Paul Dirac

British physicist

• Born: August 8, 1902; Bristol, England
• Died: October 20, 1984; Tallahassee, Florida

British born theoretical physicist Paul Dirac made invaluable contributions to quantum mechanics, the branch of physics that deals with atoms and atomic particles. Among his most important achievements was his application of Albert Einstein’s theory of special relativity to quantum mechanics and his resulting discovery of the antielectron, the first evidence of antimatter. He received the Nobel Prize in Physics in 1933.

Primary field: Physics

Specialties: Theoretical physics; quantum mechanics

Early Life

Paul Adrien Maurice Dirac was born on August 8, 1902, in Bristol, England, the son of Swiss-born Charles Dirac, a French teacher, and his British wife Florence. Dirac was an introverted child who preferred studying to social interaction, and while in primary and secondary school, he excelled at anything involving numbers. Dirac enrolled at the University of Bristol in 1918 and graduated with a degree in electrical engineering in 1921.

Dissatisfied with engineering and unable to find a good job in the field, Dirac returned to Bristol to study mathematics. He remained there for two years and then accepted a grant from St. John’s College, Cambridge, in 1923, where he became a research student in mathematics.

At Cambridge, Dirac worked with quantum theorist Ralph Fowler, who helped stimulate Dirac’s interest in quantum mechanics, a new branch of physics that had come into being at the turn of the twentieth century. Quantum mechanics studies the behavior of the smallest building blocks of matter—the atom and atomic particles—as they move and interact and as they absorb and release energy in discrete packets called quanta.

An atom consists of one or more negatively charged electrons moving in an orbit-like motion around a nucleus of positively-charged protons and neutrons, which have no electrical charge. The behavior of the atom and atomic particles cannot be predicted or explained by laws of classical physics that govern objects of larger sizes. For example, electrons usually maintain their orbits instead of decaying, as would be expected according to classical laws of physics that govern the macro world.

In addition, the position and momentum of atomic particles cannot be measured at the same time, unlike larger objects. The more accurate the measurement of an atomic particle’s position, the more inaccurate the measurement of its momentum will be, and vice-versa. The act of measuring the position of the atomic particle affects its momentum. Therefore, the position and momentum of atomic particles can only be determined in terms of probabilities.

Dirac was fascinated by the novelty of the atom. As a research student, he published eleven papers on quantum theory in which he generalized current schools of thought on the subject, reorganizing them into a version of quantum mechanics that was much more complete and enduring. He obtained his PhD from St. John’s in 1926.

Life’s Work

Dirac traveled to Copenhagen, Denmark, in the fall of 1926 to study with Niels Bohr, a physicist who made important contributions to the quantum theory of the atom. In Copenhagen, Dirac devised his transformation theory of quantum mechanics, which unified different mathematical forms of quantum mechanics popular at the time and showed how they each produced the same results. It was also during this time that Dirac began to formulate his quantum theory of electromagnetic radiation.

In 1927, Dirac moved to Göttingen, Germany, where he studied with theoretical physicist Werner Heisenberg. He also became friends with physicist J. Robert Oppenheimer, considered the father of the atomic bomb. Dirac fine-tuned his quantum theory of electromagnetic radiation in Göttingen.

An important concept in quantum mechanics is wave-particle duality, which proposes that matter and light behave like particles and like waves when measured. In his quantum theory of electromagnetic radiation, Dirac explored the wave-particle duality of the photon, the elementary particle that comprises light, when he applied quantum principles to electromagnetic fields.

Dirac became a fellow at St. John’s College in 1927 and was asked to participate in the 1927 Solvay Conference in Belgium, a meeting of experts in the fields of chemistry and physics, to discuss quantum theory. Dirac, whose peers at the conference included Albert Einstein, Marie Curie, Niels Bohr, and Werner Heisenberg, was twenty-five years old and the youngest participant.

The Solvay Conference encouraged Dirac to find a way to apply Einstein’s special theory of relativity (theories for the behavior of objects moving close to and at the speed of light) to quantum mechanics. By 1928, Dirac had successfully developed a relativistic theory of the electron in the form of a mathematical equation called the Dirac equation.

The Dirac equation was inspired by physicist Erwin Schrödinger’s wave equation, devised in 1926, which described the movement of atomic particles such as the electron as waves instead of particles. Schrödinger’s wave equation was nonrelativistic, meaning that it only concerned the behavior of electrons moving at slow speeds. It also failed to fully explain certain qualities of the electron, such as electron spin.

Dirac concluded that the positively charged electron was actually a new type of atomic particle, an antielectron. He determined that the antielectron is identical to an electron in all ways but charge, and that each electron has an antielectron counterpart. He further theorized that in the collision of an electron and an antielectron, both would be destroyed. The Dirac equation presented the first evidence of antimatter.

The antielectron, or positron as it would be renamed, was strictly theoretical until 1932, when physicist Carl Anderson identified one in a photograph of cosmic rays.

Problematically, the Dirac equation calculated that electrons move from their normal positive energy state with a negative electrical charge to a negative energy state with a positive electrical charge and then back again. However, at the time, relativity theory did not allow for negative energy states. Dirac speculated that the positively charged electron described by his equation was actually a proton, but this theory was debated for several years until it was finally disproved by other physicists in 1930.

In 1930, Dirac became a fellow of the Royal Society of London and was appointed Lucasian Chair of Mathematics at Cambridge University, an honor last bestowed upon Sir Isaac Newton. Dirac posited that if the electron had an oppositely charged counterpart, other atomic particles had corresponding antiparticles as well. Twenty years later, the existence of other antiparticles was also verified.

In 1933, Dirac (with Erwin Schrödinger) was awarded the Nobel Prize in Physics for his discoveries and contributions to quantum theory over the course of his career. After winning the Nobel Prize, Dirac dedicated himself to creating a unified theory of quantum electrodynamics, which concerned the interactions of electrons with other electrons. Dirac was never able to create this theory, and in the late 1940s a team of quantum physicists devised a standard theory, which offended Dirac because it was not mathematically elegant.

During a trip to the United States in 1935, Dirac met Margit Wigner, the sister of Hungarian physicist Eugene Wigner. Dirac and Wigner were married in 1937. They had two children: Monica, born in 1940, and Florence, born in 1942, as well as two children from Wigner’s previous marriage.

Impact

Dirac was one of the first scientists to work in the field of quantum mechanics. Through pure mathematics, he discovered the existence of the positron and antimatter. The positron would not be observed for a few more years, and it wasn’t until 2010 that scientists were able to capture and observe the first antimatter in the form of antihydrogen atoms.

For his work, Dirac was awarded the Royal Medal and the Copley Medal from the Royal Society of London, and was elected a fellow of the society in 1930. He was elected a member of the Pontifical Academy of Sciences, an organization supported by the Vatican, in 1961. Dirac remained Lucasian Chair of Mathematics at Cambridge until 1969. In 1972, he accepted a professorship at Florida State University in Tallahassee, Florida, to be closer to his daughter. He lived in Tallahassee until his death on October 20, 1984, at the age of eighty-two.

Bibliography

Farmelo, Graham. The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom. New York: Basic, 2009. Print. A biography of Dirac and his work. Explores his personal and professional life.

Kragh, Helge. “Paul Dirac: The Purest Soul in an Atomic Age.” From Newton to Hawking: A History of Cambridge University’s Lucasian Professors of Mathematics. Eds. Kevin C. Knox and Richard Noakes. Cambridge: Cambridge UP, 2003. 387–424. Print. A collection of biographies for the Lucasian Professors of Cambridge University, including Paul Dirac.

Peacock, Kent A. The Quantum Revolution: A Historical Perspective. Westport, CT: Greenwood, 2008. Print. A non-technical explanation of quantum mechanics, including explanations of Dirac’s contributions.