Grand unified theories, also called GUT's, attempt to unite three of the four fundamental forces that are thought to govern the universe. These three forces, from the weakest to the strongest, are (1) the weak nuclear force, (2) electromagnetism, and (3) the strong nuclear force. The weak nuclear force is responsible for the radioactive decay of many kinds of atomic nuclei. Electromagnetism is the force that holds electrons to the nucleus. The strong nuclear force binds the nucleus together. Gravitation, the fourth fundamental force, is not included in GUT's. The effect of gravity on a single atom or nucleus is so weak that scientists have never been able to detect it.