## What is color?

Color has the nice property that you need three different colors: red, green and blue, to get back something neutral, white, - which is very similar to what we observe in the proton, where we also need three different "colors" to form something that is going to be neutral in the end: A red quark, a green quark and a blue quark.

[Picture: Intersecting Color plains]

[READ MORE: In fact, protons and neutrons are not really white, or color-neutral. The nuclei of atoms consist solely of positively charged protons and sometimes also neutrally charged neutrons. Hence the electromagnetic interactions cannot hold the nucleus together, but would tear them apart instead. Thus, there must be some very strong force to keep them together, which is where the name of the strong force came from in the first place. The strong force affects both the neutrons and the protons, because they are so close to each other that they will not only "see" the charge of the others as a whole, but also the partial color-charges of the quarks.]

The mediators of the strong force are called the gluons (because they bind particles together very tightly, just like some kind of super-glue). But the strong force is very different from the electromagnetic one in various aspects. Speaking-of the mediators instead of the forces, this means that the gluons are different from the photons in a very fundamental way: they carry color, unlike photons, which aren’t electrically charged themselves. This means that gluons can participate in their own interaction, which makes the strong interaction very special.

## The strong force is special!

One consequence of this is that the strong interaction does not decrease with distance. Two electrically opposite charged objects will attract each other less when their distance increases. The strong interaction of two particles of different color, on the other hand, will increase rapidly as you tear them apart, and eventually grows so strong that the energy you needed to remove the particle any further would be sufficient to create new colored particles instead (due to Einstein’s famous relation E=mc², allowing us to transform energy into mass).

[Cool animation showing color string snapping and quark pair creation]

This process is the reason why we have up to now never observed an isolated quark - they only appear in groups of three or two (as in the animation, although we have not explained how this happens for pairs yet, and this will have to wait until we explain antimatter). The property of quarks to stick together so tightly and to never show up alone is called “confinement”.

## Outlook

We have come pretty far. We know how the protons and neutrons are made from quarks, and we know what holds together the protons and electrons to form atoms. This is a great achievement, as it allows us to explain about everything that holds together the building blocks of natures. But it is not the whole story.

Sometimes, thinks break apart - and it was only in the 20th century that radioactivity was discovered - the breaking apart of nuclei, emitting highly energetic and dangerous radiation. We cannot explain this kind of thing happening until now, and this will be what the next chapter is about.