Tuesday, May 7, 2013

History of Science -- Part Twenty-One: The Strong Nuclear Force

Hideki Yukawa
The attraction of the positive charged nucleus holds the negative electrons in their paths around the atom, similar to how the gravitational attraction of the sun holds the earth in its orbit.

However, the repulsion of the “like” charges in the nucleus creates a paradox of the existence of the nucleus itself. What holds it together? The nucleus is compact. Its positive electrical charge due to the many protons within it should cause it to fly apart. How can these protons, experiencing such intense electrical repulsion, manage to congregate? Why doesn’t the nucleus self-destruct?

The fact that it doesn’t gives an immediate clue to the existence of an additional, “strong” attractive force felt by the protons and the neutrons, which is powerful enough to hold them in place and resist the electrical repulsion. This strong force is one of a pair that act in and around the atomic nucleus. They are short range forces that just work on the nuclear scale and are not immediately familiar to our gross senses, yet they are essential to our existence. The strong force is so named since it is stronger than the electric force (and much, much stronger than gravity which is non-consequential at the atomic scale).

The nuclear force has been at the heart of nuclear physics ever since the field was born in 1932 with the discovery of the neutron by James Chadwick. The traditional goal of nuclear physics is to understand the properties of atomic nuclei in terms of the “bare” interaction between pairs of nucleons, or nucleon–nucleon force.

The strong force is 137 times stronger than the electric force. This number, “137,” appears in many quantum formulas and is called “alpha” or "α." Alpha is another physical constant of the universe such as “c,” the speed of light, or Planck’s constant.

For thousands of years we were only aware of the two forces, gravity and electromagnetic, until the “atomic age” increased our understanding of these two additional forces so well hidden away in the nucleus of the atom. Even in 2013, we are still making important and fundamental discoveries about these forces, although the latest experiments are actually based on theories put forward during the 1960’s.

The stability of the nuclei of atomic elements can be a delicate balance between the competing strong attraction and the electrical repulsion. You cannot put too many protons together or the electrical disruption will make the nucleus unstable. This is the source of certain radioactive decays, where the nucleus will split into smaller fragments. All of the elements with 84 or more protons are radioactive and there are a couple of naturally radioactive elements with less: technetium with 43 protons and promethium with 61.

Neutrons and protons feel the strong force equally; however, only the protons feel the electrical repulsion. This is why the nuclei of all elements other than hydrogen contain not just protons, but have neutrons to add to the strong attractive stability of the whole.

Uranium-235, for example, is so called due to the 92 protons that make it the uranium element and 143 neutrons making a total of 235 nucleons in all.

Actually, too many neutrons seems to lead to instability too. The extra mass of a neutron relative to the proton (recall the neutron is a small amount more massive than the proton) underlies an intrinsic instability of neutrons, whereby they can decay, turning into protons and ejecting an electron — the so-called “beta” particle of “beta rays.”

In 1934, Hideki Yukawa made the earliest attempt to explain the nature of the nuclear force. According to his theory, massive bosons (mesons) mediate the interaction between two nucleons. Although, in light of modern quark theory (QCD), meson theory is no longer perceived as fundamental, the meson-exchange concept continues to represent the best working model for a quantitative nucleon force potential.

Hideki Yukawa was born in Tokyo, Japan, and grew up in Kyoto. In 1929, after receiving his degree from Kyoto Imperial University, he stayed on as a lecturer for four years. After graduation, he was interested in theoretical physics, particularly in the theory of elementary particles. In 1933 he became an assistant professor at Osaka University, at the age of only 26.

In 1935 he published his theory of mesons, which explained the interaction between protons and neutrons, and was a major influence on research into elementary particles. In 1940 he became a professor in Kyoto University and he won the Imperial Prize of the Japan Academy, and in 1943 the Decoration of Cultural Merit from the Japanese government. In 1949 he became a professor at Columbia University, the same year he received the Nobel Prize in Physics, after the discovery by Powell, Occhialini, and Lattes of Yukawa's predicted pion in 1947. He was the first Japanese Nobel Laureate. Yukawa also worked on the theory of K-capture, in which a low energy electron is absorbed by the nucleus.

In ordinary matter, the strong force acts only in the nucleus and, at its source, we now think it is due to the presence of quarks, the ultimate basic particle from which protons and neutrons are formed. (Protons and neutrons, each contain three quarks.)

It was discovered in the 1970’s that protons and neutrons were not fundamental particles, but were made up of constituent particles called quarks. The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together inside the protons and neutrons.

As the electric and magnetic forces are effects arising from electric charges, so is the strong force ultimately due to a new variety of charge, which is carried by quarks, but not by leptons such as the electron. Hence electrons are not affected by the strong force at all.

Thus the strong nuclear force (or actually the “residual strong force”) binds the nucleus together. There is energy in this bonding which is released when large atoms are split or when small atoms such as hydrogen are fused. This is the energy released in nuclear power and nuclear weapons, although fusion, the process that takes place in the sun, has only been used for weapons. If fusion power could be developed, that would be a great advancement due to the lack of radioactive by-products.

The force is powerfully attractive between nucleons at distances of about 1 femtometer (fm) between particle centers, but rapidly decreases to insignificance at distances beyond about 2.5 fm. (A femtometer is 10-15 meters or 10-13 cm.) That is about the diameter of a medium sized nucleus. Again, this limited range is why the larger atoms are not stable and are radioactive.

At very short distances, less than 0.7 fm, the force becomes repulsive, and is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows.

The nuclear force is now understood as a residual effect of the even more powerful strong force, or strong interaction, which is the attractive force that binds particles called quarks together to form the nucleons themselves. This more powerful force is mediated by particles called gluons. A gluon is a type of gauge boson. Gluons hold quarks together with a force like that of electric charge, but of far greater power.

I’ve got more to say about quarks and this new kind of charge and a theory called Quantum Chromodynamics. So there’s more to cover from QCD to the weak force and radioactivity. That’s next.

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