Quarks Inside Proton And Neuton
It is often said that a proton contains two “up” quarks and one “down” quark… And that a neutron contains two “down” quarks and one “up” quark… But, this is not entirely true. There are six different flavors of quarks, which we have decided to call “up”, “down”, “charm”, “strange”, “top”, and “bottom.”
Charges On Each Quarks In A Proton
If we define the electric charge of a proton as +1, then three of the quarks each have an electric charge of +2/3, and the other three quarks each have an electric charge of -1/3.
Each quark has an associated anti-matter equivalent, called an “anti-quark”, containing the same mass but the opposite electric charge. The electric charge is responsible for the electromagnetic force.
On the other hand, the Strong Nuclear force is due to a new phenomena, which has no analogy in classical physics. We call this new phenomena the “color” charge, even though it has no relationship to visible color in classical physics.
Color Charge Of Quark
The color charge of each quark can take three possible values. Let us invent labels for these three values and call them "Green", "Red", and "Blue."
Color Charge Of Anti-quark
The color charge of an anti-quark can also take three possible values, which we will call, "anti-Blue", "anti-Red", and "anti-Green."
Besides quarks and anti-quarks, the only other type of particles that contain this so called “color” charge are particles called “Gluons.”
Color Charges Of Gluon
Each gluon possess two color charges: One “Color” and one “anti-Color” of a different type, such as “Red” and “anti-Green.” Gluons do not possess any electric charge.
Strong Nuclear Force
We now know that electric and magnetic fields do not actually exist, and that the electromagnetic force is really the result of the fact that particles with an “electric” charge exchange photons with one another.
Similarly, the strong nuclear force is the result of the fact that particles with a “color” charge exchange gluons with one another. It is this exchange of gluons that holds the quarks together inside protons and neutrons.
Conservation Of Color Charge
When a quark emits or absorbs a gluon, its flavor does not change, meaning that an “up” quark stays an “up” quark, and a “down” quark stays a “down” quark. But, the color of the quark changes. A color is cancelled out by its associated anti-color, and the total amount of “color” charge is always conserved, just as the total amount of “electric” charge is always conserved.
All composite particles, such as protons and neutrons, are what we call “color neutral.” This means that if we count the colors of all the quarks and gluons inside, we will always find the amounts of red, green, and blue to be exactly equal to one another.
Each system is in a quantum superposition of different possible states, and each quark and each gluon is in a quantum superposition of different possible colors.
A gluon can split up into a quark and its associated “anti-quark”, provided that the total color charge is conserved.
These events can also happen in reverse, with a quark and its associated anti-quark annihilating each other to produce a gluon.
Therefore, neutrons and protons don’t just each have three quarks, since many virtual “quarks” and “anti-quarks” are constantly being created and annihilated inside both neutrons and protons. But, for simplicity, neutrons and protons are typically drawn as just having three quarks each.
Protons and Neutrons have the ability to emit a virtual composite particle consisting of one quark and one anti-quark, which can then be absorbed by another Proton or Neutron.
It is the exchange of these virtual composite particles, consisting of one quark and one anti-quark, that creates the force binding Protons to Neutrons inside the nucleus of an atom.
However, these virtual composite particles consisting of one “quark” and one “anti-quark” can not exist for very long, which is why the force binding Protons to Neutrons works only over a very short distance. The Protons and Neutrons in a nucleus therefore have to be very close together for this binding force to work, and to overcome the repulsion from the electromagnetic force.
Electric Field Flux And Gluon Flux
The electromagnetic force acts on all objects with an electric charge, through the exchange of photons.
Photons do not themselves possess an electric charge, and photons will just pass through each other.
On the other hand, gluons do possess a color charge, which means that unlike photons, gluons can absorb and emit other gluons, allowing gluons to attract one another.
Therefore, whereas the imaginary “electric” field flux looks like this…
If we were to imagine a “gluon” flux, the gluon flux lines would attract each other to look like this.
A consequence of the shapes of these lines is that whereas the electromagnetic iforce decreases as we increase the distance between charged particles…
The strong nuclear force between quarks and anti-quarks does not decrease as we increase the distance between them.
If we try to separate quarks by applying a large amount of energy, then this energy will be converted into the creation of a new “quark” and an “anti-quark.” But each quark and each anti-quark we create will still be bound to another quark or to another anti-quark, and we will not observe individual quarks in isolation.
Why we don’t observe individual quarks in isolation, hypothetically
To understand it suppose we lived in a Universe where the electromagnetic force was much, much stronger. In this Universe, all electrically charged particles would clump together in a way such that we would get accustomed to the idea that all composite particles must always be electrically neutral.
This is the situation we have in our own universe with regards to the strong nuclear force, and we have gotten used to the idea that all composite particles must always be “color” neutral.
A composite particle, such as a proton or neutron, is color neutral if it contains the same amount of “red”, “green”, and “blue.” A composite anti-matter particle, such as an anti-proton or anti-neutron, is color neutral if it contains the same amount of “anti-red”, “anti-green”, and “anti-blue.” And a composite particle consisting of one quark and one anti-quark is color neutral if it contains the same amount of a “color” and its associated “anti-color.”
An individual quark can’t be color neutral, and therefore we have never seen an individual quark by itself.
An individual gluon also can’t be color neutral, and an individual gluon also can’t exist on its own, as it would never escape the attraction of other gluons. But, a combination of two or more gluons could be color neutral, and could therefore exist independently of quarks as what we call a “glue-ball.”
Many exotic composite particles consisting of quarks and gluons can be created, provided that they are color neutral, as we do not need to be limited to just the familiar “up” and “down” quarks. Though, these exotic particles are not very stable.