Paul Dirac’s discovery of the connection between quantum mechanics and relativity had a profound impact on modern physics, and his rigorous personality and brilliant work helped him break new ground in the field.
“When you pluck a flower on Earth, you move the stars at the end of the universe.” This is how Paul Dirac, a British-born theoretical physicist, described gravity. At first glance, it’s a very romantic and poetic statement, hard to imagine coming from the mouth of a physicist. But he’s also known for the following anecdote. One day, Dirac was waiting for someone in a castle. When the other guest said, “This castle is haunted at midnight,” Dirac replied, “When you say midnight, is that Greenwich Mean Time or Daylight Saving Time?” From this anecdote, Dirac seems like a cold person with no empathy. However, he was a scientist who accomplished a great deal in the field of physics, especially quantum mechanics, and his life was filled with many anecdotes. What was he like and what impact did he have on physics, especially quantum mechanics?
Paul Dirac was born in England in August 1902, and the early 20th century was a time when the order of classical physics, established since Newton, was being completely overturned and new theories and laws were being born. Problems with classical physics were being reported all over Europe, and physicists were struggling to solve them. Dirac grew up with a very strict father. His father forced him to speak only French at home, and since he didn’t know much French, Dirac naturally developed a reclusive and isolated personality. He later said that he first realized the importance of parents loving their children when he watched his father grieve over his brother’s suicide.
Dirac studied electrical engineering at the University of Bristol, but found it uninteresting as an electrical engineering student. He then went on to study physics at Cambridge University. In 1925, he read a paper by Heisenberg that Professor Fowler mailed to him, which led him to publish his life-changing quantum theory. Dirac’s quantum theory suggested that visual representations of atomic models were meaningless and could only be described through mathematical calculations. It was a development of Heisenberg’s theory and connected the then-nascent quantum theory to classical mechanics. The recognition from leading physicists encouraged Dirac to continue his research.
At the time, the physics community was struggling to find a connection between relativity and quantum theory. Relativity is the theory that applies when the speed of an object is close to the speed of light, while quantum theory was introduced to understand physical phenomena that cannot be explained by classical mechanics at the atomic scale. These two great theories had been developing independently for about 30 years without much connection. Before Dirac, Schrödinger tried this process first, but failed to describe the motion of electrons and published a non-relativistic formulation. While studying Schrödinger’s equation, Dirac realized that relativity could be applied to describe the structure of the electron and proposed the Dirac equation in 1928. The Dirac equation described the physical behavior of fermionic particles with half-integer spin, such as electrons, protons, and neutrons, and predicted the existence of the then-unfamiliar anti-particle.
In quantum mechanics, as developed by Dirac, the physical state of a particle or system of particles can be represented as a function of space and time, and this state contains all the information about the system that can be obtained through observation. Dirac treated the electromagnetic field as a collection of oscillators, each of which could represent a single photon. In this way, Dirac transformed the classical Maxwellian electromagnetic field into quantum mechanical oscillators, each of which was shown to satisfy its own Schrödinger wave equation. This breakthrough was the beginning of what is now called “quantum field theory” and is now widely used by high-energy particle physicists. By introducing different oscillators to represent electromagnetic fields, Dirac replaced the intractable continuous quantities of quantum mechanics with discrete quantities. Anecdotally, Dirac was able to come up with the right equation by staring into a brazier and thinking about what if the coefficients in Schrödinger’s equation were matrices instead of numbers. Schrödinger’s six months’ worth of work had been developed in a matter of hours. In 1932, the experimental discovery of the positron, one of the antiparticles predicted by Dirac’s equation, shocked the physics world. The discovery of antiparticles opened up new horizons in physics, which studies the nature of matter, and set a new milestone in the theoretical study of particle physics, which explores smaller elementary particles.
Dirac’s prediction of antiparticles and the discovery of a new physical formalism to describe the theory of the atom made a major contribution to the development of quantum mechanics, and he was awarded the Nobel Prize in Physics in 1933 at the young age of 31 for his work. When he heard he was going to win the Nobel Prize, he tried to turn it down because he didn’t like the media attention, but Rutherford told him that turning it down would bring him more attention, so he accepted. In 1932, he succeeded Joseph Lamarr as Lucasian Professor of Mathematics at Cambridge University, a position held by Newton and Stephen Hawking and one of the most prestigious university professorships in the UK. He continued his research in Fermi-Dirac statistical theory, a statistical treatment of the properties of microscopic particles with semi-integer spin, quantum field theory, and quantum electrodynamics. In 1971, he moved to the United States to work at the Center for Theoretical Physics at the University of Miami, and later devoted himself to research and teaching at Florida State University, where he died on October 20, 1984, at the age of 82. In honor of his work, the International Center for Theoretical Physics (ICTP) awards the Dirac Medal to theoretical physicists.
Nobel laureate Richard Feynman, known for his public lectures, said of Dirac after he became famous for his work on quantum electrodynamics. “I don’t know what all the fuss is about when Dirac has already done it all!” Another paper said, ”Dirac’s achievements are on a par with Bohr’s, and only Einstein can surpass them!” Dirac didn’t want to be in the spotlight, and he was a reclusive person. As one of the founders of quantum mechanics, a theory that shaped modern physics, Dirac is arguably the greatest scientist in human history, opening up a new world of physics that will never be equaled.
