These theories explain the world beneath the surface of what we see and touch. Theories of matter explain the building blocks of the world. Theories of energy explain the forces which animate the world.

During the 19th century theories of matter and energy followed two separate but converging paths.  While Dalton,  Mendelov, Thompson and others were discovering atoms and electrons, Oersted, Faraday, Maxwell. Hertz, and others were discovering magnetism, electricity, electromagnetism and radio waves.

The two paths met around 1900 in two seemingly intractable problems.

One problem had to do with "black  body radiation". According to the classical theories of the time, such a body would produce an infinite amount of energy - which clearly was impossible.

The other problem was with the atomic model proposed by Ernest Rutherford.  Based on Maxwell's equations, the electrons in Rutherford's model atom could not remain in position; they would collapse to the center of the atom - also clearly impossible.

The discovery of quantum theory resolved these problems and introduced others.