In this post we’re going to start a new series in exploring the four fundamental forces, which is how all stuff interacts with itself and everything else [source]. The four fundamental forces of physics are:
Gravity is perhaps the most familiar of the forces, but before we get to it, we will start looking at the strong force, before delving into gravitation, weak and electromagnetism.
So, what is the strong force? Well, as the name implies, it is the strongest force of the four, at approximately 137 times as strong as electromagnetism, a million times stronger than the weak force and 1038 times stronger than gravity [source]. The strong force is responsible for binding together the fundamental particles – particles that cannot be split into smaller parts – of matter to form larger particles [source]. What this means is that the strong force is what holds the atom together so that the nuclei does not just fly apart. This was strange, because it seemed like they would, due to the repulsive electromagnetic force between the positively charged protons in the nucleus. In fact, the strong force keeps both the protons and neutrons in the nucleus through something called the color force. You see, neutrons and protons are both a type of particle called a hadron, where hadrons are made up of quarks.
Quarks have a property called color, which is not the same of as how we think of color – we can’t even see quarks. So, when physicists say color, it means how they describe the three quantum states that quarks can exist in. Those three states are Red, Blue and Green.
The reason why quarks are assigned colors is because hadrons are required to be colorless, meaning that the color components of the quarks have to cancel each other out. When you mix the colors above, you get white.
Particles, Poems and Beer
Let’s rewind a bit and get back to the word quark. The first physicist that predicted the quarks was Murray Gell-mann. Gell-Mann named them quarks from the poem of Finnegans Wake, by James Joyce:
Three quarks for Muster Mark!
Sure he hasn’t got much of a bark
And sure any he has it’s all beside the mark.
The three quarks are the recipes of protons and neutrons, and Gell-Mann reimagined the line as a call for drinks at a bar [source]. Was this story necessary for this blogpost? Perhaps not, but it’s important to know that even physicists try to have a good time and enjoy a cold one at the bar every now and then.
Protons and Neutrons
As we discussed, neutrons and protons are made up of three quarks with different colors, but where it becomes a bit more complicated quarks constantly change color and the process that lets them do this is also what holds them together. To find out more on this topic, check out this link [source].
What this means is that each fundamental force has its own special force carrier that’s exchanged between particles that are controlled by that force. The force carrier for the strong force is called gluon. It has no mass or electric charge, but it does have color.
So, when gluons go from quark to quark, they change color of the quark that they leave and the one that they go to
As you see in the gif, the colors of the quarks always cancel each other out, thus keeping the hadron neutral.
Because gluons are attracted to both quarks and other gluons, the strong force is concentrated along a line connecting two quarks, almost looking like a “rubber band”, as we see in the image below.
Gluons can move around in many directions inside the hadron, but if they happen to move too far away, the color force pulls them back with tremendous force. This is the reason why you don’t observe quarks by themselves, floating around, and why the three quark-hadrons are exceptionally stable [Understanding the Universe: From Quarks to the Cosmos – By Don Lincoln]. This stable force is the reason why the whole nucleus holds together. That specific strong force is called the nuclear force.
The nuclear force is very strongly repulsive at very short distances, around 0.7 femtometer, so the nuclear forces are what keeps the protons and neutrons away from each other. That’s kind of interesting considering that neutrons are electrically neutral and don’t really mind who they are near.
At slightly larger distances (around 1-2.5 femtometers), on the other hand, nuclear force becomes strongly attractive, [https://en.wikipedia.org/wiki/Nuclear_force]. This means that the repulsive (and attractive) force determines the physical size of the nuclei.
There you have it, one of the four fundamental forces that determine the interaction of the universe. Stay tuned next time, when we look into the Weak Force, to further understand the basis of all physics around us. Also, check out this clip, which has been an inspiration for this post.