33.1 The yukawa particle and the heisenberg uncertainty principle  (Page 2/5)

Yukawa's proposal of particle exchange as the method of force transfer is intriguing. But how can we verify his proposal if we cannot observe the virtual pion directly? If sufficient energy is in a nucleus, it would be possible to free the pion—that is, to create its mass from external energy input. This can be accomplished by collisions of energetic particles with nuclei, but energies greater than 100 MeV are required to conserve both energy and momentum. In 1947, pions were observed in cosmic-ray experiments, which were designed to supply a small flux of high-energy protons that may collide with nuclei. Soon afterward, accelerators of sufficient energy were creating pions in the laboratory under controlled conditions. Three pions were discovered, two with charge and one neutral, and given the symbols ${\pi }^{+}\text{,}{\pi }^{-}\text{, and}{\pi }^0$ , respectively. The masses of ${\pi }^{+}$ and ${\pi }^{-}$ are identical at $\text{139}\text{.}6\text{MeV/}{c}^2$ , whereas ${\pi }^0$ has a mass of $\text{135}\text{.}0\text{MeV/}{c}^2$ . These masses are close to the predicted value of $\text{100}\text{MeV/}{c}^2$ and, since they are intermediate between electron and nucleon masses, the particles are given the name meson    (now an entire class of particles, as we shall see in Particles, Patterns, and Conservation Laws ).

The pions, or $\pi$ -mesons as they are also called, have masses close to those predicted and feel the strong nuclear force. Another previously unknown particle, now called the muon, was discovered during cosmic-ray experiments in 1936 (one of its discoverers, Seth Neddermeyer, also originated the idea of implosion for plutonium bombs). Since the mass of a muon is around $\text{106}\text{MeV/}{c}^2$ , at first it was thought to be the particle predicted by Yukawa. But it was soon realized that muons do not feel the strong nuclear force and could not be Yukawa's particle. Their role was unknown, causing the respected physicist I. I. Rabi to comment, “Who ordered that?” This remains a valid question today. We have discovered hundreds of subatomic particles; the roles of some are only partially understood. But there are various patterns and relations to forces that have led to profound insights into nature's secrets.

Summary

• Yukawa's idea of virtual particle exchange as the carrier of forces is crucial, with virtual particles being formed in temporary violation of the conservation of mass-energy as allowed by the Heisenberg uncertainty principle.

Problems&Exercises