Section 2–4 Nuclei and particles

(Nuclear force / Elementary particles / Elementary interactions)

Feynman starts this section by explaining how a bigger explosion is due to smaller particles, nuclei. That is, a greater amount of nuclear energy released in an atomic bomb has to with changes inside the nucleus, whereas a relatively lesser amount of chemical energy released in a TNT explosion has to do with changes of electrons on the outside of the atoms. This is related to stable nuclei that are held together by enormous nuclear forces. In this section, the three main ideas discussed are the nuclear force, elementary particles, and elementary interactions.

1. Nuclear force:

“… The question is, what are the forces which hold the protons and neutrons together in the nucleus? (Feynman et al., 1963, section 2.4 Nuclei and particles).”

According to Feynman, Yukawa suggested that forces between neutrons and protons have a field that behaves like a particle when it “shakes.” In 1935, Yukawa predicted that the particle should have a mass of two or three hundred times of an electron, and it could be discovered in cosmic rays. The first candidate for Yukawa’s prediction, initially called a μ-meson, was discovered in 1936 by Carl David Anderson and others through cosmic rays interactions. This particle was later proved to have the wrong properties and it is later renamed as a muon. (Note: Feynman’s statement “it was called a μ-meson, or muon” is potentially confusing.)

In 1947, Powell, Lattes, and Occhialini discover pion (previously known as π-meson) in products of cosmic rays that satisfied Yukawa’s criterion. The delay in discovery is due to the World War II that ended in August 1945 as physicists gradually returned to research. However, calculations involved in the theory of quantum nucleodynamics are so difficult that no one was able to figure out the consequences of the theory and to check them against experiments for almost twenty years. Although Feynman mentions quantum nucleodynamics which could be appropriate to describe the theory of nuclear force, it has been replaced by the term quantum chromodynamics (QCD).

Yukawa’s idea of a new particle was novel because it was thought that the nuclear force could be resulted from the exchange of electron and neutrino. The situation was also messy in the 1930s because Bohr proposed that energy is not conserved and Heisenberg thought that electron might be the particle involved in the nuclear force. Furthermore, it was unclear whether the nuclear force is a fundamental force or a secondary force that can be derived from the electromagnetic force. Importantly, Yukawa (1982) explains that “I had this idea of a nuclear force field very early. Looked at from the quantum mechanical viewpoint, a field of force, almost by necessity, implies that there is a particle accompanying that field. We actually infer the existence of the photon as the particle accompanying the electromagnetic field… (p. 194).” Yukawa was awarded the Nobel Prize in Physics in 1949 for the prediction of pions.

2. Elementary particles:

“… Several particles have been omitted from the table. These include the important zero-mass, zero-charge particles, the photon and the graviton, which do not fall into the baryon-meson-lepton classification scheme… (Feynman et al., 1963, section 2.4 Nuclei and particles).”

Feynman has discussed problems in defining elementary particles. First, he asks whether new particles discovered in 1961 are really particles because their lifetimes are very short and they could be considered as some kind of “resonance” interaction of a certain definite energy. Next, the photon and the graviton do not fall into the baryon-meson-lepton classification scheme, however, there are also problems in defining baryon, meson, and lepton. Originally, baryon, meson, and lepton mean “heavy-weight particle,” “middle-weight particle,” and “light-weight particle” respectively. Moreover, Okun (1962) introduces the term hadrons that mean strongly interacting particles. Currently, hadrons are classified as two families: baryons as bound states of three quarks and mesons as bound states of a quark and an anti-quark. The term lepton is a misnomer because a muon (a member of the lepton family) is relatively heavy.

Historically speaking, Feynman (1948) writes that “it appeared that we now know, with varying degrees of certainty, of at least eleven so-called elementary particles. First, there are the familiar electrons, neutrons, protons, and photons… (p. 8).” In addition, in an article titled partons, Feynman suggests that “[w]e need a name for what one would call the ‘elementary particles’ of the theory. We do not know if there are any such particles in the end, but we will start by supposing that there are because otherwise we would have no field theory at all. I will call these things ‘partons’ (Feynman, 1970, p. 519).” On the other hand, Gell-Mann received the Nobel Prize in physics in 1969 for his work on the theory of elementary particles and he coins the term quarks which mean the fundamental building blocks for particles such as neutron and proton.

Note: Gell-Mann does not agree with Feynman’s theory of “partons” and rename them as “put-ons” because Gell-Mann reasons that these particles do not obey quantum field theory (Wilczek, 2008). Interestingly, Gell-Mann is an enthusiastic bird-watcher and has learned many languages such as Latin, French, and Spanish. However, one of Feynman’s stories about his father is that the name of a bird does not matter: “You can know the name of a bird in all the languages of the world, but when you’re finished, you'll know absolutely nothing whatever about the bird... So let’s look at the bird and see what it’s doing – that’s what counts. I learned very early the difference between knowing the name of something and knowing something (Feynman, 1988, p. 14).”

3. Elementary interactions:

“… there seem to be just four kinds of interaction between particles which, in the order of decreasing strength, are the nuclear force, electrical interactions, the beta-decay interaction, and gravity (Feynman et al., 1963, section 2.4 Nuclei and particles).”

Feynman states that the four kinds of interaction between particles which, in the order of decreasing strength, are the nuclear force, electrical interactions, the beta-decay interaction, and gravity. In addition, he mentions that the photon is coupled to all charged particles, gravity is coupled to all energy, and provides a table that compares the strength of the four interactions. However, physicists currently describe the four interactions as the strong force, electromagnetic force, weak force, and gravitational force; the strong force is mediated by gluons and weak force is mediated by W bosons and Z bosons. More important, we can compare the strengths of the four forces by using a graph instead of a table because the strengths of these forces are distance-dependent.

Feynman summarizes that all the rules outside the nucleus seem to be known, whereas inside the nucleus, the principles of quantum mechanics are still valid. He adds that the rules of the game are the quantum-mechanical principles, and those principles apply to the new particles as well as to the old. Importantly, Feynman admits that physicists have limited understanding of the nuclear (strong) force. Recently, Wilczek (2007) explains that “[i]n principle, the equations of QCD contain all the physics of strong internucleon forces. But in practice, it is extremely difficult to solve the equations and calculate those forces (p. 156).” He clarifies that nucleons are complicated objects and the statement “protons and neutrons are made from three quarks” contains an oversimplified truth.

It is worthwhile to ponder Yukawa’s thinking process pertaining to the nature and strength of nuclear force as follows, “[i]s the new nuclear force a primary one? Or is it a secondary one, derived from the gravitational and electromagnetic forces? That is the fundamental question. Now, gravity cannot be applicable in this case, because the gravitational force between such minuscule objects as protons and neutrons is unimaginably small. It is much too weak to bind together such a compact and strongly aggregated composite as a nucleus. Electromagnetic forces, on the other hand, are much stronger than gravitation; but they are still too weak to be the source of the nuclear force. Not only that: electromagnetism appears to give only forces of repulsion, rather than attraction. Because the neutron is electrically neutral, there should not be any large electrical force between it and other particles; and protons repel each other (Yukawa, 1982, p. 194)!”

Questions for discussion:

1. Is the nuclear force a fundamental force or can it be derived from the gravitational or/and electromagnetic forces?

2. What are the elementary particles?

3. How do physicists compare the relative strengths of the four fundamental interactions?

The moral of the lesson: there is a Mendeleev-type chart for the so-called new elementary particles, but the next question is whether these so-called elementary particles are made of sub-quarks, preons, or new fundamental building blocks.

References:

1. Feynman, R. P. (1948). Pocono Conference. Physics Today, 1(2), 8-10.

2. Feynman, R. P. (1973). Partons. In Brown, L. M. (ed.), Selected papers of Richard Feynman (pp. 519-559). Singapore: World Scientific.

3. Feynman, R. P. (1988). What Do You Care What Other People Think? New York: W W Norton.

4. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

5. Okun, L. B. (1962). The Theory of Weak Interaction. Proceedings of 1962 International Conference on High-Energy Physics at CERN. Geneva.

6. Wilczek, F. (2007). Particle physics: Hard-core revelations. Nature, 445(7124), 156-157.

7. Yukawa, H. (1982). Tabibito “The Traveler” (Translated by L. Brown & R. Yoshida). Singapore: World scientific.