 One of the key differences between the EM force and the strong force is that the EM force involves an electromagnetic force field, whereas the strong force involves a gluon force field. You'll recall from our previous chapter on elementary particles that quark theory predicted the existence of the omega particle, which was eventually discovered. One of the particle configurations turned out to have three strange quarks. Like two electrons in the ground state orbital for atoms, this presented a problem. These are fermions and follow the Pauli exclusion principle. So an extra property was needed to explain the combinations. For electrons it was spin with two values, up or down. For quarks it was color charge with three values, red, blue, or green. The fact that no charge has ever been seen in the mesons and hadrons made from quarks indicates that the three charge colors neutralize each other in these configurations. This led to the idea to use red, green and blue because they neutralize each other when combined. Our rule for allowing quark combinations was that they had to add up to a whole unit of electric charge. We can now add the rule that they have to add up to no color charge at all. Another even more dramatic difference is that gluons carry color charge as well as quarks. Where quarks carry a red, green or blue charge, gluons carry two charges. One is a color and the other is an anti-color. Here's an example of how this works. We have two quarks, one with a green charge and another with a blue charge. When the green quark disturbs the gluon field it creates a gluon. This gluon carries away a green charge and an anti-blue charge. This turns the green quark blue. When the gluon encounters a blue quark it is absorbed and the gluons anti-blue and green charge turns the blue quark green. The actual functioning of the quark-gluon relationship follows the mathematical model called SU3. The math was invented in the late 1800s and was the foundation for today's abstract algebra. A hundred years later it turned out to be very useful for particle physics. But using color is quite helpful. In fact the study of quarks, gluons and their color charges is called quantum chromodynamics or QCD for short. It is a very active area of research and changes in our understanding are expected as we learn more. Our very idea of what a proton looks like has now shifted from a point particle to a three part particle to a whirlwind of elementary particle activity. In fact it is very difficult to distinguish between the disturbances that represent virtual particles and disturbances that represent actual particles in a plasma like this. But for our purposes we can view a proton as a cloud of gluons holding three quarks together.