BOSTES HSC Physics 2015 Question 27

Question: Maxwell’s theory of electromagnetism explained the nature of light and predicted the existence of other electromagnetic waves. Explain how Hertz performed experiments that validate Maxwell’s theory.

Marking Guidelines

Criteria	Marks
• Describes how Hertz performed experiments to test Maxwell’s theory and his predictions. • Describe how Hertz’s experiments validated Maxwell’s theory.	6

(Source: https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-physics.pdf)

Comments: 

This question is about an understanding of Hertz’s experiments that validate Maxwell’s theory. Students are expected to describe how Hertz performed experiments to test Maxwell’s predictions. However, we could provide the following points: 

1. Oscillator: To produce electromagnetic waves, Hertz made an oscillator - an induction coil connected by two brass knobs - that generates sparks in an air gap between the two knobs under high voltages.

2. Detector: To detect electromagnetic waves, Hertz simply took a piece of copper wire, bent it into a circular shape, and created a short air gap between its two ends. 

3. A “surprising” phenomenon: When a spark is generated at the terminals of the induction coil, we could observe another spark in the air gap of the wire when the oscillator is reasonably close to the detector.

4. Theoretical explanation: According to Maxwell’s theory, electromagnetic waves are produced while the sparks are being generated. In addition, the electromagnetic waves propagate to the detector and set up oscillating electric and magnetic fields in the wire of the detector. (In other words, there is a resonance between the oscillator and the detector.)

5. Properties of electromagnetic waves: Hertz’s experiments show that electromagnetic waves have the following properties of light: 1. reflection; 2. refraction; 3. interference; 4. diffraction; 5. polarization.

6. Mathematical validation: Hertz was able to set up a stationary wave pattern by using a metal reflector, and determine the wavelength (λ) of the electromagnetic waves by measuring the distance between two nodes. Furthermore, by determining the oscillating frequency (f) of the electric current, Hertz calculated the speed of electromagnetic waves in air (v = fλ) which equals to the speed of light.

Essentially, Hertz developed the first primitive radio that can transmit electromagnetic waves. In a sense, Hertz’s experiments validate Maxwell’s equivalence of light and electromagnetic wave. Interestingly, Duhem, for example, proposed that Helmholtz’s electrodynamics could be another alternative to explain Hertz’s experiments (O’Rahilly, 1965). Thus, Hertz’s experiments do not validate Maxwell’s theory completely and conclusively. It is worth mentioning that Hertz intended to demonstrate that Maxwell’s theory is incorrect. In Hertz’s (1893) words, “I reflected that it would be quite as important to find out that electric force was propagated with an infinite velocity, and that Maxwell’s theory was false, as it would be, on the other hand, to prove that this theory was correct, provided only that the result arrived at should be definite and certain (p. 8).”

Feynman’s insights or goofs?:

Feynman has objections on the correctness of Maxwell’s theory. For instance, Feynman proposes that point charges interact only with other charges, but the interaction is related to both advanced and retarded waves. In addition, he explains that “[l]ight behaves like photons. It isn’t 100 percent like the Maxwell theory. So the electrodynamics theory has to be changed. We have already mentioned that it might be a waste of time to work so hard to straighten out the classical theory, because it could turn out that in quantum electrodynamics the difficulties will disappear or may be resolved in some other fashion. But the difficulties do not disappear in quantum electrodynamics (Feynman et al., 1964, section 28–5 Attempts to modify the Maxwell theory).” Essentially, light has both particle-like and wave-like characteristics. However, there are still difficulties in Maxwell’s theory even after modifications are made with quantum mechanics.

Feynman’s another objection is Maxwell’s ether. In an invited talk presented at the symposium The Past Decade in Particle Theory, Feynman (1970) elaborates that “in the case of ‘the ether never being found,’ it was ultimately realized that there wasn’t any ether at all, the ether was one of these scaffoldings to create a theory. It was later realized that the ether was an irrelevant complication and it may be that the partons are also nonexistent (p. 812).” In other words, Feynman believes that it is difficult to define the ether which cannot be detected conclusively. However, Einstein (1920) writes that, ‘[m]ore careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether (p. 13).” In a sense, the concept of the ether does not fade away, but it is currently expressed by the term, fields (Wilczek, 1999).

Note:

1. In an article titled The Persistence of Ether, Wilczek (1999) writes that “[h]ow did I provoke Feynman? I asked him, ‘Doesn’t it bother you that gravity seems to ignore all we have learned about the complications of the vacuum?’ To which he immediately responded, ‘I once thought I had solved that one. I had a slogan: "The vacuum is empty. It weighs nothing because there’s nothing there."’ It was then he got wistful. I was deeply impressed to realize that Feynman had been wrestling with the problem of the cosmological term already in the 1940s, long before it became a widespread obsession and frustration. You have to admit that his slogan is catchy. So just maybe, despite everything I’ve said up to this point, eventually we really may have to do without ether (p. 13).”

2. Wilczek (1999) writes that “they are all surface manifestations of a single more basic entity, the electron field, an ether that pervades all space and time uniformly (p. 13)”.

References:

1. Einstein, A. (1920). Ether and the Theory of Relativity. In A. Einstein (1922). Sidelights on Relativity. London: Methuen. 

2. Feynman, R. P. (1970). Partons. In R. P., Feynman & L. M., Brown (Eds.), Selected Papers of Richard Feynman: With Commentary (pp. 773-813). Singapore: World Scientific.

3. Feynman, R. P., Leighton, R. B., & Sands, M. L. (1964). The Feynman Lectures on Physics, Vol II: Mainly electromagnetism and matter. Reading, MA: Addison-Wesley. 

4. Hertz, H. (1893). Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity Through Space. London: Macmillan.

5. O’Rahilly, A. (1965). Electromagnetic Theory: A Critical Examination of Fundamentals, vol. 1. New York: Dover. 

6. Wilczek, F. (1999). The Persistence of Ether. Physics Today, 52(1), 11-13.