I've been reading through (and listening to) a collection of lectures form Richard Feynman's Lectures on Physics. In lecture #2, titled "Basic Physics", he makes the following statement:
Although the forces between two charged objects should go inversely as the square of the distance, it is found, when we shake a charge, that the influence extends very much farther out than we would guess at first sight. That is, the effect falls off more slowly than the inverse square.
This statement is in the context of a discussion of the attraction between objects with opposite [static] electrical charges. I am aware of the inverse square law as it applies to static electrical fields, but I was under the impression that the inverse square law also describes oscillating electrical fields, that is, electromagnetic radiation.
Assuming a simple point-source omnidirectional EM radiator, the amplitude of the outwardly propagating EM field (a.k.a. EM "wave") should also fall off according to inverse square law in free three-dimensional space, correct?
Thus, I do not understand his statement "the effect falls off more slowly than the inverse square". Did Fenyman simply misspeak or am I missing something (perhaps embarrassingly obvious) here?
The full text of the lecture in question can be found here. You can search for the phrase "more slowly than the inverse square" if you'd like to see the immediate context.
Answer
I am aware of the inverse square law as it applies to static electrical fields, but I was under the impression that the inverse square law also describes oscillating electrical fields, that is, electromagnetic radiation.
The inverse-square law applies to the intensity of electromagnetic waves that are propagating outwards. You can see this by drawing an imaginary sphere of radius $r$ around a point source of radiation. Since the power crossing that sphere is the same, regardless of the size of the sphere, it must be the case that the intensity of the waves (power per area) is proportional to $1/r^2$.
But the intensity $I$ of an electromagnetic wave is proportional to the square of its electric field amplitude $E$. Since $I \propto E^2$ and $I \propto 1/r^2$, we conclude that $E \propto 1/r$ for a point source of EM radiation.
(This result can also be proven by much more rigorous means, but this is a quick-and-dirty heuristic to see why this must be so.)
EDIT: In response to a couple of comments in the extended discussion (now moved to chat): It is true that Feynman never actually mentions the electric field in the above paragraph. However, he is talking about the force between two charged objects, and the force on the stationary charge will be equal to the amount of charge it carries times the electric field created by the oscillating charge. Since the electric field falls off as $1/r$, the force felt by the stationary charge will be proportional to $1/r$ as well, i.e., "the effect will fall off more slowly than the inverse square."
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