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33.2 The four basic forces

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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.

Properties of the four basic forces
Force Approximate relative strength Range +/− + attractive; ‑ repulsive; + /− size 12{+/ - {}} {} both. Carrier particle
Gravity 10 − 38 size 12{"10" rSup { size 8{ - "38"} } } {} ∞ size 12{ infinity } {} + only Graviton (conjectured)
Electromagnetic 10 − 2 size 12{"10" rSup { size 8{ - 2} } } {} ∞ size 12{ infinity } {} + / − size 12{+/ - {}} {} Photon (observed)
Weak force 10 − 13 size 12{"10" rSup { size 8{ - "13"} } } {} < 10 − 18 size 12{<"10" rSup { size 8{ - "18"} } } {}  m + / − size 12{+/ - {}} {} W + , W − , Z 0 size 12{W rSup { size 8{+{}} } ,`W rSup { size 8{ - {}} } ,`Z rSup { size 8{0} } } {} (observed Predicted by theory and first observed in 1983. )
Strong force 1 < 10 − 15 size 12{<"10" rSup { size 8{ - "15"} } } {}  m + / − size 12{+/ - {}} {} Gluons (conjectured Eight proposed—indirect evidence of existence. Underlie meson exchange. )

The first image shows a photon traveling between positive charges to transmit a repulsive electromagnetic force. The second image is the same except that an eye detects the photon before it arrives at the second positive charge, thereby obstructing the passage of the photon and preventing the force from being transmitted.
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.

A diagram is shown in which time proceeds along the vertical y axis and distance along the horizontal x axis. Two positive charges are shown approaching each other, exchanging a virtual photon, then moving apart.
The Feynman diagram for the exchange of a virtual photon between two positive charges illustrates how the electromagnetic force is transmitted on a quantum mechanical scale. Time is graphed vertically while the distance is graphed horizontally. The two positive charges are seen to be repelled by the photon exchange.

Although these four forces are distinct and differ greatly from one another under all but the most extreme circumstances, we can see similarities among them. (In GUTs: the Unification of Forces , we will discuss how the four forces may be different manifestations of a single unified force.) Perhaps the most important characteristic among the forces is that they are all transmitted by the exchange of a carrier particle, exactly like what Yukawa had in mind for the strong nuclear force. Each carrier particle is a virtual particle—it cannot be directly observed while transmitting the force. [link] shows the exchange of a virtual photon between two positive charges. The photon cannot be directly observed in its passage, because this would disrupt it and alter the force.

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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