There are four fundamental forces of physics. Gravitation, electromagnetism, the strong force, and the weak force. These forces, also known as fundamental interaction, are modeled in fundamental physics as patterns of relations in physical systems, evolving over time, that appear not reducible to relations among entities more basic. In this post, only one will be covered, the strong force.
The strong force is not something you interact with every day, but it is what holds all atoms together. So, it's kind of a big deal. Not only does it hold the atom's nucleus together, but it also keeps the quarks inside the protons and neutrons from separating. Sometimes, this is called color force.
Neutrons and protons are both a type of particle called a hadron. Hadrons are made up of even smaller particles called quarks. Quarks are a fundamental constituent of matter, and by fundamental I mean that they cannot be broken down into other particles. They are a fundamental constituent. Quarks, and their friends leptons (including electrons), are the most basic components of matter in the universe.
Among a quark's many weird traits, they all have a property called color. Color is how physicists describe the three different quantum states that quarks can exist in. One we call red, one we call blue, and one we call green. All hadrons are colorless, meaning that the color components of the quarks have to cancel each other out. This is analogous to the way that when we mix red, blue, and green light, it makes white light. Protons and neutrons are each make of three quarks, which means that protons and neutrons can only contain one quark of each color at one time.
To keep things annoying, quarks are constantly changing color, and the process that lets them do that is also what holds the quarks together. It's by exchanging some particles called gluons. Each fundamental force has its own special 'force carrier' that is exchanged between particles that are controlled by that force. The gluon is the force carrier for the strong force. It has no mass, no electric charge, but it does have color. So as gluons are passed between quarks they change color. They do this in such a way so that the color of the quarks always cancel each other out.
Color force doesn't work like other forces. Color force acts on the quarks as though the gluonic exchange were forming rubber bands between them. Quarks can move around inside the hadron, but if they stray too far, the color force becomes very strong and pulls them back. This is why quarks cannot be observed by themselves. It also explains why protons and neutrons are extraordinarily stable.
The strong force is so strong that a residual effect of this is that it's able to hold the whole nucleus together. That is the nuclear force, and that's what we're going to cover next.
If you look at an atom of helium, its nucleus consists of two protons and normally two neutrons, and the thing is, those protons hate each other. Those protons want to get away from each other more than anything else because they are both positively charged. But yet, they exist right next to each other in probably the tiniest space you could ever think of. They do it because of strong force. The strand of this force working in the nucleus is called... you guessed it. The nuclear force.
The nuclear force is strongly repulsive at very short distances. This repulsion helps give atomic nuclei their size. The nuclear force is also strongly attractive at slightly larger distances. So the protons and the neutrons are being both kept apart AND bound together by the strong nuclear force. This is why so much energy is released when an atom is split.
Nuclear force is seen as a residual force brought on by the even stronger version of the strong force, called color force. Sound familiar? There is so much energy in the nucleus that it basically forms another kind of force carrier outside of the hadron called a pion. Pions themselves are made of two quarks.
NOW because color force is being exerted within the pions, and the pions are being exchanged between the hadrons, the pions essentially make it possible for color force to be exerted between hadrons. Pions, though, transmit a scaled down version of the strong force.
This is weird, because hadrons shouldn't partisipate in color interaction. So a pion is like a bus that drives a highschool football team from one county to the next, facilitating a game that would ever happen, otherwise.
Except in this case, the game is keeping the entire universe together, essentially.
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