The QCD theory, which constitutes the part of the Standard Model’s explanation of the Strong Interaction, is built on a large number of new assumptions, unmatched among known physical theories. QCD introduced the notion of the 3 colors, the idea that some particles “cannot exist separately” because their color is not “white”, etc. Rare were the theories throughout Physics history that did not grow out of a progressive historical development, let alone the need to make up so many assumptions in order to render it consistent with reality.
By the way, the fact that a theory requires so many new assumptions, which in addition lead to the establishment of an exclusive, new language, is a sign of weakness. A good theory requires a minimum of assumptions, which are as intuitive as possible. On the other hand, as we are all somewhat romantic, we’re happy to discover that nature behaves in such a surprising way.
What was known at the time
The birth of the QCD theory in the late sixties and seventies of the last century and its global acceptance as an unshakeable theory, is the outcome of a fascinating historical evolution.
In the 1960s and 70s, scientists had already discovered the quarks as constituents of the nucleon (protons and neutrons), as well as other particles composed of quarks called Hadrons. When physicists tried to understand the nature of the force holding quarks together inside Hadrons, they were had to confront quite a few questions, including some essential questions specified below:
– Why do nucleons contain precisely 3 quarks?
– Why are there no particles with 2, or 4 quarks?
– How come there are Mesons composed of a quark-antiquark pair?
– How does the force holding quarks together behave?
– What’s the explanation for “the Baryon Conservation Law”? This law states that the number of baryons in physical processes remains constant. The nucleons are part of the baryon family.
– How can one account for the existence of the “omega –” and “delta++” particles? (This topic will be clarified in another article.)
– How can one account for the similarity between the Nuclear Force and the van der Waals force?
At that time, it was not yet known that less than half of the linear momentum of a nucleon moving at a very high speed is carried by quarks. This means that most of the Nucleons’ mass is actually not in these quarks. If this information was available for scientists at that time, they may have had less difficulty adopting a theory assuming the existence of massive objects inside the nucleons. But this was only discovered after QCD was recognized by a large number of scientists.
What Comay’s theory says
Comay’s model assumes that the Strong Interaction is essentially similar to the Electrical Force, which means that particles with an oppositely signed “Strong Charge” attract each other. According to Comay’s model, nucleons have a positively strong-charged core, and that the quarks’ strong charge is negative. Thus the core attracts the quarks, and the quarks repel each other, in an analogy to the action of the electric force in the atom. The magnitude of the nucleon core’s strong charge is 3 units, and the quark charge is one (negative) unit. Thus, without having to generate any other major hypothesis, the questions physicists are confronted with are readily answered:
– The nucleon’s core’s charge has the same magnitude as the charge of three quarks but with an opposite sign since nucleons are neutral for strong charge.
– Two quarks repel each other and this is why there are no particles containing only 2 quarks.
– Quarks and antiquarks attract each other and this is the reason for the existence of mesons. The idea that an anti-particle carries a charge opposite in sign to the particle has been an inalienable asset to particle physics since over a half of a century. Here Comay assumes that this feature works for the strong charge as well.
– The Baryon Conservation Law results from the fact that every baryon has a core, which means that there are as many baryons as there are cores.
– This model serves as baseline for understanding the similarity between the van der Waals forces and the Strong Nuclear Force, both being residual forces of fundamental interaction characterized by the attraction between opposite charges and the repulsion between identical charges.
There is a subtlety to the theory: it seems like there are additional quarks in inner shells of the nucleon, in analogy to an atom filled by electron shells with three electrons on the external shell. This is an important assumption because it offers an explanation to quite a few phenomena that QCD is unable to account for, such as the increase in the elastic (as well as total) cross section curve in higher energy proton collision. This matter will soon be discussed in a separate article.
Comay’s clear and complete formulation of the Strong Interaction’s behavior clarifies phenomena which contradict the existing models. This too will be discussed in separate articles.
What QCD says
In contrast to the assumption of the existence of the nucleon’s core, building the QCD theory required the invention of a long series of new assumptions, some of them look quite fantastic. In order to account for the key issues around the Strong Interaction, QCD states that:
– The force is related to 3 colors (a new idea first presented when QCD was conceived)
– Every quark has a different color
– Only particles containing equal amounts of the 3 colors (of total color “white”) can be physically measured. The other combinations are “forbidden” and can’t exist as separate entities.
– The force is carried between the quarks by means of gluons, which are massless particles.
– Every gluon has a non-white combination of color and anti-color (for that reason no individual gluon can be found in a physical measurement).
– One of QCD’s results is the existence of an attractive force between the quarks, which increases as quarks move away from each other (unlike any other known elementary force in nature).
After QCD was conceived, it turned out that most of the nucleon mass is not carried by quarks. Therefore, scientists decided that the gluons are the ones carrying this missing mass. This would mean that the gluon’s behavior is fundamentally different than that of another particle- the photon- who has no rest mass as well.
The unasked questions
According to the QCD equations, the intensity of the force between 2 quarks grows as they move away from each other. It is therefore unclear why a quark on the moon is not attracted to a quark on earth with a tremendous force. QCD assumes that the force stops – this assumption is called “cutoff” – but for QCD, the explanation why cutoff occurs is still open.
With regard to the force acting between nucleons – the Strong Nuclear Interaction – this turns out to be a borderline issue between the fields of Elementary Particles and Nuclear Physics, and there is no single coherent theory admitted by the entire scientific community. A widespread and justified view qualifies the Nuclear Interaction as a “residual force”, which means that it is the consequence of the action of the Strong Interaction on the quarks, which emphasizes its resemblance to the van der Waals force. However, the van der Waals force exists only because the Electric Force has a simple structure of attraction between opposite charges and repulsion between identical charges. On the other hand, the structure of the force described by QCD is highly complex and does not explain the “residual” nature of the nuclear force in the context of nucleons in the nuclei.
Some physicists still hold the explanation that Yukawa provided in 1935 to the strong nuclear force, even though it is not related to any residual force.
In this state of affairs, QCD does not explain why the quarks within one nucleon inside an atomic nucleus are not powerfully attracted to the quarks inside the neighboring nucleon. It is also not clear how the potential curve mentioned in here is formed. There is no explanation to the fact that the nucleons’ density inside heavy atoms is constant. And as we already mentioned, there is no satisfactory explanation to the first EMC effect.
A note of “mathematical” nature regarding van der Waals force and the Strong Nuclear Interaction: the most prominent feature of van der Waals force and the Strong Nuclear Interaction is the fact that when the particles move away from each other the force rapidly decreases. In the case of van der Waals force, the phenomena is explained by the “Screening Effect”, i.e., when looking at the atom from a distance, electrons and protons carrying opposite charges cancel each other when the electron distribution is spherical, and nearly cancel each other when the electron distribution is not spherical. This effect has a profound mathematical reason: the intensity of the original force decreases according to the inverse square law.
Comay’s model indeed describes 2 forces: repulsion between quarks and attraction between each quark and the nucleon’s core. These forces are equivalent to electric forces, and also decrease according to the inverse square law. On the other hand, in the case of QCD, which is built of different equations, there is no analogy to van der Waals force.
Further details of QCD’s failures are described in additional articles.