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Chapter 33 | Particle Physics 1483
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.
33.2 The Four Basic Forces
Learning Objectives
By the end of this section, you will be able to:
• State the four basic forces.
• Explain the Feynman diagram for the exchange of a virtual photon between two positive charges.
• Define QED.
• Describe the Feynman diagram for the exchange of a photon between a proton and a neutron.
The information presented in this section supports the following AP® learning objectives and science practices:
• 3.G.1.1 The student is able to articulate situations when the gravitational force is the dominant force and when the electromagnetic, weak, and strong forces can be ignored. (S.P. 7.1)
• 3.G.1.2 The student is able to connect the strength of the gravitational force between two objects to the spatial scale of the situation and the masses of the objects involved and compare that strength to other types of forces. (S.P. 7.1)
• 3.G.2.1 The student is able to connect the strength of electromagnetic forces with the spatial scale of the situation, the magnitude of the electric charges, and the motion of the electrically charged objects involved. (S.P. 7.1)
• 3.G.3.1 The student is able to identify the strong force as the force responsible for holding the nucleus together. (S.P. 7.2)
As first discussed in Problem-Solving Strategies and mentioned at various points in the text since then, there are only four distinct basic forces in all of nature. This is a remarkably small number considering the myriad phenomena they explain. Particle physics is intimately tied to these four forces. Certain fundamental particles, called carrier particles, carry these forces, and all particles can be classified according to which of the four forces they feel. The table given below summarizes important characteristics of the four basic forces.
Table 33.1 Properties of the Four Basic Forces
Force
Approximate relative strength Range +/−[1] Carrier particle
Gravity + only Graviton (conjectured)
Electromagnetic Photon (observed)
Weak force (observed[2])
Strong force Gluons (conjectured[3])
Figure 33.4 The first image shows the exchange of a virtual photon transmitting the electromagnetic force between charges, just as virtual pion exchange carries the strong nuclear force between nucleons. The second image shows that the photon cannot be directly observed in its passage, because this would disrupt it and alter the force. In this case it does not get to the other charge.
1.
2. 3.
+ attractive; ‐ repulsive; both.
Predicted by theory and first observed in 1983.
Eight proposed—indirect evidence of existence. Underlie meson exchange.