BOSTES HSC Physics 2015 Question 34

Question: This question is asked in the context of particle physics. Students are expected to assess the impact of three advances in knowledge relating particles and forces on an understanding of the atomic nucleus.

Marking Guidelines:

Criteria	Marks
Provide three advanced knowledge relating particles and forces. Assess their impact on the understanding of the atomic nucleus.	6

(Source: https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-physics.pdf)

Comments:

This question is about an understanding of particles and forces that are related to the structure and behavior of the atomic nucleus. The answer may be summarized as shown below:

1. Strong interactions: The strong force is responsible for holding nuclei together and it is mediated by gluons.

2. Weak interactions: The weak force is responsible for nuclear transformation or radioactive decay processes and it is mediated by W and Z particles.

3. Electromagnetic interactions: The electromagnetic force is responsible for repulsive forces among protons in nuclei and it is mediated by photons.

4. Standard Model: A proton is composed of two up (u) quarks and one down (d) quark, whereas a neutron is composed of two down (d) quarks and one up (u) quark.

5. Asymptotic freedom: The strength of the strong interactions decreases as the quarks approach one another, and increases as they separate.

6. Nuclear stability: Knowledge of forces and particles helps to explain how nuclei are stable despite the repulsion due to the electromagnetic interactions of protons.

Furthermore, one may prefer to include a mathematical equation to explain quarks and gluons in the nuclei. For instance, one may rewrite Einstein’s famous equation as m = E/c² and elaborate that energy is the source of mass and it is due to the energetic but massless quarks and gluons. However, much work is still needed to investigate the nature of nuclear forces.

Feynman’s insights or goofs?:

The Feynman Lectures on Physics is slightly outdated for this question. For example, in Feynman’s words, “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. The photon is coupled to all charged particles and the strength of the interaction is measured by some number, which is 1/137. The detailed law of this coupling is known, that is quantum electrodynamics. Gravity is coupled to all energy, but its coupling is extremely weak, much weaker than that of electricity. This law is also known. Then there are the so-called weak decays — beta decay, which causes the neutron to disintegrate into proton, electron, and neutrino, relatively slowly (Feynman et al., 1963, section 2–4 Nuclei and particles).” Currently, we use the terms strong interaction and weak interaction instead of meson-baryon interaction and beta-decay interaction. Moreover, the law involved is quantum chromodynamics rather than quantum electrodynamics.

Importantly, Feynman mentions that “There is another question: ‘What holds the nucleus together?’ In a nucleus, there are several protons, all of which are positive. Why don’t they push themselves apart? It turns out that in nuclei there are, in addition to electrical forces, nonelectrical forces, called nuclear forces, which are greater than the electrical forces and which are able to hold the protons together in spite of the electrical repulsion. The nuclear forces, however, have a short range — their force falls off much more rapidly than 1/r² (Feynman et al., 1964, section 2–4 Nuclei and particles 1–1 Electrical forces).” Note that physicists have a better understanding of the strong interaction in 1973 when Frank Wilczek, David Gross, and David Politzer propose the concept of asymptotic freedom. Furthermore, the nuclear force is sometimes explained to be a residual effect of the strong interaction.

On the other hand, Feynman adds that “[t]he origin of the forces in nuclei leads us to new particles, but unfortunately they appear in great profusion and we lack a complete understanding of their interrelationship, although we already know that there are some very surprising relationships among them. We seem gradually to be groping toward an understanding of the world of subatomic particles, but we really do not know how far we have yet to go in this task (Feynman et al., 1963, section 2–4 Nuclei and particles).” Currently, we still do not have a complete understanding of everything in particle physics. For instance, a mysterious bump in experimental data at CERN’s Large Hadron Collider in 2015 could generate over 500 theoretical papers. However, the bump could be simply explained as a noise instead.

More importantly, Wilczek (2007) explains that “[o]ur quest to understand the force that holds atomic nuclei together has turned out to be a glorious adventure. Along the way, we have found quarks, the colored gluons that mediate the strong nuclear force, and a wonderful theory — quantum chromodynamics, or QCD. This theory has guided experimental research at the high-energy frontier, inspired dreams of ‘unified field theories’ that would embrace all nature’s forces, and allowed theoretical physics to penetrate into the cosmology of the early Universe. In all this, the original problem of understanding nuclear forces has rather fallen by the wayside (p. 156).” Our understanding of nuclear forces is still incomplete. Physicists have assumed that nuclear forces are a residual effect of strong interaction without a good mathematical model.

Note: 

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. Ishii and colleagues’ breakthrough calculation required sophisticated algorithms, running on the biggest and fastest massively parallel computers currently available. Why are the calculations so difficult? The main reason is simply that nucleons are complicated objects. It is often said that protons (and neutrons) are made from three quarks. That statement contains a kernel of truth, but it is a gross oversimplification (p. 156).”

References: 

1. 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.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1964). The Feynman Lectures on Physics, Vol II: Mainly electromagnetism and matter. Reading, MA: Addison-Wesley.

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