been observed, but the latest generation of particle accelerators has the potential to detect such objects, if indeed they exist.

A cornerstone of a robust theory is the ability to make testable predictions – how does string theory measure up?