Nuclear Strong Interaction Theory

01/06/1935View on timeline

The strong interaction is the mechanism responsible for the strong nuclear force (also called the strong force, nuclear strong force, or colour force), and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. In the context of binding protons and neutrons together to form atomic nuclei, the strong interaction is called the nuclear force (or residual strong force). In this case, it is the residuum of the strong interaction between the quarks that make up the protons and neutrons. As such, the residual strong interaction obeys a quite different distance-dependent behavior between nucleons, from when it is acting to bind quarks within nucleons. The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons. Work (energy) is required to bring charged protons together against their electric repulsion. This energy is stored when the protons and neutrons are bound together by the nuclear force to form a nucleus.

In 1935, Japanese physicist Hideki Yukawa reasoned that since the strong and weak nuclear forces had never been detected, they must act over a range smaller than the diameter of the atomic nucleus. Yukawa was interested in the strong nuclear force in particular and found an ingenious way to explain its short range. His idea is a blend of particles, forces, relativity, and quantum mechanics that is applicable to all forces. Yukawa proposed that force is transmitted by the exchange of particles (called carrier particles). The field consists of these carrier particles.

Hideki Yukawa.

This suggested that the virtual particles that transmit the nuclear forces must have a mass, unlike photons, the particles that transmit the electromagnetic force. This is because it takes more energy to produce a virtual particle with mass, and the more energy needed, the less time a virtual particle can exist according to Heisenberg’s uncertainty principle, hence its short range.

The pion is created through a temporary violation of conservation of mass-energy and travels from the proton to the neutron and is recaptured. It is not directly observable and is called a virtual particle. Note that the proton and neutron change identity in the process. The range of the force is limited by the fact that the pion can only exist for the short time allowed by the Heisenberg uncertainty principle. Yukawa used the finite range of the strong nuclear force to estimate the mass of the pion; the shorter the range, the larger the mass of the carrier particle.


Specifically for the strong nuclear force, Yukawa proposed that a previously unknown particle, now called a pion, is exchanged between nucleons, transmitting the force between them, illustrates how a pion would carry a force between a proton and a neutron. The pion has mass and can only be created by violating the conservation of mass-energy. This is allowed by the Heisenberg uncertainty principle if it occurs for a sufficiently short period of time. The Heisenberg uncertainty principle relates the uncertainties E in energy and t in time by

Et ≥ h/(4π)

where h is Planck’s constant. Therefore, conservation of mass-energy can be violated by an amount E for a time t ≈ h/(4π∆E) in which time no process can detect the violation. This allows the temporary creation of a particle of mass m, where E = mc2. The larger the mass and the greater the E, the shorter is the time it can exist. This means the range of the force is limited, because the particle can only travel a limited distance in a finite amount of time. In fact, the maximum distance is d ≈ ct, where c is the speed of light. The pion must then be captured and, thus, cannot be directly observed because that would amount to a permanent violation of mass-energy conservation. Such particles (like the pion above) are called virtual particles, because they cannot be directly observed but their effects can be directly observed. Realizing all this, Yukawa used the information on the range of the strong nuclear force to estimate the mass of the pion, the particle that carries it.

Yukawa developed the first quantum field theory of the strong force, with newly discovered particles known as ‘mesons’ acting as the force carrying virtual particles. Yukawa produced evidence of mesons in experiments where he bombarded protons with neutrons. 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. Short-lived particles were emitted. Yukawa calculated their maximum lifetime, and hence their minimum mass, and found them to be at least 200 times more massive than the electron.

In 1936, American physicists Carl David Anderson and Seth Neddermeyer discovered another new particle in cosmic radiation that seemed to have a similar mass to Yukawa’s meson. It was soon shown that Anderson’s new particle penetrated matter too easily, and was therefore not massive enough to be the same particle Yukawa had predicted. It was suggested that there might be two types of mesons. The first evidence for this came in 1947, when Brazilian physicist Cesar Lattes and his team conducted a high altitude cosmic-ray experiment. Their results showed that Yukawa’s heavier meson decays into Anderson’s lighter ones. Yukawa’s heavier particle was renamed pi and became known as the pi-meson or pion. The lighter particle was named mu and became known as the mu-meson or muon. It was later shown that the pion is composed of smaller particles but the muon is an elementary particle with a spin of 1/2, similar to the electron but more massive. It’s no longer considered a type of meson. In 1938, British physicist Nicholas Kemmer had predicted that there are three types of pions: a neutral pion, and pions with a negative and positive charge. This was similar to Heisenberg’s idea that protons and neutrons are charged and neutral versions of the same particle.

Differences in the binding energy of the nuclear force between different nuclei power nuclear fusion and nuclear fission. Nuclear fusion accounts for most energy production in the Sun and other stars. Nuclear fission allows for decay of radioactive elements and isotopes, although it is often mediated by the weak interaction.

Hans Bethe - Yukawa's meson prediction
Particle Physics - The Pion: What Is It?
Calculating the Mass of Meson Particle
Mesons | Nuclear Physics

Hideki Yukawa and the Pi Mesons


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Published in 1/10/2018

Updated in 19/02/2021

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