Figure 26-10: Unification of the Four Forces The strength of the four fundamental forces depends on the energy of the particles that interact. As shown in this schematic diagram, the higher the energy, the more the forces resemble each other. Also included here are the temperature of the universe and the time after the Big Bang when the strengths of the forces are thought to have been equal. At energies between 1014 and 1019 GeV, the forces might be described by a grand unified theory (GUT), and at energies above 1019 GeV, the forces might all appear similar in a supergrand unified theory.