I include here another part from the rather long chapter “From Dirac to the Sixties Crisis” which describes the events that led to the invention of Quantum Chromodynamics.
Gell-Mann, Zweig, and Quarks
In 1964, Murray Gell-Mann and George Zweig separately published what would later become the Quark model, each man publishing a slightly different version. Here I only mention the topics most important for our purposes.
According to Gell-Mann, the proton is composed of three sub-particles, which he named quarks: two u (up) quarks and one d (down) quark. The u quark has an electric charge of ⅔, and the d quark has an electric charge of -⅓. The neutron, however, has two d quarks and one u quark.
In addition, once a proton is bombarded, another kind of quark may also form, known as s (strange), that possess an electric charge of -⅓. This s quark rapidly transforms into either a u or d quark. Many years later, it turned out that there were three other types of quarks, marked by the letters c (charm), b (bottom), and t (top).
Gell-Mann and Zweig’s ideas allow us to understand the deluge of new particles discovered as combinations of these three u, d, and s quarks. Baryons are composed of three of these quarks, and mesons are composed of a quark and an antiquark. Each possible combination has several combinations of its own, just as a hydrogen atom can be in its ground state or at higher energy levels. At that point in time there was still no quark-dependent computational model available that could show how the energy levels of baryons and mesons are achieved.
An examination of Gell-Mann and Zweig’s articles would indicate that there is only one possible way to describe a baryon. Baryons, according to Gell-Mann and Zweig, are made up of three quarks, and no consideration is given to the possibility that they might contain other massive particles.[44,45]
At that time scientists were yet unaware of the fact that half of the proton’s mass is not ascribed to quarks (or to the other quark-antiquark pairs). Therefore, from a historical point of view, Gell-Mann and Zweig’s approach seemed reasonable. Today, however, we can say that the possible models describing the structure of baryons are as follows:
A: Three quarks attracting each other.
B: Three quarks, some attracting and some repelling one another.
C: The quarks are attracting one another. There are three quarks in the outer shell and additional quarks in inner shells (similar to the attraction between protons and neutrons inside the atomic nucleus).
D. A core attracts all three quarks, while the quarks themselves repel one another (similar to an atom with a single shell).
E. A core attracts several quark shells. There are three quarks in the outer shell. The quarks repel one another (similar to the shells of larger atoms).
In each of the C, D, and E baryon models all baryons have the same internal structure, and they differ only in their three outermost quarks.
In the 1960s only models A and B were examined, while the others were disregarded. The B model was proposed by Schwinger, who combined it with the concept of magnetic monopoles (Dirac had a similar concept that addressed the same issue). This idea did not gain any support, probably because its implications would be that neutrons should have a substantial electric dipole moment, and that would be at odds with what we know. That left the A model as the only viable option.
In 1974, physicists were astounded  to realize that half of the proton’s mass is not carried by the quarks. Paradoxically, instead of discrediting or at least doubting the veracity of the A model and reconsidering the suitability of the C, D, and E models, these models were never brought up for debate.
Even after the quarks concept was published in 1964, Gell-Mann was not convinced that those were genuine particles. In the late 1960s James Bjorken and Richard Feynman devised the necessary mathematical tools needed for calculating the behavior of quarks. And so it was that the experiments conducted with the largest particle accelerator at the time, in the Stanford Linear Accelerator Center (SLAC), corroborated Feynman and Bjorken’s predictions, and showed that quarks too were elementary particles with spin ½, and therefore quarks had to comply with the Pauli principle.
However, the Δ++ baryon was already known to science when the quark concept was first published. If the quark concept is correct, it would then follow that this particle is composed of three u quarks. The Δ++ baryon’s spin was known to be 3/2 and its parity is even.
For some reason physicists assumed, and many of them still assume, that the three quarks of the Δ++ baryon are s-waves at the lowest energy level—namely, situated in the 1s shell. Therefore, since the s-wave does not possess orbital angular momentum, the sum of these quarks’ spin is equal to the total spin of this baryon. It follows, then, that in their view a Δ++ baryon contains three u quarks of the same quantum state.
And here we arrive at a contradiction to Pauli’s principle. This is why physicists in the 1960s decided that physics at the time was unable to describe what was happening inside the proton, and that was the reason for the invention of QCD, a theory that rests on assumptions hitherto unknown to physics.
Let’s take another look at the A, C, D, and E models, and see if we can find one among them that can describe the Δ++ particle.
Even the A model, which results in a contradiction, can explain the nature of this particle if we remember that, according to multiconfiguration theory, which accurately describes a particle made up of three or more sub-particles, such a particle would always constitute a mixture, and it cannot consist solely of s-wave particles. That is the same conclusion we arrived at in the previous chapter on spin.
As for the D model, it too can describe the Δ++ particle with even greater ease. Considering the fact that it postulates a baryon composed of four sub-particles (a core and three quarks), it therefore follows that we would need a greater number of effective configurations in order to describe the Δ++ particle.
The C and E models depict a Δ++ even without understanding the configuration concept. They both describe a baryon with closed shells, and so there is no problem with assuming that the three outermost quarks occupy a p-wave, or any other wave that isn’t an s-wave. This is why the quarks possess angular momentum. The E model even explains why the Δ++ particle’s quarks all prefer to be in the same direction, in a manner akin to Hund’s rule of atomic physics.
Another assumption made by physicists was that the proton’s quarks are all s-wave quarks. As we will see later, it is increasingly accepted that the proton’s quarks possess orbital angular momentum—namely, they are not solely restricted to s-waves.
The D and E models, according to which the strong force is similar in nature to electric force, also explain why baryons have exactly three quarks. According to these models, the baryon’s core (D model), or the core added by the internal shells (E model), have a strong charge of +3 and a total electric charge of zero. Each quark has a strong charge of -1, and each antiquark has a strong charge of +1. Considering the fact that the baryon must be neutral in terms of its strong charge (like an unionized atom), it must have three quarks in its outer shell.
In summary, the QCD theory, which is founded on innovative and fantastic ideas, was created in order to explain a “crisis” that is really no crisis at all.
In the cave allegory I mentioned, after pools were discovered by scientists, they claimed that these pools did not contain any water. One would think that I was just exaggerating when I portrayed the scientists in a ridiculous light, right? Well, let’s see if the corresponding story from real-life particle physics is no less improbable:
– In the 1960s scientists thought that there were only three quarks inside the proton and the other baryons and that these particles had no additional quarks.
– Because protons and baryons only had three quarks, these were situated in their innermost 1s shell, which didn’t have enough room for three quarks of the same type whose spins were aligned in the same direction.
– Therefore, in order to abide by the Pauli principle and explain the properties of Δ++, they had to come up with QCD, a theory based on ideas hitherto unseen in physics.
– But when it was discovered that there exists a substantial mass of a non-quark matter at the proton, scientists claimed that QCD was nevertheless true and continued to ignore models C, D, and E.
So which one sounds more unlikely in the readers’ opinions, the cave allegory or the true story of particle physicists?
Following certain discoveries made in the 1980s, many scientists acknowledged the fact that the proton’s exterior quarks do not behave as expected of quarks situated in the innermost 1s shell. These conflicting data are known as the proton spin crisis, and we will discuss them further later. And yet, mainstream physicists are not willing to take the risk and see whether the flaws found in QCD’s premises justify a reexamination of the theory.
 Murray Gell-Mann, A schematic model of baryons and mesons, Phys. Lett. 8 (1964) 214–215. “Baryons can now be constructed from quarks by using the combinations (qqq), (qqqqq-bar), etc.”
 George Zweig, An SU3 model for strong interaction symmetry and its breaking. January 17, 1964. “Both mesons and baryons are constructed from three fundamental particles called aces… Each ace carries baryon number 1/3 and fractionally charged.”
 Julian Schwinger, A magnetic model of matter, Science Vol 165 (1969)
 J. T. Londergan, Nuclear resonances and quark structure, International Journal of Modern Physics E 18 1135 (2009). “A major surprise occurred with the quantitative understanding of the distribution of the proton momentum.”
 Murray Gell-Mann, A schematic model of baryons and mesons, Phys. Lett. 8 (1964) 214-215. “It is fun to speculate about the way quarks would behave if they were finite particles of finite mass…”
 F. Halzen and A. D. Martin, Quarks and Leptons (Wiley, New York, 1984). “When implementing the quark scheme, however, one runs into a trouble… The uuu configuration correctly matches the properties of Δ++ baryon… Its spin 3/2 is obtained by combining the three identical u quarks in their ground state… Such a state is of course forbidden…”