How can we construct successful new scientific theories? If there’s some phenomenon in the Universe that current theories don’t seem to explain — like dark matter, dark energy, neutrino oscillations or quantum gravity — it’s tempting to throw out our current theories, and start over from scratch. Unfortunately, this gives us too much freedom
in constructing new theories. Historically, a more successful approach has been to use existing theories to identify useful overarching principles that can guide the development of new theories. So, for example, classical mechanics led to the principle of conservation of energy and the ideas of Hamiltonian mechanics, ideas that played an important role in the development of quantum mechanics, even as classical mechanics itself was superseded.

Since 1954, one of the most important guiding principles in physics has been that our description of the world should be based on a special type of classical field theory known as a Yang-Mil ls theory. With the exception of gravitation, all the important theories of modern physics are quantized versions of Yang-Mills theories. These include
quantum electrodynamics, the electroweak theory of Salam and Weinberg, the standard model of particle physics, and the GUTs (grand unified theories) proposed in the 1970s as extensions of the standard model. The most important of these theories is the standard model of particle physics, which is our current best theory of how matter works. People sometimes describe the standard model as a Yang-Mills theory with an U (1) × S U (2) × S U (3) gauge symmetry.