Physics in the Yukawa era and the meson theory (3) Interaction
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Universal gravitation and electromagnetic force The main topic of discussion so far has been the infinitesimal elementary particles that constitute all matter around us. This article focuses on the interaction that acts between elementary particles. At the time of Yukawa’s birth, universal gravitation (gravity) and electromagnetic force were the only two “forces” that were known to exist in nature. The universal gravitation was discovered by Newton. The episode of Newton discovering the gravitation from watching an apple fall from the tree is quite famous. From watching an apple fall, Newton discovered that the universal gravitation acts between any two bodies having masses, including planets such as Earth. The electromagnetic force is an electrical force that occurs between electric charges. The magnetic force is also an electromagnetic force. For example, the electrostatic shock that occurs when a person touches a metal in winter and the magnet’s attraction of iron are caused by the electromagnetic force. The existence of the electromagnetic force was known years before Newton’s discovery of the universal gravitation. However, it was not until the 1860s, less than 50 years before Yukawa was born, that Maxwell developed the equations for describing the electromagnetic force and that his fundamental theory was understood as a law of physics. According to Maxwell’s theory of electromagnetic force, the universe is filled with something that behaves like an elastic rubber band, which was called an electromagnetic field. A rubber band is flat and thin, but when you think of an electromagnetic field, you need to imagine rubber that fills a three-dimensional space. The electromagnetic force is transmitted to an electric charge at a distance due to its distortion caused by the electric charge. If the electric charge is oscillating at high speed, the surrounding electromagnetic field will vibrate and the electromagnetic field will undulate just like a rubber band does. The oscillating electromagnetic field is called an electromagnetic wave. The speed of electromagnetic wave is 300,000 km per second. The light we can see with our eyes is also a type of electromagnetic wave, and radio waves used by mobile phones such as smartphones are also electromagnetic waves. For your reference, the universal gravitation can be understood to be caused by the distortion of the field called a “gravitational field” by the masses of substances. Waves generated by the oscillation of a gravitational field are called gravitational waves, and their existence was predicted by Einstein in 1916. The discovery of the gravitational waves in 2015, about 100 years from Einstein’s prediction, is still vivid in the minds of many people. |
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Yukawa’s inspirations that led to the theory of mesons Let’s examine the interaction that acts between the protons and neutrons that compose the nuclei. The proton has a positive electric charge, and that was a known fact even at the time of its discovery. The neutron, on the other hand, has no electric charge. The electromagnetic force that acts between particles having the electric charges of the same sign (+ or -) is a repulsive force. Therefore, if the electromagnetic force was the only force to act between protons and neutrons, the protons in the nucleus would repel from each other, resulting in an instantaneous separation. In that case, the nucleus would not be able to exist. Although the gravitational force is attraction, the strength of the force is only about 1/100,000,000 of that of the electromagnetic force; thus, the gravitational force is not strong enough to keep the protons together against the electromagnetic force’s repulsive force. However, the nuclei made up of protons and neutrons do exist stably. Amazingly, the protons and neutrons are enclosed in the infinitesimal space within the atom, which would be the size of a ping-pong ball if the atom were enlarged to the size of the Tokyo Dome stadium. How can the protons and neutrons exist stably without being torn apart by the electromagnetic force? This question was the starting point of Yukawa’s meson theory. Almost all physicists before Yukawa seem to have never seriously thought of that question. In the era when the atom was full of mysteries, the physicists probably couldn’t indulge in thinking about the nucleus which is much smaller than the atom as described by the above analogy. Notwithstanding, Yukawa addressed the pioneering question squarely and deepened his thoughts. Yukawa’s profound thinking and theorizing at that time eventually led to his glorious achievements such as the meson theory and Japan’s first Nobel Prize in Physics. I will give Yukawa’s answer first. There is an attraction force other than the gravitational force and electromagnetic force and that force is stronger than the repulsive force of electromagnetic force and acts between the protons and neutrons in the nuclei. The nuclei can exist stably because of that attraction force. The force is now called the “strong force.” Things were not that simple, however. The new force must be a strong attraction force powerful enough to overcome the repulsive force of electromagnetic force acting in the range of distance as small as the size of a nucleus, but such force was not observed in the natural phenomena that occur around us. This implies that the strong force acts only when the distance is extremely short. Physicists have the liberty to assume the existence of such a convenient force, but construction of hypotheses alone will not achieve anything in physics. In what kind of situations or environments would that kind of convenient force be allowed to exist? If such force exists, how could their existence be confirmed? Yukawa tackled each of these questions and deepened his thoughts, eventually making great achievements. |
(Written by Masakiyo Kitazawa)
