Quantum mechanics

Finally, I would like to discuss the theory of mesons, but before that, I need to mention about the laws of physics that govern the micro-world, namely quantum mechanics.

As introduced in the previous section, Maxwell’s theory of electromagnetic field completed in the 1860s verified that light is an electromagnetic wave. At that time, it was known that light displays phenomena unique to waves, such as refraction and diffraction, so the fact that light is a wave was readily accepted.

However, from about 1900, the results of experiments indicating that light, which was thought as a wave, displayed the characteristics of particles were reported one after another, and the foundation on which physics had been built started crumbling. For instance, in 1900 Planck explained the phenomenon called black-body radiation based on the postulate that the energy of electromagnetic field takes only “one of the discrete energy levels.” In 1905, Einstein showed that a phenomenon called the photoelectric effect could be explained logically based on a similar hypothesis.

Quantum mechanics explains the “wave-particle duality.” According to quantum mechanics, the composing elements of matter are waves and particles at the same time. That probably sounds odd, but the quantum mechanics developed in 1925 by Heisenberg and Schrödinger described the wave-particle duality mathematically using equations.

According to quantum mechanics, not only waves are particles but the electrons and protons that had been believed to be particles also have the characteristics of waves. In 1929, the wave nature of the electron was experimentally verified, and quantum mechanics solidified its position as one of the fundamental theories of physics. When electromagnetic waves behave as particles, they are called photons. The photon is known to have no mass.

Just about the time quantum mechanics was established in 1926, Yukawa entered Kyoto Imperial University, Faculty of Science, and started pursuing physics. Overseas, physics dealing with the micro-world was advancing rapidly, but the latest information seldom reached Japan since there was no Internet or effective distribution system in those years. Notwithstanding, during his years as an undergraduate student and graduate student, Yukawa obtained cutting-edge information of physics through “self-study” in collaboration with Shin-ichiro Tomonaga and others and expanded his knowledge. I wonder what kind of stimulation Yukawa received from the limited amounts of fragmented information materials available at that time and how he was allured into the abysses of the micro-world.