One of the most profound goals of modern theoretical physics is to describe the four fundamental forces in nature -- gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force -- with a single mathematical law. In 1915, Einstein published his theory of general relativity, which described gravity as not just a Newtonian force, but as a curvature in space-time. He then sought to broaden general relativity to encompass electromagnetism, the only other fundamental force known at the time, thereby simplifying scientific understanding of the universe.

By the 1930s, while Einstein was still searching for this unifying theory, scientists probing the structure of the atom discovered two more fundamental forces: the strong nuclear force, which binds protons and neutrons together in the nucleus of an atom, and the weak nuclear force, which turns neutrons into protons, giving off radiation in the process. The field of quantum mechanics now provided a framework for understanding the universe on a very small scale, while general relativity explained behavior on a very large scale. However, scientists were at a loss to reconcile how the two different sets of laws describing nature at completely different scales could both be right. For example, to properly understand black holes required both general relativity and quantum mechanics, not simply one or the other.

While Einstein ultimately failed at unifying gravity with electromagnetism, scientists in quantum mechanics were able to unify electromagnetism with the strong and weak nuclear forces. They discovered elementary particles of force called messenger particles; the exchange of these messenger particles creates what we experience as force. That the strong and weak nuclear forces and electromagnetism each possess messenger particles -- called gluons, weak gauge bosons, and photons, respectively -- meant these three forces could be described using the same mathematical language. Thus, quantum mechanics tells us how all of nature's forces work at the subatomic level -- except for the force of gravity. Even today, no one has ever discovered a similar messenger particle for gravity (a "graviton"), which would explain how gravity -- the odd man out -- operates at the level of atoms and subatomic particles.

Learn in this NOVA classroom activity about the four fundamental forces and the interactions they govern.