There are four basic forces of physics that are believed to underlie all phenomena in the universe. Listed in order of strength they are: the strong nuclear force, the electromagnetic force, the weak nuclear force and the gravitational force (or gravity). It is thought likely that, in extremely high energy conditions such as occurred near the beginning of the Big Bang, the four fundamental forces of nature are actually unified in a single theoretical framework (known as the Grand Unified Theory). According to quantum field theory, the forces between particles are mediated by other particles, and the fundamental forces can be described by the exchange of virtual force-carrying particles: the strong nuclear force mediated by gluons; the electromagnetic force by photons; the weak nuclear force by W and Z bosons; and gravity by hypothetical gravitons.