Tuesday, May 7, 2013

History of Science -- Part Twenty-Two: The Weak Nuclear Force

Enrico Fermi
We learned in the last chapter that neither protons nor neutrons are fundamental particles. That is, they are made up of something even more basic: quarks. Both nucleons are made of a combination of three quarks, but the recipe is different. Protons are sort of like cake and neutrons are like pie. Similar ... both for dessert ... but different things in the mix.

There are several implications of these different recipes. For one thing, protons have a positive charge while neutrons are electrically neutral. Also, neutrons are just a little bit more massive than protons. But there is another difference. Protons are fundamentally stable, while neutrons are prone to disintegrate or fly apart.

Neutrons can decay, turning into a proton and ejecting an electron is what is called “beta radioactivity.” The force that destroys a neutron is the “weak force,” so called because it appears weak by comparison to the electromagnetic and the strong force at normal temperatures. The weak force disrupts neutrons and protons, causing the nucleus of one atomic element to transmute into another through beta radioactivity. It plays an important role in helping convert the protons — the seeds of the hydrogen fuel of the Sun — into helium. This is the process by which energy is released, eventually emerging as sunshine.

I stated earlier that gravity was too weak to be felt at the atomic level. That is not true when there is a massive amount of atoms, such as in a large planet or sun. The effect on gaseous atoms in large bodies like the sun is to pull the atoms together under tremendous pressure driven by gravity. This closeness causes a large amount of bumping and banging, atomic motion that we call “heat.” So the pressure builds up the heat to a point that the electrons are stripped off leaving bare nuclei. This is a fourth state of matter called a “plasma.” (The other three states are solid, liquid, and gas.)

This is what the core of our Sun and other suns similar to ours is like. It is a big ball of hydrogen gas under great pressure producing heat and making an atomic plasma. This plasma then undergoes fusion, converting hydrogen to helium and creating a lot of energy.

Not only is energy produced, but in some stars, other elements are created such as carbon, oxygen, and iron, all the way up to the highly radioactive elements such as uranium and plutonium. When these stars explode, they spread these heavier elements out throughout the galaxy. That is the basic mechanism whereby the original element of hydrogen produced in the “big bang” becomes the heavier elements we find on the earth and in our bodies. That’s right! We are made up of atoms from exploding stars. We are literally star stuff!!

Here are the details. Interesting that they involve all four of the known forces of nature. That is what keeps us in existence, an intricate dance of the forces of Nature. There are many theories that if these various physical constants such as the weak and the strong force were slightly different, then the universe and life could not exist. A lucky accident? I don’t think so!

The gravitational attractions among the multitudinous protons in the Sun pull them inwards until they are nearly touching. Occasionally two move fast enough to overcome their electrical repulsion momentarily, and they bump into one another. The weak force transmutes a proton into a neutron, the strong force then clumps these neutrons with the protons, after which they build up a nuclei of helium.

Energy is released and radiated courtesy of the electromagnetic force. It is the presence of these four forces and their different characteristics and strengths that keeps the Sun burning at just the right rate for us to be here.

In the Standard Model of particle physics the weak interaction is theorized as being caused by the exchange (that is, emission or absorption) of W and Z bosons; and as such, is considered to be a non-contact force, like the other three forces. The best known effect of this emission is beta decay, a form of radioactivity. The Z and W bosons are much heavier than protons or neutrons and it is the heaviness that accounts for the very short range of the weak interaction.

Since the mass of the Z and W particles is on the order of 80 GeV, the uncertainty principle dictates a range of about 10-18 meters which is about 0.1% of the diameter of a proton. It is the search for the mechanism that creates these heavy particles that points to the existence of the Higgs boson that was experimentally demonstrated just in the last year.

The discovery of the W and Z particles in 1983 was hailed as a confirmation of the theories which connect the weak force to the electromagnetic force in electro-weak unification. The theory suggests that at very high temperatures, where the equilibrium kinetic energies are in excess of 100 GeV, these particles are essentially identical and the weak and electromagnetic interactions are manifestations of a single force.

The weak force was originally described, in the 1930s, by Fermi's theory of a contact four-fermion interaction: which is to say, a force with no range (entirely dependent on physical contact.) However, it is now best described as a field, having range, albeit a very short range. The theory of the weak interaction can be called Quantum Flavordynamics (QFD), in analogy with the terms QCD and QED, but in practice the term is rarely used because the weak force is best understood in terms of electro-weak theory.

Enrico Fermi, born in 1901, was an Italian theoretical and experimental physicist, best known for his work on the development of Chicago Pile-1, the first nuclear reactor, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics.

Along with Robert Oppenheimer, he is referred to as "the father of the atomic bomb.” He held several patents related to the use of nuclear power, and was awarded the 1938 Nobel Prize in Physics for his work on induced radioactivity and the discovery of transuranic elements. Throughout his life Fermi was widely regarded as one of the very few physicists who excelled both theoretically and experimentally.

Fermi left Italy in 1938 to escape racial laws that affected his Jewish wife Laura, and emigrated to the United States, where he worked on the Manhattan Project during World War II. Fermi led the team that designed and built the Chicago Pile-1, and initiated the first artificial self-sustaining nuclear chain reaction when it went operational in December 1942. He was present at the Trinity test on July 16, 1945, where he used one of his experiments to estimate the bomb's yield.

His estimate of the strength of the atomic bomb detonated at the Trinity test, based on the distance traveled by pieces of paper dropped from his hand during the blast, was 10 kilotons of TNT. This was remarkably close to the now-accepted value of around 20 kilotons, a difference of less than one order of magnitude, and all based on a simple experiment.

Enrico Fermi was born in Rome, the third child of Alberto Fermi, a division head in the Ministry of Railways, and Ida de Gattis, an elementary school teacher. As a young boy, he shared his interests with his brother Giulio. They built electric motors and played with electrical and mechanical toys. He was largely self taught working his way through several very difficult and advanced science texts.

Fermi graduated from high school in 1918 and applied to the Scuola Normale Superiore in Pisa. The school provided free lodging for students but candidates had to take a difficult entrance exam, which included an essay. The given theme was "Specific characteristics of Sounds." The 17-year-old Fermi chose to derive and solve the partial differential equation for a vibrating rod, applying Fourier analysis. The examiner that interviewed Fermi concluded that his entry would have been commendable even for a doctoral degree. Fermi achieved first place in the classification of the entrance exam.

During his years at the Scuola Normale Superiore, Fermi's knowledge of quantum physics reached such a high level that his professor asked him to organize seminars on the topic. During this time Fermi learned tensor calculus, a mathematical technique that was needed to demonstrate the principles of general relativity. Fermi initially chose mathematics as his major, but soon switched to physics. He remained largely self-taught, studying general relativity, quantum mechanics, and atomic physics

Fermi was the first to warn military leaders about the potential impact of nuclear energy, giving a lecture on the subject at the Navy Department in March 1939. In August 1939, three Hungarian physicists — Leó Szilárd, Eugene Wigner, and Edward Teller — prepared the “Einstein-Szilárd letter,” which they persuaded Einstein to sign because of his reputation, warning President Franklin D. Roosevelt of the probability that the Nazis were planning to build an atomic bomb.

Eventually, the first artificial nuclear reactor, Chicago Pile-1, was constructed at the University of Chicago, by a team led by Enrico Fermi, in late 1942. By this time, the program had been pressured for a year by U.S. entry into the war. The Chicago Pile achieved criticality on December 2, 1942 at 3:25 PM. The reactor support structure was made of wood, which supported a pile (hence the name) of graphite blocks, embedded in which was natural uranium-oxide “pseudospheres” or “briquettes.”

This experiment was a landmark in the quest for energy, and it was typical of Fermi's approach. Every step was carefully planned, every calculation meticulously done. Thus the first self-sustained nuclear chain reaction was achieved.

In the summer of 1944, Robert Oppenheimer persuaded Fermi to join his “Project Y” in Los Alamos, New Mexico. Arriving in September, Fermi was appointed an associate director of the laboratory, with broad responsibility for nuclear and theoretical physics, and was placed in charge of “F Division,” which was named after him.

After the war, Fermi served on the General Advisory Committee of the Atomic Energy Commission, a scientific committee chaired by Robert Oppenheimer that advised the commission on nuclear matters and policy.

Following the detonation of the first Soviet fission bomb in August 1949, Fermi, along with Isidor Rabi, wrote a strongly worded report for the committee, opposing the development of a hydrogen bomb on moral and technical grounds. Although Fermi was later proven wrong on the technical objections, the moral issues are still present in today’s world.

Also still present is our distrust of those that object to progress. Fermi was among the scientists who testified on Oppenheimer's behalf at the Oppenheimer security hearing in 1954 that resulted in denial of Oppenheimer's security clearance.

The hearing was a product of longstanding doubts about Oppenheimer's loyalty, and suspicions that he was a member of the Communist Party and might even have spied for the Soviet Union. The concerns about him were exacerbated by personal conflicts between Oppenheimer and others in the atomic community, including Lewis Strauss, chairman of the Atomic Energy Commission, and Edward Teller, with whom he had clashed over the development of the hydrogen bomb.

Oppenheimer was born in New York City in 1904, son of Julius Oppenheimer, a wealthy Jewish textile importer who had immigrated to the United States from Germany in 1888, and Ella Friedman, a painter. He had studied in England and he applied to the Cavendish Laboratory with Ernest Rutherford, although Rutherford considered him more of a theorist than an experimentalist, Oppenheimer was accepted to the laboratory by J.J. Thomson.

His German ancestry should not have been an issue since, by the time of Oppenheimer’s trial, the war with Germany was over and they were considered our national allies. However, our former war ally, Russia or the USSR, was now our cold war enemy. Just how much national origin or even ethnic and religious background played in the drama is not certain. These were very troubling times in the U.S.: suspicion and prejudice was prevalent.

The official result was that “Oppenheimer was unusually discrete with atomic secrets, yet he was a security risk” and his clearance was withdrawn and he never worked for the government again. The father of the atomic bomb was found to be a security risk. Government … don’t you just love it?

Oppenheimer's hearing is the subject of a play, and several article and book-length studies. Oppenheimer's trial, which marked the end of his formal relationship with the government of the United States, generated considerable controversy regarding whether the treatment of Oppenheimer was fair, or an expression of anti-Communist hysteria.

Fermi died in 1954 and Oppenheimer in 1967, but theoretical work on the weak interaction continues. In 1968, the electromagnetic force and the weak interaction were unified, when they were shown to be two aspects of a single force, now termed the electro-weak force. The hope is to, someday, combine all four forces into a unified field theory. That may be far off in the future, but small steps are made each day.

The discovery of quarks and the concept of even more mysterious forces beyond them means that, unlike the pronouncement of Lord Kelvin at the turn of the last century, there is plenty more exciting discoveries to be made. We’ll start that discussion in the next chapter.

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