Section 2–2 Physics before 1920

(Electrical force / Electric field / Electromagnetic waves)

In this section, Feynman briefly discusses classical physics. He pictures physics before 1920 as something like this: First, the universe is in a three-dimensional space of Euclidean geometry and another dimension called time. Second, the elements in physics include particles and there are 92 kinds of atoms, which have different properties. Third, particles have properties such as inertia and there are two kinds of force (electrical and gravitational). However, Feynman discusses at greater length on the electrical force, electric field, and electromagnetic waves.

Note: According to Feynman, quantum mechanics was discovered just after 1920.

1. Electrical force:

“… this new force (which is the electrical force, of course) has the property that likes repel but unlikes attract. The “thing” that carries this strong interaction is called charge (Feynman et al., 1963, section 2.2 Physics before 1920).”

Typical physics textbook authors simply write that “like charges repel and unlike charges attract.” Although Feynman shortens it to “likes repel but unlikes attract,” he elaborates the situation when a negative charge carrier is placed at an appreciable distance away from two oppositely charged particles that are very close together. He explains that the negative charge carrier would feel practically no electrical force because the attraction from a positively charged particle and the repulsion from a negatively charged particle balance out. Currently, some physicists explain that electric charge refers to an attribute or property of an object. They prefer not to use the term electric charge as an object, and state that there are two common kinds of charge particles in daily life: electrons (negative) and protons (positive).

On the other hand, if a positively charged particle is placed nearer to a large neutral object, it is possible that attraction arises because protons are repelled to the other end of the neutral object, whereas electrons are attracted to the positively charged particle and move closer to this particle. Thus, the repulsion is less than the attraction because the electrical force between the positively charged particle and electrons is relatively stronger when they are closer to each other. Simply put, when they come closer together, the protons and electrons are rearranged such that they have a stronger interaction. However, in Feynman words, “the thing that carries this strong interaction is called charge.” This description is potentially confusing because the term strong interaction means meson-baryon interaction in section 2.4 of this chapter. Section 2.2 is mainly about the electromagnetic interaction that is mediated by photons.

Importantly, Feynman adds that all things including human beings are made of strongly interacting positive charge carriers and negative charge carriers. In daily life, we may accidentally rub off electrons from our clothes such that there are attractions and repulsions among these charge carriers. This may more likely happen if we wear wool sweaters or sit on a chair that is made of certain fabric during the dry winter. As a result, “mini lightning” and “mini thunder” occur at home. Physics teachers should discuss how to avoid these static electricity problems by wearing cotton clothes, using antistatic sprays, or other ways.

2. Electric field:

“… This potentiality for producing a force is called an electric field. When we put an electron in an electric field, we say it is ‘pulled’ (Feynman et al., 1963, section 2.2 Physics before 1920).”

Feynman visualizes a charged particle creates (or distorts) a “condition” in space, such that when we put another charged particle there, it feels a force. The condition in space that produces an electrical force is called an electric field. In short, there are two rules: (a) a charged particle creates an electric field, and (b) additional charged particle that is placed in the electric field experiences a “pull” and move. Furthermore, the moving charged particle also experiences a magnetic field. To be more precise, the charged particle is not under the influence of electric field that exactly follows the inverse square law. This is due to a delay in action because the influence cannot travel faster than the speed of light. However, some textbook authors only define an electric field as a region in space in which a charge experiences an electric force.

Feynman has an insight analogy that further explains the nature of electric field: In a pool of water that has two floating corks, we can push a cork by giving another cork a push. If we look only at the two corks, we can see one cork “directly” moves in response to the motion of the other cork. Physicists explain that there is a disturbance in the water caused by a cork, and the water then disturbs the other cork. One may develop a “law” on how the water causes the motion of an object nearby. If the first cork is farther away, then the second cork would appear stationary because we move the water locally (no distant influences). If we oscillate the cork continuously, a new phenomenon is involved in which water waves travel a much longer distance. This oscillatory influence is different from the idea of direct interaction between the two floating corks. We can replace the term “direct interaction” by the existence of a pool of water or specifically, the electromagnetic field.

On the contrary, Feynman has developed quantum electrodynamics by using the concept of “particle” instead of “field.” During his Nobel lecture, Feynman (1965) explains that “electrons cannot act on themselves, they can only act on other electrons. That means there is no field at all. You see, if all charges contribute to making a single common field, and if that common field acts back on all the charges, then each charge must act back on itself. Well, that was where the mistake was, there was no field. It was just that when you shook one charge, another would shake later. There was a direct interaction between charges, albeit with a delay. The law of force connecting the motion of one charge with another would just involve a delay. Shake this one, that one shakes later. The sun atom shakes; my eye electron shakes eight minutes later, because of a direct interaction across (p. 10).” The direct (inter-particle) interaction between two charged particles can be mediated by a photon (particle).

3. Electromagnetic waves:

“… The electromagnetic field can carry waves; some of these waves are light, others are used in radio broadcasts, but the general name is electromagnetic waves (Feynman et al., 1963, section 2.2 Physics before 1920).”

Electromagnetic waves may be distinguished as radio waves, light waves, ultraviolet waves, infrared waves, X-rays, and gamma rays. The main difference between light and oscillatory electromagnetic waves such as those used in radio broadcasts is in the frequency of oscillation. In other words, if we shake a charge carrier more and more rapidly, we can pick up a whole series of different kinds of effects, which are all unified by specifying only one number: the number of oscillations per second. Interestingly, Feynman considers electromagnetic waves that oscillate at a very low frequency behave like a field because they are almost not oscillating. Thus, he identifies three rough behaviors in the electromagnetic spectrum: field (very low frequency of oscillation), waves, and particle (very high frequency of oscillation).

During a British Broadcasting Corporation interview, Feynman (1994) explains that “[t]he radio waves are just the same kind of waves, only much longer waves. Then there’s the radar from the airplane which is looking at the ground to figure out where it is, which is coming through this room too, plus X-rays, cosmic rays, all these other things which are exactly the same kind of waves, just shorter and faster, or longer and slower - it’s all the same thing. So this big field, this big area of irregular motions, this electric field, this vibration contains a tremendous information (p. 132).” However, Feynman may be perceived as sloppy because he uses the terms longer waves and electric field instead of longer wavelengths and electromagnetic field.

Questions for discussion:

1. Electrical force: What happen if a positive charge carrier is placed considerably nearer to a large neutral object?

2. Electric field: What is the nature of electric field?

3. Electromagnetic waves: How do the behaviors of electromagnetic waves change from very low frequency of oscillation to very high frequency?

The moral of the lesson: an electric field is a condition in space that can produce an electrical force, whereas the electromagnetic field can carry electromagnetic waves.

References:

1. Feynman, R. P. (1965). The development of the space-time view of quantum electrodynamics. In Brown, L. M. (ed.), Selected papers of Richard Feynman. Singapore: World Scientific.

2. Feynman, R. P. (1994). No Ordinary Genius: The Illustrated Richard Feynman. New York: W. W. Norton & Company.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.