Although classical mechanics is largely compatible with other " classical physics " theories such as classical electrodynamics and thermodynamics, some difficulties were discovered in the late 19th century that could only be resolved by modern physics. When combined with classical thermodynamics, classical mechanics leads to the Gibbs paradox in which entropy is not a well-defined quantity. As experiments reached the atomic level, classical mechanics failed to explain, even approximately, such basic things as the energy levels and sizes of atoms. The effort at resolving these problems led to the development of quantum mechanics. Action at a distance was still a problem for electromagnetism and Newton's law of universal gravitation, these were temporary explained using aether theories. Similarly, the different behaviour of classical electromagnetism and classical mechanics under velocity transformations led to the Albert Einstein 's special relativity.